89 12.
F I E L D SURVEY PROCEDURES
12.1.
General General
Effective sampling of any surficial media requires well-trained personnel capabl e of recognizing and describing the correct sample material and the sample site characteristics. Samplers should be able to recognize and, if possible, avoid situatio ns where contamination from human activity or changes in the natural physicochemi cal ca l conditions can produce spurious or unusual results, in most situations, these sampli ng duties can be undertaken by trained technical personnel under the supervision of a so m geochemist or geologist with adequate geochemical eploration eperience. !n som e surveys "e.g. where identification of the correct sample material !s critical, as in biogeochemical or glacial till sampling programs#, it is prudent to employ qua lified specialists "e.g. botanists and $uaternary geologists# to both conduct orientation surveys and instruct and supervise the sampling teams. th e field situation. Sampling tools vary according to the medium and the %oncontaminating equipment is essential and care should be eercised in not only choosi ng fo r shovels, trowels, augers etc. but bu t also in ensuring that non-contaminating steels for any associated lubricants, adhesives, welds, and solders will not cause problems. &eaded gas can sometimes constitute a potential problem in field vehicles when samples are transported in proimity to leaking containers. 'his awareness of geochemical cleanliness etends to the dress of the sampler who wh o should avoid wearing metal buckles, rings, e tc. and handling coins which might lead to contamination by chipping or transfer of metal on fingers. 'he same caution is necessary in the choice of sample contai container ners. s.
(raf (raftt p
aper "with non-contaminating water-proof glue water-proof glue and closures#, olefin, and plastic bag containers of appropriate size are frequently used. (raft and olefin allow samples to be dried witho ut
transfer. )lastic bags are commonly used for fo r larger samples. *ore rigid polypropyle ne and special glass bottles can be utilized in water sampling and a variety of sam pling devices, many of them patented, are available for the sampling of gases and particulate s. !t is strongly advised that all samples be allocated simple unique sequ ential numbers which at least include a pro+ect "or regional office# designator prefi a nd a sample type designator suffi. 'hese are best provided by pre-numbered ssa s sa y eochemical Sample 'ag 'ag /ooks. 'he potential for error and misunderstanding is thereb y minimized and problems in subsequent data management and interpretation are avoided . Some form of coordinates should also be assigned to every sample in order to a ssist sample location and computer plotting of sample locations and analytical data. !n t he case of widely spaced regional reconnaissance samples "e.g. stream sediment# the 0niversal 'ransverse *ercator "0.'.*.# grid location of each site can be determined using topographic base maps of suitable scale or possibly, a locator instrument "e.g. *agellan# .
90
In more detailed studies tine U.T.M. grid c a n be b e used to define t h e area bo und un d ar i e s, whils t individual samples ar e located by reference to a local grid.
12.2. Sampl Sa mpl e Media Med ia of thee pote Some discussion of th of available geochemical sample media in the th e po tenti ntial al role of available b e e n prov exploration sequence h a s be pr ovid ided ed in prev pr evio ious us chapters. Media selection will o f course be b e decided on t he bas b as i s of orientation studies which will in turn be b e influenced by th e local environment a s well t h e nature of t h e exploration prob pr oble lem. m. Reiter Rei terati ating ng earli er statements concerning t h e applicability of t he more widely used sample medi a in reconnaissance studies, t h e methods used might include d r a i n a g e surveys sampling stream or la#e sediment, stream or la#e water, groundwater, etc.$ !ii" glacial deposit surveys sampling of till, etc.$ !iii" roc# surveys$
!i"
g
!iv" soil surveys this approach is be b e c o mi ng increasingly popu po pula lar r at samplin densities a s low a s % sample per pe r &' km�.
(ollow)up studies of prom pr omis isin ingg leads detected in t h e reconnaissance ph p h a s e might involve dia di a
!i" closer spaced sampling of o n e or more of t he above mentioned me and*or$ stream ba b a n # !residual soil or colluvium" surveys$ !ii" !iii" biogeoc biogeochemi hemical cal surveys$ soil +v" g a s surveys, or more rarely$ !v" geob ge obot otan anic ical al surveys$ !vi" part pa rtic icul ulat atee surveys, a n d !vii" microorganism microorganism surveys.
xotic techniques such a s surveys ba b a s e d on animal tissue sampling, are ar e curre ntly primarily primar ily of academic interest, a n d unli#ely to prov pr ovid idee solutions to actual pract pr act ical exploration pr oble ob le ms. ms .
12.2 2.2.1,
Rock Rockss
-eochemical exploration surveys ba b a s e d on sys te mat ic bed roc# ar e in ro c# sampl sa mpl ing are essence a n extension of routine pr o sp e ct i ng ba b a s e d on the th e collection a n d analysis of relatively small numbers of /specimens/ or roc# chip /samples/ from potent pot ential ially ly interesting t h e former be b e d r o c # exposures. 0o aim to achieve 0owe weve ver, r, consistently representativ e material, a n d ar e generally capable of detecting a nd interpreting far more s ubtle expressions of t he poss po ssib ible le pr p r e s e n c e of minerali1ation than t h e /character/ sampli ng ou t in pr normally carried out p r os pe c t in g . Unless exposure is exceptionally good, sample 91
spacing in geochemical rock surveys tends to be less consistent than that achieved in, in, say, soil surveys. s with other types of geochemical surveys, the sampling procedures and the sample material collected in geochemical rock surveys should be standardized as much as possible. 2owever, considering the large number of variables that can be introduced by the processes of weathering and oidation, the ideal of collecting similarly weathered material is sometimes impractical. %evertheless, the geologist or the geochemist ail sample sites are conducting the survey should ensure that individual samples at ail essentially comparable and that observed variations in weathering intensity are properly -ecorded for interpretation purposes. eochemica l rock roc k samplin sam plino o necessarily must take into account the geological environment and the type of mineral deposit of interest to the eplorer. 'he precise scale of sampling necessary for detection of svnaenetic and epiaenet epia enetic ic pattern patt ernss will be
determined by orientation surveys "see 4hapter 8#. 5etection of syngenetic patterns may necessitate the regional sampling of individual plutons or more detailed sampling of specific parts of an eposed stratigraphic section. 'he latter patterns will require a different approach. Surveys designed to detect leakage anomalies will focus on systematic sampling of fault or fracture zones and, possibly, bedding structures. !n contrast, the preferred geochemical rock sample material for the detection of diffusion naloes is likely to be unfractured and the scale of sampling much more detailed. !n all instances, analysis of geochemical rock survey material has the potential of delimiting dispersion patterns beyond visible alteration associated with mineralization. 'able 16.1 summarizes the elements determined and the sampling densities used in past eploration programs for a variety of mineralization types. good eample of a regional approach capable of discriminating between productive and barren intrusions is provided by the work of arrett "197#, which was based on whole rock analysis of samples from felsic intrusions in the ukon 'erritory, 4anada ":ig. 16.1#. 0sing a variety of techniques, including residual scores from a multivariate statistical analytical procedure "principal component analysis#, comparisons of metal concentrations ":ig. 16.6# and degree of skewness of frequency distributions, he was able to demonstrate that most plutons associated with mineralization could be recognized, and certain additional plutons with no known mineralization merited further ;nvestigation. !n strong contrast the work of 4hurch et al "197<# demonstrates the potential value of district scale geochemical rock sampling programs in the detection of vein and replacement deposits. 'heir case history study was carried out in an area of /ritish 4olumbia, 4anada which includes the *esozoic volcanic sequence hosted Sam oosly replacement "=# massive sulfide deposit, and the 0pper 4retaceous andesitic volcanic sequence hosted /randina vein type occurrences ":ig. 16.#. /oth types of mineralization are reflected by large s and somewhat more limited 4u anomalies ":ig. 16.>#.
Scale
@egional
'arget
identification of productive massive plutons sulphides vein and replacement
&ocal and *ine
porphyry massive sulphides
Elements???Sampling density
(, @b. Sr, /a, 0, %aA, 4aB :e. %a, *g, *n, %a, "(#, "4a#, "/a# s. Sb, 'a. /iB (. 4a, @b, Sr. *n, "*# :e, *n. %a, (.
e.g. 4u, )b, Cn, Sn. D. *o, 0. %i 4u. Cn. ")b#
min. intrusion but see ppendi , 0.2-5/krT?
e.g. 4u, )b, Zn u, g
l-1knF
4u. Cn. *o. S
6-kr'G
4u, )b. Cn. "S#
1H-6 rn !nterval
vein and replacetnent
4a, *g, "@b#. "Sr# (h�O).
H-1 m interval
e.g. 4u, )b, 6n. ii, g
'/&E 16.1 Summary of elements to be dstermined and surface sampling density for different targets in regional. and local and mine scale eploration. Elements in parentheses have been shown to be useful in some cases but have uncertain statusI elements with asterisk are epected to be useful but there are little data. Si should be determined in all cases where petrological variation is epected to cause variation to the content >< >!J of other elements. "ovett,4��198#C�4a HP96i
9
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134=
130=
FIG 12.1 4retaceous granitoids in the 4anadian 4ordillera sampied by arrett, 197.
/lack circles are granitoids containing mineralization of the following typeN 6 O g-)bN O 4u-SbI >OuI H O u-)bI
• $ —jhost -=irl0I:a �rocks &(Up.! "taeo#s & � � •• • • ) •* • - � •raliatio+ � • .... e (ocatio+ !
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5istribution of mean Cn content in granitoids in northwest 'erritories "i*.D.'.# and ukon 'erritory ".'.#, 4anada. "ovett, 198#
of roc# s a m p l e s , -oosly)+wen 4a#e area, 5ritish 6olumbia,
6anada,
!-ovett, %278"
?@
12.9
>istribution of ?s and 6u in roc#s 6olumbia, 6anada. !-ovett, %278"
around
t h e 5radina
and Sam
- o o s i y deposits, 5ritish
9H dditional indications of some possible roles rock geochemistry might play in eploration are provided by the discussion in 4hapter 8 of the large primary haloes associated with various types of mineralization. *any of these haloes should be readily detectable by systematic geochemical rock sampling, if there is sufficient outcrop. 'he hot spring-type gold mineralization "i.e. @ound *ountain - :igure 8.11# and the sediment hosted fine disseminated gold deposit ")inson - :igure 8.16# eamples are of particular relevance in terms of current eploration priorities. 'he etensive nature of the geochemical haloes commonly associated with sediment hosted fine disseminated gold deposits are also illustrated by data from the 4arlin 5istrict. %evada, which was obtained by Evans and )eterson "198<# in the course of a routine geological mapping program ":igs. 16.Ha, 16.Hb and 16.Hc#. 'hey provide further evidence of the potential value of systematic geochemical rock surveys in eploration for this type of mineralization. n interesting review of the application of bedrock geochemistry in mineral eploration is provided by ovett "1989#. more recent discussion of the sub+ect by :ranklin and 5uke "1991# is primarily concerned with 4anada, but their conclusions regarding the need for close geological control and the potential value of parallel mineralogical studies are of universal relevance. 5espite the potential advantages of rocks as sample media in many types of geochemical eploration program, their use is often precluded by a lack of sufficient eposure andor a need for composite samples representing substantial areas. 2ence
attention must frequently be concentrated on their surficlal derivative products "e.g. soils, stream sediments, etc.#. C
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98 12.2.2.
>E+SIT
@eoloCical DeCe+(j
S%&l'
Soils vary considerably in composition and appearance according to their genetic,
climatic, and geograpiiic environment. 4lassified into residual and transported types according to their relationship to their substrate, soils are mitures of mineral and biologic matter and may be distinctively differentiated into a series of soil horizons. Soils are most often sampled alona traverses or grids in the follow-up or detaile d prospecting stages of geochemical programs. !n rugged terrain initial follow-up surveys of reconnaissance stream sediment anomalies is sometimes most readily achieved b y sampling soils along ridge and spur ":ig. 12.6() andor base of slope ":ig. 12.*(
traverses, in recent years increasing attention has been given to low densitv s oil sampling (i.e.�1 sample per km�) in geochemical reconnaissance survevs and aeochemica l mapping. s has been previously stressed, orientation programs define criteria such as sample depth or soil horizon to be sampled, sample interval, and the size-fraction f or analysis. !t is essential that these criteria be observed resolutely through the survey. @esidua l soils characteristically contain detectable dispersion patterns developed during the weathering of mineralization in the underlying bedrock, and these patterns are revealed by careful sampling of appropriate soil horizons. s might be epected, in vie w of the size of the deposits and associated primary geochemical haloes, near surfa ce sediment hosted fine disseminated gold deposits in semi-arid areas, such as %evada, are commonly reflected by etensive geochemical anomalies in the immature residual soils. !t is therefore not surprising that geochemical soil "generally 4 - horizon# sampling h as assisted in the discovery of a number of these deposits "e.g. lligator @idge, Qe rritt 4anyon, etc.#. Some indication of the size and nature of soil anomalies which might be epected in the vicinity of such mineralization is provided by /agby, et al., +1,84( i n a study of soils over the 5ee 5eposit, %evada. 'he minus 80 mesh sieved fraction of 1 ",
4 horizon soil samples were analyzed for a number of elements including u, s, Sb, g , and 2g. 'he resultant geochemical data display anomalous patterns over and in th e immediate vicinity of the known AshallowA +-100 feet - 40 m# and AdeepA +-300 feet - 1 20
m# ore zones ":igs. 12.8 and 12.,(. 'hese could be readily detected in ro utine geochemical soil surveys based on, say, a 30 m grid.
5eeply weathered residua l soils can also provide useful geochemical sampling media. n eample of use of lateritic soils as a regional geochemical reconnaissanc e sample medium is provided by &ewis et al +1,8,(. &ateritic soils were sampled on a 4 00
m grid in a 0% eploration reconnaissance program for rchean metavolcanics hoste d massive sulfide "i.e. R*S# mineralization in the Dest frican nation of /urkina :a so. :ollow-up soil sampling on a 2" m grid spacing of small weak anomalies detected in t he reconnaissance phase ":ig. 12.10 ( confirmed the eistence of a dist inct ""0 by 2"0 m 6n $$$3 •O • • anomaly "!.e. -200 ppm# ":ig. � • 12.11(. Subsequent drilling result j
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0eavy)metal content of colluvium !ppm" 9
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BGample o! ridCe-a+d-sp#r soii-sampli+C patter+, eH# /roject, Rep#Hlic o! /hilipI pi+es. ata o+ -SJ-mesh !ractio+. (Rose. aKiLes, M NeHH, 1 9%9O
1
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:! 16.8 Spatial variation of anomalous soil samples. Symbols represent the histogram groups. Jutlined samples are those that have element concentrations in the upper two histogram groups and are considered anomalous for this sample population. 5eep ore O N shallow ore [[[[ and . "/agby et al., 198>#
11
FIG 12.,
%D&
# Jutlines of areas with samples that contain anomalous values. 'he large outlined area contains samples that are anomalous for more than one element, whereas the two smaller areas contain samples that are anomalous in only one element. Jre zones are shown as in :igure 16.8. /# Jutlines of the anomalous areas from :igure 16.9a with the addition of smaller anomalies defined by areas containing three or more samples with concentrations in the uppermost histogram group. "/agby et al, 198>#
(I- %&.%D Regional soil geochemistry !On" in th e area around th e Eer#oa >eposit, 5ur#ino (aso. Jalues in ppm. !4ewis, et al.. %272"
(I- %&.%% >etailed geochemistry !On" in the vicinity of the Eer#oa >eposit, 5urP #ino (aso. Jalues in pp m. !4ewis, ei al., %272"
%D8
-eochemical soil sampling can also constitute a useful technique in goQd exploration over lateritic terrain. The potential effectiveness of this approach is illustrated by a recent study of surface pisolitic laterites over th e Saddlebac# -reenstone 5elt, in Gestern ?ustralia !Smith, %272". The data reveals that the large !3' million tonnes at %.7 g* t ?u" 5oddingion gold deposit, which comprises extensive laterite, saprolite and supergene resen*es over a primary volcanogenic massive sulfide source, displays a surface chalcophile element halo measuring some 8D #m by 3 #m !(ig. %&.%&b". In contrast the surface gold halo extends over an area of s o m e 8 #m by % #m !(ig. %&.%&a". Th e large si1e of the chalcophile element anomaly in the surface laterite is thought to reflect the retention of ?s, Sb, 5i, etc. in the (e)oxyhydoxides and Sn and G in resistant minerals during th e laterite soil profile development, and subsequent lateral mechanical dispersion !(ig. %&.%8". !Thus th e soils ar e no t completely residual, in th e strictest sense." The /mushroom)form/ of th e 1one of gold concentration in the subsurface saprolites !(ig. %&.%8" is thought to reflect leaching and supergene enrichment during post)laterito modification of th e weathering profile. 5oth low density !i.e. N % per km�) reconnaissance and higher density follow)up geochemical soil sampling techniques could obviously play useful roles in exploration for 5oddington)type gold occurrences. The 5oddington example also demonstrates th e need for care when using residual soils as geochemical exploration sample media. 0ydromorphic dispersion can sometimes produce epigenetic soil anomalies located some distance from the bedroc# source. ? similar /soil/ sampling method developed in recent years for deeply weathered semi)arid areas with long weathering histories, utili1es th e surface residual concentrations of hard roc# fragments !generally siliceous a n d * o r ferruginous" which remain after most of th e fines have been blown away. In ?ustralia this sample media is #nown as /lag/ !6arver, et al., %27", and has been successfully used in exploration for gold and b ase metal sulfides. (igure %&.%3 illustrates the far larger si1e of a gold anomaly defined by /lag/ samples from a 3DDx'D m reconnaissance grid, compared to that displayed by follow)up bul# soil samples from a %DDx&D m grid in the astern -oldfields Erovince of Gestern ?ustralia. Some success in comparable terrain !?ustralia and 5otswana" has also been claimed !(arrell, %273" on bo th regional and local scales for a geochemical
exploration method b a sed on the heavy mineral concentrate fraction of soils !i.e. /loam/ concentrates". Trans por ted soi ls pr ese nt especially difficult sampling pro blems , bu t meaningful surveys ar e possible in many areas once th e genetic origins of the transported cover ar e nderstood. in glaciated areas, for example, soils derived from glacial dispersion trains can present far larger targets than the suboutcropping source minerali1ation. ?t 5uchans, Aewfou nd land , extensive soil anomalies, overlying tills, reflect glacial dispersion trains which extend for s o m e miles /down)ice/ !i.e. southwestward" from suboutcropping volcanogenic massive sulfide minerali1ations !(ig. %&.%'". ?nomalous /hot)spots/ reflect ocally enriched !or bet ter exposed" po rt io ns of the dispersion train which ar e often far 1>
ustralia, showing dispersion patterns about tine /oddington u deposit. "Smitii, 1989#
:! 16.1
r
5iagrammatic cross-section depict-ing retention of chalcopliile elements such as s, Sb, /i, !n, *o, and perhaps e in the :e-oyhydroides and Sn and D in resistant minerals in tateritic duricrust, whereas u has undergone ieaching and supergene enrichment during post-laterite modification of the weathering profile. "Smith, 1989#
1H
&S >E
<E i
SJ!&S >E
<E
1 km H%
-Z >9%
U H ppb u i0 H- &
I
\ -8H /
FIG 12.14
X8H
[ ! U H pp b u
m H -> >-7
X 7
4omparison of u anomalies in -< Z 6 mm lags and -< mm soils, Eastern oldfields )rovince, Destern ustralia. "4arver et al., 1987#
G X 1
]
6!%4 !% SJ!&S 1 ppii [ + /ft4(SfiJ0%4 anomalous S4J /
$
;
MODERATE-STO!"k-#$
kJ'gN !lVc 5!' @E&'E 'J *!20S
1<
from their bedrock source. !n this type of situation, unless the nature of the surficial environment is fully appreciated, time and effort could be wasted in fruitless searches for mineralized bedrock sources in th e immediate vicinity of many of the anomaiy ApeaksA. !n some areas with barren eotic overburden the soi l humus horizon "alternatively known as mull. o, or h material# constitutes an effective geochemical sampling medium. s discussed below in th e section dealing with geochemical surveys based on vegetation sampling, the root penetration of plants sometimes eceeds the thickness of barren cover and obtains nutrients from underlying mineralized bedrock andor anomalous ground water. !n glaciated areas, deep rooted plants can sometimes obtain nutrients from anomalous till dispersion trains "related to nearby suboutcropping mineralization# which are obscured by barren oveburden. 'he seasonal fall of leaves an d needles transfers some of th e accumulated metals to the surface soil where they are incorporated in the humus. n early demonstration of the potential effectiveness of mull sampling in gold eploration was provided by 4urtin et al. "1971# in a research study of the Empire mining district, 4olorado. u distribution patterns in mull ":ig. 16.1<# were found to more dearly reflect glacial sediment covered gold bearing quartzsulfide veins than the u distribution patterns for the 4 horizon soils ":ig. 16.17#. *ore recently, considerable attention has been given to the use of humus as a geochemical sample medium in the 4anadian Shield. leeson and Sheehan "1987# report an absence of humus response around the 5oyon gold deposit in $uebec )rovince, presumably due to the presence of 1 m of relatively impermeable glaciolacustrine clay and silt that overlies the 1 m to 6 m of anomalous, locally derived till. t th e Dilliams property, in the 2emlo district, generally poor response is reported fo r A/A horizon soils due to the presence of several meters of eotic calcareous till. 2owever, humus samples show well defined gold anomalies ":ig. 12.18( over gold occurrences and associated anomalous glacial dispersion trains ":ig. 12.1,(. 'hey conclude that in the 2emlo district humus sampling is an effective geochemical eploration medium over terrain underlain by up to H m of permeable overburden "eotic or otherwise#, whilst A/A horizon sampling should be confined to areas where the till cover is thin or absent "i.e. U1 m#. ttention is drawn to a useful check list for the organization of soil surveys "'able 16-6# which also has some relevance to other types of sample media. 12.2.3.
Srea/ Se&/en'
Stream sediment is one of the more commonly used media fo r regiona l geochem ica l surveys. 'he sediment at any point in a stream is a natural composite sample of erosional materials from upstream in th e drainage basin and can include clastic, anomalous
hydromorphic, and biogenic contributions from any weathering mineralization present. 'he length of anomalous dispersion trains will vary with the nature of the mineralization, source, and the physicochemical environment of the field area or drainage basin. !n base metal humid, actively oidizing environments, dispersion trains from sulfide-rich * ? g deposits may etend downstream for some miles. [
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:!E&5 )@' numbers, composition, eperience, leader '@!%!% when, where, by whom /SE *)S appropriate scale, topography, etc. %0*/E@!% S42E*ES simple unambiguous, avoid comple alphanumerics make sure they are taken correctly :!E&5 %J'ES collect field duplicate samples and insert, with standards, in batch submitted to laboratory 4J%'@J& $0&!' must tXe simple and direct. Jnly designated personnel should actually give instructions to the 4J**0%!4'!J%S D!'2 &/J@'J@ laboratory. must accompany every consignment sent to the laboratory S2!))!% &!S'S !%S'@04'!J%S give clear unambiguous instructions to the laboratory @E'0@% J: 5' check duplicates, standards, etc. for quality of analytical data, request reanalysis when in doubt manual or computer aided. Dhat procedures are best for your pro+ect= 5' 2%5&!% !%'E@)@E''!J% *)S prepared 4o summarize geochemical features !%'E@'!J% J: :!E&5 used to qualify interpretation of geochemical data %J'ES need to be able to retrieve for reinterpretation S'J@E J: 5' @42! RE J: S*)&ES at laboratory, office or warehouse !%'E@'!J% D!'2 ensure good communication with management and other pro+ect personnel J'2E@ EY)&J@'!J% )@J4E50@ES author of report m!t be familiar with field program @E)J@'!%
'/&E 16.6 4hecklist for the organization of a geochenriical soil survey.
"'homson, 1987#
s has been recently pointed out ")lant, et al., 1989# stream sediment samples fall into two broad categoriesN "i# "ii#
representative samplesI samples designed to enhance patterns or anomalies related to specific mineral deposit types.
@epresentative samples are the basis of most regiona l geochemica l mappina programs conducted by national survey organizations, as well as some regional geochemical eploration surveys undertaken by mining companies. 'hey commonly use active stream sediment "i.e. material constantly or frequently washed by stream waters# that is most representative of catchment erosion products, including petrogenic elements, in a wide variety of climatically influenced weathering environments. !n most of these survey programs, approimately H to 1 g of fine-grained material is collected from the upper few inches of the sediment near the center of a drainage, avoiding sites that may be contaminated or influenced by bank collapse. !n most situations samples are best collected with the aid of a "non-contaminating# steel shovel or plastic scoop, /allantyne "1991# recommends use of the latter in flowing streams as the scoop walls help minimize loss of fines.
11
@!RE@
:!. 16.6a
@!RE@)&!%
Dater discharge of a river under ordinary conditions with normal amounts of water. Jtteson et, al., 1989#
FIG 12.20 Dater discharge of a river during a ma+or flood. JverbaniU
sedimentation takes place on the river plain. "Jtteson et. ai., 1989#
Dhere active stream sediment is unrepresentative due to localized nature of current fluvial erosion ":igs. 16.6 and 16.61#, as in many parts of Scandinavia and else-where, vertical com posi te sam ples of AoverbankA "i.e. levee or flood plain# materia have proven more effective "Jtteson, et al., 1989#. 'hese are derived from many episodes of flood sedimentation and are therefore far more likely to constitute a representative sample of the whole catchment than regular active sediment samples. voidance of more recent "i.e. near surface# sediment in and around industrialized areas helps minimize possible effects from industrial contamination. 111
:!. 16.61
d i a g r a m m a t i c depiction of ho w geochemical dispersa patterns for ac ti ve s tr ea m s ed im en t a nd o ve rb an k sediment ma y be influenced b y mineralization and sediment sources. !n tine stream on the right hand side, the active stream sediment is d o m i n a t e d b y sediment source %o . 1, a reason why the anomaly can be detected only in the overbank sediment, in the middle river, where no active sediment sources eist in the upper part, a stream-sediment a n o m a l y ha s developed w he re t he stream crosses the mineralization due to influence from paieo-sources and a presently small, diffuse sediment production occurring along th e stream bed. 'his anomaly !s diluted by sediments from source . "Jtteson et al., 1989#
@egional geochemical mapping programs b ase d on representative stream sediment samples generally cover areas of thousands or even tens of thousands of square miles. ma+ority of these surveys employ sampling densities greater than 1 per H km�& although th e recent %ordkallott )ro+ect in %orthern Scandinavia used a sa mple density of 1 sample per km�. Even lower density sampling "i.e. 1 sample per H km�) was applied in a recent geochemical mapping survey ":ig. 16.66# of the whole of %orw ay based on AoverbankA sampling "Jtteson, et al., 1989#. s mentioned above, representative stream sediments are frequently also used a s geochemical sample media in regional rec onn ais san ce ep lor ati on pro gra m s. 'he results of a fairly smail but successful survey based on this medium are described by Debs ter
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11
and Skey, 1979#. eochemicai analysis of stream sediment samples collected at a density of to H am*#e+km� over an area of 4ambrian volcanic rocks in northwestern 'asmania ":ig. 16.6#, resulted in the discovery of the $ue @iver massive )bCn sulfide deposit. :requently, anomalous response from target mineralization types can be enhanced by sub+ecting stream sediment samples to selective analytical methods, as in the /ulk &each Etractable old f/&E# or /ulk 4vanide &each "#C$ technique which has been used etensively in ustralian gold eploration in recent years "Elliott and
'owsey, 1989 1989#. #. &arge &arge "often H-1 kg# samples of active stream sediment "minus the coarser fractions# are are eposed to a weak cyanide solution which leaches out accessible of the leachate can then be analyzed. 'he method is etremely gold. 'he gold content of the sensitive and helps minimize Anugget effectsA. Jbviously it can only be used where gold is freely accessible "i.e. in fine particulate form, eposed on mineral surfaces, etc.# io the are free leachate, and is most effective in deeply weathered areas and in samples which are of refractory materials and the gold is not occluded. eochemicai eploration efficiency can in many cases be increased by collection and analysis of specific fractions of active stream sediments or even alternative drainage sediment components which, in certain circumstances, display more distinct and more consistent !ndications of the presence of target mineral deposits than do representative of the better eamples of this of this type of samples of active stream sediments. )robably one of the concentr ate s. s. 'hese improve contrast for approach is provided by heavy he avy mine mi nera ra l concentrate elements such as tungsten "e.g. 'uriel, et al, al, 1987#, barium "4oats, et al., 1981# and gold ":letcher, 198H and *auhce, 1991# when they are held in resFstate mineral phases. 'hey are are sometimes also useful in lateritic terrain where elements of interest are are held in iron oides. !n some areas "e.g. southeastern 0.S..# selective analysis of manoanese of manoanese and iron hydroide hydr oide coatings coatin gs on stream sediment particles "boulders down to fines# is an effective method of detecting hydromorphically dispersed ore and pathfinder elements ":igs. 16.6>a and 16.6>b# which have been adsorbed and concentrated by these coatings "4hao and 'heobald, 197
#. 2owever, interpretation of the resultant data can sometimes be difficult. 'he ultra fine sieved fractions "e.g. minus 6 6 mesh - minus 7H microns# of stream sediments have been shown to be effective geochemicai eploration sample medium in some arid and semi-arid environments. Jbserved advantages in both base metal in ustralia "*azzuchelli, 198I /eeson, 198>#, and gold eploration in %evada eploration in ustralia "*ehrtens, pers. comm. 198<# include more etensive and consistent anomalous dispersion trains than those provided by other sample media such as heavy mineral concentrates and the coarser sieved coarser sieved sediment fractions. !n contrast *oeskops and Dhite -18 mesh "Z.H to -1. mm# sieved coarse fraction to be "198# found the ZH to -18 ustralia, whilst Ceegersespecially effective in a base metal eploration program in South ustralia, %%3
"e,-�e+T##-,# Re^u_s in p p m
) 20� 06I+3 digestion ? ?S ?nolysts
Filometres
(I - %&.&8
scale
-eochemical results for the %2D)% Tasmania. !Gebster and S#ey, %22"
Y
mile
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stream sediment survey.
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mine area
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@ue River Erospect,
.r
FIG
12.24a
FIG
12.24
11<
fo r *agruder *ine area, eorgia. Sample location map ma p for "*eyer et a.. 1979#
5ownstream dispersion from zinc, copper, and lead in minus-8-mesh stream sediments and oide coatings. *agruder *ine area. "*eyer et al., 1979#
et al. "198H# recommended use us e of the th e Z 6H mesh "Z<6 micron# fraction in desert areas to avoid problems with dilution by fine eolian sand. $roa $r oani nicc drai dr aina nage ge samp sa mple less have b e en u s e d in northern Scandinavia an d elsewhere d u e t o lack ":ig. 16.6H# and of normal sedimentary material fo r long distances in stream channels "&arsson, th e samples 197<#. !n Scandinavia the compri comprise se organ organic ic debris in various humif ific icat atio ion n and and often ften stages of hum th e living ro roots of pene penetr trat ated ed by the various bo g plant species. Elsewhere other potential drainage sample media h a ve al s o b e e n e a m i n e d . :or eample, aquatic mosses were studied an d *odreski "198># to by Erdman and determine whether they might provide effective geochemical sample media in
ments in the Rehkavaara district. "&arsson, 197<#
area,
)a+ala
areas where steep terrain prevented accumulation of stream sediment fine fractions. 'his clearly constitutes a biogeochemical eploration method and is therefore described in more detail in Section 16.6.<. 2owever, an interesting variant of the aquatic moss biogeochemical technique was described by Smith "197<# following a limited study of mineralized areas in %orway. *oss-trapped stream sediment material was found to provide similar but higher contrast geochemical patterns to those produced by normal stream sediment samples. !n the regional reconnaissance prospecting mode, stream sediment surveys can be designed to systematically cover areas up to several thousand square miles. ver age sampling densities tend to be significantly higher than those employed in geochemical mapping programs as the emphasis is on detection of dispersion trains related t o individual mineral districts andor deposits, rather than broad mineral provinces. 5ensities are frequently in the range 1 sample per 1- km�& whilst 1 sample per 6 km �
would be considered unusually low. s
has
been
discussed
previously,
in
all surveys
in
new areas,
the critic
al parameters of sample interval, sediment size fraction, appropriate analytical procedures, significant anomaly contrasts, and background levels are determined through orientation surveys. !n areas where no previous eperience eists, a short interval of 1H ft. "H m# over an initial downstream distance of 1H ft. "H m# is recommended. 'his interval should then be progressively epanded with distance from the metal source to the limits of the known or anticipated dispersion pattern. Samples must also be collected from nonreconnais but also within the :ennoscandian Shi eld sance mineralized areas to establisii the technique, and the 4ordilleran and background range and sufficient material particularl ppalachian should be collected at each site to allow y regions of %orth merica. 'he ide for the determination of optimum size within th al e 4anadi terrain for this fractions, analytical techniques, and other technique is where l factors listed in 'ables 11.1, and 11.>#. an )reca akes mbrian S are common, conditions are swa 16.6.>. LaJe Se&/en' hield, mpy, andor where stream drainages &ake sediment sampling has been are into an effective developed geochemical
inaccessible or poorly developed "4oker goWn et al., 19 u r+lfiiuni 79#. !n low relief regions T X1EEE H1 - 1EEE e , the lak 11 - HEE W H1 - 1EE sediment medium is d 11 ` SJ tc S ` ependen % -< " t on the ?? 0-f ! deposits hydromorphic dispersion of metals into the lake environment through ground waters and the adsorption of this metal onto hydrous oides and the organic rich muds
117
m
1-cm Eye f or attaching line 'hreads fo r attaching rigid rods Jutlet vent for water forced through valve /all-and-socket valve
;H m
Sharpened end of tube fo r cutting sample
:! 16.6<
4ut-away section of sample bailer for lake-sediment sampling. "@ose et al, 1979#
"i.e. gytia# being deposited on the lake bottoms. 'he sampling generally focuses on the collection of these organic muds using specially designed sampling devices :ig. 16.6<#. !n more mountainous areas, fine grained clastic dispersion into the lake sediment becomes a more important factor. !n most areas satisfactory sample locations are found well away from lake shores and are reached using boats, float planes or helicopters. 2owever, near shore materials have been successfully used in some programs in the northern part of the 4anadian Shield although these are generally subaqueous equivalents of glacial and postglacial sediments on the margins of lakes and not true lake sediments. &ake water samples see Section 16.6.7.# are commonly collected at the same sites as the lake sediments. 'he lake sediment technique has successfully indicated the presence of several important forms of mineralization as the following eamples clearly demonstrate. !n Saskatchewan the (ey &ake and @abbit &ake uranium mineralizations and associated anomalous glacial dispersion trains are reflected by etensive lake sediment anomalies ":igs. 16.67 and 16.68#. Equally impressive anomalies are found in the vicinity of the
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12.2*
5istribution of 0 in lake sediments in the vicinity of the (ey &ake 0-%i deposit, Saskatchewan. "4oker et a., 1979#
FIG
12.28
0ranium "ppm# in lake sediments near the @abbit l?ake uranium deposit. Saskatchewan. &ocation of deposit shown by solid triangle. "4oker et al., 1979#
application su lf ide grFcola m as si ve deposit, %orthwest 'erritories ":ig, 16.69#, and in fact assisted in its original discovery "4oker, 1979#. *ore recently a number of authors have reported on the
of lake sediment
geochemistry to gold eplora tion. *c4onnell and 5avenport "1989# carried out etensive in orientation studies %ewfoundland based on the geochemical analysis of organic sediment collected from lake centers. !t was determined that
119
most, but not all known u occurrences were distinguished *E'RJ&4%!4S
;
by anomalous u concentra tions in nearby take sediments ":igs. 16. and 16.1#. )athfinder elements "Sb, s, )b, 4u and Cn# display inconsistent :! relationships to gold mineraliza tion and it was concluded that
18-A � 7 n %esrshore
[
l8iWNe *E'SE5!*E%'S .
sediments
.9E
.@,%!'ES `eological
�
boundary
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16.69
5istribution of Cn "ppm# in nearshore lake bottom materials. grFcola &ake area, %.D'. "4oker et a, 1979#
u is the only universal indicator. 'hey suggest that for detailed eploration a sampling density of at leas t 1 sample per >-H km� is necessary. 0seful reviews
of the
application
neral eploration in 4anad riske "1991#. 16.6.H.
of lake sediment geochemistry
in
mi
a are provided by 2ornbrook "1989# and :
Gla5&al Se&/en' T
Etensive $uat ernary glacial deposits occurring over mos / t of 4anada and th e northern 0nited States, northern Europe, northern sia, eenland, and a number of hi gh elevation areas in the southern hemisphere have presented ma+or challenge s to eploration. s a better understanding of the origin and formation of these gl acial sediments has grown, their blanketing presence has become progressively less formidable and effective eploration techniques have been developed. XM
G - We
*ineralized boulde r tracing in glaciated regions is an established technique of th e traditional prospector in Scandinavia and parts of 4anada. !n Scandinavia, dogs h ave been trained to assist the prospector by sensing SJg released from oidizing sul fide boulders at shallow depths below the surface. !n :inland, methods were developed for 16
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g@)JR(Ni% ! fo J/ "# a r$taftte %& ffr$ntfota ro'*f 6 otiti' se/tfft (f*o é&sf)ttufinr C a ,m< OI�OOviCian 1 long sngt ntM/r' - +ttrn&sf, Co&pf%#
'n
old in lake sediment. 4ape @ay :ault area. "*c4onnell and 5avenport. 1989#
FIG 12.30
C:�;C4�++S8/ O�recfVfT fA fOC�f. A
iJAJJ X1
%ewfoundland.
1
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fault
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to
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t
CA�aO!I=E7O>S � 5WWr &ak 4oup.- eistitc W e F m t n i F y rocki 5ERJ%!% 6 1 1 = o aod"XsaW7 ac h 99 f 7 S!! &*W !nin+fthv Su(JN /7877�. fonaui�. +aFQfirffl S!&0@!% < Soos r m 4roup- seif.irft#r0f fo'*1 ie2!3i' an a tiiifi't m�iic voMcviMc rociks CAMeROO�DO?iCIA! H SJ*lfXBrn Dhtiv /a y tlochlhon ionBht9 §/ 9� sc ft is t. m�tagré>fw»c»tm. méi�nq* , 4i,XnWv r m 4roui �7@4i'7A,/B477 BUiUt cU>iti> ro'i )@E4*/@!% 0�?' Wn"!e S 5i :rerXcn-4ii(l /%/7,%�Oii'. 6. 6t$nn# 1 +nvn+boM(p.. p>^]iro ��ci09¡ca t&8naéf9f
r .A S.U 7f ; ,
9E 9W
:t;it
'l
Gta'i1i-tion 00re'rro/i
old in lake sediment, Dhite "*c4onnell and 5avenport. 1989#
vX[
'3
-
/ay area. %ewfoundland.
161
:! 16.6
reverse Simplified sketch of the circulation drilling system "'hompson, 1979#
sampling tills in the 19HMs, an d this technique is now the preferred sampling method in iiiost :innish geochemical eploration programs. Esker sampling and till sampling for distinctive heavy mineral suites have been used for kimberlite and diamond prospectin g &r the 4anadian Shield. pproimately 7 of lodge men t till is locally derived,
and most of th e e
arly m+ccess with till sampling wa s in areas of shallow till cover "less than ft or 1 m# wh ere i'e sample medium is reasonably accessible. !n th e 19<Ms, lightweight percussion drill s (+ch as th e )ion+ar and 4obra models, were adapted to collect small samples of till fro m i r m e d i a t e i y above the suboutcropping bedrock to geochemically categorize anomaQous geophysical features at depths of up to 7 to 8 ft "6 to 6H m# "leeson et al., 1 971#. Jverburden drilling technology, particularly reverse circulation ":ig. 16.6# and son ic T�'ifing, advanced rapidly with the utilization of larger drills in programs for uranium, ba so metal and gold deposits in glaciated areas. /ecause most types of geophysical methods, lodgement ft has been routinely used poo m# he teily 198Ms. !n these programs are !
166
gold deposits are not detectable by conventional till sampling using overburden drills to depths of in prospecting fo r gold in the 4anadian Shield since t large samples of till "approimately 6 lb or 1 kg #
LOGIUDIML
SECIO M
FIG 12.33
N
C
"
!dealized geochemical dispersion modei for lodgement till, "*iller, 198>#
generally recovered and the heavy mineral fraction is separated and eamined both visually and chemically for gold and other metals. !t !s essential that the whole of the lodgement till section is routinely sampled as indicator trains tend to rise down-!ce along smear or thrust planes within individual till formations as shown in :igure 16.. Even in this simple eample, significant parts of the dispersion train will not necessarily b e detected if attention is restricted to till immediately ad+acent to bedrock. dequate sampling becomes even more critical when there are several lodgement tills related to distinct glacial episodes in an area with pronounced bedrock topography ":ig. 16.>#. !n the hypothetical eample shown in the figure three mineral deposits suboutcrop beneath lodgement tills which have been sampled by si vertical drill holes. Samples of till from hole 1 will not contain anomalous indications because the site is up-ice of mineralization. 2ole 6 will contain anomalous material related to *ineralization in AbasalA till "6#. 2ole will contain a strong anomaly in AbasalA till "1# related to *ineralization / and a weaker, distal anomaly in an AupperA till "6# related to *ineralization . 2ole >, because of the effect of bedrock topography and the constriction of till deposition and possibly accelerated erosion of earlier till over the bedrock escarpment, will contain no anomalous material related to either *ineralizations or /. 2ole H will contain anomalous material related to *ineralization 4 in its AbasalA till "># but hole < with
!4E *JRE*E%' 6
>
H
@EDJ@(E5 '!&&S. 2J@!CJ%S E'4. 1,6-. JRE@/0@5E%
&
2.34
16>
in
r&0R!J4l4MQ*.
5@!&&
2J&E SF'ES
Onar&%. D&a>ra//a&5 %erBren r%&&e' &n l&e M&&& 5laA el)
no lodgement till will not provide material suitable for sampling. 'he correct interpretaticr of till data is obviously dependent on a thorough understanding of local glacial sedimen stratigraphy and provenance. 'he technique is epensive, with combined drilling, am*� treatment, and analytical costs ranging from 6 to per foot "<<-99 per m#, but N is cost effective in deep overburden covered environments where other eploratior methods have not been as successful, especially in gold eploration.
0p to now the ma+ority of the glacial overburden drilling programs in %orth merica have primarily relied on geochemical analysis of the heavy mineral concentrate fraction of overburden samples. Several significant gold deposits have been discovered in 4anada by this method. :or eample, in the 4asa /erardi area of $uebec the technique was successfully used in follow-up of favorable stratigraphic zones delineated by geophysica methods "Sauerbrei, et al.. 1987#.!nitially, orientation studies of glacial overburder overlying a portion of this stratigraphic sequence known to contain pyrite-arsenopyritenative u bearing quartz-carbonate vein mineralization "olden )ond deposit# had enabled determination of optimum geochemical procedures for the district. 'argets were first tested with overburden drill holes located 6H-1 m down-ice at -> m intervals along strike ":ig. 16.Ha#. 4loser spaced overburden drilling was used to further define anomalous dispersion trains prior to diamond drilling of bedrock ":ig. 16.Hb# which resulted in the discovery of the olden )ond East zone. 'he gold content of heavy mineral concentrate samples from the overburden were determined both visually and by analysis. 'he samples were also analyzed for s and Sb. !t should be noted that in this particular area glacial dispersion of gold is only of the order of 6 to > m. 'he degree of compleity sometimes observed in till stratigraphy and glacial dispersion, which was previously alluded too, is clearly shown by the work of /ird and 4oker "1987# in the vicinity of the Jwl 4reek gold mine, 'immins, Jntario. 'hey defined four distinct glacial episodes each with different ice movement directions. !n the lowest "older# till, which directly overlies bedrock, gold dispersal "as determined by analysis of heavy mineral concentrates# is very limited, as it is truncated against a bedrock ridge ":ig. 16.<#. 'he highest gold concentrations are located ad+acent to the subcropping gold occurrence. 'he overlying till has not been in contact with the mineralization and has derived itMs gold from the lower till. 'his disperal train is longer "approimately < m, and maimum gold values in heavy mineral concentrates are displaced m down-ice from the mineralization ":ig. 16.7#. successful gold eploration program using both geophysical and geochemical techniques is described by 2arron, et al. "1987#. round 9 overburden sampling drill holes were completed to test specific geophysically defined targets in an area virtually devoid of outcrop. Jn the first pass, sampling of overburden drill holes at 8 to 16 ft intervals perpendicular to the ice transport direction yielded anomalous gold values in the heavy mineral concentrate fraction of till and carbonatized quartz pyrite-rich bedrock chips. Subsequent induced polarization-resistivity surveys defined the areal etent of a pyritic carbonate alteration zone. second pass till sampling and shallow bedrock 16H
(m
E.. @
GOLD
EMVY
IERML
COCERMES
K Mn%/al%B' - 2. 0
lU
% %
FIG 12.3"a
I
C%nB5%r
%
eaJlA
/ Mn%/al%B' 0. "
Mn%/al%B'
<0."
! 2. 0
/
/
)lan showing results of reverse-circulation drilling from the olden )ond orientation survey and eploration follow-up east and west of olden )ond, $uebec. nomaly classification is based on the highest heavy mineral concentrate "2*4# gold assay from the bottom three samples. "Sauerbrei et al.. 1987G
>
1
1EEE %
71>SH 71>6
G%l &n eaA &neral C%n5enrae' K MB - 2. 0 / % MB 0. " ! 2. 0 / MB < 0. " / %
FIG 12.3"
�Discovery
I
P?a'e I P?a'e II
)lan showing the results of the initial and follow-up phases of reversecirculation drilling at olden )ond East, $uebec. "Sauerbrei, et alK 1987#
167
sampling program was then +sed to determine the gold potential of this zone. 'he Aieavy mineral concentrate faction of the till samples delineates an anomalous "i.e. Meportedly �2000 ppb# gold dispersion train of fairly limited etent as shown in :igure 16,8 "i.e. etending for hundreds rather than thousands of feet down ice#. 'he short dispersion train is thought to be related to the presence of a bedrock ridge down ice from the auriferous veins. 5espite the
&ggE2J
P■ ICIULQU' C*r- =ESISW CS eSRE= %
DIMNMSe X SEDI=ER
3
*0@)2-2J&E Q.R. $LOER ILL GOLD MOMLY a GEOPYSICS '!**t*S @E, J%'@!J
1320' Y6
eploration $. V.) &//ln' area) Onar&%) Br?A!%Ale Canaa. G%l &n n& an%/alA l%e &n rela&%n % :! 16.8 &nB5e %lar&a&%nZre'&'&lAan ?%r&%nal l%% ele5r%/a>ne&5 'BreA re'Bl'. +arr%n) e al))
use of heavy mineral concentrates fromresulting is the successes from till, caution recommended. !n view of the variable nature of target m in er al iz at io ns and the weathering history of the glacial overburden, heavy mineral concentrates will not necessarily always constitute the optimum sample medium for geochemical analysis. 'here have in fact been a number of reports of the successful use of the ultra fine sieved fractions "e.g. U6 microns# of glacial overburden samples in 4anada and Scandinavia "4oker and 5i&abio, 1989, Shilts, 198>, %ikkarinen, et al., 198>#. !t is thought probable that in weathering tills the fine grain size phyllosilicate and secondary minerals act as scavengers, and adsorb trace metals released during the breakdown of sulfide and other minerals. 2owever, in view of the practical problems associated with the preparation of sufficient U6 micron material for analysis, the U< micron "U6H mesh# is more commonly used. Shelp and %ichol "1987# demonstrate, using data from the 2emlo u district and the area containing the Jwl 4reek u deposit, Jntario 4anada, that the U< micron fraction can be a more effective geochemical sample medium than heavy mineral concentrates, at least when target mineralizations contain ultrafine gold. t 2emlo the 2*4 gold anomaly decays far more rapidly than that associated with the sieved fines ":ig. 16.9#. t Jwl 4reek they are broadly comparable in etent ":ig. 16.>#. !t should be noted that Scandinavian eplorers place a greater routine reliance on the minus < micron "minus 6> mesh# fraction of till than do the 4anadians. 'his fraction has successfully indicated the presence of several types of mineralization, including gold, in Scandinavia.
heavy)mineral fraction of till assocated with minerali1ation at 0emlo. +ntario. !Siieip and Aicliol. %27"
mineral concentrate an d the )C8 fr acti on of tills from +wl 6ree#, +ntario. !Shelp and AichoQ, %27"
169
In view of the general compleity of glacial sediments and the need for correc t
identification of the materiaf being sampled, effective aeochemical eploration in glaciated terrain requires the particioation of $uaternary geologists or at least geologists with some training in $uaternary aeoloov f4oker. 1991#.
)robably the best recent reviews of geochemical eploration in glacial terrain are provided by 4oker and 5i&ablo "1989#, 4oker "1991# and Shilts "1991#. 12.2.6. Ve>ea&%n
Early scientific observers dating from the eighth and ninth centuries recorded that the morphology and distribution of certain plants were affected by the presence of metals in the soils. Such visible variations in a plant species are referred to as oeobotanica l indicator s. *any other plants, while not showing any visible variations, are capable of concentrating metals in their tissues and the presence of anomalous metals in the soils or ground waters is often reflected in the metal content of leaves, twigs, or other plant organs. 'hese invisible metal concentrations are known as biooeochemica l indicators
"/rooks, 1976#. eobotanica l and biooeochemica l indicators are of greates t potential interest as mineral eoloration tools n areas where soi l sampling !s ineffective "e.g. +n areas with barren transported overburdenR 5eep penetrating root systems can sometimes provide surface evidence of bedrock and ground water geochemistry "i.e. they allow the prospector to Asee throughA the overburden#. 4onsequently, these techniques, in particular biogeochemistry, have been applied with varying degrees of success in glaciated regions of %orth merica "/oyle et al., 19<9#, Europe and sia, and in arid and semi-arid areas, like the Southwestern 0nited States, where pediment, colluvial, and alluvial cover is etensive "4haffee, 1977#. lthough a number of papers and books on geobotany have been published over Nhe years, there is little evidence of etensive direct surface application in mineral eploration field surveys. 'he bulk of the published studies are of an academic nature "e.g. 4annon, 1979# rather than case histories of successful eploration programs. 'his presumably, at least to some degree, reflects the fact that effective application of geobotany requires highly developed botanical skills which are unlikely to be found in the ma+ority of eploration groups. !n addition, effective programs for large areas are difficult r# design as the results of orientation studies are often likely to have only restricted acpticability due to the wide variety of environmental factors which can influence plant growth. )robably the greatest potential value geobotanical features have in mineral is indirect. Suitably enhanced satellite imagery may sometimes detect M eploration listinctive spectral responses related to vegetational associations, together with other ia�ae features, whose distribution patterns %i�#,e significant "i.e from a mineral tetploration point of view# regional structural and lithological features "4ole, 198#. !n 140
addition anomalous plant communities associated with mineralization may sometimes be recognized on conventional air photographs. Some of the few well documented eamples of the use of geobotany in a mineral eploration program are provided by 4ole and &e @oe "1978#, and 4ole "198#. initial air and ground reconnaissance and orientation surveys of large areas with hot and semiarid climate in South Dest frica and /otswana revealed distinctive vegetation associations that distinguished areas of near surface )roterozic bedrock from those with thick cover of (alahari Sand and calcrete. 'he recognition of anomalous plant communities ":igs. 16.>1a, 16.>1b and 16.>6# at one of these locations with thin cover resulted in the discovery of sede-type copper mineralization, it should be noted that the mineralization is also reflected by distinctive soil anomalies ":ig. 16.># which could also have assisted in the discovery.Subsequently, regional eploration of some 1 km strike length of thinly covered potential host strata, using a combination of geobotanical, biogeochemical and geochemical "soil# techniques, resulted in the discovery of a number of similar mineral occurrences. n interesting application of geobotany in gold eploration in :innish &apland is described by )uikkinnen, et al. "1989#. 'hey found that host rocks for gold mineralization "i.e. carbonatized and mica-altered zones within a volcanic sequence# support a distinctive vegetation despite the presence of glacial overburden "mainly lodgement till#. 2ence potential target areas could be effectively delineated during follow-up of anomalies
detected n the course of regional geochemical mapping based on till and drainage sampling. lthouch bioaeochemistry has found wider acceptance than geobotany in mineral eploration, it is still generally far less popular than the techniques described in the preceding sections "i.e geochemical methods based on rock, soil, stream sediment, etc., sampling#, mainly due to the difficulties associated with program design and data interpretation. )lants are comple organisms and so is their metabolism. 5ifferent species respond differently to the same conditions and consequently some species are more effective biogeochemical indicators than others. :or eample, deep-rooted plants "e.g., the mesquite# are much more effective prospectors of the deeper ground waters than the shallow-rooted flora of the southwestern 0.S. deserts and are, therefore, generally preferred species in biogeochemical work in this particular region. %evertheless, shallow rooted plants growing in transported cover may reveal meaningful patterns in some desert regions. Evapotranspiration has been suggested as a mechanism for movement of metals into the nutrient depth of these plants. 181
1
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Scattered occurence of 2etichrysu m leptolepi s
ssaciot ion of rist ido congesto , Eragrostis denudata and nthe phor o pubesc ens
reas of Stip ogro stis unipl umis
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arly ?ugust
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FIG 12.43
4opper values in surface soi in area shown in :ig. 16.>6 "4ole, 198#
Some species preferenti %' %D &3 %% '8 %3 %7 a lly concentrate metals i n specific tissues such a %C &' %% %& 3& ' %D %8 s leaves, % %3 2 %% CC %8 N' %8 twigs, bark, or wood. ! %7 &% %D 37 87 7 %3 t is therefore very importa Mean; &7 %D % C2 %7 7 %3 n t to establish the most f !D.'C" !D.&" !%.87" !D.83H !D.2" !D.3" !D." a vorable tissues for sampling once a useful species has been identified. 'his compleity is accentuated by the fact that metal uptake may vary with aspect and season "'able 16.#. !n temperate forest regions, accelerated uptake and higher concentration commonly occurs during the spring growth following a dormant winter season. !n hot desert regions, following the ehaustion of available near-surface water during the dry season, deep rooted plants will tap the deeper ground- waters. /ecause of these seasonal variations, biogeochemical surveys must be completed quickly in the optimum period"s# defined by the orientation studies. %3
%&
%
%7
88
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7
7
'hese variables make biogeochemical sampling a very specialized eercise. Some epertise in botany as well as eploration geochemistry is essential for both the orientation studies and the supervision of vegetation surveys. Jn the other hand the basic field equipment required for biogeochemical sampling is very simple "5unn, 1991#N
1AE
?sh yield of dry twigs is about & pe rcent$ ?sh yeield of dry leaves is ' ) C percent.
&YDB 12. A
Sea so+a l cha+Ces i+ the Cold co+te+t o! ashed aider tKiCs a+d leaes. (#++, %22%"
1<
in pa rentnon-contaminating to dr y brass-free# a pair of Jalues he se s have be en recalculated weight basis. anvil-type e.g. pruning shears. preferably 'eflon coatedI "ii# fairly large sampling bags "e.g. 6 cm#. 0se heavy duty coarse brown paper bags when conditions are dry and cloth bags when conditions are wetI "iii# roll of masking tape or stapler to close bagsI "iv# very large pack. lthough samples are not heavy they are etremely bulkyI "v# hunting knife or hatchet for bark sampling.
"i#
5unn "1991# also provides a useful summary of the procedures to be used and the precautions to be observed when conducting a biogeochemical survey "'able 16.>#.
5unn "1987# has pointed out that with some species, sampling of bark "a dead tissue# can be an effective technique that is not sub+ect to seasonal metabolic variations. 'he potential value of humus as a geochemical sampling medium has already been discussed in the section on soil surveys. 'his again avoids the effects of seasonal variations as the accumulated material constitutes an integrated sample. Deathering, leaching, and bacterial decomposition will work to diminish the metal contents, but signatures in mull are generally preserved. !n view of current interest in gold eploration, some eamples of recent biogeochemical studies in gold areas are probably particularly appropriate. 0nfortunately, the few case histories published to date on the application of biogeochemistry in gold eploration over alluvial and colluvial covered semi-arid areas in the western 0nited States are mostly somewhat inadequate. Jne by /usche "1989# suggests the presence of possibly anomalous concentra-tions of gold in the leaves of creosote bushes over both eposed and colluvium covered epithermai quartz stockwork vein gold at Standard 2ill in the *o+ave 5istrict, 4alifornia ":ig. 16.>>#. s no soil data are provided it is uncertain whether the apparent anomalous geochemical response is confined to the plant cover. nother test survey has been described by Erdman, et al. "1988# of the 0.S..S. &imited studies were made of sage-brush geochemistry over skarn-, disseminated gold and silver-, and hot springs-type mineral occurrences in %evada. 'he results are again far from definitive, but apparently anomalous gold was detected in ashed stem and leaf sagebrush samples from one traverse ad+acent to known gold mineralization ":igs. 16.>H and 16.><#. gain no information is provided on the geochemistry of soils along this traverse )ublished data for the 4anadian Shield are somewhat more comprehensive. :or eample, 4ohen et al. "1987# describe an etensive investigation in the 2emlo old 5istrict designed to establish the potential usefulness of biogeochemistry in gold eploration over areas with shallow eotic till cover. /alsam fir was shown to have an uneven, if not erratic, distribution along the tree within all organs ":ig. 16.>7#. !n contrast, gold distribution in *ountain *aple displayed generally higher concentrations in the base
sect on
an n t e crown.
1A%
not prevent delineation of distinct
2owever, this
5asic Rules %.
6ollect same species.
&.
Reasons a different very species has chemical composition, and trace element and requirements tolerances.
6ollect same plant organ.
ach plant organ has different capacity to store trace elements.
8.
6ol#ct same amount !i.e. age" of growth, from same area of tree !e.g. chest height", pr eferably from all sides.
There ar e chemical variations along a twig !see Table 3", 0eterogeneity in ba r# scales can be minimi1ed by scraping from around the tree.
3.
Try to collect samples from plants of similar age an d appearance,
This is the basic inter)site that is consistency required for any geochemical sample medium.
'.
If living tissue is the selected medium, collect at same time of year !i.e. conduct survey in &)8 wee# peri od".
There
>ead tissue bar# " can be any time
Ao appreciable seasonal change
C.
%87
!e.g, outer collected at
>o not return to a previously sampled tree and expect to obtain exactly the same analyses.
HioCeochemical s#re>.
&YDB 12. Yasic
r#les
to He
ar e seasonal significant changes in plant chemistry.
This is unrealistic in view of the heterogeneity
of
element
distributions an d seasonal variations in composition !and to a leser extent annual 5e variations". satisfied if an anomaly is the same order of magnitude.
(#++, 1991O
applied at
each sampli+C statio+
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co+d#cti+C
a
.9
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i
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(I- %&.33 Sample sites and contoured ?u values for the Standard 0ill area, 6alifornia. -old in plants determined by IA??.
!5usche, %272"
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Cre4�k
Roc# 6(eeii Ranch %2 V %' %8 n 3�1�� I &D I5 %C U %& %D 7 C
�
3 &
(I- %& .3' 5ase map showing site locations along traverses ? and 5, and prospect s, adits, and shafts of the -old Run mining district, AeV*ada. !rdmann, et al. !%277"
9�� a
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.
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-old levels in ashed stem)and) leaf samples of sagebrush along traverse ?. Samples with traces of ?u !reported as less than the limit of determination" ar e given a value of 3 ppb and shown as N th ose in which no ?u w as detected are indicated by A. @a alluvium, @g gravels and tp Erebie (ormation. !rdmann, etal.. %277"
8D S;
%D
linit aT det:rnMna.twn
7 ?.
I
%*
growing over mineraii6ation, sampled at varying heigiits. "4ohen, 198<#
geochemical patterns in the vicinity of mineralization. :or eample, in areas of pooN drainage or eotic overburden the needles of balsam fir and the leaves of birch and alder display far more etensive anomalous response than the local soils ":igs. 16,>8 anc 16.>9#. !n well drained areas both soils and plants display good geochemical response for gold ":igs. 16.H and 16.H1#. @esponse for most of the pathfinder elements "!.e. *o, Sb and /a#, ecept s, is good in both poor and well drained areas. 2owever, the multielement data do not provide unique information and their main value is probably as
backup n
e even o un e ec e
problems w
e gold analysis. e a a suggest that in this district biogeoc hemistry would be more e ffective than geochemical s oil sampling in poorly drained are as. !n well drained areas geo chemical soil sampling would probably be preferable as biog eochemistry appears to offer n o clear technical advantages and is certainly more epensi ve.
)JJ@& 5@!%E5 S)@04E
8 ,
6 8
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1
E
bailB
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1
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X
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i 1 !
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1
1
0
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1
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8$
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:! 16.>8
2emlo 5istrict, Jntario. 4omparison for poorly drained ground between the u
contents of needles and bark of /alsam :ir "biles balamea# and Spruce ")icea glauca, ). mariana# and / horizon soils and humus. "A*A represents location %H mineralization#, "4ohen, et al., 1987# )JJ@& 5@!%E5 /!@42
u
ppb
rnslrea
�
:! 16.>9
12
2emlo 5istrict. Jntario. 4omparison for poorly drained ground between the MB contents of ider leaves, first year twigs and bark, Dhite /irch "/etula papyrifera# leaves and bark and / horizon soils and humus. "A*A represents location % mineralization#. "4ohen, et al., 1987#
(R4B >R?IA> SERU6
(I- %&.'D
0emlo >istrict, +ntario. 6omparison for freely drained ground be tween tfie ?u contents of needles and tarlN of 5alsam (ir !?bies bala mea" an d Spruce !Eicea glauca, E. mariana" and 5 hori1on soils. !/M/ represents location of minerali1ation.". !6ohen, et al., %27"
(R4B
>R?IA> ?4>R
(I- %&.'%
0emio >istrict, +ntario.
6omparison for freely drained ground betwee n ?u contents
o ? er !?Inus rugosa, ?. crispa" leaves, first year twigs and bar# and 5 or 1on soils. !/M/ represents location of minerali1ation". !6ohen, et al., %27"
1>
5unn "1989# provides an interesting demonstration of the potential value of 5iogeochemistry &n regional gold reconnaissance programs. nalytical data for samples of the outer scales of spruce bark, collected on sample density of only 1 '&e .er "0 k#� over H km� &n %ova Scotia, broadly define the known gold districts, 'he best ndications of these districts are provided by u, s, Sb and Se ":igs. 16,H6a, b, c, and p#.
n interesting variant of biogeochemistry based on the analysis of aquatic mosses n drainage channels has been the sub+ect of a number of studies, 'hese suggest that rese mosses might provide effective alternative geochemical sample media in areas ,iere steep terrain prevented accumulation of stream sediment fine fractions. :or iample, Erdman and *odreski "198># found good correlation between the 4u and 4o 6ta for aquatic moss and regular stream sediment samples in the vicinity of the !ron 4Meek stratabound 4u4o occurrence in &ehmi 4ounty, !daho. 'hey concluded that in re absence of stream sediment, aquatic mosses might be a suitable alternative ;rconnaissance sample medium. Qones "198H# reached a similar conclusion following a ady of the u, g, s, 4d and Sb content of aquatic bryophytes from the 5olgellau gold +strict in %orth Dales, 2owever, arsenic, rather then gold, appears to be the most E�ive pathfinder element for gold mineralization in this particular area. Jther studies Mie been carried out for uranium in the northwestern 0,S,, "Shacklette and Erdman, M86# and for base metals in laska "Smith, 198<#. 1
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arsenic, !c" !next page" antimony, !d" !next page" selenium !>unn, %272"
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1>< Ecellent reviews of recent developments in eploration biogeochemistry are provided by 5unn "1989 and 1991#. lthough it is now a little dated, a useful bibliography on the use of plants in prospecting for gold was compiled by Erdman and Jlson "198H#.
16.6.7. Dater 'he detection and interpretation of aqueous dispersion haloes in surface and groundwaters related to mineralization form the basis of hydrogeochemical prospecting. eochemical surveys, especially regional reconnaissance based on water sampling offer potential theoretical advantages in many environments "'able 16.H#, especially in the case of highly mobile elements such as uranium and zinc. 2owever, where there are alternative effective geochemical sample media "e.g. soils, stream sediments, etc.# these commonly tend to be used in preference to water. 'his is due to certain potential difficulties associated with geochemical eploration programs based on water samples,
n particularN relatively large samples generally required. 'hese can present transportation difficulties. "ii# chemical instability of untreated samplesI the frequent need for ultra sensitive analytical methodsI "iii# the etreme sensitivity of aqueous dispersion processes to a variety of interacting "iv# P - C dU 9 P 0O -t environmental factors, A$ ) � S � some seasonal in nature and not all of A - ' of which might� be g <-sl 1 # -C . + e o � which will be necessarily e (0 U Pg !l 7J < M EE •74Qk (Precognized. "i#
S+SA cao U) ..ff > 0\ �1 -S Dater samples are generally in clean acid rinsed H collected � : i ) <;; g � are k.lU !ticj1 E< A 7Emore 0 C 2 Mas these than glass 5 a s
ml or 1 ml bottles under conditions. 2owever, several !mportant trace metals are incorporated in plastics during the manufacturing process and are often present in teachable form. 'his applies (Q f particularly to zinc. E'herefore #� thorough cleaning prior to use iiisc arequired. )rolonged is. N i X9 M a. soaking in H hydrochloric or nitric acids followed by repeated rinses with distilled or deionized water is recommended. )rior to sampling in the field Athe should be J !!l fbottles X B rinsed seve /7, r. +ic ral times with the water to be sampled. Samples that are to be analyzed for @ acidified to trace eleme A> i E nts should ? be aU Bnitric # 1 or hydrochloric acid to p2 U6 with metal free � 0O c Q Q i to ? S J from precipitating or adhering walls. 'urbid water #the Q. 1V bottle J. J keep the di 0L0A ?S PA6c ssolved metals C + # filtered should be using a .>H micron membrane before addition of the acid, otherwise •0 leaching o 0$ ; f elements from the suspended sediment might occur. Dhen both trace and ma+or elements are to be determined, at least two samples must be collected so that material without introduced cations and anions is available for the ma+or element analysis. !n order to minimize growth of algae "*iller, 1979# in samples prior to analysis they should be stored in a cool place away from sun light. !f this is not possible, chloroform should be added. p2, conductivity and certain other measurements are commonly made at the sample site. $
E
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Jne the more effective applications of hydrogeochemistry has been in regionaN reconnaissance programs based on lalUe water "and lake sediment# sampling, particularly within the 4anadian )recambrian Shield, but also within the :ennoscandian Shield anc the 4ordilleran and ppalachian regions of %orth merica. s in the case of lake sediment sampling the ideal terrain for this technique is where lakes are common. conditions are swampy, andor where stream drainages are inaccessible or poorly developed "4oker et al., 1979#. !n low relief regions, the lake water medium is dependent on the hydromorphic dispersion of metals into the lake environment through ground waters. !n areas with higher relief surface hydromorphic dispersion could also be an important factor. n early eample of a hydrogeochemical survey based on lake water sampling was provided by 5yck et al. "1971# and 5yck "1979# in the course of a eperimental study of a variety of sample media over a H square mile area including the /eaverlodge 0ranium 5istrict in Saskatchewan.Surface lake water samples were collected at an average density of 1. samples square mile. 'he resultant anomalous uranium distribution patterns clearly outlined known uraniferous zones as well as additional areas of potential interest ":igs. 16.Ha and 16.Hb#. !t was concluded that organic sediments, bicarbonate and p2 control the migration of 0 in the surface environment. 'he presence of high organic concentrations severely restricted uranium dispersion. f5??t,
@egional reconnaissance for selected metals can also be achieved by sampling the waters �4 actively flowing streams where metal is dispersing in solution. prospecting approach similar to the sampling of stream sediments is necessary. Sampling of ground water seepage sites is an integral part of stream water surveys. !n view of the paucity of recently published eamples of stream water sampling, it is again necessary to refer to fairly old studies. :or eample 5yck et al. "1971# sampled stream water in addition to take water "see above# in their eperimental uranium eploration program in the /eaverlodge5istrict of Saskatchewan. Stream water "and stream sediment# samples were collected at an average density of 1 samplesquare mile. Sample temperature was recorded at the sample site, whilst radon, p2 and alkalinity were determined in a field laboratory. 'he samples were then acidified and transported to a central laboratory for uranium and other trace element analyses. lthough the regional uranium distribution patterns ":igs. 16.H>a and 16.H>b# were found to be broadly similar to those displayed by the lake waters, it was concluded on economic grounds "i.e. ease of sample collection# that the latter were the preferred sample medium. roundwater can also play a useful role in mineral eploration, especially when the targets and large potential target hosts are obscured by post mineral overburden or unmineralized bedrock, and target andor associated pathfinder elements are mobile in the prevailing groundwater environment. !nteresting eamples of the attempted application of groundwater geochemistry in mineral eploration are provided by work carried out by the 0.S..S. "2uff, 197#, and various companies in the southwestern 0.S.. in the 19<Ms and 197Ms. 2ere the primary interest was in locating porphyry copper deposits under pediment and alluvial sediment cover. 'he eploration technique
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%'% �va) ba sed on the #nown mobility of molybdenum, a significant component of most 1orphyt copper deposits, in the neutral to al#aline conditions prevalent in local groundwater. Samples were generally collected from domestic and irrigation wells, and X�atural springs. arly results, as for example in the Eima Mining >istrict, showed anomalous molybdenum was indeed present in the groundwaters around #nown deposits e.g. (ig. 12.EEO and elsewhere, but the anomalies were so large !i.e. tens or even Z$)uii_ftfNbi8i/siuarfeTrtiWe<%rTi source location in routine surveys was not apparently economically feasible. In addition, anomalous contrast appeared to often correlate with total dissolved solids, a feature presumably unrelated to minerali1ation. 4ater studies by ~3JSt and Trautwein (19%EO indeed found molybdenum concentration in groundwater to correlate strongly with conductivity !an indirect measure of total dissolved solids", wea#ly :vrth p0 and not at aQQ with h !(ig. 12.EFO. +n theoretical grounds they concluded that /�i) reflected a relationship between the solubility of molybdate salts, such as CaM,O�& /0e to an increase in ionic strength and corresponding decrease of the activity 8cefficients. They suggested utili1ing the ratio of log Mo*F in conunction with a plot of og Mo*F vs. log Mo to determine the proximity of a buried oxidi1ing porphyry copper 1eposit. The possibility of applying sophisticated hydrogeochemlcal models to mineral fxploration purp os es was discussed by Runnells and 4indberg (19:1O. They determined Tat the saturation index is a reliable indicator of the presen ce of uranium minerali1ation,
rut t e requirement or r e a e rrfficulties in routine surveys.
measurements could present certain practical
32.2.:. @ases Under certain conditions, mineral deposits produce gaseous emanations which can /e detected by speciali1ed measurements. There are two broad categories of soil gas sampling technique !i" instantaneous$ !ii" integrated. *9*WWh the instantaneous method, specific volumes of soil gas are extracted through a 8n>be, from some predetermined optimal depth which is normally somewhere in the range 1r 20 to 0 inches (E0 to 100 cm". These can then be either !i" analy1ed on site by a field instrument !e.g. CO� and analy1ers or one of the new generation of portable micro gas chromatographs". Mobile mass spectrographic systems, such as that used by 0oward Mc6arthy of the U.S.-.S., should probably be classified more as research systems at this time in view of high capital and operating cost as well as interpretational uncertainties !ii"
or placed in a special container for transportation to an analytical laboratory !e.g. 5arringer Technique, etc."$
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4orrelation plots for groundwaters in southern rizonaI "a# p2 vs. log *o, suggesting a slight correlation between the molybdenurr content and p2I "b# Eh vs. log *o, showing poor correlation between molybdenum content and EhI "c# log ( vs. log *o, suggesting a good correlation between increasing conductivity and increasing molybdenum content. "'rost and 'rautwein, 197H#
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or adsorbed onto a special collector material and transported to laboratory for analysis.
'he main potential problem with the instantaneous methods is that they are sub+ect to aA
T
s or erm gas u variability w c might occur. s cou prove particularly ser ous large surveys during periods of climatic instability. !nstantaneous methods are alsN inappropriate when dealing with gases occurring in concentrations at or near B#/� detection limits of the available analytical equipment. 'he main advantage of thMS instantaneous methods is that they only entail one visit to the sample site. Dith the integrated method of soil gas sampling, special adsorbent or reactive material, including molecular sieves or porous polymer sorbents "e.g. 4lifton, 198>a arN 198>b, etc.#, or activated carbon "e.g. A)etreA system#, is buried in the soil at ea-� sampling site for specific periods of time "generally for several weeks#. t the end of thIO time the collectors are recovered and sent to the laboratory for analysis. lternatively, 0)� can be made of natural soil gas collectors, including specific soil fractions "e.g. clays AdvolA system#. 'he main potential advantage of the integration methods of soil gas sampling is that results are unlikely to be influenced by short term fluctuations in the so gas flu. 2owever, two visits to each sample site are required, ecept in the case of ti-re natural soil collectors. problem with some of the collector systems "i.e. molculaB sieves# is that gas desorption in the laboratory requires heating to very high temperatures which will likely modify the nature of some of the volatile species. *uch of the published information on the application of gas geochemistry M mineral eploration relates to research and orientation studies. 2owever, numerous geochemical gas surveys were carried out some years ago during the last uraniuA AboomA. 'hese were mainly concerned with measurement of radon produced during t�L radioactive decay of uranium and radium. lthough some radon in stream and lake watestudies were undertaken "e.g. 5yck et al., 1971#, most of the programs were based or integrated measurements of radon in soil gases. Some of the better known procedures "e.g. A'rack EtchA# use detectors which respond to alpha particles emitted during /a%,� decay "e.g. ingrich, 198>#. 'he A'rack EtchA procedure utilizes small pieces of radiatioM sensitive film. 'hese are processed in an etching solution to provide visible track-like images of the alpha particles which can then be counted to provide an indication of the average amount of radon present during the eposure time ":ig. 16.H7#. &ater versiorts include plastic filters to prevent eposure of the detectors to thoron. !n A'rack Etcf surveys the integrative detectors are buried at shallow depth "i.e. normally around .H r in inverted plastic cups ":ig. 16.H8# and left for a specific period of time. !n the /ake&ake area in %orthern 4anada ":uchs et al., 1986# cups were buried for the full winteseason at 1 ft intervals along traverses with 6 ft separation. 'he unusually lone eposure produced improved results over those obtained with detectors left in place foM shorter periods during the summer. :ollow-up of several high contrast anomalies resulted in the discovery of the &one ull 0ranium 5eposits ":ig. 16.H9#.
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*ercury-bearing minerals which can include sphalerite and other sulfides, ofter release mercury vapor during oidation. 'his vapor can be measured in soil gas directly "i.e. generally using integrative collectors which are analyzed in the laboratory# or adsorbed 2g can be released from conventional soil samples by heating to 61-9: "1-64!MM4# "&anda, 1978#. Jver the past twenty or so years there have been many studies of the possible application of mercury vapor surveys to mineral eploration, but the published results have often been poorly documented and frequently inconclusive. :edikow and mor "199# recently described their evaluation of commercially available mercury detectors "i.e. AureA# following test studies at a number of 4anadian mineral deposits. 'he detector consists of a thin silver wire in an open ended glass vial ":ig. 16.<#. 'hese are buried in overburden at a depth of > cm ":ig. 16.<1# for a period of 6 days. 'hey concluded that the system does not measure mercury in soil gas in a consistent manner in proimity to mercury enriched base and precious metal deposits in the rest areas. 'he oidation of moist sulfides leads to the generation of CS� and 4JS "'aylor, et al., 1986#. Jakes and 2ale "1987# describe an eperimental eploration technique basec on the selective thermal desorption of 4JS from the U1H micron fraction of overburden materials and quantitative determination by a rapid gas chromatographic method. t Qohnson 4amp, rizona the surface microlayer of the soil was sampled on < m grid. nalysis disclosed distinct 4JS anomalies over suboutcropping sediment hostec replacement sphaleritechalcopyrite mineralization despite the presence of considerable thicknesses of pediment gravel and alluvium ":ig. 16.<6#. Elsewhere comparable
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anomalies were delineated over sulphide mineralizations covered by a variety transported overburden, sometimes eceeding 9 m thickness and ranging from arid highly porous sand and gravel to moist, clay-rich glacial till. pro e ranspor e sI /ecause of the consumption of oygen in the oidation process, the atmospheric proportions of 4JgiJg change in the vicinity of oidizing sulfides and these imbalances can be measured in the soil gas "&ovell, et al,, 198#. &ovell and @eid "1989# used this approach in northern rizona to evaluate the subsurface potential for structurally controlled mineralization in 86 collapse breccias. 'hey collected a little under H soil gas samples with
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by gas chromatography. lthough the presence of buried sulfide accumulations was found to be reflected by strong 4J6 anomalies :ig. 16.<#. these anomalies were etremely sensitive to climatic change "e.g. rainfall#. !n recent years 4Jg and Jg in so= gas surveys have also been applied in a number of eploration programs for sedimert hosted gold deposits in the western 0.S.. 'hese surveys have reportedly resulted in the determination of lithology changes, location of faults and the presence of alteration beneath transported overburden "Qaacks, 1989#. @ecent studies in Sweden "*almqvistet al., 198<# have identified an upward fluinc of EJS in the near-surface sections of the earth. 'his EJS enters the grounc waters as dissolved air. 'he circulating meteoric waters and changing pressure conditions in the subsurface cause the EJS to rise as small streaming bubbles. 'he bubbles contain other gases and can also collect metallic ions and particles that can be trapped in collectors set out in the surface soil. nomalous patterns in EJS have
een recor e over m nera za on transported cover.
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c nesses up o
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!t should be noted that biogenic activity in the soil can produce methane "J2Q. hydrogen "2g#, carbon monoide "4J#, hydrogen sulfide "2gS#, dimethyl sulfide ""42#6S#, dimethyl disulfide ""42#6S6#, methyl mercaptan "42S2#, carbonyl sulfide "4JS#anc carbon disulfide "4Sg# "'aylor, et al., 1986#. 'herefore, near-surface detection of these gases can be suspect. *ethane is found at depth in several types of mines, but s genetic association with mineralization is not always clear. 1H9
:! 16.<
4p+ content of soil air over mineralized breccia pipe, rizona. "&ovell and @eid, 1989#
recent volume of the Qournal of eochemicai Eploration "(esler, 199# provides a useful review of a number of soil- and rock-gas geochemistry studies. 12.2.,.
Par&5Blae'
Solid particles down to the size range of large molecules are present in the atmosphere. Deiss "1971# developed an airborne geochemicai prospecting technique %r arid terrain based on the collection of dust particles suspended in the atmosphere. ?e !@'@4E and S0@'@4E techniques of /arringer @esearch &td. were designed to sample a variety of particulates in the lower atmosphere including spores, pollen, dust, microorganisms, organometallics, and hydrocarbon complees. 'he collected particulates BF
ever m nera deposits. @eproducibility o e technique in the search for mineral deposits hampered by variable weather conditions and temperature inversions. 'he S0@'@4E method was designed to overcome these climatic problems by sampling 1<
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o
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)lot of log /. cereus "colony forming unitsgram of soil# in I[+and / horizon soils overlying and ad+acent to u-quartz veins !n the Empire *ining 5istrict, 4olorado "from )arduhn and Datterson, 198>#. ")arduhn, 1987#
particulate matter from the ground surface "i.e. microlayer# using a helicopter-based or manually transported system. 5espite the fact they have been available for a numt+er ofyears neither method has yet been used etensively in routine metallic minera eploration programs. !@'@4E was used in some large scale hydrocarbon surveys in the 198Ms. 16.6.1. &5r%%r>an&'/' )arduhn and Datterson "198># and )arduhn et al. "198H# have demonstrated tha the population of the common microorganism, /acillus cereus, increases with natura increases in the base and precious metal content of soils in the vicinity of known minera deposits. !t has also been noted that the increased antibiotic resistance of these bactera correlates with increased metal concentrations in soils "Datterson et al., 198<#. Jn the basis of very limited sampling )arduhn and Datterson "198># reported anomalous /acillus cereus populations over quartzgoldsulfide veins, overlain by 1H-6 feet of glada overburden, near Empire, 4olorado ":ig. 16.<>#. 2owever, earlier work by 4urtin, et a.
%C%
f%2%", determined that although the 5 hori1on soils were not particularly effective sam ple /ledia in this district, the ?u, 6u and 5i content of the forest humus layer !mull" dear ly WWefine the minerali1ed veins. Ttius no particular advantage appears to be provided by the 5acillus cereus data in this example. The same appears to be true in certain other t est ireas !e.g. Jista Eit area, Mesquite >eposit, Imperial 6ounty, 6alifornia" studied by arduhn !%27".
?n interesting alternative approach to the possible application of microbiology to mineral exploration is discussed by Michaels and Riese !%27C", They sug gest determination of the metal tolerance of bacteria by the addition of suitable met al ancentrations to culture media prepared from stream sediment, stream vi*ater, and organisms on stream water surfaces. Eeliminary data indicate bacteria populations are more metal tolerant in minerali1ed areas.
IJIineral exploration methods based on microbiological features are still in their
rfancy and should be approached with extreme caution.
%&.&.%%.
&+imal iss#es
?nimal tissues have not been used extensively as a geochemical medium. Garr
en St al. !%2%" analy1ed 2C trout livers from locations in 5ritish 6olumbia and identified a general correlation between the 1inc and copper contents of these livers and #nown
