GUIDE TO THE EPITHERMAL ENVIRONMENT
VEIN ATTRIBUTES, HYDROTHERMAL ALTERATION AND ORIGIN
Renato E. Bobis Joel S. A!ino
February, 1995
214055452.doc
1
INTRODU"TION
In an epithermal field, the explorationist should be familiar with two important attrib attribute utess of any hydro hydrothe therma rmall syste system m !ein !ein textur textures" es"min minera eralo# lo#yy and altera alteratio tionn assembl assembla#es a#es.. $hese $hese attributes attributes pro!ide clues on the character character of the minerali%in minerali%in## fluids and position !is a !is the principal minralised conduits. $he purpose of this #uide is to pro!ide an illustrated reference to the common !ein textures and hydrothermal alteration assembla#es in epithermal systems, to summarise models for distribution of alteration and !ein textural assembla#es within simple !ein systems and emphasise characteristics of %ones coincident with #old mineralisation. &uch of the information on the detailed description of the !arious textures in this #uide are based on the detailed study of the 'cott (ode #old)sil!er, deposit in the *a+i *a+in# n#oo epit epithe herm rmal al fiel field, d, uee ueens nsla land nd -ob -obis is,, 1992 1992// whic whichh was was cond conduc ucte tedd in con+unction with the study on textural %onin# in epithermal uart% !eins -&orrison et al., 1991/ and the ad!anced ar#illic alteration system at i%on porphyry copper)#old deposit in the *hilippines -uino, 1993/. !aluation of uart% uart% textures was mainly performed by systematic description of !ein outcrops and intersections in diamond drill holes. "ON"EPTS
eald et al -1967/ and ayba et al. -1965/ ha!e subdi!ided the !olcanic)hosted epithermal deposits into two classes accordin# to the predominant roc8 alteration assembla#e adularia)sericite: and acid)sulphate:-fi#.1/. onham -196;/ used the terms low sulphur: sulphur: and hi#h sulphur: correspondin#ly. correspondin#ly. er#er and enley enley -1969/ recommended replacin# the fluid term acid)sulphate: with the mineralo#ic term 8aolinite)alunite: to achie!e a consistent basis with the adularia)sericite: #roupin#.
$he term >ad!anced ar#illic? alteration was initially coined by emley and @ones -19;4/ to indicate a special type of clay alteration representin# extreme base leachin# under acidic to !ery acidic conditions. conditions. $ypical minerals associated associated with this alteration are uart%-silica/, alunite, 8aolinite, pyrophyllite, andalusite and corundum formin# o!er a temperature ran#e from surface conditions to abo!e 400 oA. ad!anced ar#illic alteratio alterationn are commonly commonly texture destruct destructi!e i!e and per!asi!e per!asi!e.. From then on, other other synon synonym ymous ous terms terms ha!e ha!e been been introd introduc uced ed li8e li8e >acid) >acid)sul sulpha phate? te? syste systems ms -after -after the process of acid leachin# and presence of sulphatesB systems -due to the hi#h sulphur and oxy#en fu#acitiesB edenuist, 1967/.
214055452.doc
2
INTRODU"TION
In an epithermal field, the explorationist should be familiar with two important attrib attribute utess of any hydro hydrothe therma rmall syste system m !ein !ein textur textures" es"min minera eralo# lo#yy and altera alteratio tionn assembl assembla#es a#es.. $hese $hese attributes attributes pro!ide clues on the character character of the minerali%in minerali%in## fluids and position !is a !is the principal minralised conduits. $he purpose of this #uide is to pro!ide an illustrated reference to the common !ein textures and hydrothermal alteration assembla#es in epithermal systems, to summarise models for distribution of alteration and !ein textural assembla#es within simple !ein systems and emphasise characteristics of %ones coincident with #old mineralisation. &uch of the information on the detailed description of the !arious textures in this #uide are based on the detailed study of the 'cott (ode #old)sil!er, deposit in the *a+i *a+in# n#oo epit epithe herm rmal al fiel field, d, uee ueens nsla land nd -ob -obis is,, 1992 1992// whic whichh was was cond conduc ucte tedd in con+unction with the study on textural %onin# in epithermal uart% !eins -&orrison et al., 1991/ and the ad!anced ar#illic alteration system at i%on porphyry copper)#old deposit in the *hilippines -uino, 1993/. !aluation of uart% uart% textures was mainly performed by systematic description of !ein outcrops and intersections in diamond drill holes. "ON"EPTS
eald et al -1967/ and ayba et al. -1965/ ha!e subdi!ided the !olcanic)hosted epithermal deposits into two classes accordin# to the predominant roc8 alteration assembla#e adularia)sericite: and acid)sulphate:-fi#.1/. onham -196;/ used the terms low sulphur: sulphur: and hi#h sulphur: correspondin#ly. correspondin#ly. er#er and enley enley -1969/ recommended replacin# the fluid term acid)sulphate: with the mineralo#ic term 8aolinite)alunite: to achie!e a consistent basis with the adularia)sericite: #roupin#.
$he term >ad!anced ar#illic? alteration was initially coined by emley and @ones -19;4/ to indicate a special type of clay alteration representin# extreme base leachin# under acidic to !ery acidic conditions. conditions. $ypical minerals associated associated with this alteration are uart%-silica/, alunite, 8aolinite, pyrophyllite, andalusite and corundum formin# o!er a temperature ran#e from surface conditions to abo!e 400 oA. ad!anced ar#illic alteratio alterationn are commonly commonly texture destruct destructi!e i!e and per!asi!e per!asi!e.. From then on, other other synon synonym ymous ous terms terms ha!e ha!e been been introd introduc uced ed li8e li8e >acid) >acid)sul sulpha phate? te? syste systems ms -after -after the process of acid leachin# and presence of sulphatesB systems -due to the hi#h sulphur and oxy#en fu#acitiesB edenuist, 1967/.
214055452.doc
2
C!er the years, explorationists ha!e reco#ni%ed the occurrence of economic #old and copper #rade #rade mineralisation mineralisation within ad!anced ad!anced ar#illic ar#illic systems. systems. 'ilica)enar#ite ± #old ore bodies ha!e been mined in Dansatsu, @apanB Ahinuashih, $aiwanB (epanto, *hillippines and *ueblo Eie+o, ominican epublic to name a few. ecent ad!ances in #eothermal research -e.#., &atsuhisa et al., 1991/ and isotope systematics of alunite -ye et al., 1992/ increased the understandin# of the processes and en!ironments of formation of ad!anced ar#illic systems -Fi#.2/. In mineral exploration, ad!anced ar#illic alteration is belie!ed to form the upper parts of epithermal and "or porphyry hydrothermal systems as result of oxidation of 2' #as released by deep boilin# fluids or by 'C 2 disproportionation of a de#assin# ma#ma. $his $his is clea clearly rly demo demons nstra trate tedd by uch uchan anan an:s :s mode modell -196 -1961/ 1/ of a >sin >sin#l #lee pass pass?? ephitermal system -Fi#.2.a/ where silica G alunite G 8aolinite alteration caps a deeper adularia)s adularia)serici ericite te systems systems.. (i8ewise (i8ewise the the spatial spatial relation relationships ships of of ad!anced ad!anced ar#illic ar#illic alteration o!er selected selected porphyry systems systems in western De!ada ha!e ha!e been demonstrated demonstrated by udson -1963/ -Fi#.2.b/. the abo!e mechanisms mechanisms in!o8e a deeper heat and fluid source for the formation formation of ad!aced ar#illic alteration. alteration. owe!er, this may may not always be the case since acid)sulfate weatherin# products can be de!eloped by surficial oxidation of pyrite and other sulphides -Fi#. 2.1c/.
214055452.doc
3
ADULARIA%SERI"ITE
A"ID%SULPHATE
are hypo#ene alunite Holinite subsidiary Do pyrophyllite dularia Do enar#ite Aommon selenides are tellurides are i)phases &n phases in #an#ue Fluorite, chlorite Eariable *b, n, -Au/ (ow acti!ities C2 J '2 lower ∑' Dear neutral p eep circulatin# meteoric
xtensi!e hypo#ene alunite &a+or hypo#ene 8aolinite Aommon pyrophyllite Do adularia nar#ite = pyrite = co!ellite co!ellite Do selenides 'ome tellurides 'ome i)phases &n minerals rare are Au biproduct i#h acti!ites C2 J '2 hi#h ∑' (ow p &a#matic !olatiles
Figure 1. Comparison of the two classes of volcanic-hosted volcanic-hosted epithermal deposits (after Morrison, 1991).
214055452.doc
Lo- S!l&i$ation (A$!la*ia%se*i#ite
uchanan:s -1961/ model -Fi#s. 1 and 2.a/ represents low sulphidation !eins scaled as a sin#le pass boilin# system and the %onin# patterns are interpreted in terms of the beha!iour of fluid under#oin# boilin#. $his models is based on empirical obser!ation on many epithermal systems in wester K'. $hus, this model does not apply !ery well in systems where ma#matic in!ol!ement and telescopin# are si#nificant -e.#. island arc settin# li8e the *hillipines/. $he outer, more per!asi!e propylitic alteration #rades to adularia)bearin# and intermediate ar#illic alteration near the principal !ein structure. $he propylitic) adularia)intermediate ar#illic alteration pro#ression towards the principal !ein is almost always obser!ed around !eins hostin# the ore shoots. $he wid of the alteration en!elope is #enerally proportional to the thic8ness of the !ein structureB with the intensity and width enhanced by the series of pre)mineralisation faults occurrin# in the footwall and the han#in#wall of the main !ein structure.
VEIN TETURES IN LO/ SULPHIDATION VEINS
Don)metallic and metallic mineral constitutin# the different hands in uart% !eins and ociated wall roc8 alteration and chemistry ha!e been described in detail in
4
Lo- S!l&i$ation (A$!la*ia%se*i#ite
uchanan:s -1961/ model -Fi#s. 1 and 2.a/ represents low sulphidation !eins scaled as a sin#le pass boilin# system and the %onin# patterns are interpreted in terms of the beha!iour of fluid under#oin# boilin#. $his models is based on empirical obser!ation on many epithermal systems in wester K'. $hus, this model does not apply !ery well in systems where ma#matic in!ol!ement and telescopin# are si#nificant -e.#. island arc settin# li8e the *hillipines/. $he outer, more per!asi!e propylitic alteration #rades to adularia)bearin# and intermediate ar#illic alteration near the principal !ein structure. $he propylitic) adularia)intermediate ar#illic alteration pro#ression towards the principal !ein is almost always obser!ed around !eins hostin# the ore shoots. $he wid of the alteration en!elope is #enerally proportional to the thic8ness of the !ein structureB with the intensity and width enhanced by the series of pre)mineralisation faults occurrin# in the footwall and the han#in#wall of the main !ein structure.
VEIN TETURES IN LO/ SULPHIDATION VEINS
Don)metallic and metallic mineral constitutin# the different hands in uart% !eins and associated wall roc8 alteration and chemistry ha!e been described in detail in many deposits and in some, form the basis of the !ein para#enesis -e.#. Hamilli and Chmoto, 1977B Ei8re, 1965B Aasade!all and Chmoto, 1977B 'lac8, 1960/. owe!er, textures at hand specimen scale and microscopic structures in uart% within, abo!e, below and away from the ore hori%on are rarely described. ecause #old occurs mainly in nati!e form and in microscopic dimensions in most of the epithermal deposits, it is a worthwhile research to establish if assembla#es of !ein attributes in hand specimen and microscopic scales can be used to discriminate economic, subeconomic and barren epithermal !eins. $hus, distinct !ein textures mi#ht differentiate hi#h #old #rade %ones !is a !is barren ones. owlin# and &orrison -1969/ established a scheme in distin#uishin# uart% !eins emplaced in the different mineralisation en!ironments -i.e, epithermal, porphyry, plutonic and slate belt/. $hey ha!e considered multiple #enerations of chalcedonic, microcrystalline and comb)textured uart% disposed as crustiform and colloform bands and coc8ade o!er#rowths as characteristic of epithermal !eins. $hus, the epithermal en!ironment #enerated a relati!ely uniue set of uart% textures.
214055452.doc
5
Fi#ure 2. n!ironments and processes of formation of ad!anced ar#illic systems. / represent low sulphidation !ein scaled as a simple pass boilin# system G uchanan -1961/B / ad!anced ar#illic abo!e porphyry Au systems G udson -1963/B A/ ad!anced ar#illic as surficial weatherin# products of sulfide)rich deposits -fter uino, 1993/.
&orrison et al. -1990/ and on# et al.-in press/ based on systematic description of uart% textures in different ueensland epithermal deposits, established a formal definition, classification of epithermal uart% textures and %onin# of textures in epitehermal !eins. $his proposed nomenclature of uart% sullplemented with the classification of dams -1920/ formed the framewor8 on which this re!iew was drawn. $he classification is primarily dwscripti!e but textures are #rouped into #enetic classes primary #rowth, replacement and recrystallisation textures. *rimary #rowth textures indicate precipitation in open space, replacement textures results fr om silica products partally or completely pseudomorphin# earlier mineral precipitates -e.# carbonates, sulphates, adularia/ and the recrystallisation textures encompass morpholo#ical attributes resultin# from the transformation of essentially metastable phases - e.# chalcedony, silica #el, opal, cristobalite, opal)A$/ to uart%. $hree !ein textural super%ones -Fi#.3/, broadly correspondin# to below boilin#, within boilin# and abo!e boilin# inter!als of the uchanan:s -1961/ model - on# et al., in pressB &orrison et al., 1991/ constitute assembla#es which can be identified at hand specimen scale. $hus proper identification of these textures can be usefil in predictin# the !ertical position of an epithermal !ein sample within a !ein system.
&orrison et al. -1990/ and on# et al.-in press/ based on systematic description of uart% textures in different ueensland epithermal deposits, established a formal definition, classification of epithermal uart% textures and %onin# of textures in epitehermal !eins. $his proposed nomenclature of uart% sullplemented with the classification of dams -1920/ formed the framewor8 on which this re!iew was drawn. $he classification is primarily dwscripti!e but textures are #rouped into #enetic classes primary #rowth, replacement and recrystallisation textures. *rimary #rowth textures indicate precipitation in open space, replacement textures results fr om silica products partally or completely pseudomorphin# earlier mineral precipitates -e.# carbonates, sulphates, adularia/ and the recrystallisation textures encompass morpholo#ical attributes resultin# from the transformation of essentially metastable phases - e.# chalcedony, silica #el, opal, cristobalite, opal)A$/ to uart%. $hree !ein textural super%ones -Fi#.3/, broadly correspondin# to below boilin#, within boilin# and abo!e boilin# inter!als of the uchanan:s -1961/ model - on# et al., in pressB &orrison et al., 1991/ constitute assembla#es which can be identified at hand specimen scale. $hus proper identification of these textures can be usefil in predictin# the !ertical position of an epithermal !ein sample within a !ein system.
214055452.doc
7
EPITHERMAL VEIN TETURES
Belo- Boilin+ Inte*)al ("*0stalline S!&e*1one
*late I and 2 *ro#radin# fluids which ha!e not under#one !i#orous boilin# is characteri%ed by wea8ly banded crystalline comb uart% as the dominant texture. eplacement textures -those texture mainly inherited from silica #el/ are !irtually absent. $his %one is also characteri%ed by poorly de!eloped crustiform bandin# in dominantly crystalline uart%. lso !eins in this %one carries sparse sulphides -e# #alena, sphalerite, chalcopyrite/ with sporadic anomalous #old #rades. I$enti2i#ation
Arystalline comb uart% -*late I/ texture is easily reco#ni%ed by parallel euhedral prisms of uart% perpendicular to the walls and pro+ectin# into the free space. Indi!idual crystals are #enerally %oned with each %one defined by multitudes of fluid inclusions and impurities. Indi!idual crystals could also ha!e alternatin# clear and mil8y %ones. Alosely)spaced uart% terminations from opposin# walls produce a do#)tooth: texture. O*i+in
$he coarse #rained and faceted nature of the uart% indicate slower rate of precipitation, #radual coolin# of the fluids and silica saturation controlled by uart%. $hus, relati!ely slow chan#in# conditions in an open space durin# crystal #rowth. *late 1 Arystalline comb uart% with wea8 crustiform texture.
214055452.doc
6
Plate 4
Plate 5 /it'in Boilin+ Inte*)al ("*!sti2o*3%"ollo2o*3 S!&e*1one
214055452.doc
9
*late 3)12 oilin# of hydrothermal fluids ha!e been to be a principal menchanism in precipitatin# precious metals and associated #an#ue in epithermal deposits. $he !i#orous boilin# inter!al in characteri%ed by increase in p, coolin#, loss of !olatiles and a faster rate of deposition. Indicators of hydrothermal boilin# include the presence of coexistin# liuid and #as)rich fluid inclusions in minerali%ed uart%, extensi!e !ein brecciation, !ein adularia, bladed calcite and pronounced crustiform) colloform bandin# in uart% !eins. $his inter!al #enerally coincides with the locus of economic mineralisation. $his %one is characteri%ed by consistent banin#. $here is a spectrum of textures and minerals within the boilin# inter!al but they are almost always entirely within bands. I$enti2i#ation
$he indi!idual bands could !ary from L10 µm to se!eral centimeters in thic8ness. ands are mainly composed of the different !arities of silica -i.e., chalcedonic, microcrystalline to crystalline uart%/, sulphides, clays -illite, mixed layer I"', 8aolinite/, adularia, and calcite. $he bands may simply be crustiform -*late 3/ and defined by !ariable composition and physical morpholo#y -#rain si%e, shape, colour/. Aomplex inter#rowths of sulphides and sulphosalts includin# sphalerite, #alena, chalcopyrite, pyrite, hessite and electrum could occur as bands intersitial to, and inter#rown with uart%, and blebs within uart% -*late 4/.
214055452.doc
10
&oss texture has an o!erall appearance similar to moss"dendritic !e#etation. Knder plane polari%ed li#ht, this texture occurs as fine #ranular a##re#ates of apparently spheroidal #rains outlined by concentric %ones of unwor8able fluid)rich and decrepitate inclusions and"or concentration of impurities on the rims of the uart% crystals. on# -1990/ and &orrison et al. -1990/ attributed the rounded forms to represent recrystallisation of silica #el preser!in# the ori#inal structure and impurities in the uart% crystals. $he adularia in epithermal uart% !eins exhibits characteristic textures which form an important component in differentiatin# the textural assembla#es -on#, 1991/. on# -1992/ has reco#ni%ed four textural types of adularia in epithermal uart% !eins -i/ coarse #rained crystalline, -ii/ fine #rained crystalline, -iii/ microcrystalline or moss, -i!/ needle pseudomorphs. lthou#h #enerally pseudomorphed by uart%, 8aolinite and illite, the adularia has distincti!e morpholo#ies such as bladed or needle)li8e pseudoorphs, rhombic trains and wed#e)li8e terminations that are typical of adularia in epithermal !eins -e.#. *late 10/. In hand specimen, the adularia in !eins occurs as euhedral to subhedral crystals disposed in sel!a#es alon# uart% bands, as a##re#ates of blaed or needle)li8e pseudomorphs or inter#rown with uart%. Aoarse #rained -M1mm/ adularia is dominantly associated with crystalline comb uart% -*late 7/. dularia crystals form euhedral, rhombic trains sometimes with wed#e)li8e terminations mainly disposed alon# !ein mar#ins. Aoarse #rained adualria also forms as a component in coc8ade bands. Fine #rained adularia which usually form tabular crystals is #enerally inter#rown with crystalline uart% -*late 7/. ##re#ates of acicular adularia form discrete bands in chalcedony. adiatin# a##re#ates of relic fine adularia needles also trans#ress chalcedony bands. O*i+in
$he repetiti!ely banded uart% fillin#s in ore hori%ons are indicati!e of open)space deposition and reflect fluctuatin# hydrothermal fluid conditions durin# mineral precipitation. anded uart% is so common in precious metal deposits that it has been considered a dia#nostic feature of epithermal deposits -uchanan, 1961/. Ro+e*s -1917/ first coined the term colloform: to describe the assembla#e of
spherical, botryoidal, remiform, stalactitic, and mammillary forms. $he colloform texture is thou#ht to be precipitated as a primary bandin# while in the #el state or by diffusional or rhythmic precipitation throu#h colloidal media -dams, 1920/. *reser!ation is thou#ht to occur !ia concentration and multista#e diffusion formin# the colloform textures. $hus, the uart% bands showin# colloform bandin# could indicate uart% #rowin# on earlier colloform bands or recrystallisation of primary banded silica #el with possibilities of silica #el)chalcedony)uart% order of transformation.
214055452.doc
11
S&!* -192;/ called the coc8ade texture as orbicular !eins: and su##ested that the
fra#ments were supported by hi#hly concentrated: or dense solution and successi!ely precipitated bands around the fra#ments from the enclosin# fluids. $his model is unli8ely in epithermal systems since hydrothermal fluids responsible for mineralisation are typically of !ery low salinity -i.e., !ery dilute/ and under low pressure. more li8ely explanation would be the deposition of uart% around pried off wallroc8 durin# hydrothermal brecciation, tri##ered by rapid pressure drops and release of !olatiles, could account for the open space: conditions around newly created clasts and subseuent silica precipitation. 'udden rupturin# would then result in decompressional boilin# and the emplacement of boilin# fluids into the resultin# fractures and breccias. Resi$en#e o2 Gol$
Nold is #enerally present in the ore inter!al as nati!e #old and electrum. Irre#ular #old and electrum #rains of different si%es occur as free #rains, #rains associated and within base metals, sil!er telluride, tetrahedrite, pet%ite and pyrite and in fractures -*lates 11 and 12/. /it'in Boilin+ 6one 7 List o2 Plates
*late 3 Arustiform uart% G alternatin# bands of chalcedonic uart%, fine clear and mil8y uart% and clear)crystalline comb uart%. $he other half of the core point to simple symmetrical bandin# parallel to the !ein wall. *late 4 ase metal)rich crustiform and colloform banded uart% !ein. ands consist of mil8y microcrystalline to crystalline uart%, coarse sulphides and adularia -pseudomorphed by hydrothermal uart% and 8aolinite/. *late 5 Aolloform bands G finely banded chalcedony with botryoidal surfaces. Aolloform bands alternate with mil8y and clear crystalline uart% bands. *late ; Aoarse crystalline sphalerite, #alena, chalcopyrite, hessite, tennantite)tetrahedrite and electrum occurrin# as bands as, matrix to a !ein breccia 6.01 #"t u o!er 1.0m. *late 7 reccia in the boilin# inter!al. Alasts of crustiform)colloform banded uart%)sulphide -center and upper ri#ht/ and !eined silicified wallroc8 -left/ o!er#rown by crustiform bands of fibrous chalcedony -#rey/ and saccharoidal uart% -white/ 7.92 #"t u o!er 1.0m. *late 6
214055452.doc
12
Aoc8ade texture G coc8ade rims of crustiform banded uart% around wallroc8 fra#ments. Fine bands of mil8y and clear crystalline uart% pro#resses to crystallinecomb uart% with do# tooth: texture. *late 9 adiatin# a##re#ate of acicular -needle)li8e/ ca!ities trans#ressin# crustiform) colloform banded chalcedonic uart%. Aa!ities contain uart% and 8aolinite pseudomorphin# adularia needles which #rew ori#inally with the banded uart%. *late 10 Aoarse #rained euhedral adularia -now lar#ely 8aolinite/ alon# !ein sel!a#es of mil8y crystalline uart%. $ypical adualaria in deep !eins. *late 11 Aomplex inter#rowths of #alena -#n/, chalcopyrite -cpy/, hessite -hs/ and electrum -elec/. Irre#ular electrum #rains occur as inclusions in #rains of hessite 6.91 #"t u o!er 1m. field of !iew O 0.17mm. *late 12 ##re#ates of coarse electrum #rains -finenessO;50/ formin# bands and clots within !ein uart%. Field of !iew O 1.02mm.
214055452.doc
13
Plate 8
Arustiform uart% G alternatin# bands of chalcedonic uart%, fine clear and mil8y uart% and clear)crystalline comb uart%. $he other half of the core point to simple symmetrical bandin# parallel to the !ein wall.
Plate 9
ase metal)rich crustiform and colloform banded uart% !ein. ands consist of mil8y microcrystalline to crystalline uart%, coarse sulphides and adularia -pseudomorphed by hydrothermal uart% and 8aolinite/.
214055452.doc
14
Plate :
Aolloform bands G finely banded chalcedony with botryoidal surfaces. Aolloform bands alternate with mil8y and clear crystalline uart% bands.
Plate ;
Aoarse crystalline sphalerite, #alena, chalcopyrite, hessite, tennantite)tetrahedrite and electrum occurrin# as bands as, matrix to a !ein breccia 6.01 #"t u o!er 1.0m.
214055452.doc
15
Plate <
reccia in the boilin# inter!al. Alasts of crustiform)colloform banded uart%)sulphide -center and upper ri#ht/ and !eined silicified wallroc8 -left/ o!er#rown by crustiform bands of fibrous chalcedony -#rey/ and saccharoidal uart% -white/ 7.92 #"t u o!er 1.0m.
Plate =
Aoc8ade texture G coc8ade rims of crustiform banded uart% around wallroc8 fra#ments. Fine bands of mil8y and clear crystalline uart% pro#resses to crystallinecomb uart% with do# tooth: texture.
214055452.doc
1;
Plate >
adiatin# a##re#ate of acicular -needle)li8e/ ca!ities trans#ressin# crustiform) colloform banded chalcedonic uart%. Aa!ities contain uart% and 8aolinite pseudomorphin# adularia needles which #rew ori#inally with the banded uart%.
Plate 4?
Aoarse #rained euhedral adularia -now lar#ely 8aolinite/ alon# !ein sel!a#es of mil8y crystalline uart%. $ypical adualaria in deep !eins.
214055452.doc
17
Plate 44
Aomplex inter#rowths of #alena -#n/, chalcopyrite -cpy/, hessite -hs/ and electrum -elec/. Irre#ular electrum #rains occur as inclusions in #rains of hessite 6.91 #"t u o!er 1m. field of !iew O 0.17mm.
Plate 45
##re#ates of coarse electrum #rains -finenessO;50/ formin# bands and clots within !ein uart%. Field of !iew O 1.02mm. Abo)e Boilin+ Inte*)al ("'al#e$oni#, "a*bonate%Re&la#e3ent S!&e*1one Plates 48 an$ 49 Chalcedonic quart, !laded pseudomorphs and massive car!onate characterie this te"tural one (Fig.#). Massive and$or !laded car!onate ma% or ma% not remain, depending on whelther the s%stem is isolated from later fluids effectivel%. I$enti2i#ation
214055452.doc
16
Aommon replacement textures include uart% a##re#ates mimic8in# bladed carbonate and adularia precursors. Ksually, massi!e chalcedonic uart% has patches or crude bands of lattice blades. *late 13 shows calcite blades replaced by elon#ated uart% crystals. $he extreme thinness of blades, lamellar partin#s, rhombohedral clea!a#e and wed#e)li8e terminations are characteristics of primary bladed carbonate -&orrison et al., 1990/. 'imilar blades composed of an8erite occur as late infill in !eins. O*i+in
dams -1920/ noted that the uart% laminae do not #enerally pseudomorph the rhombohedral clea!a#e directions of the pre)existin# calcite, but replace alon# a set of parallel planes. $hese planes represent the prominent lamellar partin# parallel to the basal pinacoid of carbonate minerals. eplacement thus proceeds more easily alon# these planes than planes normal to them. $he uart% formin# the parallel blades exhibits !ariable #rain si%es or with different contents of impurities. $his selecti!e replacement produces a set of #enerally closely)spaced parallel planes. *late 13 (attice blades G partial to complete replacement of carbonate blades by a##re#ates of uart% #rains. *late 14 Ahalcedonic uart% G massi!e chalcedonic uart% with >phantom? !einlets. Faint multidirectional L0.2 cm wide !einlets trans#ressin# chalcedonic uart%. Knder the microscope, these !einlets either disappear or only shom subtle outlines of the !einlet mar#ins.
214055452.doc
19
Plate 48
(attice blades G partial to complete replacement of carbonate blades by a##re#ates of uart% #rains.
Plate 49
Ahalcedonic uart% G massi!e chalcedonic uart% with >phantom? !einlets. Faint multidirectional L0.2 cm wide !einlets trans#ressin# chalcedonic uart%. Knder the microscope, these !einlets either disappear or only shom subtle outlines of the !einlet mar#ins. A"ID SULPHATE (ADVAN"ED ARGILLI" SYSTEMS
d!anced ar#illic alteration appears to be an inte#ral part of the upper portions of ma#matic hydrothermal systems -udson, 1963B eald, 1967B 'illitoe, 1992/. owe!er, they are not exlusi!e to these systems since assembla#es somewhat similar to ad!anced ar#illic assembla#es can form under super#ene"weatherin# conditions as exemplified at Areede, Aolorado -'te!en and aton, 1975 and eth8e and ye, 1979/, ound &ountain, De!ada -'ander, 1966/, Hidston and &t. (eyshon, ustralia -ird et al., 1969/. &ore recently, llibone -1993/ in his wor8 at $emora mine, am"(ilydale and *ea8 ill in eastern ustralia has shown that ad!anced ar#illic
214055452.doc
20
alteration may also de!elop synchronously with re#iaonal deformational e!ents and be focused alon# ma+or shear %ones. In porphyry copper systems, ad!anced ar#illic alteration has been #enerally obser!ed to o!erprint the porphyry system. udson -1963/ interprets this as a late sta#e process in porphyry e!olution. In contrast 'ilitoe -1964/ su##ested that it may ha!e formed in the early sta#es of porphyry history but be of lon#er duration, conseuently o!erprintin# the other porphyry)related alteration %ones. 'patially, ad!anced ar#illic alteration occurs from 0 to M 18m abo!e the porphyry systems implyin# a deep to shallow o!erprintin# relationship -Eilla and 'illitoe, 1991B 'illitoe, 1992/. $his deep o!erprintin# can be explained by the retreat of the exsol!in#, coolin# pluton -udson, 1963/ or alternati!ely, the #ra!itational collapse of the system coupled by the rapid de#radation of the paleosurface by erosion coe!al with hydrothermal acti!ity -'illitoe, 1991/. In this scenario, the ad!anced ar#illic alteration is related to the de!elopment of the porphyry system, therefore a #radation from deeper le!el hi#h temperature ad!anced ar#illic assembla#e -andalusite, corundum/ to intrmediate temperature -pyrophyllite, diaspore/ to shallow le!el low temperature assembla#e -8aolinite, chalcedonic silica, alunite/ is expected. t the deeper le!els of the ad!anced ar#illic alteration, relics of early porphyry alteration -potassic, phyllic)ar#illic and proplitic/ can be reco#ni%ed. Furthermore, the #eometry and morpholo#y of these porphyry)related ad!anced ar#illic systems should be broadly compatible with the shape of the exsol!ed !apour plume which is rou#hly circular -udson, 1963/. Aorollary to this concept of porphyry)related or intrusi!e) centred ad!anced ar#illic alteration is that structures feedin# the ad!anced ar#illic system should be broadly dippin# towards the heat source. owe!er, these feeders could also be multidirectional compatible, with the de!elopment of deeper le!el potassic"'AA !eins, but still centred around the porphyry system. d!anced r#illic $extural and Aommpositional onin# bo!e Nold)ich *orphyry Aopper eposit $he #old)rich porphyry copper deposit is located on the southern flan8 of the currently acti!e &t. *inatubo strato!olcano, in the pro!ince of ambales, (u%on island, *hilippines. It contains a pro!en reser!e ot 190 million tones at 0.355P Au, 0.7; #"t u and 2.0 #"t u based on 0.20P Au cut)off. Late*al "o3&ositional 6onation
$he main sta#e ad!anced ar#illic at the i%on porphyry copper deposit in the *hillipines exhibits textural as compositional %onation as depicted in Fi#. It can be di!ided into an upper per!asi!e hori%ontal blan8et and lower %one of structurally controlled !eins and sel!a#es. $he per!asi!e portion is about 150 to 200 metres thic8 where it forms an undulatory surface or fin#er)li8e bodies drapin# and o!erprintin# the 'AA, potassic and propylitic alteration %ones -*late 15/. Dear the present surface
214055452.doc
21
the ad!anced ar#illic alteration is per!asi!e, texture destructuti!e and completely obliterates all pre!ious alteration. $he per!asi!e blan8etli8e portion of the main sta#e ad!anced ar#illic alteration exhibits lateral compositional %onin# -Fi# 4/. It is centred on massi!e silica pods which are dominantly composed of microcrystalline uart%, alunite and late diaspore. $hese pods are >chalcedonic? in character and are porous, !u##y and brecciated -*late 1;/. n!elopin# these pods is an inner pyrophyllite dominant en!elope -*late 17/ and an outer 8aolinite en!elope -*late 16/. $he pyrophllite dominant en!elope is composed of fine #rained uart%, pyrophyllite, illite, 8aolinite while the outer en!elope is mostly fine uart% and 8aolinite. $he boundaries between these %ones are transitional. In #eneral as one mo!es away from the pods, the alunite and uart% proportions diminishes, the pyrophyllite)illite ratio decreases and the intensity and the crystallinity of 8aoline -i.e dic8iteQ/ ad+acent to the pods to less ordered but more abundant away from the pods. *yrophyllite is intimately associated with massi!e silica pods but away from them, illite appears and is inter#rown with pyrophyllite. 'imilarly, the abundance of pyrophyllite decreases with increasin# distance from the pods. Cre minerals pyrite, chalcopyrite, tetrahedrite)tennantite, pyrite ± enar#ite are preferably associated with the massi!e silica pods. Ve*ti#al "o3&ositional 6onation
$he !ertical %onation in the main ad!ance ar#illic alteration at i%on is defined by a transition from predominant uart%)musco!ite"illite)8aolinite ± base metal assembla#e at depth to a predominant uart%)pyrophyllite"illite G 8aolinite)alunite)diaspore assembla#e in the upper portions of the system. In the field this transition is mar8ed by the per!asi!e, texture destructi!e of roc8s in the upper %one narrowin# down to discrete fractures of the uart%)musco!ite %one at depth. $he transition between the two %ones is mar8ed by the coexistence of pyrophyllite G illite, the common presence of ore minerals G pyrite, chalcopyrite, marcasite and tennantite)tetrahedrite and the compositional chan#e of tennantite)tetrahedrite. $he apperance and coexistence of illite)pyrophyllite within the immediate en!elope of the massi!e silica pods occurs both laterally and !ertically. $he ore minerals pyrite, chalcopyrite, marcasite and tennantite)tetrahedrite are common to both the deep and shallow portions of the system. I$enti2i#ation
&assi!e uart%)alunite alteration is often mista8en for silicification because in surface outcrop it forms lar#e hard boulders. $he presence of alunite can often be confirmed simply by scratchin# a freshly bro8en surface with the point of a pic8B alunite -hardness 4/ scratches to a white powder where as silica dosen:t scratch and a sil!er strea8 of the pic8 is left across the roc8. $he alunite is commonly coarse
214055452.doc
22
enou#h to be reco#ni%ed with a hand lens, it has a characteristic crystalline appearance with numerous rectan#ular faces which #lint in the sun: -after a8er, 1993/. field tes, de!eloped by 'challer -from INA $echnical Dotes, !.1, no. 3, 1965/?R boil the powdered sample with water or with hydrochloric acid for se!eral mi+utesB after allowin# the powder to settle, pour off the liuid and repeat the operation to insure the remo!al of all soluble sulphates. ry the powder and heat to a dull red. #ain boil in water and, after settlin#, pour offsome of the clear liuid. $o this add a small fra#ment or a solution of barium chloride. If the mineral is alunite, a hea!y white precipitate will form. arium sulphate causes precipitation of barite. e sure the water used in the test does not contain sulphate -usually sweetish in taste/?. *late 15 Field photo of steeply dippin# ad!anced ar#illic cross)cuttin# potassic alteration. *late 1; epresentati!e hand specimen of massi!e silica ± alunite pod showin# !u##y, porous texture. In some cases the !u#s are filled with euhedral uart%, alunite and pyrite.
214055452.doc
23
Plate 4:
Field photo of steeply dippin# ad!anced ar#illic cross)cuttin# potassic alteration.
Plate 4;
epresentati!e hand specimen of massi!e silica ± alunite pod showin# !u##y, porous texture. In some cases the !u#s are filled with euhedral uart%, alunite and pyrite.
214055452.doc
24
Plate 4<
epresentati!e hand specimen of 8aolinite dominant en!elope.
Plate 4=
epresentati!e hand specimen of an ad!anced ar#illic alteration -uart%,pyrophyllite"illite, 8aolinite/ with relics of porphyry)related alteration. HYDROTHERMAL ALTERATION IN A LO/ SULPHIDATION BONAN6A VEIN SYSTEM $he *a+in#o epithermal field in ueensland, ustralia encompasses around ;5 s. 8m. $he only producin# !ein is the 'cott (ode which contains 1.23 million tons of 9.4 #"t #old and 36.0 #"t sil!er -about 12 tons of #old and 47 tons of sil!er/. mon# the !olcaic)hosted epithermal #old)sil!er deposists, 'cott (ode !ein is bonan%a with shoots !ery hi#h #old)sil!er #rades -M100 G 2000 #"t uB M50 to 5000 #"t #/. round 17P of the resource contains more than 30.0 #"t u. $he 'cott (ode #old) sil!er deposit pro!ides an example of an epithermal deposit of the low sulphidation type proposed by
214055452.doc
25
$able 1. shows the mineralo#ical components of the !arious alteration types. $he !arious alteration assembla#es are best exhibited in andsitic !olcanics. $he stable hydrothermal mineralo#ical components occurrin# in the wall roc8s ad+acent to the principal !ein is illustrated in Fi#. 5. $able 2 presents a summary of alteration types, assembla#es and their relation to the principal !ein. P*o&0liti#
$he propylitised roc8 units retain primary textures and fabric. I$enti2i#ation
$he propylitic assembla#e in the andesitic roc8s is characteri%ed by partial to complete albite)chlorite)uart%)epidote)calcite replacement of the pla#ioclases, chlorite)calcite)an8erite)epidote)actinolite alteration of pyroxenes, pyrite)pyrrhotite) chalcopyrite)rutile replacement of primary ma#netite"ilmenite and an ele!ated content of calcite as replacement and as cross)cuttin# !einlets -$able 1/. 'electi!e replacement of alteration minerals within this assembla#e per!erse primary phenocrysts and #roundmass textures. $he labradorite to andesine pla#ioclase #rains are mainly replaced by nearly pure albite startin# at the rims and alon# clea!a#e planes and fractures. Aoarse boo8s of chlorite locally with pyrite, epidote and calcite replace mafic phenocryst. *la#ioclase and mafic phenocrysts are partially replace by epidote. pidote locally occurs as radiatin# fans, in ca!ities or in !eins with calcite and uart%. ctinolite with chlorite and calcite partally replace mafic phenocrysts. $he #roundmass consists of albitised pla#ioclases, chloritised pyroxene prisms, primary apatite needles and minor rutile, pyrite, pyrrhotite and chalcopyrite after primary ma#netite. @l!i$%3ine*al E!ilib*ia D!*in+ P*o&0liti# Alte*ation
eplacement reactions from pla#ioclases and pyroxenes to epidote, chlorite, and albite point to essentially isochemical beha!iour of the reactions. $he ori#inal constituents -pla#ioclase, pyroxene/ are altered with addition of water, AC 2 and little addition or remo!al of other elements. For example, alteration of labradorite to epidote reuires Aa =2 to be added to the reaction. $his Aa =2 could be con!eniently be deri!ed from further labradorite replacement by albite.
214055452.doc
2;
Alte*ation T0&e
Asse3bla+es
e0 Tet!*e
*ropylitic
Ahlorite)albite) epidote) uart%=actinolite=pyr ite)pyrrhotite)rutile) chalcopyrite
lbite after primary *er!asi!eB increasin# pla#ioclaseBchlorite,c intensity towards the alcite, epidote after principal !ein structure pyroxenes
(ow) temperature potassic
uart%)adularia) calcite)chlorite)2& musco!ite)pyrite) chalcopyrite)rutile
dularia after albite
Intermediate r#illic"'ilicic
uart%)illite)mixed) uart, illite, mixed) n!elopes the precious layer I"' =8aolinite) layer I"' after metal)bearin# !einsB pyrite)chalcopyrite feldspar and chlorite per!asi!e and intensely de!eloped in the ore iner!alB silicic %ones form inner haloes to the principal !ein
(ate)sta#e Haolinite
Haolinite)uart%) pyrite
Haolinite o!erprint xtensi!e o!erprint on on illite, mixed)layer all alteration types I"'
'iderite)an8erite) calcite=pyrite
'iderite after all. hydrothermal alteration minerals
(ate)sta#e Aarbonate
Relation to P*in#i&al Vein
t'e
*er!asi!e below the intense inter ar#illicB does not contain si#nificant minerali%ation
*er!asi!e distributionB intense alon# permeable units
Lo- Te3&e*at!*e Potassi# (a$!la*ia
$he adularia)bearin# alteration is desi#nated as low)temperature potassic -adularia/ to emphasi%e the difference with the potassic or H)silicate alteration type encountered in the porphyry copper en!ironment. $he low temperature monoclinic H)feldspar which crystalli%ed from hydrothermal solution particularly as open)space or !ein fill in the epithermal en!ironment is termed adularia -eer, owie and ussman, 19;;B Aerny and Ahapman, 196;/. $he classic potassic alteration in the porphyry copper en!ironment is characteri%ed by the hi#h temperature -M300oA/ assembla#e of orthoclase, secondary biotite, anhydrite, apatite, pyrite, chalcopyrite, ma#netite and bornite -$itley and eane, 1975/.
214055452.doc
27
I$enti2i#ation
$he alteration assembla#e includes adularia, uart%, chlorite, calcite, musco!ite -2& mica/, pyrite, chalcopyrite and rutile"anatase -$able 1/. $his altertion type is conspicuous in hand specimen le!el as coalescin# reddish pin8 en!elopes around the uart%)adularia)calcite !einlets -*late 19/. 'odium cobaltinitrite stainin# of the host roc8s employin# procedures recommednded by utchison -1979/ confirmed the presence of per!asi!e H)feldspar -*late 19/. $he distincti!e texture of this alteration is the partial to complete replacement of pla#ioclase phenocryst and the alteration albite -from earlier propylitic/ by nearly pure adularia. dularia is almost always accompanied by uart%, musco!ite -2& mica/, chlorite, calcite, rutile"anatase and pyrite. Cptically, the adularia #rains are clear #rey with distincti!e anomalous, mottled extinction formin# a tle:)li8e appearance. $he adularia replaces primary pla#ioclase and albite alon# #rain boundaries and clea!a#e planes and de!elops in fractures -*late 20/. dularia may also occur as o!er#rowths: alon# ed#es of pla#ioclase #rains or as micro!einlets within, ad+acent to and cuttin# primary pla#ioclase sites. $he albite #rains and albitised pla#ioclases show pro#ressi!e replacement by adularia particularly in areas where there is e!idence of enhanced fluid flow -e.#. proximal to faults, uart% !eins/ and fa!ourable litholo#ies. 'econdary uart% is more prominent alteration product of the #roundmass relati!e to the propylitic altertion. $he distincti!e pla#ioclase microlites preser!ed in the propylitic alteration are replaced by fine to medium #rained eui#ranular a##re#ates of uart% and sericite -*late 20/. $he mica)li8e minerals associated with adularia show sharp symmetrical reflection in the S)ray diffracto#ram at 10 Å confirmin# 2& mica structure. $his 001 crystallo#raphic trace is different from the broader reflection of illite associated with intermediate ar#illic alteration. Illite also has lower birefrin#ence than sericite. *yrite and occasional chalcopyrite are present mainly as disseminations and are occasional in !einlets with adularia. $he sulphides associated with the low temperature potassic alteration #enerally attain less than 2 !olume percent. $hus the 8ey textures to this alteration transition include the adularia replacement of albite and the dramatic increase in silicification of prominent pla#ioclase laths in the #roundmass by medium #rained eui#ranular uart%. @l!i$ Mine*al E!ilib*ia D!*in+ A$!la*ia Alte*ation
$he replacement of albite to adularia indicates reactions which are lar#ely potassium metasomatism reactions accompanied by silicification. *otassium metasomatism and silicification are common for ascendin# and coolin# solutions affectin# roc8s in or
214055452.doc
26
close to ma+or upflow %ones in acti!e #eothermal systems and many epithermal deposits -Ni##enbach, 1964B uchanan, 1961/. rowne -1979/ has related abundance of adularia with increasin# permeability and increased fluid roc8 ratios in the ma+or upflow re#ions. $he transition of propylitic to low)temperature potassic is also related with coolin# of the pro#radin# fluid. $he probable mechanisms for coolin# includeB i/ mixin#, ii/ coolin# by massi!e incursion of #roundwaters, iii/ boilin#, or i!/ conducti!e heat loss attendin# fluid con!ection. *late 19 (ow temperature potassic -adularia/ altered andesite porphyry intrusi!e. istincti!e pin8ish alteration on hand specimen scale. $he yellowish stains in the !einlets and !einlet en!elopes are effects of the sodium cobltinitrate stainin# which confirms thepresence of per!asi!e H)felspar. *otassic assembla#e consists of adularia)chlorite) calcite)uart%. *late 20 *hotomicro#raph. Hey replacement texture in potassic -adularia/ alteration. *la#ioclase and albitised pla#ioclase -lab/ partially replaced alon# rims by adularia -adu/. Field of !iew O 1.06 mm.
Plate 4>
214055452.doc
29
Plate 5? Inte*3e$iate A*+illi#
$he intermediate ar#illic alteration type is per!asi!e. It is superimposed on propylitic and low temperture potassic -adularia/ alteration types. $he distribution of intense intermediate ar#illic alteration where primary textures are almost always completely obliterated is shown in fi#ure 5. I$enti2i#ation
$he intermediate ar#illic alteration is a texture)destructi!e a##re#ate of essential ilite -showin# broad 10 T reflection in S)ray diffracto#ram/, and"or mixedGlayer illite"smectite, uart%, and pyrite -$able 1/. Alay composition su##ests that the alteration is best classified as intermediate and not as ad!anced ar#illic -emley and @ones, 19;4/. $he relic phenocryst sites -i.e., of pla#ioclase, alteration albite and adularia/ in andesitic !olcanics and co#enetic intrusi!es are now masses of L50 micron)si%ed fla8es of illite and"or mixed)layer illite"smectite, and uart% -*late 21 and 22/. $he essential clays dominantly occupy pore spaces between uart% #rains pointin# to its hydrothermal ori#in. In hand specimen, the clay)uart% are li#ht #rey to pale #reen in colour. $he #roundmass is altered to patches of clays, anhedral uart%, rutile"anatase -after ma#netite"ilmenite/, pyrite and occasional chalcopyrite. Haolinite o!erprints intermediate ar#illic alteration and is therefore mainly late)sta#e. Sili#i#
214055452.doc
30
$he outer intermediate ar#illic en!elopes to the principal !ein structure and the low temperature potassic -adularia/ types pro#ress inward to a silicic %one characteri%ed by moderate to stron# de!elopment of multidirectional uart% !eins"!einlets and almost monomineralic alteration of the host roc8s -Fi#. 5/. Cutside the principal !ein structure, stron#ly silicified host roc8 also forms metre)scale haloes on discontinuous auriferous uart% !eins or stowc8wor8 %ones. $he silicic haloes #rade outward to the more per!asi!e en!elopes of intermediate ar#illic alteration. 'ilicification are also locally de!eloped alon# the faulted contact betwwn the uart%one sandstones and andesitic !olcanics. I$enti2i#ation
Alays of the intermediate ar#illic alteration and relic pla#ioclases by fine to medium mosaic uart% -*late 21/, illite and 8aolinite. Illite and subordinate 8aolinite are seen to be inter#rown with uart%. 'ilicic alteration renders almost total obliteration of the primary textures lea!in# only #host: textural features. *yrite occurs as dissemination in silicificied roc8 and may attain 3)5 !olume P. 'ilicic alteration also hosts disseminated sphalerite, #alena and chalcopyrite.
@l!i$%3ine*al E!ilib*ia D!*in+ Inte*3e$iate A*+illi# an$ Sili#i# Alte*ation
$he intermediate ar#illic assembla#e indicates intense hydro#en metasomatism of earler minerals. $he transition from potassic -adularia/ to intermediate ar#illic represents a shift of p, temperature, cation " = ratio in the mineral)fluid euilibria. $he shift to intermediate ar#illic stability field can be accomplished by coolin# or mixin# -=attendant coolin#/ with more acidic fluids. *late 21 $ypical intermediate ar#illic)altered andesitic la!a. *artial to complete obliteration of primary textures. *la#ioclase phenocrysts replaced by a##re#ates of uart%, mixed) layer I"' and pyrite. *late 22 *hotomicro#raph. Hey replacement texture in intermediate ar#illic alteration. Illite) uart% assembla#e replacin# pla#ioclase phenocrysts. Nroundmass consists of uart%, illite. (ate siderite replacin# clays and feldspars. Field of !iew O 1.15mm.
214055452.doc
31
Plate 54
Plate 55
*hotomicro#raph. Hey replacement texture in intermediate ar#illic alteration. Illite) uart% assembla#e replacin# pla#ioclase pehnocrysts. Nroundmass consists of uart%, illite. (ate siderite replacin# clays and feldspars. Field of !iew O 1.15mm. Late%sta+e aolinite
Haolinite has been obser!ed to occur e!en to depths of ;00 metres below the present surface. It is emphasi%ed here that the presence of per!asi!e 8aolinite does not
214055452.doc
32
indicate typical ad!anced ar#illic alteration such as alunite, cristobalite, nati!e sulphur, !u##y uart% and copper sulphosalts are absent. I$enti2i#ation
$hin section studies point to dominant 8aolinite o!erprintin# the clays associated with silicic and intermediate ar#illic alteration -plate 23/. Haolinite is #enerally accompanied by uart% and pyrite. Haolinite alson occurs as a replacement of bladed, needle)li8e minerals -adulariaQ/ within uart% !ein sel!a#es. Haolinite is seen as low bire firin#ent clay and characteri%ed by a sharp pea8 at around 7.10 T below the base of oxidation su##estin# #ood crystallinity of 8aolinite -A. Auff, pers.comm.., 1990/ has been employed to indicate probable hydrothermal ori#in. 'uper#ene 8aolinite #enerally shows a broad S pea8 due to poor crystallinity. owe!er, S diffracto#rams of 8aolonite in post)mineralisation @urassic co!er at the Aracow epithermal #old deposit in central ueensland also show similar sharp reflections -&.
eplacement of illite and chlorite by 8aolinite indicates increased hydro#en acti!ity and decreasin# temperature. &ixin# of the deep ascendin# chloride waters with the cooler, acidic #roundwaters will mo!e the fluid in euilibrium with the intermediate ar#illic alteration to the 8aolinite stability field because of the decrease in p and temperature of the fluid. $he abundant 8aolinite input is thou#ht to be related to the continuous collapse of the hydrothermal system tri##ered by massi!e incursion of coolin# acidic fluids alon# ma+or structures. $hus the extensi!e 8aolinite o!erprint is interpreted to be tri##ered by acidid waters descendin# alon# permeable of the hydrothermal acti!ity. *late 23. Hey texture in late)sta#e 8aolinitic alteration. *henocrysts, lithic are totally replaced by 8aolinite, uart% and pyrite. $exture destructi!e.
214055452.doc
33
Plate 58.
@e**oan "a*bonate
$he propylitic, low temperature potassic -adularia/, intermediate ar#illic, silicic and 8aolinite alteration types are o!erprinted by per!asi!e reddish brown to oran#e brown)coloured carbonate alteration. $he carbonates clearly replace earlier clays in the phenocryst sites and occur in !eins cuttin# earlier uart% !eins. Ferroan carbonates also cement late)mineralisation hydrothermal breccias. S and microprobe analyses point to carbonates consistin# of calcite, an8erite -AaU,Fe =2, &nV UAC3V2/ and siderite -FeAC3/. $he dominant carbonate !arieties howe!er are siderite and an8erite based on S analyses. Knder the microscope, an8erite and siderite replace earlier calcite, clays and other relic hydrothermal minerals. SYSTHESIS O@ ALTERATION PARAGENESIS
model for the pro#ression of alteration mineral assembla#es related with the epithermal mineralisation is presented in $able 3. ltered litholo#ies contain se!eral o!erprintin# hydrothermal minral assembla#es as well as super#ene minerals. Neometry, o!erprintin# relationships of minerals and cross)cuttin# relationships of coe!al !eins"!einlets allow the seuence of alteration assembla#es to be established. lteration on andesitic !olcanics best exhibits the para#enetic relationships. Fi#ure summarises the mineralo#ical components occurrin# in the wall roc8s ad+eacent to the principal !ein at the 'cott (ode. C!erprintin# and crosscuttin# relationships su##est propylitic is earliest, pro#ressi!ely followed by the low temperature potassic -adularia/, then intermediate ar#illic and
214055452.doc
34
silicic alteration. &ain sta#e ore)bearin# !ein emplacement immediately followed adularia and was probably lar#ely coe!al with silicic and intermediate ar#illic alteration based on clasts of the latter in the !ein structures and similarity in mineralo#ical components. Haolinite and ferroan carbonate occur as late)sta#e alteration products post)datin# the main precious metals mineralisation. 'uper#ene minerals consist of alunite, #ypsum, 8aolinite, and smectite. &ain sta#e ore)bearin# !ein replacement is interpreted to immediately follow and probably be lar#ely coe!al with silicic and intermediate ar#illic alteration based onB 1/ clasts of the latter in the !ein structures, 2/ similarity in #an#ue mineralo#ical components i.e., uart% inter#rown with illite -and"or mixed)layer I"'/ and locally 8aolinite, and 3/ silicic and intermediate ar#illic alteration form symmentrical %oned en!elopes around the auriferous !eins. irect e!idence for dominant late)sta#e 8aolinite alteration is seen from pseudomorphous replacement of !ein adularia alon# !ein sel!a#es and 8aolinite replacement of illite and"or mixed)layer I"'. 8aolinite also replaces phenocryst sites within the silicic %one whilst secondary uart% in the #roundmass remains unaffected. $he ferroan carbonate alteration assembla#e o!erprints all alteration types. Cne or more carbonate phases are often seen clearly replacin# and enclosin# remnant 8aolinite in phenocryst sites. $he ferroan carbonate alteration type is clearly late) sta#e and postdates the main sta#e uart% !ein emplacement as indicated by the followin# features -i/ carbonate in !eins and as breccia matrix contain fra#ments of !ein uart%, -ii/ !eins containin# uart%, illite and adularia are refractured, rebrecciated and filled by carbonates, and -iii/ carbonate replaces clays and adularia.
ORIGIN O@ THE CUART6 SULPHIDATION VEINS
TETURES
IN
LO/
$he ori#in and %onation of the uart% textures can be interpreted in terms of fluid conditions and physico)chemical processes affectin# silica and calcite solubilities
214055452.doc
35
prior to and coe!al with !i#orous boilin# of hydrothermal fluids -Fi#. ; and 7/. $he %onin# of uart% -=calcite/ textures depends on the 8inetics of dissolution and rate o f precipitation of the !arious forms of silica and calcite. Aoolin# is the most si#nificant factor controllin# the solubility of the !arious polymorphs of uart% under epithermal conditions. $he continuous drop in temperature increases silica saturation of the fluids. $he morpholo#y of the precipitated silica depends on the pre!ailin# hydrothermal conditions and the de#ree of silica supersaturation of the solution.
*ro#radin# fluids which ha!e not under#one boilin# is characteri%ed by wea8ly banded crystalline comb uart% as the dominant texture. $he coarse #rained and faceted nature of the uart% indicate slower rate of precipitation, #radual coolin# of the fluids and silica saturation controlled by uart% -Fi#. ; and 7/. Aoolin# as a conseuence of boilin#, had been indicated earlier to ma8e the solution undersaturated with respect tocalcite and thus will hinder calcite precipitation. owe!er, the effect of AC 2 loss is much #reater than the effect of coolin#. Further loss of AC2 will not cause precipitation of calcite but will lead to its dissolution. eplacement of calcite -or other carbonates/ by uart% producin# bladed uart% pseudomorphs is a conseuent of retro#rade solubility of silica. $hus boilin# presents an effecti!e mechanism of precipitatin# calcite from epithermal fluids. $he continuous loss of 2C to the !apour phase durin# prolon#ed boilin# will increase the concentration of 'iC 2 in the remainin# fluid, thus more silica will be precipitated. $he crustiform bands reflect episodic fluid batches and repeated boilin# episodes. Eein breccias with clasts of earlier)formed banded uart% also point to multi)#eneration boilin# e!ents. $he sealin# of the fluid channelways producin# locali%ed o!erpressure because of restriction to fluid circulation can also #enerate the !ein breccias ubiuitous in ore shoots.
214055452.doc
3;
'i#nificant and rapid coolin# because of adiabatic boilin# and the continuous loss of water to the !apour phase can ma8e the fluid hi#hly supersaturated with respect to amorphous silica and cause the formation of silica #el -on#, 1992/. $he textures that are probably inherited from silica #el include colloform, moss and related recrystallisation textures. $he increased le!els of silica supersaturation may result in the deposition of amorphous silica at the surface in the form of sinters or silica"ad!anced ar#illic caps common in many epithermal deposits. $hus, !eins with hi#h #rade u #rade in the boilin# inter!al will #enerally be associated with well) de!eloped crustiform)colloform bandin# -=sulphide bands/, stron# !ein brecciation, and pronounced moss and needle adularia. TRANSPORT AND METAL PRE"IPITATION ME"HANISMS
Fi#ure 6 presents the results of chemical euilibrium computations done to model transport, precipitation and dissolution of the si#nificant mineralo#ical components associated with adiabatic boilin# in a low sulphidation !ein deposit. $he fluid composition used are #i!en in obis -1994/. $he results presented fi#s. 6a to 6e illustrates the profound effects of adiabatic boilin# on the total molalities of the si#nificant metals ions, molalities of important dissol!ed #ases, and precipitation of #an#ue minerals and metals -expressed in the more readable #"t unit of the precipitated solids/. lso presented is the acti!ity of =. all uantities presented in Fi#s. 6a)6e are per8ilo#ram of fluid:. $he initial fluid at 300oA is ta8en to be in euilibrium with uart%, H)feldspar, albite, musco!ite, chlorite, pyrite and calcite. t 300oA, the homo#enous aueous phase is satured with respect to #old, *b', and n' but respect to sil!er. $his homo#enous aueous phase is simulated to adiabatically boil successi!ely from 300oA. For example, a 26 wt P loss of fluid due to boilin# induces the fluid to cool from 300 oA. For example, a 26 wt P loss of fluid due to boilin# induces the fluid to cool from 300oA to 161oA. apid loss of the more !olatile components li8e AC2, Al and 2' -Fi#. 6a/ results in the increase of p -fi#. 6b/. $he loss of AC 2 in early boilin# si#nificantly increases the p. $his increase in p leads to the deposition of potash feldspar, carbonate and pyrite -Fi#. 6c/. howe!er, with continuous coolin#, calcite will dissol!e and most li8ely be replaced by the deposition of adularia in the initial sta#es of boilin#. $he beha!iour of pyrite precipitation !irtually parallels that of adularia -Fi#.6c/. uart% constitutes more than 96 wt P of the total minerals precipitated and the amount deposited increases with decreasin# temperature -Fi#.6c/. multiple e!ents of uart% deposition due to repeated boilin# of hydrothermal fluids results in the crustiform banded uart%. 'ince u -'/2 G is the dominant #old complex. $he loss of 2' from the fluid will cause #old to precipitate but the p increase will stabili%e u-'/ 2). $he increase in p from 20 oA temperature drop and resultin# ; wt P loss of fluid will not precipitate #old -see also Fi#.6b and 6d/ because the solution becomes more undersaturated in #old. Cnly after the fluid has boiled to temperatures below 275oA will #old and sil!er precipitate -Fi#. 6d and 6e/. t temperatures below 275oA processes that will reduce
214055452.doc
37
a 2' will cause #old precipitation. t these temperatures, a 8ilo#ram of fluid is capable of depositin# 10)11 #"t u and 240)2;7 #"t # of the precipitated solids -Fi#.6d/. n increase in p will destabili%e the sil!er chloride complex. Aoolin# by boilin# will cause the hydrothermal fluids to be saturated with respect to sil!er. t temperature immediately below 275 oA sil!er, li8e #old, will start to precipitate -Fi#s. 6b)6d/. $hus the initiation of two)phase separation will not immediately deposit sil!er. t 200oA, sil!er solubility is approximately 1 ppb representin# a decrease in sil!er solubility by 100 times. Aoolin# to 242 oA !ia boilin# will precipitate 91P of sil!er out of solution. $he increasin# p destabili%es the sil!er chloride species but increase the stability of # -'/2). $o form a hi#h #rade #old deposit, the decrease in #old solubility should be !ery si#nificant and efficient o!er a restricted !ertical inter!al. Cne of the most efficient mechanism for dramatically decreasin# #old solubility should be boilin# abo!e throttle points:. $hrottlin# is an irre!ersible adiabatic process causin# rapid expansion of an aueous fluid, as it passes throu#h constrictions in a !ein system -arton and $oulmin, 19;1/. $he much more dilated en!ironment for boilin# and"or open system #as remo!al because of the lar#e pressure decrease. Aonclusion B “Multa fiunt eodem, sed nod semper in eodem mod” -&any thin#s are formed to loo8 the same but not always in the same way/ -amdohr, 19;9/.
214055452.doc
36
PROPYLITI"
LO/% TEMPERATURE POTASSI"
INTERMEDIATE ARGILLI"
SILI"I"
AOLINITI"
"ARBONATE
Andesitic Volcanics
*la#ioclase
*yroxene
&a#netite"ilmenite
Nroundmass
Cpen 'pace"Eeins
lbite=epidote= uart%=adularia= Illite=mixed uart%=mixed mica layer=uart%=8aol layer chlorite=calcite 2& inite=calcite =uart% =pyrite =chlorite=pyrite I"'=illite=8aolinit e Ahlorite=epidot Illite=mixed layer Illite=mixed layer uart%=mixed e=actinolite=cal I"'=uart%=8aoli I"'=uart%=8aolin layer nite=calcite ite=calcite cite=pyrite I"'=illite=8aolinit e *yrite=chalcopy *yrite=chalcopyri *yrite=rutile"anat *yrite=rutile"anat rite=pyrrhotite= te=rutile"anatase ase=chalcopyrite ase=chalcopyrite rutile"anatse
Haolinite=uart%= 'iderite) pyrite an8erite=calcite
lbite=chlorite uart%=adularia= Illite=mixed uart%=mixed =calcite=uart% pyrite=chalcopyri layerI"'=uart%=8 layer te=rutile"anatase aolinite=calcite =adularia I"'=illinite=8aoli nite pidote=calcite uart%=adularia= uart%=pyrite=illi uart%=pyrite=ba te=8aolinite =chlorite=uart calcite=chlorite se metal % sulphides=electru m
Haolinite=uart%= 'iderite) pyrite an8erite=calcite
Haolinite=uart%= 'iderite) pyrite an8erite=calcite *yrite=rutile=chal *yrite copyrite
Haolinite=uart%= 'iderite) pyrite an8erite=calcite
$able1.&ineralo#ical components of the !arious alteration types present at the 'cott (ode low sulphidation !ein deposit, ueensland. fter obis, 1992.
P*o&0liti#
Lo-%Te3&e*at!*e Potassi#
Inte*3e$iate A*+illi#
Sili#i#
Late%sta+e aolinite
Late%sta+e "a*bonate
uart% Ahlorite lbite Aalcite pidote ctinolite dularia &usco!ite -2& mica/ Illite &ixed)layer I"' Haolinite n8erite 'iderite utile"anatase *yrite *yrrhotite Ahalcopyrite arly
(ate
$able 3. &odel for the pro#ression of alteration mineral assembla#es with the epithermal mineralisation ato 'cott (ode, $emp " A ueensland. fter obis, 1992. Fluid *hysio)chemical conditions *rocesses 100 200 300 Eery hi#h silica supersaturation acid) sulphate waters low p
*pt of amorphous silica as silica cap and sinter. Aondensation = oxidation of acidic #ases. d!anced ar#illic alteration releases 'iC2
MIING 6ONE BELO/ BOILINGe!ersible BOILING 6ONE expansion 6ONE
P*o&0liti#
Lo-%Te3&e*at!*e Potassi#
Inte*3e$iate A*+illi#
Sili#i#
Late%sta+e aolinite
Late%sta+e "a*bonate
uart% Ahlorite lbite Aalcite pidote ctinolite dularia &usco!ite -2& mica/ Illite &ixed)layer I"' Haolinite n8erite 'iderite utile"anatase *yrite *yrrhotite Ahalcopyrite arly
(ate
$able 3. &odel for the pro#ression of alteration mineral assembla#es with the epithermal mineralisation ato 'cott (ode, $emp " A ueensland. fter obis, 1992. Fluid *hysio)chemical conditions *rocesses 100 200 300 Eery hi#h silica supersaturation acid) sulphate waters low p
*pt of amorphous silica as silica cap and sinter. Aondensation = oxidation of acidic #ases. d!anced ar#illic alteration releases 'iC2
MIING 6ONE BELO/ BOILINGe!ersible BOILING 6ONE expansion 6ONE
100 Increase in silica concentration due to coolin# Increase in p and concentration of silica. ecrease in temperature (oss of AC2,2',Al,2C
apid ascension, expansion
H5S H5O
*recipitation of uart% due to rapid coolin#, precipitation of calcite due to AC2 loss
'low coolin#, moderate 'low precipitation of silica saturation, neutral coarse crystalline uart% p due to re!ersible expansion
H"l "O5
200
300
$emp " oA Fi#ure ;. ia#rammatic representation of initial of boilin# in epithermal system and depositon of silica cap"sinter. apidly risin# fluid boils and cools, Fluid *hysio)chemical #i!in# riseconditions to hi#h le!els of silica saturation. Aondensation and oxidation of acidic #ases form acid sulphate waters, causin# ad!anced *rocesses 100 200ar#illic 300 alteration. elow the le!el of boilin# slowly chan#in# conditions precipitate coarse crystalline uart%. eposition of uart% within the boilin# %one *recipitation of calcite due in calcite caused ecrease by coolin#. fter i#weed, 1991. to mixin# of fluid and solubility due to addition of AC2
#aseous AC2
'low coolin#, moderate 'low precipitation of 214055452.doc silica saturation, neutral coarse crystalline uart% p due to re!ersible expansion
MIING 6ONE
e!ersible expansion
43
100 Increase in silica concentration due to coolin# Increase in p and concentration of silica. ecrease in temperature (oss of AC2,2',Al,2C
apid ascension, expansion
H5S H5O
H"l "O5
*recipitation of uart% due to rapid coolin#, precipitation of calcite due to AC2 loss
200
'low coolin#, moderate 'low precipitation of silica saturation, neutral coarse crystalline uart% p due to re!ersible expansion
300
$emp " oA Fi#ure ;. ia#rammatic representation of initial of boilin# in epithermal system and depositon of silica cap"sinter. apidly risin# fluid boils and cools, Fluid *hysio)chemical #i!in# riseconditions to hi#h le!els of silica saturation. Aondensation and oxidation of acidic #ases form acid sulphate waters, causin# ad!anced *rocesses 100 200ar#illic 300 alteration. elow the le!el of boilin# slowly chan#in# conditions precipitate coarse crystalline uart%. eposition of uart% within the boilin# %one *recipitation of calcite due in calcite caused ecrease by coolin#. fter i#weed, 1991. to mixin# of fluid and solubility due to addition of AC2
#aseous AC2
MIING 6ONE
'low coolin#, moderate 'low precipitation of 214055452.doc silica saturation, neutral coarse crystalline uart% p due to re!ersible expansion
Increase in silica &ultiple boilin# concentration due to -throttlin#/. &ultiple decrease in temperature episodes of uart%
e!ersible expansion
43
100 H5O
precipitation due to coolin#. Ni!in# rise to crustiform uart%. Influx of fluid batches from below, renewed boilin#. Eapour becomes more 2C rich, less AC2 with
"O5
200
BOILING 6ONE
$hrottlin#
continued boilin#.
300
BELO/ BOILING 6ONE
Fi#ure 7. ia#rammatic representation of throttlin# causin# multiple episodes of uart% deposition, leadin# to the formation of crustiform uart%. &ixin# of meteoric water and AC2 leads to precipitation of bladed calcite. fter i#weed, 1991.
214055452.doc
44
Increase in silica &ultiple boilin# concentration due to -throttlin#/. &ultiple decrease in temperature episodes of uart%
100 H5O
precipitation due to coolin#. Ni!in# rise to crustiform uart%. Influx of fluid batches from below, renewed boilin#. Eapour becomes more 2C rich, less AC2 with
"O5
200
BOILING 6ONE
$hrottlin#
continued boilin#.
300
BELO/ BOILING 6ONE
Fi#ure 7. ia#rammatic representation of throttlin# causin# multiple episodes of uart% deposition, leadin# to the formation of crustiform uart%. &ixin# of meteoric water and AC2 leads to precipitation of bladed calcite. fter i#weed, 1991.
214055452.doc
44
Fi#ures 6a G 6e. esults of the chemical euilibrium computations modelin# transport, precipitation and dissolution of the si#nificant species and mineralo#ical components associated with adiabatic boilin# of a fluid saturated with respect f #old, #alena and sphalerite and undersaturated with respect to sil!er at 300 oA a/. Ahan#es in the acti!ity of dissol!ed #ases Fu#acity of 2 reflects the chan#es in the redox state of the fluid on boilin#B 2/ chan#e in p and the molalitites of total dissol!ed u, #, *b and nB c/ amount of minerals -#"t/ precipitatin# durin# boilin#, d/ amount of u, #, *b, #alena, sphalerite -#"t/ precipitatin# durin# boilin#B e/ percent fluid boiled with fall in temperature.
Nan#ue
$extures
mpty -Alays/ eolites, Aalcite Alays -#ate/ Aalcite eolites #ase 'ubrute eal#as uart% Aalcite *yrite -ar/ -Aal,Ahl,FI/ uart% Fluorite *yrite *yrrhotite *yrite rsenopyrite
epth -m/
AW'$((ID ACD$ ($$IA ( = ( ACD$ &''IE A(ACDIA &C'' = A(ACDIAMAW'$((ID
AW'$((ID M &C''=A(ACDIA
AW'$((ID K$ = K(I='K(FI -=crustiform/ AW'$((ID K$ = ACD$ -crustiform/
$ -o A/ Fi#ure 3. &orrison et al. -1991/ textural model superimposed on uchanan -1961/ model on low sulphidation epithermal !eins.
214055452.doc
47
Fi#ure 4. (ateral and !ertical compositional %onation of the ad!anced ar#illic system at the i%on fold)rich porphyry copper deposit, *hilippines.
RE@EREN"ES
dams, '.F., 1920, microscopic study of !ein uart% con. Neol., !.15. p.;23);;4. uiono, @'., 1993, 'patio)temporal relationships and e!olution of the ad!anced ar#illic alteration system at i%on porphyry copper)#old deposit, *hilippines Knpub. *h thesis, @ames Aoo8 Kni!ersity, ueensland, ustralia, 263 p. llibone, . ., 1993, *ara#enetic relations and the ori#in of ad!anced ar#illic alteration in shear)hosted hydrothermal systems of the central (achlan Fold elt, ustralia Knpub *h.. thesis, @ames Aoo8 Kni!ersity, ueensland, ustralia. obis, .., 1992, 'ettin# and ori#in of an epithermal)#old deposit at *a+in#o, Dortheast ueensland, ustralia Knpub *h.. thesis, @ames Aoo8 Kni!ersity, ueensland, ustralia. onham, .F., @r., 196;, &odels for !olcanic)hosted epithermal precious metal deposits re!iew in International Eolcanolo#ical Aon#ress, Dew ealand, *roc. 'ymposium 5, p.13)17. Aasade!all, $., and Chmoto, ., 1977, 'unnyside &ine, ure8a minin# district, 'an @uan Aounty, Aolorado)#eochemistry of #old and base)metal ore deposition in !olcanic en!ironment con. Neol., !.72, p. 1265)1320. eer, <.., owie, .., and ussman, @., 1969, n Introduction to the oc8) formin# &inerals 'ixtieenth impression -really impressi!eX/, (on#man 'cientific and $echnical, n#land. i#weed, @.&., 1991, $he #eolo#y and %onin# of epithermal #old)uart% !eins at
214055452.doc
49
Fournier, .C., 1965b, $he beha!ior of silica in hydrothermal solutions in er#er, . and eth8e, *. &. -eds./, Neolo#y and Neochemistry of pithermal 'ystem, e!iews in con. Neol. E. 2., p.45);1. Fournier, .C., 1965c, Aarbonate transport and deposition in the epithermal en!ironment in er#er, . . nd eth8e, *.&. -eds/, Neolo#y and Neochemistry of pithermal systems, e!iews in con. Neol., !. 2, p.;3)72. ayba, .C., eth8e, *.&., eald, *., and Foley, D.H., 1965, Neolo#ic, mineralo#ic and #eochemical charactristics of !olcanic)hosted epithermal precious)metal deposits in er#er, .. and eth8e, *.&. -eds./, Neolo#y and Neochemistry of pithermal systems, e!iews in con. Neol., !. 2, p.129) 1;7. eald, *., Foley, D.H, and ayba, .C, 1967, Aomparati!e anatomy of !olcanic) hosted epithermal deposits acid sulfate and adularia)sericite typesB con. Neol., !.62, p. 1)2;. emley, @.@., and @ones, <.., 19;4, chemical aspects of hydrothermal alteration with emphasis on hydro#en metasomatism con. Neol. E. 59, p.536)5;9. udson, .&., 1963, lteration and #eochemical characteristics of the upper parts of selected porphyry systems, western De!ada Knpub *h thesis, Kni!ersity of De!ada, eno. Hamilli, .@., and Chmoto, ., 1977, *ara#enesis, %onin#, inclusions, and isotopic studies of the Finlandia !ein, Aentral district, central *eruB con.Neol.,!.72, p.950)962. &atsuhisa, W., o8i, &. edenuist, @.<., -eds/, 1991, i#h temperature acid fluids and associated alteration and mineralisation extended abstracts of the 3rd 'ymposium on eep)crustal fluids, held at $su8uba, @apan, Cctober, 1990, Neolo#ical 'ur!ey of @apan report no.277, 111 p. &orrison, N.<., on# Nuoyi, @aireth, '., 1990, Field manual textural %onin# in epithermal uart% !eins &I pro+ect *247, @ames Aoo8 Kni!ersity of Dorth ueensland. Cehlers, @.., 197;, ydrothermal crystalli%ation of silica #el Neol.'oc m.ull., !. 67, p.1143)1152. ye, .C., eth8e, *.&. and
214055452.doc
50