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MAPPING ALTERED AND MINERALIZED ROCKS an introduction to THE "ANACONDA METHOD"
Marco T. Einaudi Stanford University 1997
©Einaudi, 1997
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MAPPING ALTERED & MINERALIZED ROCKS THE "ANACONDA METHOD"
I. Introduction II. Mapping Vertical Faces: trenches, road cuts, tunnels, benches A. General Aspects B. Key Features of Mapping Scheme a. The "baseline" b. Use gridded field sheets c. The rock side litho contacts, faults, veins, density (vol%) of qtz veins d. The air side Background alteration. Alteration halos. C. Organizational hints for efficient mapping a. Use a double-sided aluminum clipboard b. The importance of hard-lead color pencils c. Mapping vests d. Make several mapping passes e. Stand up, facing the rocks III. Mapping outcrop: use multiple overlavs A. Base Map. B. Alteration Overlay. C. Limonite Overlay. IV. Color Codes (Figs. 3 & 4) A. Lithologic contacts and structure (recorded on rock side, plot true strike, dip) B. Hypogene mineralization (veins/veinlets & disseminations). (Plot on rock side) sulfides/oxides (Fig. 3) Veinlet/vein fillings other than sulfides/oxides C. Leached/oxide/supergene sulfides (plot on rock side). Mineralogy Symbols for degree of leaching D. Alteration of hornblende (and/or biotite) sites (plot on air side) E. Alteration of feldspar sites (plot on air side) V. Weathering products: how to map and recognize them. A. Distinguishing between Hypogene and supergene alteration. B. Leached and oxidized outcrops. (1) Keeping track of the degree of leacbing of primary sulfude sites (2) "Glassy limonite", indigenous limonites (3) Relict sulfides locked in quartz (4) Exotic limonites VI. Reconnaissance: What to retain from the Detailed Mapping Scheme. A. Rock description, B. Quartz veins and veinlets C. Limonite assemblages D. Relative abundance of indigenous and exotic Fe and Cu oxides E. Biotite distribution patterns, especially of "shreddy biotite" F. Magnetite abundance
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VII. Posting Sheets (Fact Maps) and Interpretations: The "Folio" A. Posting sheets and follow-up interpretation B. The Folio. C. Composite maps: exploration models and drill to targeting.
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MAPPING ALTERED & MINERALIZED ROCKS THE "ANACONDA METHOD" I. Introduction Color-coded mapping of key features of alteration/mineralization, augmented by quantitative estimates of mineral/vein abundance, measurement of attitudes (strike & dip, or core-axis angle), and relative age between features (different vein-types, or veins/intrusive contacts) is critical to successful exploration, mine development, and development of accurate descriptions for a genetic understanding. This style of mapping should be used to complement standardized numerical mapping designed for computer data bases. A geologist who draws what s/he sees in the rocks has greater flexibility and freedom of thought than one who is forced to pigeon-hole everything into a numerical category. Further, at the stage of map compilation there is no substitute for the detailed, colorcoded, geological and mineralogical notes compiled on posting sheets ("fact maps"), whose color and textural distinctions allow quick visual correlation of common features between outcrops, mine benches, or drill holes. The use of standardized colors also allows a Given exploration team or research group to read and understand each other's maps. Although this tract focuses on mapping in igneous rocks of porphyry-type environments, the approach is easily modified for application in any deposit type or any geological environment. The approach presented here is a direct evolution of mapping schemes devised by Anaconda geologists at El Salvador, Chile, and Yerington, Nevada during the 1960's. What is written here represents in large part a melding of ideas generated during field work and discussions with John Proffett, John Hunt, Bill Atkinson, and John Dilles.
II. Mapping Vertical Faces: trenches, road cuts, tunnels, benches A. General Aspects The most efficient approach to mapping vertical walls is to project everything to a horizontal plane (for example, at chest height). The hundreds of strike & dip measurements that are taken during a day's mapping are all plotted directly on the map; in other words, the map is being produced as you map. Confusion about strikes of faults, contacts, etc, doesn't arise as often as it does when drawing in vertical view or when recording data in a notebook. You know exactly where to go in the next cross-cut or trench to find that fault, and geology can be drawn across the drift from one wall to the other. (NOTE: (1) some features will not project to chest height, e.g., a flat fault at ankle level; these require notes, a quick sketch, or a projection (see below). (2) When mapping an underground decline or a surface trench up a hill, continue to map at chest height; your map will be an inclined plane, which later can be corrected to a different datum plane depending on the ultimate goal). The essential idea is to record by means of a color code the various features of rock type, structure, veins, alteration minerals and ore minerals (see Figs. 1 & 2). Color
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coding is a means of reducing note-taking to some degree, but, more importantly, to force the geologist to look more critically at the rocks. Color also helps to visually stimulate the brain during mapping and afterwards during the compilation process. Another important aspect of the mapping scheme is that in mapping altered wall rocks, you are marina minerals not alteration types. This means that you are not classifying alteration types as you map (think of all the variations on the theme of advanced argillic or of potassic alteration types!) and, therefore, you are coming closer to the ideal of recording observations rather than interpretations. Map what you see. Notes are used for those features that cannot be recorded in the drawing, such as rock descriptions, relative ages between features (e.g.., between faults, veins, veinlets, intrusive contacts), percent total sulfides. percent magnetite, sulfide ratios, and veinlet abundance. Notes are written for intervals of the bench face or tunnel where such features are relatively uniform in character (Figs. 1 & 2).
B. Key Features of Mapping Scheme Figures 1 and 2 illustrate the style of mapping being described here. Figure 1 represents a map of sulfide-bearing rocks, whereas Figure 2 represents a map of the oxidized (weathered) equivalent. The various aspects of the mapping scheme are illustrated by these figures and dicussed in the paragraphs that follow. Comparison of the two figures (and Figures 3 and 5) also will allow you to visualize the results of oxidation of by pogene ores (discussed in a separate section below). (1) The "baseline" consists of the tape laid out at chest height along a drift or trench wall. This baseline is surveyed by brunton and plotted on the Field sheet (taking account of irregularities in the face relative to the straight line of the tape). (2) Use gridded field sheets to enable rapid plotting of strikes and dips with a plastic scale/protractor. The grids represent N-S and E-W lines, not lines parallel to the rock face you are mapping. Assign the E-W line to the long dimension of your map sheet (the north arrow points toward the long dimension c sheet) for ease of use of your clipboard and for internal consistency. (3) Locate your baseline in the center of the field sheet to allow working room (notes and drawing) on all sides. Start a new field sheet before you run out of room toward the edge of the sheet. (4) Before you start mapping, be sure to include coordinates, survey points, locality, scale, the date, and your name. (5) Notes and sample locations are written directly on the mapping sheet, rather than in the field notebook. This ensures that this information is never separated from the map. The baseline serves to separate your map sheets into two areas: the "air side" and the "rock side" ( see Figs. 1-5). (6) The rock side is used to record faults, vein minerals, veinlet minerals, disseminations of "ore" minerals, and lithologic contacts. All through-going features are plotted with true strike and the dip is indicated. (7) Because of the close relation between the distribution of quartz veins and CuAu grades in many porphyry-type deposits, a method of quantifying the density (vol%) of these veins is highly useful. Experiec, has shown that consistency between different geologists can be achieved by estimating (for a given set of veins and a given bench interval where the veins are of relatively constant
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spacing and width): 1) the average w width of the veins, and 1) the average spacing between center lines. Write these down in your notes as a fraction (e.g., "0.5/6" would indicate 0.5 cm average width and 6 cm average spacing between center lines). Dividing out the traction yields percent of the rock that is constituted by this vein set (0.5'6= S vol. %). This approach works well for porphyry deposits where veins occupy definite sets; the estimate is male for each set. The approach also is better than counting vein widths along a tape. because such a count has to be corrected for the true v; width and doesn't record vein widths and spacing. Clearly, the approach is difficult to apply in rocks where the v -,ins are truely random, but this is less common than is generally believed. For A-B quartz veins, which most likely represent open-space filling, you are recording the volume percent of quartz that tilled open spaces. For D veins (pyrite veins with quartz-sencite-pyrite (QSP) halos), record the "vein width" as the distance betwveen outer edges of the QSP halo: the fraction will represent the vol% of the rock that is altered to QSP. (8) The air side is used to record alteration minerals and rock type. Alteration minerals are recorded by color code in two ways. - Background alteration. Narrow "imaginary columns" along the baseline (much as the columns used for different minerals in logging core) are used to record "background" alteration minerals. "Background" alteration is defined here as any alteration minerals that occur throughout a given velum-, of rock and do not appear to be related as halos to individual veins. Pervasive biotization of andesite at El Salvador is one example of background alteration. - Alteration halos. If distinct alteration halos are present on the margins of fractures and vein .fillings, these are shown as lines drawn along the strike of the paricular vein, but on the air side of the map sheet. For example, a sericitic envelope on a pyrite vein would be shown as a brown line on the air side..
C. Organizational hints for efficient mapping 1. Use a double-sided aluminum clipboard the size of the mapping sheet (8.5 X 11 inches in the U.S.) with leather pencil holders riveted to one or both sides. All pencils and scales are kept in this clipboard for easy access. Place rubber erasors on the ends of each pencil for easy retrieval of pencils out of their leather sleeves! 2. The importance of a hard-lead color pencils which can be sharpened to a fine point cannot be overemphasized. Pencils available in the U.S. which meet these standards include Eagle Verithin (or Berol Verithin) and Sraedtler Mars-Lurnochrom.. [Caveat: in tropical climates, leads tend to become soft; in field mapping, rain obviously places severe limitations on the quality of your drawing even if waterproof paper is used. But, try anyway! Keep a loner in your aluminum clip-board facing your map sheet, and keep the clipboard closed and in your mapping vest when not in use]. Sharpening pencils is an art: keep a sharp knife (same one you use to scratch rocks) to expose a length of lead, tape a piece of sandpaper to the back inside of your clipboard for sharpening the point, and do final sharpening by rubbing the point at a shallow angle on a piece of paper at the back of your clipboard.
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3. Mapping vests that have pockets large enough for an aluminum clipboard to tit in loosely, ar critical to the success of the mapping method described above. Loose fit is important because the mapping J method requires a contant back-and-forth between map sheet and rocks: every time you have finished marking a feature on your map sheet, the clipboard is "dumped" back into its pocket. your hands are free, and you can get back to breaking rocks. Your vest "organizes" your work environment, much as the "desktop" on your computer. The clipboard is never dumped on the ground. 4. Applying color. Features recorded on the rock side can become very densely spaced (especially in highly mineralized zones) and great care needs to be taken to maintain color separation with very sharp pencils. A key technique in this regard is to mate the Youngest features first (e.g., post-ore faults, youngest veins), then follow with mapping the older features. In this way, offsetting of older features by younger features can be shown easily as you map and much less erasing is involved! Also, as you apply color to represent a vein. apply first the color of the most abundant mineral as a dashed line; the lesser mineral colors are then applied between the dashes of the first and no color is applied on top of another. 5. Make several mapping passes for any given outcrop or length of bench face; in other words, partition your work. I find that I need at least three passes to complete all the observations and note-taking that I need. The first pass should be the one in which you get down on paper the major features of the outcrop: descriptions of lithology, lithologic contacts (indicate whether intrusive, conformable, stratigraphic, or faulted), major faults, and major through-going veins. In subsequent passes you begin to add detail. In a second pass, map veins and veinlets, diagrammatically showing the relative age of different vein types (plot the youngest veins fusty, and add alteration haloes. and background alteration. The third pass can be devoted to sulfide (or oxide) minerals, their abundance, and relative proportions. 6. Stand up, facing, the rocks, while marking a feature on vour map sheet. This reduces the odds of plotting a wrong strike, because you are oriented with your rocks and your map sheet. Fast, efficient, and accurate mapping is your goal; to achieve this goal, the best mappers do not sit down with their backs to the face. (saves time and saves your pants!) 7. In regional exploration, I recommend that prior to commencing a mapping project at a small scale (say, 1:5,000) that some key representative outcrops be mapped first at a large scale (say, 1:250). The reason is that mapping at a large scale gives the geologist the opportunity to spend some time looking at the rocks in detail and this enables him to develop an idea of the key features of a given prospect. Anned with this information, he can then move out more confidently at higher speed at a smaller scale.
III. Mapping, outcrops: use multiple overlays In mapping sub-horizontal exposures (i.e., outcrops), color codes for alteration, veins, and ore minerals (limonites) are used as above but are applied to successive overlays. Color separation is maintained by plotting: • lithologic contacts, faults, veins, and other structure on a base map (Fig. 6. Base Map); • pervasive (or background) alteration and alteration halos on the first overlay (Fig. 6, Overlay #1)
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• and ore minerals or their oxidation products on a second overlay (Fig. 6, Overlay #2). Notes for these various features are written on their respective overlays.
A. Base Map. The limit of outcrop is sketched first on the base map (along with any additional "culture" such as trenches, paths, etc) and the major features of structure and lithology are mapped in. Rock-type symbols can be assigned to various units, and these symbols can be applied in black pencil (rather than assigning a color-code to rock types) along the outer perimeter of the outcrop outline. Veins are plotted directly on the base map, using color codes for dominent vein-filling minerals. Notes can be written outside the outcrop area.
B. Alteration Overlay. On Overlay #1, lines are used to identify alteration halos on veins shown on the base map. Care should be taken to ensure that the alteration color-code is applied directly over the vein shown on the base. This points out the need to plot the veins first on the base map, then quickly apply the alteration-halo color over that vein on overlay #1. For example, on Fig. 6, the NE-dipping qtz-(Kspar-mag) veins at the north end of the outcrop (base map) have Kspar alteration halos (alteration overlay). Background alteration not related to individual veins is shown next by color-coded dots for the minerals present. Because only one overlay is used for alteration, feldspar sites and mafic mineral sites are difficult to keep separate. .- This turns out not to be a major disadvantage, because, for example, a mix of brown dots and olive green dots implies clay in the feldspar sites and secondary biotite in the mafic mineral sites. The density of dots should reflect the relative abundance of alteration minerals seen in the outcrop. For example, in Fig. 6, background sericite alteration increases in intensity southwesterly and then declines abruptly into a zone with minor epidote and chlorite. As another example, an intensely silicified rock would be represented as a solid orange color on overlay =l (but, apply the color of any minor minerals first as dots, then color-in the orange around d the dots in order not to get superposition of colors). An alternative approach (John Dilles, pers. comm., 1997) is to place major alteration halos (color coded) on the base sheet., and save the alteration overlay for background alteration. This allows the distinction to be maintained between mafic and feldspar mineral site, in the following manner: 1) diagonal NT-SW lines represent altreration of mafics, and 2) diagonal NW-SE lines represent alteration of feldspars. The lines are color-coded following the normal codes. The degree of alteration of individual mineral sites are denoted by how heavily you apply the color: solid lines denote 100 to 80% of that mineral site is altered, dashed lines indicate 5-80% of that mineral site is altered, and dotted lines indicate <5% to trace amounts of that mineral site are occupied by hydrothemal alteration products.
C. Limonite Overlay. This is a key overlay, because ultimately it will allow you to draw a map that displays the distribution and relative abundance of the oxidation products of sulfides. The key minerals whose distribution and abundance need to be TM-Ipped include the green copper carbonates and silicates, black copper pitch (tenorite), glassy limonite (pitch
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limonite), goethite, earthy hematite, and jarosite. Together with the alteration and vein maps, the distribution of these minerals will allow you to say something about original sulfide zoning and about secondary dispersion of metals. With out such information and geological interpretation geochemical assays of soil and rock chip -,,-unples cannot be properly interpreted. The color codes and symbols used in mapping limonite minerals are summarized on the right-hard-siu'e of Figure 3 and in Figure 6 and detailed in section IV.C and VI.B (below). An interpretation of the original sulfide distribution pattem, based on the limonites and the style of alteration and veins (shown for the same outcrop on Figure 4) is illustrated on the left-hand-side of Figure 3.
IV. Color Codes (Figs. 3 & 4) The separation of the mapping sheet into air side and rock side (or overlays for outcrop mapping) allows for efficient use of colors: in the list below, 12 colors are used to record some 40 different mineralogical features and structural features. The list is instructive because it indicates those features that can be mapped continuously by bandlens inspection of freshly-broken rock surfaces in igneous rocks related to porphyry systems. Colors are identified by "Eagle Verithin" (or "Berol Ver-iLhin") numbers. Simplification of the color coding for regional mapping is discussed in Section VI below. A. Lithologic contacts and structure (recorded on rock si , plot true strike, dip) Black dark blue Black
1. Lithologic contacts: use your lead pencil (black). 2. Faults (breccias, clays, shears) and fault contacts: use indigo blue (741). 3. Foliation, joints, bedding: use your lead pencil (black).
B. Hypogene mineralization (veins/veinlets & disseminations). (Plot on rock side). Schematic representation of mineral distribution in approprite color (Fig. 3 & 4). Dots for dissenminations, random short lines for random veinlets (e.g., A-,,nlts) or fracture coatings, continuous lines for through-going veins (e.g.. B- and Dveins). Care should be taken to approximate relative vein densities and relative abundance of disseminated sulfides/oxides along the face by the density of color added to map. Plot veins and veinlets with true strike and indicate dips. Veins are drawn with color of dominant mineral; additional minerals indicated by dots along line: Z vein=tilling consisting of 50% quartz and 509o chalcopyrite, would be drawn as a dashed orange and red line. purple red dark green med yellow black gray Orange dark green olive green yellow-green
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sulfides/oxides (Firm. 3) 1. bornite: purple (752) 2. chalcopyrite: carmine red (745) 3. molybdenite: green (739) 4. pyrite: canary yellow (735) 5. magnetite, hematite: mapping pencil) 6. specular hematite Veinlet/vein fillings other than sulfides/oxides 1. quartz: orange (737) 2. chlorite: green (739) 3. biotite: olive green (739 1/2) 4. epidote: light green (738 1/2)
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C. Leached/oxide/supergene sulfides (plot on rock side). Schematic drawings of textures, abundance, mineralogy, and degree of leaching. (Fig. 3) Red pastel green dark brown reddish brown med yellow med blue
Mineralogy 1. glassy limonite (conchoid fract, red internal reflections): carmine red (745) 2. oxide Cu minerals (malachite, tenorite, etc): true green (751) 3. goethite (orange streak): brown (756) 4. earthy hematite (red streak); tuscan red ( ) 4: jarosite (yellow to honey yel'w x'als; pale yel'w streak): canary yellow (735) 5. supergene chalcocite: medium blue
Symbols for degree of leaching in former sulfide sites (the most useful minerals are glassy limonite, goethite, and hematite; jarosite and Cu oxide generally are transported/exotic and less useful): black brown brown red
1. total leaching, empty leached cavities (no Fe-oxide left): black circles 2. moderate leaching (limonite-rimmed cavities): brown circles 3. weak leaching (limonite pseudomorphs and/or boxworks): brown dots 4. very weak leaching of chalcopy sites (classy limonite): red dots Exotic oxides on fractures are denoted by random, short, lines (brown for goethite, true green for copper oxides).
D. Alteration of hornblende (and/or biotite) sites. (recorded on air side in innermost column next to baseline; if alteration occurs as a distinct halo on a fracture or veinlet, plot the alteration color as a line extending outward from base line on air side.) Fig. 4. black dark green
yellow-green olive green dark green dark brown
black
1. fresh hornblende (dark black, glassy, good cleavage visible): write lower case h’s along baseline. 2. chloritized hornblende (no shreddy texture that might imply that the bbl had first been biotitized): green 739) (pervasive chloritization use solid green line; partially chloritized use dashed green line; local cblorite use dots). 3. epidotized hornblende: use light green (738 1/2). 4. biotized hornblende (shreddy biotite occupying the bbl site): olive green (739 1/2) (solid, dashed and dotted to indicate degree of biotization). 5. chloritized biotized-hornblende (cbloritic alt'n superimposed on biotitic; this is a tough call!): olive green (739 1/2) with dark green dots (739) 6. sericitized and/or argillized mafic minerals (tan- or white-colored pseudomorphs after mafic mineral sites including mixtures of sericite, clays, leucoxene): brown (756) 7. mafic sites absent or only leucoxene visible: use lead pencil (black).
E. Alteration of feldspar sites. (recorded on air side, outer colum; intensity of color application in this column denotes degree of alteration; if alteration occurs as a distinct halo on a fracture or veinlet, plot the alteration color as a line extending outward from base line on air side.) Fig. 4. magenta med yellow
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1. of plagioclase to secondary K-spar (pinkish-lavender hue in groundmass and in plagioclase sites): magenta (759) 2. of orthoclase and plagioclase to secondary Na-spar (bard, white feldspars with cleavage preserved: yellow (735)
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(NOTE: feldspar color is not in every case diagnostic of feldspar type! Use thinsections as back up. Even if that white feldspar turns out to be Kspar, you will have recorded the distribution of white Kspar!). yellow-green dark brown none dark brown
3. of feldspars to epidote: use light green (738 1/2) 4. of feldspars to sericite and/or clays: use brown (756) 4a. fresh feldspars: if feldspar is hard, clear, glassy, dark, good cleavage: leave column blank. 4b. incipiently ser'd feldspars: if moderately hard, pale-colored, good cleavage (e.g., "bleached", but hard): sparse brown (756) dots.
(NOTE: incipient alt'n of feldspars to "clays" is difficult to distinguish from albitization and these two alt'n types can occur together; use thin-section back-up) dark brown dark brown
dark brown
4c. moderately ser'd feldspars: if partially soft, white to pale colors, cleavage present: closely spaced brow-n (756) dots. 4d. highly ser'd feldspars: if soft, white to pale colors, no cleavage, but outline of original feldspar is preserved (rock-textufe preserved): continuous brown (756) line applied lightly. 4e. pervasive and total hydrolysis: if soft, white to pale colors, rock texture largely obliterated: continuous brown (756) line :plied heavily.
(NOTE: in rocks containing both plagioclase and orthoclase phenocrysts, because these react differently to acidic solutions, keep track of orthoclase sites separately (in a third column). This allows the distinction to be made between intermediate argillic and advanced argillic alteration).
V. Weathering products: how to map and recognize them. In mapping altered rocks in surface exposures, most of the time we are struggliny, to read through surface, weathering to understand 1) the degree to which metals have been leached, transported, and redeposited by surface waters, and ?) the original hypogene (hydrothermal) distributions of wall-rock alteration and ore minerals. Ho:." do we read through all that punky clay?
A. Distinguishing between Hypogene versus supergene alteration. It is especially difficult to differentiate between hypogene and supergene alteration types in weathered rocks that contained abundant pyrite. This is because the sulfuric acid generated by oxidative weathering of pyrite attacks minerals (especially plagioclase) and converts them to various new mineral assemblages that can ^e similar to forms of hypogine intermediate argillic alteration (e.g., montmorillonite, kaolinite) or even acid-sulfate (advanced argillic) alteration (e. g., kaolinite, alunite). The latter is especially true in rocks that originally contained pyrite veins with qtz-ser-py halos (e.g.., D-veins), but where the halos did not overlap. On weathering, the rock inbetween the halos can be converted to a supergene qtz-kaolinite assemblage and alunite may precipitate in open fractures. The end result, a rock containing quartz, kaolinite, sericite, and alunite, can be mistaken for bydrothermal advanced argillic alteration. How, then, can one distinguish between hydrothermal argillic alteration and argillic weathering? Althouah not in every case definitive, the following observations can help in making the distinction:
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1. in igneous rocks with original alkali-feldspar, the presence of relic cores of alkali f-tld:; phenocrysts would suggest that the rock had not undone pervasive advanced argillic (or even pervasive serici ic) alteration. Alkali feldspar does not survive either pervasive sericitic alteration or advanced argillic alteration at hydrothermal temperatures. However, even in very acidic weathering environments, alkali feldspar commonly survives (in contrast with plagioclase, which goes readly to montmorillonite or kaolinite). One allways needs to consider veins halos and background alteration separately in making these distinctions. 2. in igneous rock lacking alkali feldspar, the call is much more difficult. In such rocks, the presence of moderate to abundant amounts of montmorillonite (rather than sericite-kaolinite) would indicate l of intense hypogene sericitic or advanced argillic alteration. The montmorillonite could be the result either of hypogene intermediate argillic alteration or weathering. Again, make separate observations for halos and background alteration. 3. The presence of magnetite in punky clay-rich rocks is suggestive of clay alteration due to weathering because hypogene clay alteration (e.g., intermediate argillic) generally converts magnetite to hematite+rutile and/or pyrite. This underlines the importance of mapping magnetite abundance in all outcrops (also serves as a basis for interpretation of geophysical data). 4. In rocks that originally contained relatively coarse-grained biotite, the presence of freshlooking brown biotite in otherwise argillized rock is suggestive of weathering. Like magnetite, biotite can survive weathering relatively unscathed, but is readily converted to chlorite+clays during intermediate argillic alteration at hydrothermal temperatures. 5. Intense sericitic alteration occurring, as halos generally can be recognized even in intensely weathered outcrops. This is because the mixture of sericite and quartz in such halos is very resistent to weathering (it is stable in acid environments) and stands out as resistent, gray ribs in punky clay-altered rocks. On first inspection, these ribs may look like gray quartz veins, but recognition of relict rock texture and the fact that it can be scratched (though much harder than punky argillized rockc) gives them away. 6. the presence of high-temperature forms of clay minerals. such as di cki T (well-crystallized kaolinite) and pyrophyllite are diagnostic of hydrothermal advanced argillic alteration because they are stable only at temperatures above about 270`C. This underlines the importance of submitting samples for mineral identification (do your own preliminary mineral separation by plucking out clay-rich portions of the rock or actual feldspar sites, rather than submitting a whole-rock for XRD). 7. hypogene versus supergene alunite. Textures and association of alunite can be diagnostic of by pogene versus supegene origin of this mineral.: FEATURE Veins
alteration halos
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SUPERGENE alunite in open fractures without other mineral. (e.g., monomineralic alunite, possibly with chalcedonic or opaline silica & jarosite. lack of halos on alunite
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HYPOGENE alunite in association with hydrothermal minerals in veins (e.g, with quartz, pyrophyllite, barite, etc) presence of hydrolytic
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veins sulfides
textures color
lack of evidence of former sulfides associated with alunite massive, porcelanous white, yellow, mixed with jarosite
alteration halos on alunitebearing veins evidence that sulfides were present intergrown with alunite fine- and coarse-grained white, yellow, pink
B. Leached and oxidized outcrops. In addition to the factors outlined above, there are techniques focused on the "limonites" that are very useful in broadly outlining original, hypogene patterns of alteration and mineralization. These follow directly from the geochemistry of leached and partly leached outcrops, as discussed in Einaudi (199-5). 1. Keeping track of the degree of leaching of primary sulfude sites is useful in order to reconstruct both hypogene sulfide zoning and alteration zoning. Sericitic zones leach to a greater degree than potassic zones. The degree of leaching can be recorded during mapping (see section IV.C): increasing degrees of leaching are recognized by the sequence: • • • • • • • •
glassy limonite: lowest degree of leaching; copper still present in glassy limonite and in malachite and/or tenorite; indicates absence of abundant pyrite and neutral surface waters; potassic or propylitic alteration typical. goethite pseudomorphs: low degree of leaching of Cu and Fe in near-neutral environments associated with potassic protores (or propylitic fringes, but these with less or no classy limonite, lack of A,B veins, etc); goethite boxworks: leaching increasing partly leached cavities (rimmed with goethite or hematite): indicative of high pyrite: chalcopyrite ratios, likely that sericitic alteration is present; Cu-oxides and carbonates unlikely. partly leached cavities. increasing hematite:goethite ratios indicates increasingly acid conditions; all Cu leached, most of the Fe leached. leached cavities (in some cases filled with jarosite or alunite) represent high degree of leaching in very acid environments: sericitic or advanced argillic alteration, acid-sulfate zones, silica-pyrite-alunite ledges, vuggy silica; Cu-oxides & carbonates absent.
All of these forms of limonites (but mainly the -oethite) are termed "indigenous", on the basis of texture as indicating in-situ oxidation of original sulfide sites. 2. "Glassy limonite" is a term applied to amorphous Fe-hydroxide that commonly contains copper; this phase is important because it denotes very low degrees of leaching (copper still present) and is characteristic of weathering of Potassic Prot res (lots of K-spar and little or no pyrite, hence little acid generation). Mapping the distribution of glassy limonite can help to delineate the chalcopyrite-(bornite) zone
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and commonly this represents the zone of highest hypogene Cu-(Au) grade. It is an indigenous limonite. Glassy limonite has the following characteristics: • • • • •
glassy looking, like obsidian conchoidal fracture dark blackish brown to black bright ruby-red internal reflections in sunlight Grain size and morphology that mimics chalcopyrite.
3. Relict sulfides locked in unbroken quartz An aid in delineating original distributions of sulfide assemblages is to make polished sections of quartz collected throughout the leached cap. Study under the microscope in reflected light may reveal unoxidized sulfides that have survived the leaching process. 4. Exotic limonites are all the limonites that do not represent original sulfide sites. The iron has been transported in solution in surface waters and precipitated along fractures in the rock. Exotic limonites can be distinguished from indigenous limonites by the lack of pseudomorphs or boxworks after sulfide, by their presence on random fractures that are part of the regolith and that cut all hydrothermal fractures, and by their characteristic appearance as massive coatings and "paints", commonly with botryoidal and chatoyant surfaces (if goethite). Some exotic goethite takes on a glassy appearance, but it can be distinguished from glassy limonite by the fact that the glassy material is only on the surface of the coating (you, can't "see" into it). An important point is that mapping of total abundance of limonites does not reveal the original sulfide content of the rock (a rock with 1090 exotic limonite contained less sulfide than a rock with with 1% indigenous limonite); keep track of relative abundance of indigenous and exotic limonites and their mineralogy.
VI. Reconnaissance: What to retain from the Detailed Mapping Scheme Reconnaissance mapping for porphyry-type deposits needs to focus on the standard features of lithology and structure and on some additional key features. These are listed below in order of importance. The list largely is based on those features that survive weathering, even in highly acid-generating environments. Wall-rock alteration, especially the "argillic" types, needs to be de-emphasized! Recon mapping focused on porphyry targets can be done efficiently with only five color pencils: blue for faults, red for porphyries, orange for quartz veins, green for shreddy biotite, and brown for limonites. A. Rock description, especially "Productive" porphyries e, including color, textures, and grain size and 90 of each mineral in the rock. In porphyry exploration, the characteristics of the "productive porphyry" have to be understood and looked for: ~50% fine-grained (<0.2 mm) aplitic (more rarely aphanitic) groundmass, ~50% phenocrysts ranging from 1 to 3 mm, if quartz phenos are present they are rounded and embayed (qtz eyes). The significance and
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importance of this rock texture needs more emphasis. It is so important, that you could consider assigning a special color (red?) to this rock type! B. Ouartz veins and veinlets, including their abundance and structural attitudes. At 1:5000 scale, one obviously cannot "map all the veinlets", but the key sets have to be identified and representative strikes and dips plotted on the map. Abundance can be estimated and written down for each outcrop. The importance of quartz veins and veinlets in regional recon stems from two factors: one, we know the close correlation between grade and quartz veins in porphyry-type deposits, and two, quartz veins survive weathering and remain in outcrop as unambiguous evidence of hydrothermal activity. Quartz veins are so important that the,; also are worthy of a special color during mapping (orange). C. Limonite assemblages need to be emphasized. Each outcrop should be assessed for proportions of glassy limonite, goethite, earthy hematite, jarosite, tenorite, and green Cu carbonates/silicates. Proportions can be visually recorded by a color code for each of these minerals, or by assigning ratios in pre-assigned order. In recon, I would choose the latter approach. In mapping a prospect that is being drilled, I would use color codes. Limonites typically are well-zoned and represent an excellent targeting tool. D. Relative abundance of indigenous and exotic Fe and Cu oxides also needs to be estimated. Each outcrop needs to receive a number that indicates the geologist's assessment of whether the Cu assays represent transported copper or "in-place" copper. E. Biotite distribution patterns, especially of "shreddy hiotite" are useful to delineate zones of potassic alteration, which in many porphyries can represent the ore target. Biotite in tine-Qrained biotized andesite may not out-live weathering, but coarsergrained biotite that has replaced hornblende in hornblende andesites or in bblbearing tonalite porphyries commonly survives weathering. In the case of bbl sites, if the biotite doesn't survive. its characteristic "shreddy" texture may survive. F. Magnetite abundance needs to be recorded by visual estimate and magnetic suseeptibility measurement.s.
VII. Posting Sheets (Fact Maps) and Interpretations: The "Folio" Field sheets are transferred to three separate posting sheets. A base posting-sheet serves as the basis for drawing an interpretive geological map with lithologies, structure, and veins; an alteration posting-sheet serves as the basis for drawing an alteration map, and a "limonite" or "ore" posting-sheet serves as the basis for drawing a mineralization map. The posting sheets and the interpretive maps drawn from the posting sheets should retain all of the structural information shown on the original field sheets. Transfer of dips of veins and faults to the alteration and mineralization overlays is especially important, as this allows the geologist to document the third dimension and to explore the structural control of alteration/mineralization patterns and the possibility of post-ore offsets on faults. A map without any strike and dip symbols is not a geological map.
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A. Posting sheets and follow-up interpretation are two steps that go hand-in-hand with field mapping. Both of these steps need to be taken routinely and in a timely fashion. The mapping project should not be considered complete until such time as the posting sheet has been used to construct interpretive geologic base maps and relevant overlays. A rough estimate is that 3 days of field mapping requires 1 day of transferring the mapping to a posting sheet and doing the interpretive work. (1) Posting sheets should be kept up-to-date on a daily (or at least weekly) basis (2) once a significant area has been mapped, but well before mapping of the chosen area has been completed, the geologist should begin to make interpretive overlays. These will be working by that will aid him/her as s/he continues to map. (3) interpretation based on posting sheets should be done at the ;,tmQ scale as the posting sheets and should retain all the structural detail recorded in the field. Ultimately, these working sheets will be reduced in order to generate a smallerscale map, but the important structural details need to be preserved at all scales (I have seen too many geological maps that have no dip symbols, and that display photo linears instead of faults actually mapped in the field). (4) The Interpretive maps should be done by hand in full color. (5) The hand-drawn posting sheets, fully color-coded, represent a major investment of time and money. They should be carefully archived, and the name(s) of the geologist(s) and dates of work should be written on each. B. The Folio. The following types of information should be displayed in a folio s derived from the field mapping phase: 1. base map: lithologies, strike and dips of bedding, faults, contacts, major veins. 2. each overlay is drawn on a gray-scale version of the base map, so that they "stand alone" e.g., lithologic contacts, faults, etc, are visible without having to overlay the overlay on the base map). 3. vein overlay: all large veins and representative veinlet sets (color-coded to dominant mineral) plotted to true strike, illustrative dips indicated; vein abundance contoured. 4. limonite overlay: distribution of glassy limonite, goethite, earthy hematite, jarosite, tenorite, and green copper oxides. Color applied to indicate relative abundance (absent, low, moderate, high). Areas of dominantly exotic versus dominantly indigenous limonites and Cu-oxides should be identified. Based on mapping of limonites, areas of py-dominance versus cp-dominance should be outlined. 5. magnetite overlay: Illustrate the distribution of magnetite, disseminated, vein/veinlet, or replacement, and contour for abundance. 6. alteration overlay: emphasis on minerals rather than alteration types; try simply showing limits of minerals such as secondary biotite, chlorite, epidote, clays, sericite, silica ledges, jasperoids, etc. Finally, with regard to folios, a complete folio also includes topography, geochemical and geophysical data and data interpretations. In their relation to the geological maps, the following points are important: 7. the magnetite overlay produced during the field mapping phase (and which could include magnetic susceptibility measurements on the outcrop) can be used to interpret ground and airborne magnetics.
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8. the raw geochemical data on soil, stream sediment, or rock-chip samples, should be geologically interpreted and hand-drawn by the geologist who generated the geological maps and overlays discussed above. The patterns of grade distribution should reflect his/her concepts of grade control. Computer generated contour maps of assay results should be used only if there is no geological mapping available. C. Composite maps: exploration models and drill targeting. The final and very important product, or raison d'etre, of the folio is the composite of key features. Examples of this approach are given by Figures 7 (Pancho, Maricunga region, northern Chile) and 8 (Batu Hijau, Sumbawa Is., Indonesia).
Refugio district. Figure 7 is re-drawn from portions of a folio that was completed during mapping of the porphyry Au-Cu prospect at Pancho (Refugio district, Maricunga region. Chile) by John Muntean in 1994-95. The folio displays the following features: lithologies, faults, alteration types, vein abundance (A-B veinlets, banded veinlets), and hand-contoured rock-chip geochemistry for gold. We attempted to pull out of this data set the key features that would help target a drill hole into the center of the ore zone. It is clear from Figure 8 that the outer limit of banded veinlets outlines the +0.5 ppm Au zone. Additionally, the following is evident: the innermost 1/2 of the gold zone is identified by-the presence of potassic alteration and AB quartz veinlets and the outer 1/2 of the gold zone is identified by the disappearance of AB quartz veinlets and appearance of abundant banded veinlets. Further, quartz-alunite veins lie outside the zone of +0.5 ppm Au, and sericitie & intermediate argillic alteration does not serve as a useful targeting tool. The result is a composite map that displays the main features of the prospect that would help to target on the gold zone. The composite becomes a useful tool not only in further exploration at Pancho, but in further exploration in the Maricunga area and in establishing a genetic model for these unusually high Au/Cu porphyries. Baru Hijau. Figure 8 is re drawn from portions of a folio that was one of the projects completed during a mapping course held at Batu Hijau in March 1996. The folio displays the following features: lithologies, faults, three classes of quartz vein abundance, and hand-contoured soil geochemistry for copper and for gold. Additionally, we had available a 1:10.000 scale alteration map. We attempted to pull out of this data pct the, key features that would help target a drill hole into the center of the ore zone. It is clear from Figure 8 that the outer limit of abundant quartz veins (-+5 vol cc) outlines the +0.5 %Cu zone at depth. Additionally, the following is evident: distance from +0.5% Cu • outermost edge of secondary biotite 300 - 400 m • outer limit of rare quartz veins 200 m • outer limit of moderate quartz veins (1-2%) 150 m • outer limit of abundant qtz veins (+5%) 0m • presence of porphyries 0m In the absence of a drilled-out reserve, the composite map could show the limit of glassy limonite. Additional features that should be displayed on a composite would be structural directions of quartz veins, limits of sericitic alteration and other bydrothermal alteration types, and limits of indigenous limonites.
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A final exercise that can be done to aid in targetting is to generate a "coincidence" map. By this I mean that there are certain features whose coincidence in space yields stronger evidence for a target than any individual feature by itself. As an example, the following coincidences were used as indicators of the most favorable drill targets at Batu Hijau (listed in order of increasing favorability): • • •
A: coincidence of abundant quartz veins aced highest copper in soil B: coincidence of abundant quartz veins and highest gold in soil C: coincidence of A and B
The rationale for coincidence A is that a copper anomaly associated with abundant quartz veins is more likely to represent indigenous (rather than transported,) copper. Had we had the data, we would have used coincidence of quartz veins with glassy limonite and with highest rock chip copper. Coincidence C reinforce A and B. All the targets based on coincidence C overlie the +0.5 %Cu zone at depth.
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