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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