SUMMARY OF ALTERATION TYPES

This section summarizes the SWIR infrared active minerals detected in Epithermal Gold and Porphyry Copper systems.

ALTERATION SYSTEMS

One of the primary applications of SWIR Reflectance Spectroscopy is the identification and characterization of alteration minerals and their distribution within zoning patterns around mineral deposits. This brief overview presents spectra for and discusses some properties of the more common minerals detected by the PIMA™ spectroscopy technique.

Representative spectral plots from the mineral groups commonly encountered in alteration zones are compiled in Figure 1. Each group has diagnostic profiles and wavelength positions for the respective absorption features.

The common kaolinite group minerals include dickite (Figure 1, spectrum A), kaolinite (Figure 1, Spectrum B) and halloysite. It has always been difficult to differentiate kaolinite from dickite by conventional analytical methods like XRD. The SWIR doublets in the 1.4�m and 2.2�m regions are very distinctive. Differentiating these two minerals becomes important because dickite can indicate higher temperatures and potentially slightly different pH than does kaolinite.

The smectites can be characterized by their octahedral layer cations. Even with XRD, the different species are not easy to identify. Since smectites form at low temperatures and usually in alkaline pH, they provide significant information about the environment of formation. Spectrum E of Figure 1 is an aluminum-bearing beidellite, while Spectrum F is a nontronite or iron-bearing phase. Note the significant shifts in the wavelength positions for the 2.2�m and 2.25�m features as a function of the different cations.

Figure 1 - Common alteration minerals: [A] Dickite from Tombstone, AZ; [B] Kaolinite from Cuprite, NV; [C] illite from Twin Creeks Mine, NV; [D] "sericite" from Silver Belle, Az; [E] beidellite from the Black Jack Mine, ID; [F] Nontronite from Colorado; [G] Alunite from Cuprite, NV; [H] jarosite from Tombstone, AZ; [I] Dolomite from Idaho; [J] Calcite from Alligator Ridge, NV; [K] Mg-chlorite from Yerington, NV; [L] Chamosite from Maine.

Illites are particularly diagnostic in alteration suites as they show a crystallinity change with temperature and the shift in their 2.2�m absorption feature can be related to the aluminum content of the octahedral layers. These two factors can be used to trace changes in illite composition through an alteration envelope. Spectrum D, Figure 1, shows a higher temperature sericite from hypogene alteration, while Spectrum C is an illite from a lower temperature disseminated gold deposit.

Sulfates, such as alunites and jarosites, are very common in oxidized hydrothermal environments and have very distinctive spectra. Like illites, alunites have the potential to delineate zoning based on chemical and temperature changes in the hydrothermal system. Spectrum G of Figure 1 is a potassium-bearing alunite from Cuprite, Nevada, while Spectrum H shows a jarosite from Arizona.

In carbonate diagenetic environments, SWIR can readily distinguish calcite (Figure 1, Spectrum J) from dolomite (Figure 1, Spectrum I). Percent mixtures of calcite with dolomite can be algorithmically determined on the basis of wavelength shift.

Chlorites are another very useful mineral group. Their spectra change quite dramatically through the magnesium�iron substitution series. In Figure 1, spectrum K shows a magnesium-bearing clinochlore from Yerington, Nevada, while spectrum L illustrates the major feature shifts and profile changes in Fe-bearing chlorite species.

ALTERATION MINERAL SUITES

Knowing the mineral associations almost immediately in the field can assist the geologist in his interpretation of the type of alteration system and potential deposit under investigation. This knowledge will increase the efficiency of the sampling and drilling program and therefore greatly reduce laboratory analytical time and costs.

The spectral plots in Figures 2, 3, 4 and 5 show generic examples of alteration suites including argillic, advanced argillic, and propylitic, and a cross section of alunites from both gold and porphyry copper systems. These spectra have been taken from actual field samples, most from well known deposits.

PROPYLITIC ALTERATION:

Propylitic alteration is usually found in the outer zones of epithermal gold and copper porphyry deposits. It is readily distinguished from more argillized alteration types by the occurrence (commonly) of chlorite, epidote, smectite and carbonate (usually calcite), and may contain an amphibole such as actinolite. Zeolites may also be present. Example spectra are shown in Figure 2: (A) actinolite, (B) tremolite, (C) calcite (D) montmorillonite, (E) nontronite, (F) clinoptilolite, (G) epidote , (H) Mg-chlorite, and (I) Fe-chlorite.

Cation substitution, especially aluminum, magnesium and iron, can be spectrally tracked. Note the spectral shifts and profile changes between the chlorites and the smectites.

ARGILLIC ALTERATION:

The Argillic Suite is perhaps the most common alteration type encountered in western United States deposits, and occurs in disseminated gold deposits with host sedimentary rocks. The alteration of plagioclase produces kaolinite, illite, smectite, mixed layer illite/smectite, and halloysite. Calcite can also be present.

All of these clay species are very sensitive to environment. Figure 3 shows how the structural changes between illite (A), illite/smectite, (B) beidellite (C), montmorillonite (D), nontronite (E) and their distribution can usually indicate temperature/chemical differentials in the hydrothermal fluids.

Rectorite, (B) in Figure 3, is a 1:1 mixed layer illite/smectite. The percent illite in an illite/smectite clay can also be used to show differences as phases towards the illite end member are probably higher temperature. Degree of ordering and amount of smectitic interlayers can be used as a temperature guide. Changes in the aluminum composition of illites and sericites, as shown by a shift of their spectral features (specifically the 2.2�m absorption minimum), can be used to track changes in alteration environment. Kaolinite (F) and halloysite (G) are also found in argillic zones, as is jarosite (H).

ADVANCED ARGILLIC ALTERATION:

This is one of the more complex assemblages and also produces very interesting and diagnostic zoning. It is also one of the most difficult mineral suites to differentiate in the field with just a hand lens. Knowledge of it at the outcrop, however, can provide invaluable information to the explorationist in the process of determining what system is present and where one is located within the system. This suite can be found in porphyry copper and epithermal gold deposits.

A generic compilation of advanced argillic minerals is shown in Figure 4. The common minerals are [A] alunite-K, Cuprite, NV, hot springs; [B] alunite-Na, Sulfur, NV, epithermal gold; [C] dickite, Arizona porphyry copper breccia pipe; [D] topaz, Brazil; [E] pyrophyllite, Summitville, CO, acid sulfate, gold; [F] zunyite, Nevada and [G] diaspore, Goldfield, Nevada, epithermal gold deposit. Pyrophyllite, zunyite, dickite, diaspore and topaz tend to favor higher temperature deposits.

ALUNITES

Alunites are very important in both gold and porphyry copper systems. They are usually pathfinders and often host for the precious metals. The degree of ordering in their structure and their composition can both be utilized to define their paragenetic environments. Alunites with a doublet towards 1.5�m contain higher sodium and therefore tend towards natroalunite, while the potassium bearing species have a doublet closer to 1.48�m. The profiles will also change with temperature and crystal structure order. This has been documented internally with microprobe analyses by Anne Thompson of PetraScience, Vancouver, BC.

Alunites can occur in a supergene environment. They can appear at the top of a deposit in an oxidizing environment and in acid pH systems through a deposit. Empirical observations (by Spectral International and numerous explorationists) indicate that magmatic alunites usually tend to be of the potassic variety, while the sodic-bearing phases may result from ground water weathering and/or a mixing of meteoric and magmatic waters.

Figure 5 compiles alunites from some well known deposits in North and South America. These include [A] Marysvale, UT; [B] Cuprite, Nevada; [C] La Coipa, Chile; [D] Virginia City, NV; [E] Cooper Pedy, Australia; and [F] Sulfur, Nevada. The progression is from potassic [A] to sodic [F]. Note the shift in the 1480nm.

ALUMINUM CONTENT FROM SWIR DATA

The aluminum content of the aluminum-bearing minerals can be estimated from the SWIR 2.2�m feature, which shifts relative to percent aluminum. There appears to be a deposit specific correlation, when illite/sericite/muscovite ( QAS [quartz-adularia-sericite], or QSP [quartz-sericite-pyrite]) alteration is present, with higher amounts of aluminum apparently associated with the ore zone or close to it. This shift in aluminum composition has also been observed in lower temperature systems for kaolinites, smectites and mixed layer illite/smectite species. This is a new technique, still under development, and should be used carefully at this stage.

Selected samples of micas and illites from various hydrothermal and sedimentary environments have been plotted in Figure 6 to illustrate this spectral shift. These range from about 30-36% Al2O3. In the plot, the illites have considerably more water as seen from the deeper 1.9�m feature, and they generally have more rounded profiles.

The following plot is compiled from chemical X-ray fluorescence data in the US Geological Survey Records (Clark, et al., 1993) and work done by Post and Noble (1994). This diagram shows how approximate aluminum values can be estimated from the shift in the wavelength position of the 2.2�m absorption feature.

To give these data perspective, feature positions for a cross section of muscovites and illites from the SPECMIN� database range from 2.198-2.208, with the majority falling within 2.2-2.204�m. Muscovites range from 2.196 to 2.212. While there is some overlap between localities, there should be consistency within a sample suite, sufficient to delineate zoning using shifts in this feature position.

Figure 6 - Selected muscovite, "sericite" and illite samples showing effects of aluminum substitution on the wavelength positions of the 2.2�m absorption feature. [A] Australia; [B] Lone Tree Mine, NV; [C] Muscovite, California; [D] Virginia City, NV; [E] Muscovite, Pancho, Pass, CO; [F] Morenci, AZ; [G] El Heuso, Chile; [H] El Indio Mine, Chile; [I] Muscovite, Ruby Hill, CA; [J] Muscovite, Colorado.

PORPHYRY COPPER ALTERATION MINERAL SUITES

Porphyry Copper deposits have very complex alteration systems containing several types and combinations of alteration suites. SWIR is therefore an ideal analytical tool to identify and characterize the zones around the porphyry.

A further advantage to the use of infrared spectroscopy is that overprints of different alteration and weathering events can usually be detected with PIMA.

ALTERATION TYPES

Each alteration type in a porphyry copper deposit has fairly distinctive mineralogy. Following is a summary of the common alteration types and the major minerals seen in each.

Reference spectral plots for porphyry copper alteration suites are included in Figures 7, 8, 9 and 10 to show the wavelength positions and spectral profiles for some of the more commonly encountered minerals. The accompanying descriptions emphasize the minerals seen by Short Wave Infrared Spectroscopy.

Propylitic, argillic and advanced argillic suites have been summarized previously as they commonly occur in gold systems. Phyllic (Figure 8), Potassic (Figure 7) Supergene (Figure 10) and Leached Cap suites (Figure 9), which are more often associated with porphyry systems, are discussed here.

POTASSIC ALTERATION

The potassic alteration zone is late magmatic and occurs in the core of the deposit. It is characterized by the formation of secondary K-silicates through potassic metasomatism. The infrared active minerals are plotted in Figure 7. This zone is dominated by feldspars, biotite [D] and phlogopite [E] with anhydrite [B], gypsum [A], illite [C] and chlorite [F, G].

The potassic zone hosts the central, relatively low-grade ore consisting of disseminated and veinlet pyrite, chalcopyrite and molybdenite. Alteration overprints in this zone can consist of chlorites and vermiculitic phases.

PHYLLIC ALTERATION

The phyllic (sericitic) zone overlies the potassic, and is usually hosted in the stockworks and dominated by illite/sericite with tourmaline and quartz.

Stockworks are very common in porphyry systems. Illite/sericite is the main phase seen associated with quartz. SWIR is very sensitive to the structural ordering of the illite. Changes in symmetry and profiles of the illite absorption features can therefore be used to delineate this alteration.

These minerals are shown in Figure 8, with muscovites in [A]&[B], and illites in [C]&[D]. Note that the water content (1.9 microns) changes. Tourmaline [F] is a very diagnostic phyllic zone mineral. Kaolinite [E] is an alteration product formed by oxidation of the pyrite common to this zone.

Phyllic alteration can host a hypogene ore characterized by disseminated and veinlet pyrite and chalcopyrite

LEACHED CAP

In exploration, the leached cap environment is often the first evidence encountered of a porphyry copper deposit. Leached caps are characterized by a variety of iron oxide minerals especially goethite, hematite and jarosite with clays and sulfates. Some of these are shown in Figure 9 and include alunite [A, B], gypsum [C], jarosite [D], kaolinite [E], opal [F], quartz [G] and scorodite [H]. Leach caps are commonly highly oxidized from the weathered and altered iron minerals with red, gold, yellow, and pink hues.

Infrared active minerals found in leach cap porphyry copper zones include [A] alunite-K, [B] alunite-Na, [C] gypsum, [D] jarosite, [E] kaolinite, [F] opal, [G] quartz, [H] and scorodite.

SUPERGENE ENRICHMENT

Supergene minerals (Figure 10) are commonly and erroneously grouped with the leach cap suite and can include some of the more exotic copper sulfates, silicates and carbonates such as brochantite [D], chrysocolla [A], antlerite [B], atacamite [C], azurite [F] and malachite[E], with coquimbite [G], chalcanthite [H] and copiapite [J].