seg newsletter 100 2015

ABSTRACTWhole-rock lithogeochemical analyses combined with short-wave infrared (SWIR) spectroscopy provide a rapid and cost-effective method for pros-pecting for porphyry-type hydrothermal systems. Lithogeochemistry detects trace metals to average crustal abundance levels and allows vectoring via gradients of chalcophile and lithophile elements transported by magmatic-hydrothermal ore and external circulating fluids that are dispersed and trapped in altered rocks. Of particular use are alka-lis in sericite and metals such as Mo, W, Se, Te, Bi, As, and Sb, which form stable oxides that remain in weathered rocks and soils. SWIR mapping of shifts in the 2,200-nm Al-OH absorption feature in sericite define paleofluid pH gradients useful for vectoring toward the center of the buoyant metal- bearing magmatic-hydrothermal plume.

INTRODUCTIONPorphyry and related epithermal Au-Ag ores are the world’s most important ore deposits outside of iron and aluminum mines, produce most of the Cu and Mo, and are the largest producers of Au and Ag globally. It has been known for over a cen-tury that metals in porphyry Cu deposits are zoned, with a central

Advancing Science and Discovery

JANUARY 2015

NEWSLETTERwww.segweb.org

NUMBER 100 Number 100

NUMBER 100 Number 100

www.seg2015.org

World-Class Ore Deposits: Discovery to RecoverySeptember 27–30, 2015Hobart, TAS, Australia

SEG 2015

to page 12 . . .

†Corresponding author: e-mail, [email protected]

graniteporphyry

dikes

magmatic�uids

spec. hemat ite

Cu > 0.2wt% (±Mo±Au)

2210 nm

2200 nm2205 nm

Wavelength of white mica SWIR (2200)

Illite-chl-smect-relic fspar

epid-chl-act-fspar

plag-act±epid

chl-fspar±calc/epid-hem

biot±Kspar

smect±Illite±kaol±chl& relic feldspar

phengiticmusc-chl-relic fspar

musc

pyroph-alun±topaz

non-magm

atic �uids

SODIC-CALCIC

PROPYLITIC

ADVANCEDARGILLIC

LATE INTERMEDIATEARGILLIC

POTASSIC

INTERMEDIATE

ARGILLIC

a.) Hydrothermal alteration assemblages

Cp-Py

Cp±Bn

Py±Cp±Sl±Ga

SERICITICPHYLLIC

Illite-chl-relic fspar

FIGURE 1. a.) Vertical cross section of a typical porphyry Cu deposit showing distribution of hydrothermal alteration and sulfide minerals. Also shown are generalized contours of the 2,200-nm peak measured in SWIR instruments.

Footprints: Hydrothermal Alteration and Geochemical Dispersion Around Porphyry Copper DepositsScott Halley, Mineral Mapping Pty Ltd., 24 Webb Street, Rossmoyne, WA 6148, Australia, John H. Dilles, Oregon State University, College of Earth, Oceanic and Atmospheric Sciences,104 CEOAS Administrative Building, Corvallis, OR 97331, United States, and Richard M. Tosdal,† PicachoEx LLC, 21 Quince Mill Court, North Potomac, MD 20878, United States

SEG 2015

See p. 29–40

for details

Advancing Science and Discovery

JANUARY 2015

NEWSLETTERwww.segweb.org

NUMBER 100

Number 100

NUMBER 100

Number 100

12 S E G N E W S L E T T E R No 100 • JANUARY 2015

zone with Cu ± Mo/Au that is enclosed in zones enriched in Zn, Pb, and Ag and, in some cases, Mn (Meyer et al., 1968). Gold-Ag may be present laterally away from (e.g., Lang and Eastoe, 1988) or above (Hedenquist et al., 1998) the porphyry Cu core.

Exploration programs for porphyry Cu deposits rely on many techniques, but from a geologist’s perspective, whole-rock lithogeochemistry and short-wave infrared (SWIR) spectrome-try have become standard tools in addi-tion to the hammer and hand lens. To assist this effort, we established the ver-tical and lateral footprint of a porphyry Cu deposit using SWIR and lithogeo-chemistry (Fig. 1). We tracked alter-ation paths ~1 to 5 km vertically (e.g., Yerington, Red Chris, Galore Creek) or

up to 7 km laterally (Butte, Christmas, Highland Valley) from the porphyry ore center. To understand the metal disper-sion outside the ore zone, we focused on the D-type veins with alteration selvages containing sericite, pyrite, and chlorite, the latter being common in hydrolytic assemblages where rocks are dominantly of intermediate to mafic compositions. These veins extend vary-ing distances vertically and laterally from the ore zone.

The distribution of SWIR recognized minerals and changes in rock composi-tions of just two of the many elements (Tl, Bi) that were mapped vertically and laterally in the Ann Mason porphyry Cu-Mo deposit in Yerington and are shown in Figure 2 as examples. Petrog-raphy, electron microprobe analysis,

and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) of the hydrolytic mineral alteration assemblages analyzed in the lithogeo-chemical and SWIR data sets further constrain the interpretation of the resulting patterns (Alva-Jimenez, 2011; Cohen, 2011).

METHODOLOGYRocks were chosen as the sample medium in order to ascertain the primary elemental and mineralogic dispersion halos. However, in many environments during initial explora-tion, soil may be nearly as effective as rocks, as soils capture a geographic aver-age that may include both weakly and strongly altered rocks.

Elevation of Highest Grade Cu

Top of Mineralized Zone

1 km

1 km

1 km

SODIC-CALCIC

PROPYLITIC

POTASSIC

SERICITIC

Na, Ca, and Sr enrichment,K, Fe, Mn, V, Pb, Zn, Cs, Cu depletion

Mo±Bi, Se, Te halo

Localized Tl, As, Bi

Depletion in As, Mn, Pb, Zn,

Cs, Sb, Tl in K silicate core

Mo±Bi, Se, Te

Outer Zn, Pb, Mnelevated compared

to core

Mo +Bi-Se-Te

Increasing Tl, As, Bi,

Se, TeLi, Zn, V, ± As, Sbelevated compared

to core

c.) Lateral distribution of elements

Deep Environment

Above Mineralized Zone

1 km

PROPYLITIC

SERICITIC

Bi-Se-Te±Mo

Tl, As, Sb, Li ±Bi

ADVANCEDARGILLIC

Polymetallicveins

magmatic �uids

spec. hemat ite

SODIC-CALCIC

PROPYLITIC

ADVANCEDARGILLIC

LATE INTERMEDIATEARGILLIC

INTERMEDIATEARGILLIC

SERICITICPHYLLIC

�ag�

agmamm gmui

titicdsam

uiididmam ccc

LALALA

A I IEATTEAI

INAR

EGER

RGTEN

ARRRNTN

CRL

ECED

LIMRM

LLLLMR D

CATAATAADI

CIAD TEEEEE

SEPHSE

H

DEDS

NCEDCEDVANCECGILLICEDCED

SADV

RGADAR

ADR

AAAAAAAA

AADVRG

granite porphyry dikes

+Li > 15 ppm+Tl > 1.5 ppm

+Sb > 4 ppm; As > 50 ppm

+Se > 4 ppm

+Te > 1 ppm

+Bi > 1 ppm

+Sn > 4 ppm+W > 5 ppm

Mo > 5 ppm

+Cu >0.1 wt%

Explanation

Flow path of external fluid

Added

Depleted

+-

zone of magm

atic hyd rothermal alteration

Flow path of magmatic-derived fluid

b.) Vertical distribution of elements

-Zn, Mn,Co, Ni, Sr, Pb, As, Tl, Cs, Rb, Li

+ Na, Ca, Sr-Fe, K, Cu , Zn, Mn, Co, Ni,

+ Na, Ca, Sr-Fe, K, Cu , Zn, Mn, Co, Ni, As, Sb

Return tobackground

or slightly elevatedZn, Mn, Pb, Sr,

Co, Ni, Li, ± As, Sb

Return tobackground

or slightly elevatedZn, Mn, Pb, Sr,

Co, Ni, Li, ± As, Sb

POTASSIC

FIGURE 1. (Cont.) b.) Vertical variations in trace elements in a porphyry Cu system. c.) Schematic changes in hydrothermal alteration assemblages and trace elements through a porphyry Cu system viewed as a series of map views through a vertical system.

Footprints: Hydrothermal Alteration and Geochemical Dispersion Around Porphyry Copper Deposits (continued). . . from page 1

No 100 • JANUARY 2015 S E G N E W S L E T T E R 13

Dominant Hydroxl-bearing Mineral (symbols reflect dominant or local presence of sericitic mineral)

Fresh rocks hornblende

SWIR 2200nM peak <2203 nM 2203-2210 nM >2210 to 2230

Na-Ca or propylitic actinolite chlorite epidote

Advanced argillic pyrophyllite

Sericitic sericite (mus or ill.) tourmaline

Potassic (drill hole) / fresh rock biotite

0 2km

39°0

0’N

119°15’W

Sing

atse

faul

t

Blue Hill

fault Ann MasonPCD

offset part of Ann Mason

PCDbeneath Blue

Hill fault

N Jurassic up

< 0.8 ppm0.8 - 2.5 ppm> 2.5 ppm

Tl concentrationc.

0 2km

39°0

0’N

119°15’W

Sing

atse

faul

t

Blue Hill

fault Ann MasonPCD


PCDbeneath Blue

Hill fault

N Jurassic up

< 1 ppm1 - 2.5 ppm> 2.5 ppm

Bi concentrationd.

Alteration Assemblages

Pyrophyllite-alunite-topazSericitic

K-silicate - ore zoneFresh rock

Sodic-calcicAlbite

Sericite-albiteAlbite-sericite

Advanc Arg (Qz-Pyrop-Alun-Topaz)

Sericite±Tourm-PySer-Chl±Felds-Hem (weak Ser)

Hydrothermal Alteration Zones

Albite-Chl-Epid±Ksp

Na-Ca (Plag-Act)Endoskarn (Plag-Cpx)

Skarn (Gar-Cpx)

Albite-Ser±Chl-Py

Chl-Epid+Felds-Hem (Propylitic)

K-silicate: Biotite (weak, <50%) & 100%

0 2km

39°0

0’N

119°15’W

Sing

atse

faul

t

Blue Hill

faultAnn Mason

PCD


PCDbeneath Blue

Hill fault

N Jurassic up

100% Biot 100% Biot

100% Biot 100% Biot

0 2km

39°0

0’N

119°15’W

Sing

atse

faul

t

Blue Hill

faultAnn Mason

PCD


PCDbeneath Blue

Hill fault

N Jurassic up

100% Biot 100% Biot

100% Biot 100% Biot

a. b.10

0%10

0%>5

0%

FIGURE 2. a.) Map of hydrothermal alteration assemblages in the region of the Ann Mason porphyry Cu-Mo deposit from Dilles and Einaudi (1992) and from J.H. Dilles (unpub. mapping, 2014) superposed on the geology from Proffett and Dilles (1984). Pale purple units are postmineral Tertiary volcanic rocks. All Jurassic rocks are outlined, but the different units are not shown to avoid a cluttered map. b.) Map of hydrothermal alteration with distribution of hydrous alteration minerals identified and 2,200-nm peak using SWIR instrument. Identified hydrous mineralogy is plotted according to the dominant mineral in color and the mineral phase in the rocks. Geochemical maps of c.) Tl and d.) Bi for rocks from Yerington, Nevada. Symbols are color coded to hydrothermal alteration assemblages mapped in the field and inverted from standard geochemical plots. to page 14 . . .


Lithogeochemical analysesCommercial laboratories provide rapid turnaround of chemical analyses including sample preparation, internal standardization, and replicates. In this study, we used ALS Global for 48 ele-ments (ME-MS61 method) that include all major elements but silica, which is lost as SiF4 gas. The best sample dissolu-tion combined with low detection limit is achieved when rock is digested by a mixture of four acids (hydrofluoric-ni-tric-perchloric-hydrochloric). This dis-solution method liberates nearly 100% of all elements except for Zr, Hf, and a small percentage of heavy rare earth ele-ments (REE) and Y contained in refrac-tory zircon. Coupled with the ICP-MS + ICP-atomic emission spectroscopy (AES) instrumental finish, the result-ing assays report elements at detection limits similar to crustal abundance. At a minimum, assay values an order of magnitude or more greater than aver-age crustal abundances in rocks (Table 1) should be viewed as anomalous and attract further attention from an explo-ration program.

Alternative sample digestion proce-dures are available but, compared to four-acid digestions, are more costly, lead to incomplete dissolutions, or have higher detection limits. Aqua regia dis-solves sulfides and oxides effectively,

but cannot dissolve silicate phases where many useful pathfinder elements reside and, furthermore, that commonly survive weathering of rocks to form soil. Lithium metaborate flux results in complete dissolution of rocks so that total silica, Zr, and Hf concentrations are obtained, but this fusion technique is expensive, increases the sample blank, and produces higher detection limits for trace elements.

SWIR analysesShort-wave infrared spectra were col-lected from rock chips from larger sam-ples analyzed for lithogeochemistry. As the SWIR spectrum is collected from a small rock surface (~0.5-cm diameter), the sample volume is much smaller than the corresponding one for litho-geochemistry. Samples were broken in the field such that the effect of surface weathering is minimized.

The hydrous minerals that are the most widespread in sericitic alteration are white micas/clays (muscovite/illite commonly called “sericite”) and chlo-rite. Each has distinctive spectra that also yield compositional information (Thompson et al., 1999). SWIR instru-ments efficiently analyze 500 to 1,000 samples per day in the laboratory, or in the field from samples archived in chip trays.

PORPHYRY COPPER DEPOSITSThe geology of porphyry Cu (Mo-Au) deposits is well described (Gustafson and Hunt, 1975; Seedorff et al., 2005; Sillitoe, 2010). Hydrothermally altered rocks, sulfides, and veins in the por-phyry Cu environment result where ascending magmatic-hydrothermal fluids escaping from a deep intrusion cool, depressurize, and react with rocks. Fluids chiefly rise vertically (Fig. 1a), but may spread laterally in their upper parts as they encounter topograph-ically driven meteoric waters in the epithermal (<2 km) environment. Mag-matic-hydrothermal alteration is char-acterized by abundant sulfides zoned from Cu sulfide rich in ore zones to pyrite rich in upper zones (Fig. 1a). The silicate alteration minerals are zoned upward (Fig. 1a) from potassic (or K-sil-icate: biotite ± K-feldspar) to sericitic (or phyllic: muscovite ± chlorite) to advanced argillic (alunite ± pyrophyl-lite ± dickite). In general, the upward zonation reflects decreasing tempera-ture and pH of fluids during ascent, but there is also a temporal evolution with widespread collapse and downward superposition of sericitic alteration on older potassic alteration (Gustafson and Hunt, 1975). Late intermediate argillic alteration (smectite-illite-chlorite or smectite-chlorite) forms at low tempera-ture and introduces little sulfide, but may extensively overprint higher-tem-perature assemblages.

Nonmagmatic fluids such as meteoric water, seawater, or sedimentary brines are common external to the rising plume (Fig. 1a). Meteoric waters dom-inate the shallow parts of continental geothermal systems at temperatures less than about 350°C. Deeply circulating formation waters or brines dominate sedimentary and volcanic sections, par-ticularly where evaporitic rocks are pres-ent. The latter in the Yerington district, Nevada, have penetrated up to 5-km depth and produced sodic-calcic alter-ation (Na-plagioclase-actinolite-epidote) and propylitic alteration at shallower depths (albite-K-feldspar-epidote-chlo-rite ± actinolite; Carten, 1986; Dilles and Einaudi, 1992). These alteration types are notably poor in sulfides, lack hydrolytic alteration unless overprinted, contain abundant feldspar, and may remove sulfides.

TABLE 1. Typical Pathfinder Elemental Ranges (ppm)

Metal Average crust abundance1 Potassic Deep sericitic Shallow sericitic

Cu 75 >200–ore 100 50Mo 1 0.5–ore 2–20 0.5–5Sn 2.5 0.5–10 2–30 bkgdW 1 bkgd 2–20 0.5–5Mn 1,400 <bkgd 1,000–5,000 400–1,000Zn 80 <bkgd 200–1,000 10–100Pb 8 50 200–1,000 10–1002

Ag 0.08 0.5–3 1–50 1–102

Ni 20 <bkgd bkgd–30 <bkgdCo 10 <bkgd bkgd–20 <bkgdSe 0.05 5–20 1 1Te 0.001 0.1 1–5 0.1–1Bi 0.06 0.05 1–10 0.05–1As 1 <bkgd 10–50 50–1,000Sb 0.2 <bkgd 1–3 3–100Li 13 <bkgd <bkgd 15–50Tl 0.36 0.2 0.2 1–50Hg 0.08 0.05 0.05 0.2–10Cs 1 <bkgd 1–10 1–20

1Background concentration (bkgd) varies by rock unit; this is the average crustal abundance (Ni and Co = average upper crust; Taylor and McLennan, 1985); alteration zones are potassic from ore zone, deep sericitic above ore zone, and shallow sericitic and associated advanced argillic near surface

2Alunite is commonly enriched in Pb, Ag, Ba, and Sr in advanced argillic alteration



HYDROTHERMAL ALTERATION: TRACE METAL DISPERSION PATTERNSThe trace element content of rocks var-ies by bulk rock composition, and com-monly somewhat within a single rock unit. We minimized the effect of hetero-geneous rock compositions by focusing on porphyry Cu systems hosted in granitic rocks with relatively limited local compositional variation, but that vary from one district to another. The lithogeochemical results suggest rela-tively consistent zoning of trace metal enrichments and depletions in the six magmatic-hydrothermal systems stud-ied (Fig. 1b).

Magmatic-hydrothermal fluids dom-inate the center of the porphyry hydro-thermal system, whereas the peripheral and upper zones are dominated by nonmagmatic fluids. In addition to the high acid and sulfur content, magmat-ic-hydrothermal fluids have distinctive trace metal suites compared to dilute metal-poor external meteoric waters, seawater, and alkaline saline sedimen-tary brines. For all fluid types, their trace metal suites are modified by addi-tion and removal of elements as a result of wall-rock alteration. The hydrother-mal fluid thus compositionally evolves along its flow path and transports ele-ments from a source area to new sites where they may be fixed in hydrother-mal minerals. Therefore, a considerable challenge is recognizing metals that are leached from the wall rock by diverse non-ore fluids and that may be added elsewhere versus those added by the ore-forming magmatic-hydrothermal fluids.

Where do trace metals occur in altered rocks?Wall-rock reactions with host minerals modify not only the rock composition but also the fluid composition as ele-ments are transferred between rock and fluid. Alteration of amphibole contrib-utes Mn, Co, Ni, Zn, Cu, V, Cr, As, and Sb to the buoyantly rising hydrothermal fluid whereas alteration of feldspars contributes Na, Ca, K, Sr, Ba, Rb, and Pb. Intense alteration of these minerals may form negative anomalies, with concentrations returning to background or potentially slightly enriched in rocks along the margins of the hydrothermal plume as many of these elements pre-cipitate upon cooling or neutralization of the acidic fluid.

Along the flow path of the magmat-ic-hydrothermal plume, many trace elements are fixed in hydrothermal minerals. Metals that form chloride complexes (that is, metals that have a common valence of +1 or +2) become more soluble as hydrothermal condi-tions become more acid. Metals that form oxyanion complexes (that is, met-als that have a common valence of +4 to +6) become less soluble as hydrother-mal conditions become more acidic. So, in the rising plume above porphyry cen-ters where the SO2 disproportionation reaction continuously produces sulfuric acid, the acidic fluid continues to leach Zn, Mn, Cu, Pb, Ag, Co, and Ni, but, in contrast, Mo, Sn, W, Bi, Te, As, Sb, and Tl are precipitated in sulfide, oxide, and silicate minerals, but in a sequence that reflects declining temperature (Fig. 1). The common chalcophile elements (Zn, Pb) routinely used in mineral explora-tion geochemistry sit on the margins of the magmatic-hydrothermal plume.

There have been many compositional studies of pyrite, but fewer of hydro-thermal sericite and chlorite. At the Yer-ington and Highland Valley porphyry systems, trace metals incorporated in sheet silicates include Ba, B, Cs, Li, Rb, Sn, Tl, and W (Fig. 4). Tungsten and tin are likely present as minute scheelite or wolframite and cassiterite grains. Seric-ite and chlorite from sericitic alteration zones are also enriched in many other transition metals (Mn, Zn, V, and Cr), but chlorite from propylitic alteration zones is enriched in the same elements, so they are not generally useful trac-ers. Lithium is more useful because it is more concentrated in sericitic zones compared to propylitic zones. Moreover, many of these other transition metals are removed from deep potassic and sodic-calcic alteration zones and so are not entirely derived from a parent mag-matic-hydrothermal fluid.

Metal dispersal patternsTrace metal dispersions are illustrated in a schematic cross section and plan section (Fig. 1b, c) views of a porphyry copper hydrothermal system. This sum-mary diagram is modeled principally on 850 whole-rock chip samples collected on a nominal 200- × 200-m grid from Yerington, Nevada, in a 2-km-wide by 5-km vertical transect over the Ann-Ma-son porphyry Cu-Mo hydrothermal zone (Fig. 2), but incorporates infor-mation from five other sites. A similar dispersion of elements is observed in

vertical profiles from Galore Creek (British Columbia), and plan views at Highland Valley and Red Chris (Brit-ish Columbia), Butte (Montana), and Christmas (Arizona). For example, at Butte the trace metal dispersion extends 6 km laterally westward along the Main Stage veins from the porphyry center in a mixed magmatic-meteoric groundwa-ter plume, and the distribution of trace elements is comparable to the vertical dispersion. In these localities and sev-eral others, the trace metals are zoned upward and outward from the >0.1 wt % Cu zone in the general sequence Mo, W, Sn, Se, Te, Bi, Sb, As, Li, and Tl. Most of these metals have subtle anomalies (>1 ppm for Te, Bi, and Tl to >50 ppm for As), hence the need for analytical methods with low detection limits. Sericitic zones also record anomalies of Cs and Rb in addition to Li and Tl. As the sequence of metals above and beyond the Cu zone closely mimics the general decrease in solubility of metal chlorides in ore fluids during cooling (Reed and Spycher, 1984), the zonal arrangement of metals or metal ratios is therefore a tool for targeting further exploration.

Comparisons of the six hydrothermal systems suggest that all contain similar magmatic-hydrothermal trace metal anomalies; however, their concentra-tions and ratios vary, as do the ore metal ratios. For example, if we consider the Yerington, Highland Valley, and Christmas calc-alkaline magmas (Cu-Mo) as a baseline, the more silica rich magmas at Butte (Cu-Mo) have sericitic zones relatively enriched in As, W, Sn, Cs, Sb, and Zn whereas the alkaline to high-K magmas at Red Chris and Galore Creek (Cu-Au) have alteration zones relatively enriched in Te, Se, and Bi (Micko, 2010).

Several elements are depleted from potassic ± sericitic alteration in the central Cu-Mo ore zone. Hypogene leaching results from hydrothermal destruction of the host igneous miner-als amphibole ± pyroxene and feldspar. Several other elements (Tl, Cs, Rb ± Li) are likely dominantly supplied by the parent magmatic-hydrothermal fluid and, because they are highly soluble in high-temperature chloride solutions, they are present in low abundance in the ore zone. Hypogene leaching in the Cu-Mo ore zone can potentially contrib-ute metals to distal polymetallic (Zn-Pb-Mn-Ag) ores, especially near the outer limits of to page 16 . . .


late sericitic alteration. Nonetheless, because leaching of different host rocks will provide different ratios of these metals, the amount and ratios will vary in the peripheral zones.

There is no indication that mag-matic-hydrothermal fluids contribute significant sulfides, ore metals, or trace metals to most of the propylitic zone that extends laterally away from the ore zone. In contrast, sodic-calcic alter-ation leaches a suite of metals (Fe, K, Cu, Zn, Ni, Li, Pb, As, Sb, and Co) that are moved upward and locally fixed in propylitic or shallower levels of sod-ic-calcic-altered zones. Magmatic fluids add trace metals to propylitic or sod-ic-calcic alteration at the interface with potassic alteration or where lateral D veins cut propylitic zones. For example, the prominent Zn enrichment zone that is common on the margins of most porphyry Cu deposits is present where minor amounts of sericite and chlorite are in the rocks.

MAPPING CHANGES IN SERICITE COMPOSITION“Sericite” is a field term that includes fine-grained white micas of indetermi-nate mineralogy (Meyer and Hemley, 1967). In most porphyry Cu environ-ments where rocks contain some potas-sium, sericite forms above about 300°C to as much as 550°C and is muscovite (KAl2(AlSi3)O10(OH)2) with a white to gray color. At low temperatures below about 300°C, sericite is fine grained, may range from white to pale green in color, and is usually the potassium-defi-cient clay mineral illite (K0.6–0.8Al2(Al0.6–

0.8Si3.4–3.2)O10(OH)2). The boundary between muscovite and illite is consid-ered to be about 300°C (Reyes, 1990); however, both minerals are chiefly 2M1 sheet silicates with an identical crystal structure, so the different names merely represent an arbitrary boundary in a sin-gle continuous solid solution (Cohen, 2011).

Sericite forms chiefly via hydrolytic alteration of feldspar and, to a lesser extent, from mafic minerals, via reac-tions such as the following:

1.5KAlSi3O8 (Kspar) + H+ ⇔ 0.5KAl2(AlSi3)O10(OH)2 (musc) + K+ + 3SiO2;

1.5NaAlSi3O8 (albite) + H+ + 0.5K+ ⇔ 0.5KAl2(AlSi3)O10(OH)2 (musc) + 1.5Na+ + 3SiO2; and

1.5CaAl2Si2O8 (anorthite) + 2H+ + K+ ⇔ KAl2(AlSi3)O10(OH)2 (musc) + 1.5Ca2+.

These reactions consume acid, release Na and Ca, and are produced by acidic fluids. Sericite (muscovite and illite) is stable over a relatively wide range of log(K+/H+) at a given temperature that reflects pH changes from acidic low log(K+/H+) to more neutral high log(K+/H+) conditions. Therefore, pH or acidity can vary considerably in the sericite field, and can be mapped by the mica composition.

Mapping pH gradients with SWIRFor the purposes of quickly mapping the mineralogic changes in the seric-ite-chlorite–altered rocks, the ability of the SWIR instruments to obtain rapid identification of minerals and their solid-solution compositional changes is of particular value (Thompson et al., 1999). In the porphyry environment, the wavelength of the 2,200-nm absorp-tion feature corresponding to the Al-OH bond energy of sericite-bearing samples is critical. In sericite, including both muscovite and illite, the wavelength of the 2,200-nm feature shifts from 2,195 nm in muscovite toward 2,220 nm in phengite, as Al is replaced by (Fe, Mg) + Si. This coupled Tschermak-type substi-tution is controlled by the pH as well as the concentrations of (Fe2+) and (K+) of the hydrothermal system via the follow-ing reaction:

2KAl2(AlSi3)O10(OH)2 (musc) + K+ + 1.5Fe2+ + 4.5SiO2 + 3H2O ⇔ 3KFe0.5Al1.5 (Al0.5Si3.5)O10(OH)2 (phen) + 4H+.

The acidity of the hydrothermal fluid helps determine the proportions of muscovite and phengite in the mica. Muscovitic white mica (including illite) means an acidic environment, whereas a phengitic composition means a more neutral environment. Therefore, the position in the white mica wavelength can be used as a hydrothermal pH indi-cator, and changes laterally and verti-cally in the porphyry Cu environment (Fig. 1a). Furthermore, the position of the 2,200-nm absorption is not very sensitive to the K content and whether the mica is muscovite or illite.

Above the core of a porphyry Cu deposit, the position of the 2,200-nm absorption shifts to lower values in the

spectra of sericite-chlorite–altered rocks as the fluid pH decreases, because acids in the rising fluid continuously disso-ciate during cooling and destroy the ability of the rocks to buffer the fluid (Fig. 1a). Upward fluid flow is rapid along the permeability fabric, whereas lateral pressure gradients are smaller as acid enters the rock via slow diffusion. Consequently, along the centers of fluid flow channels, the water/rock ratios are high and pH is low, whereas, laterally, the water/rock ratio quickly decreases and rock-buffering and neutral pH dominate. Local-scale evidence for the changes is readily evident in steeply dipping D veins with inner pervasive sericitic alteration enclosed laterally by weakly altered halos of feldspar-seric-ite-chlorite cutting rocks that may only have chlorite replacing mafic silicates.

Laterally, the position of 2,200-nm absorption increases, reflecting a tran-sition toward low fluid flux and a more neutral pH environment (Fig. 1a). In the shallow environment influenced by surface topography, the low-pH magmatic or mixed magmatic-meteoric groundwater moves laterally to produce extensive distal sericitic and advanced argillic zones. A complication to the general pattern arises, such as at Red Chris and Butte, where the lower-tem-perature acid-stable environments are telescoped upon the higher-temperature silicate alteration assemblages (Meyer et al., 1968; Norris et al., 2011).

WEATHERINGWeathering of rocks that are initially pyrite rich and contain little feldspar produces sulfuric acid and supergene leaching that removes Cu, Ag, Pb, and Zn. Nonetheless, many elements useful as magmatic tracers are not leached and remain in rocks and derivative soil. There are three suites of nonleachable elements: (1) chalcophile elements (Mo, As, Sb, Te, Se, Bi) that were trace impu-rities in pyrite or minute sulfide grains associated with pyrite, (2) lithophile ore minerals (W, Sn) that form stable oxide complexes, and (3) lithophile alkali and alkali earth elements that follow potas-sium and remain fixed in sericite and chlorite (Ba, B, Cs, Li, Rb, Tl). Hence, during surface weathering where micas and chlorite are stable, many trace ele-ments largely remain in the rock and also soil. Even extreme acid weathering



accompanied by hydrolytic destruction of sheet silicates may lead to complete leaching of the alkalis, Zn, and Tl. However, elements that form oxides are not leached and remain detectable. Understanding the behavior of elements in the weathering profile allows the explorer to use trace elements as vec-tors, even in highly weathered terranes.

CONCLUSIONSSWIR and lithogeochemistry are simple and relatively inexpensive but powerful tools for the exploration geologist, as well as for geometallurgy (Halley, 2013). Samples of rocks and soils collected on a grid spacing as broad as 500 m can be used to identify anomalies and gradi-ents in mica mineral compositions and trace metal abundances during initial exploration for porphyry Cu deposits. Sampling of rocks must be selective, preferably in altered rock selvages to veins, as fluid flow is rapidly channel-ized outside the mineralized core both upward and outward. Nonetheless, a suite of rock analyses must be used with caution for targeting, and always within the context of geology. For example, at Yerington, the mineral deposits were tilted 90° west by Cenozoic normal faulting so that they are exposed in cross section. In this case, the min-eralogic and geochemical vectoring cannot be used ad hoc. Any lithogeo-chemical and SWIR analyses must be done in concert with basic mapping of the geology, hydrothermal alteration mineralogy, and ore sulfides and oxides. Mapping D veins with sericitic selvages that extend kilometers upward and outward from the porphyry ore center remains one of the simplest geologic guides to targeting the center. Further-more, these sericite-chlorite-pyrite alter-ation zones are ideal for geochemical vectoring because they will preserve a magmatic fingerprint of lithophile trace metals in the mica structure and of chal-cophile elements within and associated with pyrite.

Lastly, there is an enhanced focus on exploring under cover and at depth; however, in many parts of the world, mineral deposits are still hidden by a leached weathering profile. In these places, new discoveries should still be possible through soil geochemistry if the program considers what works and what does not work in those envi-ronments. Traditional assay packages include Cu, Zn, Pb, Au, and Ag that are

leached at surface and therefore not useful vectors in many weathering envi-ronments, but the oxyanion elements (As, Mo, Sn, W, Te, Se) are robust and will be preserved. A cost-effective and recommended methodology is a four-acid digest to achieve a reliable dissolu-tion of several refractory elements and ICP-MS analyses to provide sufficiently low detection limits near elemental crustal abundance.

ACKNOWLEDGMENTSThis paper is a partial summary of a three-year industry project on footprints of porphyry copper deposits funded principally by Barrick, Teck, Freeport, Imperial Metals, BHP-Billiton, and Vale, with additional grant support from Geo-science British Columbia, NSERC, and the USGS. ALSGlobal is further thanked for its generous analytical support. We thank Pacmag Metals Ltd for providing access to two drill holes in the Ann Mason deposit. Numerous MDRU-UBC and OSU students made important con-tributions to the project. Comments by Marco Einaudi, John Muntean, Brock Reidell, and Peter Winterburn improved the clarity of the manuscript. MDRU contribution no. 341.

REFERENCESAlva-Jimenez, T.R., 2011, Variations in

hydrothermal muscovite and chlorite com-position in the Highland Valley porphyry Cu-Mo district, British Columbia, Canada: M.Sc. thesis, Vancouver, University of Brit-ish Columbia, 225 p.

Carten, R.B., 1986, Sodium-calcium metaso-matism—chemical, temporal, and spatial relationships at the Yerington, Nevada, por-phyry copper deposit: Economic Geology, v. 81, p. 1495–1519.

Cohen, J.F., 2011, Mineralogy and geochem-istry of hydrothermal alteration at the Ann-Mason copper deposit, Nevada: Com-parison of large-scale ore exploration tech-niques to mineral chemistry: M.Sc. thesis, Corvallis, Oregon, Oregon State University, 112 p. plus appendices.

Dilles, J.H., and Einaudi, M.T., 1992, Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada—a 6-km recon-struction: Economic Geology, v. 87, p. 1963–2001.

Gustafson, L.B., and Hunt, J.P., 1975, The porphyry copper deposit at El Salva-dor, Chile: Economic Geology, v. 70, p. 857–912.

Halley, S., 2013, Calculated mineralogy and its applications: Australian Insitute of Mining and Metallurgy Monograph 30, p. 199–206.

Hedenquist, J.W., Arribas, Jr., A., and Reyn-olds, T.J., 1998, Evolution of an intru-sion-centered hydrothermal system: Far Southeast-Lepanto porphyry and epither-mal Cu-Au deposits, Philippines: Economic Geology, v. 93, p. 373–404.

Lang, J.R., and Eastoe, C.J., 1988, Relation-ship between a porphyry Cu-Mo deposit, base and precious metal veins, and Lara-mide intrusions, Mineral Park, Arizona: Economic Geology, v. 83, p. 551–567.

Meyer, C., and Hemley, J.J., 1967, Wall rock alteration, in Barnes, H.L., ed., Geochem-istry of hydrothermal ore deposits: New York, Holt, Rinehart, and Winston, Inc., p. 166–235.

Meyer, C., Shea, E.P., Goddard, Jr., C.C., and staff, 1968, Ore deposits at Butte, Mon-tana, in Ridge, J.D., ed., Ore deposits of the United States, 1933–1967 (Graton-Sales Volume): New York, American Institute of Mining, Metallurgical, and Petroleum Engi-neers, v. 2, p. 1373–1416.

Micko, J., 2010, The geology and genesis of the Central zone alkalic copper-gold por-phyry deposit, Galore Creek district, north-western British Columbia, Canada: Ph.D. thesis, Vancouver, University of British Columbia, 359 p.

Norris, J.R., Hart, C.J.R., Tosdal, R.M., and Rees, C., 2011, Magmatic evolution, min-eralization and alteration of the Red Chris Copper-gold porphyry deposit, northwest-ern British Columbia (NTS 104H/12W): Geoscience BC, Report 2011-1, p. 33–44.

Proffett, Jr., J.M., and Dilles, J.H., 1984, Geo-logic map of the Yerington district, Nevada: Nevada Bureau of Mines and Geology Map 77, scale 1:24,000, 1 sheet.

Reed, M.H., and Spycher, N.F., 1984, Cal-culation of pH and mineral equilibria in hydrothermal waters with application to geothermometry and studies of boiling and dilution: Geochimica et Cosmochimica Acta, v. 48, p. 1479–1492.

Reyes, A.G., 1990, Petrology of Philippine geothermal systems and the application of alteration mineralogy to their assessment: Journal of Volcanology and Geothermal Research, v. 43, p. 279–309.

Seedorff, E., Dilles, J.H., Proffett, J.M., Ein-audi, M.T., Zurcher, L., Stavast, W.J.A., Johnson, D.A., and Barton, M.D., 2005, Por-phyry deposits: Characteristics and origin of hypogene features: Economic Geology 100th Anniversary Volume, p. 251–298.

Sillitoe, R.H., 2010, Porphyry Copper Sys-tems: Economic Geology, v. 105, p. 3–41.

Taylor, S.R. and McLennan, S.M., 1985, The continental crust: Its composition and evo-lution: Oxford, Blackwell Scientific Publica-tions, 330 p.

Thompson, A.J.B., Hauff, P.L., and Robitaille, A.J., 1999, Alteration mapping in explo-ration: Application of short-wave infrared (SWIR) spectroscopy: SEG Newsletter, no. 39, p. 1–13. 1

seg newsletter 100 2015

Documents