BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

VOL. 108 January–February NO.1

EXPRESS LETTER

APPLYING STABLE ISOTOPES TO MINERAL EXPLORATION: TEACHING AN OLD DOG NEW TRICKS

SHAUN L.L. BARKER,† GREGORY M. DIPPLE, KENNETH A. HICKEY, WILLIAM A. LEPORE, AND JEREMY R. VAUGHAN Mineral Deposit Research Unit, Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada

Abstract The stable isotope ratios of various elements (e.g., H, C, O, S) have numerous uses to improve the under- standing of the genesis and formation of hydrothermal and magmatic ore deposits, as well as having various applications to mineral exploration. However, stable isotope data has not been routinely collected during min- eral exploration for various reasons related to cost per sample, the speed at which analytical data can be col- lected, and uncertainty regarding the benefits of stable isotope measurements to mineral exploration. Recent advances in analytical technologies which utilize infrared absorption spectroscopy (e.g., off-axis integrated cav- ity output spectroscopy [OA-ICOS]) mean that stable isotope data can now be collected in far greater quanti- ties than has been previously possible. This advance in analytical technology, which allows for significantly more rapid and less expensive stable isotope analyses, has significant implications for the way in which stable isotope data can be collected and utilized during mineral exploration. Potential applications of stable isotope ratios to mineral exploration include delineating property- to district-scale stable isotope alteration halos and identify- ing “blind deposits” at depth, as well as vectoring toward new deposits within endowed districts. Stable carbon and oxygen isotope data collected using OA-ICOS from carbonate rocks surrounding the Screamer Carlin-type gold deposit in Nevada demonstrate that stable isotope alteration can be detected at distances of up to (and potentially more than) 3 km laterally around mineralization.

Introduction deposits. Nesbitt provided a review of previous studies that Ratios of the stable isotopes of H, C, O, and S have been showed kilometer (or larger)-scale isotopic alteration halos measured and applied to mineral deposit research since the around different mineral deposit types, in which stable iso- 1950s (Engel et al., 1958). Stable isotopes have been used to tope ratios may differ by more than 1% (more than 10‰). decipher the origin and evolution of ore-forming fluids (see One of the studies reviewed by Nesbitt (1996) included one reviews of Ohmoto and Goldhaber, 1997; Taylor, 1997). In ad- of the very few examples in scientific literature of a deposit dition, several studies have demonstrated that stable isotope discovery attributed to the identification of a stable isotope ratios are commonly altered in rocks surrounding orebodies anomaly (Naito et al., 1995). compared to rocks unaffected by hydrothermal alteration, While there is convincing scientific evidence for why stable meaning that stable isotope alteration halos can be delin- isotopes should be useful for identifying rocks which have eated. Stable isotope alteration halos are typically larger than been altered by hydrothermal fluids (i.e., prospective zones mineralogical alteration halos (i.e., visual alteration) and geo- for finding economic mineralization), stable isotopes are still chemical alteration halos (Engel et al., 1958; Taylor, 1974; rarely applied during mineral exploration. In our view, this is Criss and Taylor, 1983; Criss and Campion, 1991; Criss et al., due to the perceived significant expense involved when stable 1991; Kesler et al., 1995; Naito et al., 1995; Vázquez et al., isotope “vectoring” studies are conducted to define stable iso- 1998; Kelley et al., 2006). Thus, stable isotope ratios have the tope alteration halos around mineral deposits, which could potential to be a valuable tool for mineral exploration in order involve hundreds, or even thousands, of samples. If used in to define regions of rocks that have been altered by hydro- three dimensions, thousands to tens of thousands of samples thermal fluids. could be involved (e.g., a scale similar to that at which multi- Nesbitt (1996) discussed the applications of oxygen and element downhole lithogeochemical data is now collected hydrogen isotope ratios to exploration for hydrothermal ore during many exploration programs). In addition, the time re- quired to obtain stable isotope analyses is typically viewed as † Corresponding author: e-mail, [email protected] being too long to be useful in an exploration context, due to

©2013 by Economic Geology, Vol. 108, pp. 1–9 Submitted: September 21, 2012 0361-0128/13/4079/1-9 1 Accepted: October 2, 2012 2 EXPRESS LETTER the necessity of accessing specialist laboratories, which often include relatively low initial capital cost, low power consump- have long analytical delays and/or do not have the capacity to tion, benchtop size, lack of a high-vacuum system, no re- analyze very large numbers of samples in a timely manner. quirement for high-purity gases, and relatively simple opera- The above factors, along with the rare use of stable isotope tion. All of these factors contribute to field portability and analyses in mineral exploration case studies, have led to poor significantly reduced operating costs. industry awareness of the potential utility of such analyses to The factors outlined above mean that OA-ICOS instruments assist in identifying hydrothermal alteration and/or vectoring have many of the required factors to be deployed into differ- toward hydrothermal mineralization, particularly in areas ent mineral exploration environments (e.g., fly camps, core where mineralization does not crop out at the surface, or logging facilities, mine site assay labs, etc.) However, as yet, where visual alteration is lacking. Stable isotope data has OA-ICOS instruments have not been modified or optimized potential to be of significant assistance during mineral explo- for mineral exploration purposes. In particular, instruments ration, but due to issues surrounding cost, time to acquire will require interfaces of different kinds to turn solid mineral data, and uncertainty regarding data interpretation, it has not phases into gases suitable for isotopic analysis. Commercially yet been widely applied outside of academic studies. available OA-ICOS instruments measure several different stable isotope ratios of potential interest to mineral explo- A Paradigm Shift for the Use of Stable Isotope Data ration, including C and O isotopes in CO2 (which can be lib- in Mineral Exploration? erated from carbonate minerals via acidification) and H and Traditionally, stable isotope ratios of hydrogen, carbon, oxy- O isotopes in water and water vapor (which could be liberated gen, and sulfur are measured using gas source isotope ratio from hydrous silicate minerals and/or fluid inclusions by ther- mass spectrometry (IRMS). These instruments are capable of mal decomposition). We suggest that both of these commer- providing extremely precise measurements of stable isotope cially available techniques could be of benefit to mineral ex- ratios, and can resolve isotopic ratios that differ by as little as ploration, based upon earlier studies demonstrating C, O, and 0.01% (0.1‰), which far exceeds the precision needed to H alteration halos around different deposit types, while fur- resolve isotopic changes typically associated with hydrother- ther analytical developments may lead to the development of mal alteration. While these instruments are extremely pre- OA-ICOS systems capable of analyzing sulfur isotope ratios. cise, they are also expensive (>US$250,000), delicate (need to We believe that this revolution in analytical technology be stored in air-conditioned and vibration- and contamina- represents a paradigm shift in the way that stable isotope data tion-free laboratories), have high consumable costs, demand are collected and utilized, particularly for applications to min- frequent maintenance, and require highly trained and skilled eral exploration, which demand low-cost analyses and rapid personnel to operate them. Thus, their use is mainly re- turnaround. While substantial method development will be stricted to research laboratories in academic or government required, particularly on the conversion of solid mineral institutions. phases to gases suitable for laser-based analysis, such con- Over the last few years, new types of analytical instruments version techniques are already required for IRMS analysis for the measurement of H, C, and O isotope ratios based on and could be adapted relatively easily for use with OA-ICOS infrared absorption to measure isotopic ratios in different gas techniques. species have begun to become available commercially. One Criss and Taylor (1983) demonstrated that fossil hydrother- such infrared absorption technique is off-axis integrated cav- mal systems may produce zones of relative 2H and 18O deple- ity output spectroscopy (OA-ICOS), a form of cavity ring- tion in rocks surrounding the hydrothermal systems, due to down spectroscopy (O’Keefe, 1998; O’Keefe et al., 1999). In interaction of hydrothermal fluids (containing meteoric recent years, instruments based on OA-ICOS have become water) with rocks. Thus, OA-ICOS systems capable of mea- increasingly popular to measure trace gas concentrations and suring the H and O composition of mineral-bound water the isotopic composition of environmental water and gas (which would be extracted by heating hydrous minerals to samples in the laboratory and in the field. OA-ICOS uses a high temperature to extract mineral-bound water as water laser source which produces light at an infrared wavelength vapor) could be of significant benefit to mineral exploration, suitable for interacting with the gas species of interest. The and promising methods for measuring the hydrogen isotope laser light is admitted into a highly reflective mirrored cavity, composition of hydrous minerals utilizing OA-ICOS have in which the light is reflected thousands of times before exit- recently been described (Koehler and Wassenaar, 2012). ing the cavity. As such, strong absorptions occur as the in- Numerous workers have demonstrated that variations in frared light interacts with gas species present in the cell, sulfur isotope ratios may help identify more prospective rocks which can then be measured using photodetectors (O’Keefe, for mineral exploration (e.g., Ripley et al., 2003), and may also 1998; O’Keefe and Deacon, 1998; O’Keefe et al., 1999). By help to vector toward mineralization around various ore changing the wavelength over which the laser operates, the deposit types, including orogenic gold deposits (Hattori and concentration of different isotopologues of the same gas can Cameron, 1987), sedimentary exhalative deposits (e.g., Good- be measured, and isotopic ratios can thus be determined, fellow, 2004), and porphyry copper deposits (e.g., Deyell, commonly with precision similar to IRMS (e.g., Lis et al., 2006; Wilson et al., 2007). Experimental results have been 2008). reported for the analysis of sulfur isotopes by a different The development of OA-ICOS now offers an alternative to infrared absorption spectroscopy technique (Christensen et conventional IRMS, with the ability to measure isotopic ratios al., 2007), raising the possibility that measuring sulfur iso- in several gas species (including H2O, CO2, CH4, N2O). The topes by a commercially available infrared absorption spec- documented advantages of OA-ICOS compared to IRMS troscopy system may become a reality in the future.

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As an example of the application of infrared absorption by the same hydrothermal fluids. In addition, the rates of iso- spectroscopy to mineral exploration, we present carbon and topic equilibration between CH4 and CO2 at temperatures of oxygen isotope data collected from carbonate rocks surround- <300°C exceed 1,000,000 years (Ohmoto and Goldhaber, ing the Screamer Carlin-type gold deposit in the Carlin trend, 1997), meaning that, for the majority of intrusion-related Nevada, collected using a newly developed analytical tech- hydrothermal systems (Cathles et al., 1997), isotopic equilib- nique based on OA-ICOS (Barker et al., 2011). We assess the rium is likely never reached between CH4 and CO2 species. carbon and oxygen isotope alteration halos recorded in car- In lower-temperature hydrothermal systems, isotopic equilib- bonate rocks surrounding the Screamer deposit, and discuss rium is likely never reached between CH4 and CO2 due to the potential ways in which stable isotopes in carbonate rocks extremely slow kinetics of isotopic exchange. This means that might be used in a mineral exploration context. in order to incorporate carbon from organic carbon species (such as CH4) within carbonate minerals, oxidation of organic Mineral Deposit Types Hosted in Carbonate Rocks species (which will generally be significantly depleted in 13C Carbonate rocks are particularly amenable to isotopic relative to limestones) is required in order to allow the carbon analysis, due to the ease with which carbonate minerals can to be incorporated within CO2 and related carbonate species, be converted to CO2 suitable for isotopic analysis by IRMS or and thus precipitated into carbonate minerals. Therefore, sig- laser spectroscopy. A number of important types of mineral nificantly depleted δ13C values in low-temperature hydro- deposits are found in carbonate-rich host rocks. Examples in- thermal systems are most likely indicative of oxidation of or- clude skarn and other carbonate-replacement deposits, Mis- ganic carbon during hydrothermal fluid flow. sissippi Valley-type deposits, and Carlin-type gold deposits. Skarn deposits typically have large (up to 1,000 m) δ13C and Carbon and oxygen isotope alteration around δ18O depletion halos (e.g., Vázquez et al., 1998). Kesler et al. Carlin-type gold deposits, Nevada (1995) highlighted that oxygen isotope halos up to 3 km in In order to assess the size of the stable isotope alteration size surround some manto-style deposits, with the largest footprints and evaluate controls on fluid flow around Carlin- halos developed above and in the upper parts of deposits, type gold deposits, we have analyzed more than 5,000 sam- meaning that stable isotope halos should be particularly use- ples from the Goldstrike property (northern Carlin trend, ful for the detection of blind deposits, and are likely to have a Nevada, containing multiple gold deposits consisting of more significantly larger areal extent than mineral alteration halos. than 60 million ounces [Moz] of contained gold) and the Carlin-type gold deposits also have significant oxygen isotope Long Canyon deposit, northeast Nevada (currently ~3 Moz of depletion halos associated with and surrounding gold miner- contained gold) for their carbon and oxygen isotope composi- alization (Radtke et al., 1980; Stenger et al., 1998; Arehart tion. At Goldstrike, samples were collected from multiple and Donelick, 2006). drill holes on two E-W cross-section lines extending for ~5 Isotopic alteration of host-rock carbonates by hydrothermal km across the Goldstrike property (Vaughan et al., unpub. fluids at low temperatures (<400°C) will generally require data, 2011). In this contribution, we present isotopic values either recrystallization of host-rock carbonate minerals in the collected along a section line chosen to intersect the Screamer presence of a hydrothermal fluid (e.g., dissolution-precipita- gold deposit in the northern Carlin trend and extend as far tion and/or replacement process), or, alternatively, the pre- west from the deposit as samples from drilling existed, in cipitation of new carbonate minerals in pore space. This is order to delineate the isotopic footprint of hydrothermal fluid because diffusional exchange of both oxygen and (particu- flow both proximal and distal to the Screamer deposit. larly) carbon between mineral and fluid is exceptionally slow Some samples were collected from hand specimens from at temperatures less than 400°C (Farver, 1994). diamond drill core. However, the majority of samples were The final oxygen isotope composition of carbonate rocks collected from crushed and powdered samples or “pulps” surrounding orebodies that have interacted with hydrother- which had been produced for gold assaying and lithogeo- mal fluid will depend on the isotopic composition of unal- chemical analysis from both reverse circulation (RC) and dia- tered host rock, the isotopic composition of the hydrothermal mond core drilling. Demonstrating the application of isotopic fluid present at the time that dissolution-precipitation is analysis to pulps is of particular importance to mineral explo- occurring, and the temperature of dissolution-precipitation ration due to the ubiquity of pulps, which are produced dur- (which will affect the equilibrium fractionation factor be- ing mineral exploration across various deposit types. They tween mineral and fluid). In general, rocks that have under- may be especially useful when assessing data and conducting gone higher degrees of fluid-rock reaction, or where fluid- analyses of material collected during historical exploration rock reaction occurred at higher temperature (from a fluid programs, where pulps may have been preserved but core is with identical isotopic composition), will have lower δ18O val- not accessible or in acceptable condition for analysis. In addi- ues. Thus, carbonate rocks immediately adjacent to mineral- tion, a pulp is a sample that is commonly prepared either ization would generally be expected to have lower δ18O values from split core over an interval of various sizes (perhaps 1 to than rocks farther from mineralization. 6 m), or from samples collected at equal intervals from drill With respect to carbon isotopes, many hydrothermal fluids core or RC chips, and should thus be more statistically repre- have far greater quantities of oxygen than carbon, due to the sentative of the isotopic composition of a rock over a particu- relative abundance of H2O compared to carbon-bearing lar volume of rock than individual hand samples. However, species such as CH4 or CO2. Generally, this means that oxy- the interpretation of data collected from pulps may be com- gen isotopes are likely to show greater degrees of isotopic re- plicated if there are multiple generations of carbonate ce- setting relative to carbon isotopes in the same rocks affected ments and/or carbonate veins, and homogenization of rock

0361-0128/98/000/000-00 $6.00 3 4 EXPRESS LETTER samples via crushing will incorporate all of these cement gen- We interpret these results to indicate that hand sampling erations. In order to evaluate the data collected from pulps results in sampling bias within highly altered and mineralized versus hand samples, several drill hole comparisons were carbonate rocks because hand samples are generally collected completed using pulps prepared from 20-cm-long hand sam- from the most coherent material, whereas surrounding mate- ples and pulps collected from the corresponding 1.5-m drill rial may be less coherent, and could represent horizons either assay interval (prepared from split core). more or less susceptible to hydrothermal fluid flow and iso- Comparisons of isotopic data collected from hand samples topic alteration. with that collected from pulp samples down three drill holes Our results imply that, in general, analyses of pulps are likely encompassing interbedded limestone and calcareous siltstone to provide a more representative estimate of the isotopic rock types that have been variably altered by hydrothermal composition of carbonate rocks. Depending on the origin of fluids are shown in Figure 1. Moderate to strong correlations pulps (e.g., split core, core chipped at frequent intervals), de- (r2 = 0.32, 0.46, and 0.66) are seen between the oxygen iso- tailed logs of carbonate veining, which should be obvious in tope values for hand samples and pulp analyses (Fig. 1). The drill core (although potentially less obvious in RC chips), will drill hole (LC555; Fig. 1) with the strongest correlation be- help with the interpretation of isotopic results in intervals of tween isotopic values for pulps and hand specimens was drill core in which there are overprinting carbonate mineral located adjacent to an area of strong brecciation and gold cements potentially unrelated to the hydrothermal event that mineralization and was itself highly brecciated throughout caused the mineralization which is of economic interest. and consistently altered. The drill holes with weaker correla- The Screamer deposit is in the northern part of the Carlin tions display both stronger (LC556) and weaker (LC553) trend, in northeastern Nevada, and forms the most western alteration and mineralization, but also greater meter-scale part of the Betze-Post gold deposit (~40 Moz gold), the variability between altered and unaltered rocks. The relation- largest gold deposit in North America. Carlin-type gold de- 18 18 ship between δ Ohandsample and δ Opulp is close to 1:1 for the posits in the northern Carlin trend are mostly hosted in drill hole with consistent alteration and mineralization, Silurian-Devonian–age carbonate rocks. The Screamer de- 18 18 whereas the relationship between δ Ohandsample and δ Opulp is posit had a pre-mining resource estimate of ~5 Moz (Bettles, closer to 0.5:1 for drill holes displaying greater meter-scale 2002). The Screamer deposit consists mostly of stratigraphi- variability between interbedded rocks. A comparison of oxy- cally controlled mineralization, with ore mostly localized in gen isotope values down drill holes using both hand samples the wispy member of the Devonian Popovich Formation (Ye and pulps indicates that both record similar spatial variations et al., 2003). We collected pulps from two drill holes that in- of isotopic alteration compared to background values. tersected gold grades of greater than 15 ppm within the

LC533C

LC555C

LC556C LC533Cr2=0.32 y=0.6816x+3.7435

LC555Cr2=0.66 y=0.6822x+5.3741

(‰) LC556Cr2=0.46 y=0.7472x+4.8141

All Drillholes r2=0.66 y=0.6857x+4.9978 VSMOW O 18 δ Hand Sample Pulp

18 Drill Assay Pulp δ O VSMOW (‰)

FIG.1. Comparison of oxygen isotope compositions between drill assay pulps and hand samples, with moderate to strong correlations (r2 = 0.32, 0.46, and 0.66) between the oxygen isotope values for hand samples and pulp analyses collected from the Long Canyon deposit, Nevada. Drill hole LC555 has the strongest correlation between pulps and hand specimens, and was located adjacent to an area of strong brecciation and gold mineralization. The drill holes with weaker correlations dis- play both stronger (LC556) and weaker (LC553) alteration and mineralization but greater meter-scale variability between al- tered and unaltered rocks.

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Screamer deposit, as well as drill holes as far to the west of (Veizer et al., 1999) and within stratigraphically equivalent Screamer as drilling exists (~3 km to the west; see Fig. 2). rocks in northern Nevada (Vaughan, unpub. data, 2011). Car- Pulp samples were collected from all carbonate rock types bon isotope results show little systematic variation, either where pulp samples were available down each drill hole. The downhole or between drill holes, presumably reflecting the composite interval for pulps varied between 1.5 and 6 m. low concentrations of CO2 and CH4 thought to be present in Pulps were analyzed for C and O isotope ratios using the Carlin-type hydrothermal fluids (Cline et al., 2005). However, 18 method described by Barker et al (2011). δ OVSMOW values range between ~7 and 25‰—values which 13 18 Globally, rocks of Silurian to Devonian age have δ CVPDB are significantly different from the δ OVSMOW values of 18 values that range between –2 and +6‰, and δ OVSMOW val- Silurian-Devonian–age rocks unaffected by hydrothermal al- ues that range between 23 and 29‰ (Veizer et al., 1999). teration both in Nevada and globally. Depletion in 18O likely Rocks of this age in northern Nevada that are unaffected by reflects alteration of carbonate rocks by hydrothermal fluids, hydrothermal alteration fall within the globally defined range, which has been noted for various Carlin-type gold deposits by 13 18 with δ CVPDB between –1 and +2‰, and δ OVSMOW between several previous workers (Radtke et al., 1980; Stenger et al., 24 and 27‰ (Vaughan et al., unpub. data, 2011). 1998; Arehart and Donelick, 2006). 13 δ CVPDB values measured in carbonate rocks around the Oxygen isotope results from a series of drill holes defining Screamer deposit vary between –3 and +3‰, which is almost a cross section across the Goldstrike property are shown in entirely within the range of carbon isotope values determined Figure 3, along with the concentration of gold down drill for carbonate rocks of Silurian-Devonian age both globally holes. Plotted in Figure 4 are the downhole variations in δ18O

0 500 1000 Drillhole sampled for stable isotopes meters Cross section line fault

Roberts Mountains thrust

surface projection of gold deposits Banshee Mine workings

Meikle Quaternary alluvium Miocene Carlin Formation

Eocene intermediate to felsic dykes

Post Fault Jurassic diorite-granodiorite stocks

Jurassic lamprophyre

Devonian Rodeo Creek Formation Screamer SJ-464C SJ-390C Devonian Popovich Formation Silurian-Devonian Roberts Mountain Betze-Post Formation BZ-998C Silurian-Devonian Roberts Mountain Formation, Bootstrap Limestone unit PD-20C Ordovician Vinini Formation WM-01C

Nevada

4533000m

549000m

FIG. 2. Geologic map of the northern Carlin trend, showing surface geology and major gold deposits projected to the sur- face (adapted from Thompson et al., 2002). Samples for stable isotope analysis were taken from selected drill holes (shown as black circles), both proximal and distal to mineralization, with results shown in Figures 3 and 4. The approximate cross- section line for Figure 3 is shown as the black dotted line between drill holes. Grid reference is given in UTM NAD27 zone 11N.

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6 EXPRESS LETTER 1000 Feet Feet 1000 1000 m NE SJ-464C 0 500 1500 1000 /ton, data provided by Barrick SJ-464C 0 O values for the pulped intervals are 18 δ intersecting significant gold mineralization. gold significant intersecting athways with variable permeability and de- Miocene Carlin Formation Formation Devonian Rodeo Creek Upper Mud Devonian Popovich Fromation Devonian Popovich Formation Soft Sediment Deformation Unit Devonian Popovich Formation Parallel Laminated Unit SJ-390C 800 1000 1200 1400 (‰) BZ-965C VSMOW BZ-965C SJ-390C O Au (oz/ton) 18 δ BZ-997C 800 Devonian Popovich Formation Wispy Unit Silurian-Devonian Roberts Mountain Formation Upper Plate 1000 1200 1400 1600 BZ-998C PD-20C 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 ? 30 25 20 15 10 5 30 25 20 15 10 5 30 25 20 15 10 5 30 25 20 15 10 5 1600 1800 2000 2200 2400 2600 2800 3000 O 18 δ WM-01C PD-20C . 3. Cross section through the Screamer Carlin-type gold deposit in the northern Carlin trend, Nevada. Gold concentrations (oz IG SW F Au WM-01C shown as black symbols and lines. Note that the average amount of isotopic depletion (cf. Fig. 4) increases toward drill holes drill toward increases 4) Fig. (cf. depletion isotopic of amount average the that Note lines. and symbols black as shown The variation in isotopic values within each drill hole presumably represents the drill hole intersecting multiple fluid flow p grees of fluid-rock reaction. Gold Corporation) are plotted as gray lines and symbols within intervals of rock collected as pulps, while the corresponding 0 0.1 0.2 0.3 0.4 0.5 30 25 20 15 10 5 2600 2800 3000 2400

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30

WM01C PD20C BZ-998BZ-997 BZ-965 SJ-390 SJ-464 n=30 n=70 n=44 n=49 n=87 n=55 n=80 25

20 (‰)

15 VSMOW O 18

δ 10

1000 m 700 m 100 m 100 m 500 m 700 m

5 Au > 0.2 oz/ton

0

FIG.4. Box and whisker plots showing the distribution of δ18O values down drill holes shown in Figure 3. Note that sig- nificant variation in stable isotope values occurs within each hole. However, the average amount of depletion increases from the most distal drill holes toward drill holes proximal to mineralization, and is greatest within drill holes that intersect min- eralization (drill holes highlighted in yellow). compositions as box and whisker plots, showing the median, of rock encountered with greater degrees of 18O depletion. upper, and lower quartile and whiskers (interquartile range) We interpret this to reflect different degrees of oxygen iso- of δ18O values measured down each drill hole, along with tope alteration reflecting differences in hydrothermal fluid outliers. Figure 4 demonstrates that median δ18O values in flux controlled by variations in primary (e.g., lithological) and each drill hole decrease toward the center of the Screamer secondary (e.g., fault and fracture) permeability. It is inferred deposit, with the lowest median values in the two drill holes that the flux of hydrothermal fluids increases toward the cen- that intersect significant gold mineralization (Au > 6 ppm ter of gold mineralization, which produces lower δ18O proxi- within 6-m composites). The drill hole most distal from min- mal to mineralization compared to carbonate rocks more dis- eralization (~3 km to the west of Screamer) has a median δ18O tal to mineralization. value that is indistinguishable from background δ18O values de- termined for carbonate host rocks, although it contains some Discussion intervals that are depleted in 18O compared to background Our studies of stable isotope ratios, conducted using OA- host rocks. Drill holes within ~500 m to 1 km of Screamer ICOS, demonstrate that carbonate rocks surrounding the have median δ18O values of ~19‰, with individual intervals Screamer Carlin-type gold deposit in the northern Carlin as depleted as 12‰ and highest values of 25 to 26‰. Drill trend, Nevada, have significant O isotope alteration. These holes within 100 to 200 m of mineralization have median δ18O results support earlier studies of isotopic alteration of carbon- values between 16 and 19‰, with individual intervals having ate rock-hosted ore deposits (e.g., Megaw, 1990; Naito et al., δ18O values as low as ~7‰. In comparison, drill holes inter- 1995; Kesler et al., 1995; Stenger et al., 1998; Vázquez et al., secting gold grades of greater than 6 ppm have median δ18O 1998; Arehart and Donelick, 2006), which revealed that the values of ~16‰, with maximum δ18O values of 19 to 22‰. carbonate rocks surrounding different types of hydrothermal In summary, the oxygen isotope alteration footprint of min- ore deposits have variable intensity C and O isotope alteration eralizing fluids which formed deposits that make up the Gold- halos, of different sizes. The size and distribution of isotopic strike property appears to extend at least 3 to 4 km from the alteration is likely controlled by the total flux of hydrothermal main orebodies, with fluid flow likely controlled by faults and fluid controls on permeability in surrounding rocks. Our re- high-permeability rock types. As drill holes become more sults suggest that isotopic alteration could be used to vector proximal to mineralization, oxygen isotope values down drill toward mineralization in several different ways, depending on holes become increasingly more variable, with more intervals the deposit type of interest, and the exploration environment.

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Patterns of carbon and oxygen isotope alteration in carbon- techniques will assist in the application of stable isotope ate rocks affected by hydrothermal fluids are complex, and analyses to mineral exploration, but applied research will be are controlled by variations in permeability, mineralogy, grain required to assess the best mechanisms for utilizing stable iso- size, temperature, and fluid/rock ratios. Therefore, to inter- topes within the mineral exploration process. pret isotopic variations and delineate isotopic alteration halos with accuracy, large numbers of samples will be required. The Acknowledgments approach outlined here for assessing isotopic alteration around The authors wish to thank Paul Dobak from Barrick Gold mineral deposits is unique in that the recently developed OA- Corporation for discussions that motivated several aspects of ICOS method for isotopic analysis of carbonate minerals this study, and permission to publish geological and geo- (Barker et al., 2011) makes it logistically and financially feasi- chemical information presented in Figure 3. Doug Baer and ble to analyze the large numbers of samples required (thou- Feng Dong from Los Gatos Research Inc. are thanked for sands as opposed to the 30 to ~500 samples common in ear- assistance with various technical aspects of OA-ICOS analy- lier studies). Pulps produced for assay and lithogeochemical sis, and Barrick Gold Corporation, Teck Mining Company, analysis during mineral exploration appear to be a viable Newmont Mining Corporation, and the Natural Science and medium for assessing isotopic alteration on a bulk scale, and, Engineering Research Council of Canada (NSERC) for sup- thus, specialized sampling approaches are not necessarily re- porting various aspects of this research. This is MDRU pub- quired (e.g., carefully constrained hand sampling or micro- lication number 307. drilling), although these more constrained sampling styles will add additional constraints and information about patterns of REFERENCES hydrothermal fluid flow and associated alteration. All of the Arehart, G., and Donelick, R., 2006, Thermal and isotopic profiling of the factors outlined above—low cost, simple sample preparation, Pipeline hydrothermal system: Application to exploration for Carlin-type gold deposits: Journal of Geochemical Exploration, v. 91, p. 27–40. and simple and rapid analysis—suggest that stable isotope Barker, S.L.L., Dipple, G.M., Dong, F., and Baer, D.S., 2011, Use of laser analysis via OA-ICOS is a tool that can be routinely integrated spectroscopy to measure the 13C/12C and 18O/16O compositions of carbon- during mineral exploration. However, new and larger case ate minerals: Analytical Chemistry, v. 83, p. 2220–2226. studies are required to demonstrate the value of stable iso- Bettles, K., 2002, Exploration and geology, 1962–2002, at the Goldstrike property: Nevada Bureau of Mines and Geology Bulletin 111, p. 54–75. tope data to mineral explorations. Cathles, L.M., Erendi, A.H.J., and Barrie, T., 1997, How long can a hydro- We suggest that the overall size and magnitude of an iso- thermal system be sustained by a single intrusive event?: ECONOMIC GE- topic alteration footprint may reflect the overall flux of hydro- OLOGY, v. 92, p. 766–771. thermal fluid flow through those rocks, and thus be used to Christensen, L.E., Brunner, B., Truong, K.N., Mielke, R.E., Webster, C.R., identify areas of rocks that have the potential for greater metal and Coleman, M., 2007, Measurement of sulfur isotope compositions by tunable laser spectroscopy of SO2: Analytical Chemistry, v. 79, p. 9261–9268. endowment (due to a larger flux of fluids). It may therefore Cline, J.S., Hofstra, A.H., Muntean, J.L., Tosdal, R.M., and Hickey, K.A., 2005, be used on a regional scale. Another potential use of stable Carlin-type gold deposits in Nevada: Critical geologic characteristics and vi- isotopes is to vector toward regions of isotope-altered host able models: ECONOMIC GEOLOGY 100TH ANNIVERSARY VOLUME,p. 451–484. rock that have experienced relatively greater volumes of fluid Criss, R.E., and Campion, D.E., 1991, Oxygen isotopes study of the fossil flow (or where fluid temperatures are higher, which will lead hydrothermal system in the Comstock lode mining district, Nevada: Geo- chemical Society Special Publication 3, p. 437–447. to lower oxygen isotope values in carbonate minerals precipi- Criss, R.E., and Taylor, H.P., 1983, An 18O/16O and D/H study of Tertiary tating from those fluids); they can thus be used for property- hydrothermal systems in the southern half of the Idaho batholith: Geolog- scale vectoring. Alternatively, stable isotope analyses could be ical Society of America Bulletin, v. 94, p. 640–663. used to reveal “near misses” when exploring for ore, as stable Criss, R.E., Fleck, R.J., and Taylor, H.P., 1991, Tertiary meteoric hydrother- mal systems and their relation to ore deposition, northwestern United isotope alteration halos have relatively large footprints com- States and southern British Columbia: Journal of Geophysical Research, v. pared to other vectors toward mineralization (e.g., assay, vi- 96, p. 13,335–13,356. sual alteration, lithogeochemistry). Deyell, C.L., 2006, Sulfur isotope zonation at the Mt Polley alkalic porphyry In addition to the use of stable isotopes for direct vectoring Cu-Au deposit, British Columbia, Canada: Society for Geology Applied to toward mineralization, detailed isotopic analyses may be used Mineral Deposits (SGA), Eighth Biennial SGA Meeting, Beijing, China, Aug. 18–21, 2005, Proceedings, p. 373–376. to help understand fluid flow pathways and evaluate whether Engel, A.E.J., Clayton, R.N., and Epstein, S., 1958, Variations in isotopic fluids have migrated in a pervasive fashion through rocks or composition of oxygen and carbon in Leadville limestone (Mississippian, were channeled along a particular orientation or generation Colorado) and in its hydrothermal and metamorphic phases: Journal of Ge- of fault (or other high-permeability lithology), and how those ology, v. 66, p. 374–393. Farver, J.R., 1994, Oxygen self-diffusion in calcite—dependence on temper- pathways are connected to one another. Such information can ature and water fugacity: Earth and Planetary Science Letters, v. 121, p. be used to help interpret what role different structures may 575–587. have played in controlling fluid flow, and whether particular Goodfellow, W.D., 2004, Geology, genesis and exploration of SEDEX de- structures or rock types should be targeted during explo- posits, with emphasis on the Selwyn Basin, Canada, in Deb, M., and Good- ration. It can also help in predicting which direction fluids mi- fellow, W.D., eds., Attributes and models of some major deposits in India, Australia and Canada: New Delhi, Narosa Publishing House, p. 24–99. grated. With sufficient sampling density, it may be possible to Hattori, K., and Cameron, E.M., 1987, Pyrite of distinctive isotopic compo- generate three-dimensional models of isotopic alteration, sition from the Hemlo deposit—a potential tool to identify this type of gold which may then be used to interpret controls on fluid flow mineralization in Archean terrain: Journal of Geochemical Exploration, v. and paleofluid flow directions in three dimensions. 28, p. 85–102. Technological developments in light stable isotope analy- Kelley, D.L., Kelley, K.D., Coker, W.B., Caughlin, B., and Doherty, M.E., 2006, Beyond the obvious limits of ore deposits: The use of mineralogical, sis mean that isotopic analysis is cheaper, easier, and faster geochemical, and biological features for the remote detection of mineral- than ever before. The development of these new analytical ization: ECONOMIC GEOLOGY, v. 101, p. 729–752.

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