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1-9 Layout 1 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
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