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 understanding 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 mineral exploration for various reasons related to cost per sample, the speed at which analytical data can be collected, 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 cavity output spectroscopy [OA-ICOS]) mean that stable isotope data can now be collected in far greater quantities 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 identifying “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
Ratios of the stable isotopes of H, C, O, and S have been
measured and applied to mineral deposit research since the
1950s (Engel et al., 1958). Stable isotopes have been used to
decipher the origin and evolution of ore-forming fluids (see
reviews of Ohmoto and Goldhaber, 1997; Taylor, 1997). In addition, several studies have demonstrated that stable isotope
ratios are commonly altered in rocks surrounding orebodies
compared to rocks unaffected by hydrothermal alteration,
meaning that stable isotope alteration halos can be delineated. Stable isotope alteration halos are typically larger than
mineralogical alteration halos (i.e., visual alteration) and geochemical alteration halos (Engel et al., 1958; Taylor, 1974;
Criss and Taylor, 1983; Criss and Campion, 1991; Criss et al.,
1991; Kesler et al., 1995; Naito et al., 1995; Vázquez et al.,
1998; Kelley et al., 2006). Thus, stable isotope ratios have the
potential to be a valuable tool for mineral exploration in order
to define regions of rocks that have been altered by hydrothermal fluids.
Nesbitt (1996) discussed the applications of oxygen and
hydrogen isotope ratios to exploration for hydrothermal ore
† Corresponding
deposits. Nesbitt provided a review of previous studies that
showed kilometer (or larger)-scale isotopic alteration halos
around different mineral deposit types, in which stable isotope ratios may differ by more than 1% (more than 10‰).
One of the studies reviewed by Nesbitt (1996) included one
of the very few examples in scientific literature of a deposit
discovery attributed to the identification of a stable isotope
anomaly (Naito et al., 1995).
While there is convincing scientific evidence for why stable
isotopes should be useful for identifying rocks which have
been altered by hydrothermal fluids (i.e., prospective zones
for finding economic mineralization), stable isotopes are still
rarely applied during mineral exploration. In our view, this is
due to the perceived significant expense involved when stable
isotope “vectoring” studies are conducted to define stable isotope alteration halos around mineral deposits, which could
involve hundreds, or even thousands, of samples. If used in
three dimensions, thousands to tens of thousands of samples
could be involved (e.g., a scale similar to that at which multielement downhole lithogeochemical data is now collected
during many exploration programs). In addition, the time required to obtain stable isotope analyses is typically viewed as
being too long to be useful in an exploration context, due to
author: e-mail, [email protected]
©2013 by Economic Geology, Vol. 108, pp. 1–9
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Submitted: September 21, 2012
Accepted: October 2, 2012
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EXPRESS LETTER
the necessity of accessing specialist laboratories, which often
have long analytical delays and/or do not have the capacity to
analyze very large numbers of samples in a timely manner.
The above factors, along with the rare use of stable isotope
analyses in mineral exploration case studies, have led to poor
industry awareness of the potential utility of such analyses to
assist in identifying hydrothermal alteration and/or vectoring
toward hydrothermal mineralization, particularly in areas
where mineralization does not crop out at the surface, or
where visual alteration is lacking. Stable isotope data has
potential to be of significant assistance during mineral exploration, but due to issues surrounding cost, time to acquire
data, and uncertainty regarding data interpretation, it has not
yet been widely applied outside of academic studies.
include relatively low initial capital cost, low power consumption, benchtop size, lack of a high-vacuum system, no requirement for high-purity gases, and relatively simple operation. All of these factors contribute to field portability and
significantly reduced operating costs.
The factors outlined above mean that OA-ICOS instruments
have many of the required factors to be deployed into different mineral exploration environments (e.g., fly camps, core
logging facilities, mine site assay labs, etc.) However, as yet,
OA-ICOS instruments have not been modified or optimized
for mineral exploration purposes. In particular, instruments
will require interfaces of different kinds to turn solid mineral
phases into gases suitable for isotopic analysis. Commercially
available OA-ICOS instruments measure several different
stable isotope ratios of potential interest to mineral exploration, including C and O isotopes in CO2 (which can be liberated from carbonate minerals via acidification) and H and
O isotopes in water and water vapor (which could be liberated
from hydrous silicate minerals and/or fluid inclusions by thermal decomposition). We suggest that both of these commercially available techniques could be of benefit to mineral exploration, based upon earlier studies demonstrating C, O, and
H alteration halos around different deposit types, while further analytical developments may lead to the development of
OA-ICOS systems capable of analyzing sulfur isotope ratios.
We believe that this revolution in analytical technology
represents a paradigm shift in the way that stable isotope data
are collected and utilized, particularly for applications to mineral exploration, which demand low-cost analyses and rapid
turnaround. While substantial method development will be
required, particularly on the conversion of solid mineral
phases to gases suitable for laser-based analysis, such conversion techniques are already required for IRMS analysis
and could be adapted relatively easily for use with OA-ICOS
techniques.
Criss and Taylor (1983) demonstrated that fossil hydrothermal systems may produce zones of relative 2H and 18O depletion in rocks surrounding the hydrothermal systems, due to
interaction of hydrothermal fluids (containing meteoric
water) with rocks. Thus, OA-ICOS systems capable of measuring the H and O composition of mineral-bound water
(which would be extracted by heating hydrous minerals to
high temperature to extract mineral-bound water as water
vapor) could be of significant benefit to mineral exploration,
and promising methods for measuring the hydrogen isotope
composition of hydrous minerals utilizing OA-ICOS have
recently been described (Koehler and Wassenaar, 2012).
Numerous workers have demonstrated that variations in
sulfur isotope ratios may help identify more prospective rocks
for mineral exploration (e.g., Ripley et al., 2003), and may also
help to vector toward mineralization around various ore
deposit types, including orogenic gold deposits (Hattori and
Cameron, 1987), sedimentary exhalative deposits (e.g., Goodfellow, 2004), and porphyry copper deposits (e.g., Deyell,
2006; Wilson et al., 2007). Experimental results have been
reported for the analysis of sulfur isotopes by a different
infrared absorption spectroscopy technique (Christensen et
al., 2007), raising the possibility that measuring sulfur isotopes by a commercially available infrared absorption spectroscopy system may become a reality in the future.
A Paradigm Shift for the Use of Stable Isotope Data
in Mineral Exploration?
Traditionally, stable isotope ratios of hydrogen, carbon, oxygen, and sulfur are measured using gas source isotope ratio
mass spectrometry (IRMS). These instruments are capable of
providing extremely precise measurements of stable isotope
ratios, and can resolve isotopic ratios that differ by as little as
0.01% (0.1‰), which far exceeds the precision needed to
resolve isotopic changes typically associated with hydrothermal alteration. While these instruments are extremely precise, they are also expensive (>US$250,000), delicate (need to
be stored in air-conditioned and vibration- and contamination-free laboratories), have high consumable costs, demand
frequent maintenance, and require highly trained and skilled
personnel to operate them. Thus, their use is mainly restricted to research laboratories in academic or government
institutions.
Over the last few years, new types of analytical instruments
for the measurement of H, C, and O isotope ratios based on
infrared absorption to measure isotopic ratios in different gas
species have begun to become available commercially. One
such infrared absorption technique is off-axis integrated cavity output spectroscopy (OA-ICOS), a form of cavity ringdown spectroscopy (O’Keefe, 1998; O’Keefe et al., 1999). In
recent years, instruments based on OA-ICOS have become
increasingly popular to measure trace gas concentrations and
the isotopic composition of environmental water and gas
samples in the laboratory and in the field. OA-ICOS uses a
laser source which produces light at an infrared wavelength
suitable for interacting with the gas species of interest. The
laser light is admitted into a highly reflective mirrored cavity,
in which the light is reflected thousands of times before exiting the cavity. As such, strong absorptions occur as the infrared light interacts with gas species present in the cell,
which can then be measured using photodetectors (O’Keefe,
1998; O’Keefe and Deacon, 1998; O’Keefe et al., 1999). By
changing the wavelength over which the laser operates, the
concentration of different isotopologues of the same gas can
be measured, and isotopic ratios can thus be determined,
commonly with precision similar to IRMS (e.g., Lis et al.,
2008).
The development of OA-ICOS now offers an alternative to
conventional IRMS, with the ability to measure isotopic ratios
in several gas species (including H2O, CO2, CH4, N2O). The
documented advantages of OA-ICOS compared to IRMS
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As an example of the application of infrared absorption
spectroscopy to mineral exploration, we present carbon and
oxygen isotope data collected from carbonate rocks surrounding the Screamer Carlin-type gold deposit in the Carlin trend,
Nevada, collected using a newly developed analytical technique based on OA-ICOS (Barker et al., 2011). We assess the
carbon and oxygen isotope alteration halos recorded in carbonate rocks surrounding the Screamer deposit, and discuss
potential ways in which stable isotopes in carbonate rocks
might be used in a mineral exploration context.
by the same hydrothermal fluids. In addition, the rates of isotopic equilibration between CH4 and CO2 at temperatures of
<300°C exceed 1,000,000 years (Ohmoto and Goldhaber,
1997), meaning that, for the majority of intrusion-related
hydrothermal systems (Cathles et al., 1997), isotopic equilibrium is likely never reached between CH4 and CO2 species.
In lower-temperature hydrothermal systems, isotopic equilibrium is likely never reached between CH4 and CO2 due to the
extremely slow kinetics of isotopic exchange. This means that
in order to incorporate carbon from organic carbon species
(such as CH4) within carbonate minerals, oxidation of organic
species (which will generally be significantly depleted in 13C
relative to limestones) is required in order to allow the carbon
to be incorporated within CO2 and related carbonate species,
and thus precipitated into carbonate minerals. Therefore, significantly depleted δ13C values in low-temperature hydrothermal systems are most likely indicative of oxidation of organic carbon during hydrothermal fluid flow.
Mineral Deposit Types Hosted in Carbonate Rocks
Carbonate rocks are particularly amenable to isotopic
analysis, due to the ease with which carbonate minerals can
be converted to CO2 suitable for isotopic analysis by IRMS or
laser spectroscopy. A number of important types of mineral
deposits are found in carbonate-rich host rocks. Examples include skarn and other carbonate-replacement deposits, Mississippi Valley-type deposits, and Carlin-type gold deposits.
Skarn deposits typically have large (up to 1,000 m) δ13C and
δ18O depletion halos (e.g., Vázquez et al., 1998). Kesler et al.
(1995) highlighted that oxygen isotope halos up to 3 km in
size surround some manto-style deposits, with the largest
halos developed above and in the upper parts of deposits,
meaning that stable isotope halos should be particularly useful for the detection of blind deposits, and are likely to have a
significantly larger areal extent than mineral alteration halos.
Carlin-type gold deposits also have significant oxygen isotope
depletion halos associated with and surrounding gold mineralization (Radtke et al., 1980; Stenger et al., 1998; Arehart
and Donelick, 2006).
Isotopic alteration of host-rock carbonates by hydrothermal
fluids at low temperatures (<400°C) will generally require
either recrystallization of host-rock carbonate minerals in the
presence of a hydrothermal fluid (e.g., dissolution-precipitation and/or replacement process), or, alternatively, the precipitation of new carbonate minerals in pore space. This is
because diffusional exchange of both oxygen and (particularly) carbon between mineral and fluid is exceptionally slow
at temperatures less than 400°C (Farver, 1994).
The final oxygen isotope composition of carbonate rocks
surrounding orebodies that have interacted with hydrothermal fluid will depend on the isotopic composition of unaltered host rock, the isotopic composition of the hydrothermal
fluid present at the time that dissolution-precipitation is
occurring, and the temperature of dissolution-precipitation
(which will affect the equilibrium fractionation factor between mineral and fluid). In general, rocks that have undergone higher degrees of fluid-rock reaction, or where fluidrock reaction occurred at higher temperature (from a fluid
with identical isotopic composition), will have lower δ18O values. Thus, carbonate rocks immediately adjacent to mineralization would generally be expected to have lower δ18O values
than rocks farther from mineralization.
With respect to carbon isotopes, many hydrothermal fluids
have far greater quantities of oxygen than carbon, due to the
relative abundance of H2O compared to carbon-bearing
species such as CH4 or CO2. Generally, this means that oxygen isotopes are likely to show greater degrees of isotopic resetting relative to carbon isotopes in the same rocks affected
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Carbon and oxygen isotope alteration around
Carlin-type gold deposits, Nevada
In order to assess the size of the stable isotope alteration
footprints and evaluate controls on fluid flow around Carlintype gold deposits, we have analyzed more than 5,000 samples from the Goldstrike property (northern Carlin trend,
Nevada, containing multiple gold deposits consisting of more
than 60 million ounces [Moz] of contained gold) and the
Long Canyon deposit, northeast Nevada (currently ~3 Moz of
contained gold) for their carbon and oxygen isotope composition. At Goldstrike, samples were collected from multiple
drill holes on two E-W cross-section lines extending for ~5
km across the Goldstrike property (Vaughan et al., unpub.
data, 2011). In this contribution, we present isotopic values
collected along a section line chosen to intersect the Screamer
gold deposit in the northern Carlin trend and extend as far
west from the deposit as samples from drilling existed, in
order to delineate the isotopic footprint of hydrothermal fluid
flow both proximal and distal to the Screamer deposit.
Some samples were collected from hand specimens from
diamond drill core. However, the majority of samples were
collected from crushed and powdered samples or “pulps”
which had been produced for gold assaying and lithogeochemical analysis from both reverse circulation (RC) and diamond core drilling. Demonstrating the application of isotopic
analysis to pulps is of particular importance to mineral exploration due to the ubiquity of pulps, which are produced during mineral exploration across various deposit types. They
may be especially useful when assessing data and conducting
analyses of material collected during historical exploration
programs, where pulps may have been preserved but core is
not accessible or in acceptable condition for analysis. In addition, a pulp is a sample that is commonly prepared either
from split core over an interval of various sizes (perhaps 1 to
6 m), or from samples collected at equal intervals from drill
core or RC chips, and should thus be more statistically representative of the isotopic composition of a rock over a particular volume of rock than individual hand samples. However,
the interpretation of data collected from pulps may be complicated if there are multiple generations of carbonate cements and/or carbonate veins, and homogenization of rock
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samples via crushing will incorporate all of these cement generations. In order to evaluate the data collected from pulps
versus hand samples, several drill hole comparisons were
completed using pulps prepared from 20-cm-long hand samples and pulps collected from the corresponding 1.5-m drill
assay interval (prepared from split core).
Comparisons of isotopic data collected from hand samples
with that collected from pulp samples down three drill holes
encompassing interbedded limestone and calcareous siltstone
rock types that have been variably altered by hydrothermal
fluids are shown in Figure 1. Moderate to strong correlations
(r2 = 0.32, 0.46, and 0.66) are seen between the oxygen isotope values for hand samples and pulp analyses (Fig. 1). The
drill hole (LC555; Fig. 1) with the strongest correlation between isotopic values for pulps and hand specimens was
located adjacent to an area of strong brecciation and gold
mineralization and was itself highly brecciated throughout
and consistently altered. The drill holes with weaker correlations display both stronger (LC556) and weaker (LC553)
alteration and mineralization, but also greater meter-scale
variability between altered and unaltered rocks. The relationship between δ18Ohandsample and δ18Opulp is close to 1:1 for the
drill hole with consistent alteration and mineralization,
whereas the relationship between δ18Ohandsample and δ18Opulp is
closer to 0.5:1 for drill holes displaying greater meter-scale
variability between interbedded rocks. A comparison of oxygen isotope values down drill holes using both hand samples
and pulps indicates that both record similar spatial variations
of isotopic alteration compared to background values.
We interpret these results to indicate that hand sampling
results in sampling bias within highly altered and mineralized
carbonate rocks because hand samples are generally collected
from the most coherent material, whereas surrounding material may be less coherent, and could represent horizons either
more or less susceptible to hydrothermal fluid flow and isotopic alteration.
Our results imply that, in general, analyses of pulps are likely
to provide a more representative estimate of the isotopic
composition of carbonate rocks. Depending on the origin of
pulps (e.g., split core, core chipped at frequent intervals), detailed logs of carbonate veining, which should be obvious in
drill core (although potentially less obvious in RC chips), will
help with the interpretation of isotopic results in intervals of
drill core in which there are overprinting carbonate mineral
cements potentially unrelated to the hydrothermal event that
caused the mineralization which is of economic interest.
The Screamer deposit is in the northern part of the Carlin
trend, in northeastern Nevada, and forms the most western
part of the Betze-Post gold deposit (~40 Moz gold), the
largest gold deposit in North America. Carlin-type gold deposits in the northern Carlin trend are mostly hosted in
Silurian-Devonian–age carbonate rocks. The Screamer deposit had a pre-mining resource estimate of ~5 Moz (Bettles,
2002). The Screamer deposit consists mostly of stratigraphically controlled mineralization, with ore mostly localized in
the wispy member of the Devonian Popovich Formation (Ye
et al., 2003). We collected pulps from two drill holes that intersected gold grades of greater than 15 ppm within the
LC533C
LC555C
LC556C
Hand Sample Pulp δ18O VSMOW (‰)
LC533C r2=0.32 y=0.6816x+3.7435
LC555C r2=0.66 y=0.6822x+5.3741
LC556C r2=0.46 y=0.7472x+4.8141
All Drillholes r2=0.66 y=0.6857x+4.9978
Drill Assay Pulp δ18O 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 display both stronger (LC556) and weaker (LC553) alteration and mineralization but greater meter-scale variability between altered and unaltered rocks.
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Screamer deposit, as well as drill holes as far to the west of
Screamer as drilling exists (~3 km to the west; see Fig. 2).
Pulp samples were collected from all carbonate rock types
where pulp samples were available down each drill hole. The
composite interval for pulps varied between 1.5 and 6 m.
Pulps were analyzed for C and O isotope ratios using the
method described by Barker et al (2011).
Globally, rocks of Silurian to Devonian age have δ13CVPDB
values that range between –2 and +6‰, and δ18OVSMOW values that range between 23 and 29‰ (Veizer et al., 1999).
Rocks of this age in northern Nevada that are unaffected by
hydrothermal alteration fall within the globally defined range,
with δ13CVPDB between –1 and +2‰, and δ18OVSMOW between
24 and 27‰ (Vaughan et al., unpub. data, 2011).
δ13CVPDB values measured in carbonate rocks around the
Screamer deposit vary between –3 and +3‰, which is almost
entirely within the range of carbon isotope values determined
for carbonate rocks of Silurian-Devonian age both globally
(Veizer et al., 1999) and within stratigraphically equivalent
rocks in northern Nevada (Vaughan, unpub. data, 2011). Carbon isotope results show little systematic variation, either
downhole or between drill holes, presumably reflecting the
low concentrations of CO2 and CH4 thought to be present in
Carlin-type hydrothermal fluids (Cline et al., 2005). However,
δ18OVSMOW values range between ~7 and 25‰—values which
are significantly different from the δ18OVSMOW values of
Silurian-Devonian–age rocks unaffected by hydrothermal alteration both in Nevada and globally. Depletion in 18O likely
reflects alteration of carbonate rocks by hydrothermal fluids,
which has been noted for various Carlin-type gold deposits by
several previous workers (Radtke et al., 1980; Stenger et al.,
1998; Arehart and Donelick, 2006).
Oxygen isotope results from a series of drill holes defining
a cross section across the Goldstrike property are shown in
Figure 3, along with the concentration of gold down drill
holes. Plotted in Figure 4 are the downhole variations in δ18O
0
500
meters
1000
Drillhole sampled for stable isotopes
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
st
Po
Jurassic diorite-granodiorite stocks
ult
Fa
Screamer SJ-464C
Devonian Rodeo Creek Formation
Devonian Popovich Formation
SJ-390C
BZ-998C
Jurassic lamprophyre
Betze-Post
PD-20C
Silurian-Devonian Roberts Mountain
Formation
Silurian-Devonian Roberts Mountain
Formation, Bootstrap Limestone unit
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 surface (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 crosssection 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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3000
2800
2600
2400
δ18O
5
3000
2800
2600
2400
2200
2000
1800
1600
30 25 20 15 10
PD-20C
0 0.1 0.2 0.3 0.4 0.5
?
PD-20C
5
BZ-965C
0 0.1 0.2 0.3 0.4 0.5
18
800
5
30 25 20 15 10
1400
1200
SJ-390C
5
0
0 0.1 0.2 0.3 0.4 0.5
SJ-464C
1000 Feet
5
1000 m
NE
30 25 20 15 10
1500
1000
500
SJ-464C
0
0 0.1 0.2 0.3 0.4 0.5
SJ-390C
1000
δ OVSMOW (‰)
30 25 20 15 10
1600
1400
1200
1000
800
BZ-965C
Au (oz/ton)
BZ-998C BZ-997C
Devonian Popovich Formation Parallel Laminated Unit
Devonian Popovich Formation Soft Sediment Deformation Unit
Devonian Popovich Fromation Upper Mud
Devonian Rodeo Creek Formation
Miocene Carlin Formation
FIG. 3. Cross section through the Screamer Carlin-type gold deposit in the northern Carlin trend, Nevada. Gold concentrations (oz/ton, data provided by Barrick
Gold Corporation) are plotted as gray lines and symbols within intervals of rock collected as pulps, while the corresponding δ18O values for the pulped intervals are
shown as black symbols and lines. Note that the average amount of isotopic depletion (cf. Fig. 4) increases toward drill holes intersecting significant gold mineralization.
The variation in isotopic values within each drill hole presumably represents the drill hole intersecting multiple fluid flow pathways with variable permeability and degrees of fluid-rock reaction.
30 25 20 15 10
Au
WM-01C
0 0.1 0.2 0.3 0.4 0.5
WM-01C
SW
Upper Plate
Silurian-Devonian Roberts Mountain Formation
Devonian Popovich Formation Wispy Unit
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30
WM01C
n=30
BZ-998
PD20C
n=70
BZ-997
n=44
n=49
BZ-965
SJ-390
n=87
SJ-464
n=55
n=80
δ18OVSMOW (‰)
25
20
15
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 significant 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 mineralization (drill holes highlighted in yellow).
of rock encountered with greater degrees of 18O depletion.
We interpret this to reflect different degrees of oxygen isotope alteration reflecting differences in hydrothermal fluid
flux controlled by variations in primary (e.g., lithological) and
secondary (e.g., fault and fracture) permeability. It is inferred
that the flux of hydrothermal fluids increases toward the center of gold mineralization, which produces lower δ18O proximal to mineralization compared to carbonate rocks more distal to mineralization.
compositions as box and whisker plots, showing the median,
upper, and lower quartile and whiskers (interquartile range)
of δ18O values measured down each drill hole, along with
outliers. Figure 4 demonstrates that median δ18O values in
each drill hole decrease toward the center of the Screamer
deposit, with the lowest median values in the two drill holes
that intersect significant gold mineralization (Au > 6 ppm
within 6-m composites). The drill hole most distal from mineralization (~3 km to the west of Screamer) has a median δ18O
value that is indistinguishable from background δ18O values determined for carbonate host rocks, although it contains some
intervals that are depleted in 18O compared to background
host rocks. Drill holes within ~500 m to 1 km of Screamer
have median δ18O values of ~19‰, with individual intervals
as depleted as 12‰ and highest values of 25 to 26‰. Drill
holes within 100 to 200 m of mineralization have median δ18O
values between 16 and 19‰, with individual intervals having
δ18O values as low as ~7‰. In comparison, drill holes intersecting gold grades of greater than 6 ppm have median δ18O
values of ~16‰, with maximum δ18O values of 19 to 22‰.
In summary, the oxygen isotope alteration footprint of mineralizing fluids which formed deposits that make up the Goldstrike property appears to extend at least 3 to 4 km from the
main orebodies, with fluid flow likely controlled by faults and
high-permeability rock types. As drill holes become more
proximal to mineralization, oxygen isotope values down drill
holes become increasingly more variable, with more intervals
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Discussion
Our studies of stable isotope ratios, conducted using OAICOS, demonstrate that carbonate rocks surrounding the
Screamer Carlin-type gold deposit in the northern Carlin
trend, Nevada, have significant O isotope alteration. These
results support earlier studies of isotopic alteration of carbonate rock-hosted ore deposits (e.g., Megaw, 1990; Naito et al.,
1995; Kesler et al., 1995; Stenger et al., 1998; Vázquez et al.,
1998; Arehart and Donelick, 2006), which revealed that the
carbonate rocks surrounding different types of hydrothermal
ore deposits have variable intensity C and O isotope alteration
halos, of different sizes. The size and distribution of isotopic
alteration is likely controlled by the total flux of hydrothermal
fluid controls on permeability in surrounding rocks. Our results suggest that isotopic alteration could be used to vector
toward mineralization in several different ways, depending on
the deposit type of interest, and the exploration environment.
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Patterns of carbon and oxygen isotope alteration in carbonate rocks affected by hydrothermal fluids are complex, and
are controlled by variations in permeability, mineralogy, grain
size, temperature, and fluid/rock ratios. Therefore, to interpret isotopic variations and delineate isotopic alteration halos
with accuracy, large numbers of samples will be required. The
approach outlined here for assessing isotopic alteration around
mineral deposits is unique in that the recently developed OAICOS method for isotopic analysis of carbonate minerals
(Barker et al., 2011) makes it logistically and financially feasible to analyze the large numbers of samples required (thousands as opposed to the 30 to ~500 samples common in earlier studies). Pulps produced for assay and lithogeochemical
analysis during mineral exploration appear to be a viable
medium for assessing isotopic alteration on a bulk scale, and,
thus, specialized sampling approaches are not necessarily required (e.g., carefully constrained hand sampling or microdrilling), although these more constrained sampling styles will
add additional constraints and information about patterns of
hydrothermal fluid flow and associated alteration. All of the
factors outlined above—low cost, simple sample preparation,
and simple and rapid analysis—suggest that stable isotope
analysis via OA-ICOS is a tool that can be routinely integrated
during mineral exploration. However, new and larger case
studies are required to demonstrate the value of stable isotope data to mineral explorations.
We suggest that the overall size and magnitude of an isotopic alteration footprint may reflect the overall flux of hydrothermal fluid flow through those rocks, and thus be used to
identify areas of rocks that have the potential for greater metal
endowment (due to a larger flux of fluids). It may therefore
be used on a regional scale. Another potential use of stable
isotopes is to vector toward regions of isotope-altered host
rock that have experienced relatively greater volumes of fluid
flow (or where fluid temperatures are higher, which will lead
to lower oxygen isotope values in carbonate minerals precipitating from those fluids); they can thus be used for propertyscale vectoring. Alternatively, stable isotope analyses could be
used to reveal “near misses” when exploring for ore, as stable
isotope alteration halos have relatively large footprints compared to other vectors toward mineralization (e.g., assay, visual alteration, lithogeochemistry).
In addition to the use of stable isotopes for direct vectoring
toward mineralization, detailed isotopic analyses may be used
to help understand fluid flow pathways and evaluate whether
fluids have migrated in a pervasive fashion through rocks or
were channeled along a particular orientation or generation
of fault (or other high-permeability lithology), and how those
pathways are connected to one another. Such information can
be used to help interpret what role different structures may
have played in controlling fluid flow, and whether particular
structures or rock types should be targeted during exploration. It can also help in predicting which direction fluids migrated. With sufficient sampling density, it may be possible to
generate three-dimensional models of isotopic alteration,
which may then be used to interpret controls on fluid flow
and paleofluid flow directions in three dimensions.
Technological developments in light stable isotope analysis mean that isotopic analysis is cheaper, easier, and faster
than ever before. The development of these new analytical
0361-0128/98/000/000-00 $6.00
techniques will assist in the application of stable isotope
analyses to mineral exploration, but applied research will be
required to assess the best mechanisms for utilizing stable isotopes within the mineral exploration process.
Acknowledgments
The authors wish to thank Paul Dobak from Barrick Gold
Corporation for discussions that motivated several aspects of
this study, and permission to publish geological and geochemical information presented in Figure 3. Doug Baer and
Feng Dong from Los Gatos Research Inc. are thanked for
assistance with various technical aspects of OA-ICOS analysis, and Barrick Gold Corporation, Teck Mining Company,
Newmont Mining Corporation, and the Natural Science and
Engineering Research Council of Canada (NSERC) for supporting various aspects of this research. This is MDRU publication number 307.
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