Isotope geochemistry has long played a role in elucidating the crustal structure of the western part of North America. There is a large database of radioisotopic data available for the region as well as whole-rock oxygen isotope data. This project, funded by NSF, is focused on additional analysis of mineral oxygen isotopes (quartz, zircon) because whole-rock samples are subject to both high-temperature and low-temperature water-rock exchange, which may affect the oxygen signature of such samples (primarily the feldspar component).
In addition to oxygen, sulfur isotopes are being analyzed. Both oxygen and sulfur appear to reflect the crustal interaction and contamination of ascending magma bodies, and provide important new information on crustal architecture, evolution of western North America, and the time-space distribution of some metal deposits (DeYoung et al., 2005, 2006, & in review). Additional work along similar lines is continuing in Turkey (Boztug et al., 2005; 2007) and in northern BC/Yukon (Rasmussen et al., 2010).
The figure shows a model of the crustal architecture of the Great Basin, drawn from geochemical data.
I have been involved in determining stable isotope compositions of H, C, N, O, and S in volcanic gases from around the world. Because many of these systems are actively depositing metals, these studies are important for understanding the links between magmatic and hydrothermal ore-forming systems in volcanic terranes. Active crater lakes are analogues for acid-sulfate gold-copper systems, and geothermal systems are low-sulfidation epithermal vein systems. An important aspect of these low-sulfidation systems is understanding the origin of calcite, which can be an important indicator of whether the system is expanding or contracting. In systems such as Golden Cross, New Zealand (Simmons et al., 2000), isotopic measurements suggest that the latest calcite veins represent collapse of the geothermal system that was responsible for gold deposition there. At the Midas vein system in Nevada, geochemical measurements on the various mineral bands in the vein should provide important insights into the development of geothermal system chemistry through time. The geologic setting of the Midas system was a part of the dissertation research of Ellie Leavitt (Ph.D., 2004) (Leavitt et al., 2005).
As part of the Geothermal Resource Center, trace metal concentrations in geothermal systems across the Great Basin have been investigated with the goal of better understanding the differences between magmatically-driven systems and those that are "extensional‚" systems, that is, are driven by the elevated geothermal gradient in the extending Great Basin. Preliminary data (Arehart et al., 2003) suggest that there are significant differences in the geochemistry of elements such as As, B, Cs, and Li between the two types of system. Developing a better understanding of these differences may lead to more efficient exploration and exploitation strategies. Geothermal systems in the Great Basin are often closely associated with very young epithermal deposits; an understanding of the time-space relationships of these systems is important to exploration of new resources of both types (Coolbaugh et al., 2005).
Top photo shows the crater lake at Ruapehu, New Zealand in 1995 prior to the most recent eruptive cycle. The lake had a pH of about 2.5, molten native sulfur on its bottom, and a surface temperature near 40ºC.
Bottom photo shows the site of the crater lake at Ruapehu, New Zealand following several months of eruptive activity.
In the Great Basin, one of the major questions related to Carlin-type mineralization is the timing of that mineralization relative to the complex tectonic and igneous history of the region. My students and I have recently dated a number of important ore deposits in Nevada. Dave Tretbar utilized Rb-Sr to date galkhaite, a complex Hg-Tl-Cs-Sb-S mineral from the Getchell and Rodeo deposits, the first and only direct dates on any Carlin-type deposits (Tretbar et al., 2000; Arehart et al., 2003). Tony Chakurian examined partially and totally annealed apatite grains to elucidate the timing of mineralization in the Carlin trend (Chakurian et al., 2003). This work also has shown that there is a zoning in the annealing from south to north, providing a vector toward the intrusion that most likely was the heat engine for this productive district.
I am currently working in east-central Yukon on the comparison of an exciting new group of recently-discovered gold deposits to the well-studied Carlin-type deposits of Nevada. These new deposits have many characteristics of Carlin-type deposits, but some potential significant differences.
This figure shows an isochron from galkhaite samples, Getchell Mine, NV. This isochron yields an age of 39.5 Ma, the first date on a gold-bearing phase in a Carlin-type deposit. From Tretbar et al., 2000.
The geochronological work described above has been expanded at the Pipeline Carlin-type deposit (Arehart & Donelick, 2006) and in the Bald Mountain district (Schmauder et al., 2005). Stable isotope geochemistry coupled with low-temperature thermochronology (apatite and zircon fission-track and U/Th-He) can provide information on buried mineral deposits, because such deposits are fossil hydrothermal systems. Stable isotopes yield information on fluid flux from water-rock isotope exchange calculations, and thermochronology can yield information on the temperature of the hydrothermal fluids, as well as provide age constraints on the system. Both methods can be useful in vectoring exploration toward the center of the system, and may have extent beyond traditional geochemical indicators.
The figure at right shows a generalized geologic map of the Carlin Trend, NV. A gravity/magnetic high SW of the Trend may represent a buried (Tertiary?) intrusion that has completely reset apatite fission tracks in the southern part of the Trend (yellow) and partially annealed fission tracks in the northern part of the Trend (orange). From Chakurian et al., 2003.
Porphyry and epithermal mineral deposits are common in young volcanic terranes throughout the world, and my recent research has included studies of these deposits in the Great Basin (Leavitt et al., 2004, 2005; Coolbaugh et al., 2005), Alaska, New Zealand and Turkey (Yilmaz et al., 2005, 2006) in which the geology, geochemistry and geochronology are documented. Ellie Leavitt dated several of the epithermal veins in the Midas district, along with the enclosing volcanic and volcaniclastic rocks, providing evidence of the very close temporal relationship between volcanism and mineralization (Leavitt et al., 2004; 2005). She also documented the relationship of the vein system to the surrounding wallrocks. Studies of epithermal deposits are closely aligned with work on geothermal and volcanic systems, on which I worked in New Zealand, Nevada, and Turkey. Jessica Smith (2010) studied the Adanac (Ruby Creek) molybdenum deposit to determine the chronology of intrusion and mineralization there as well as potential links between molybdenum and gold deposits. She showed that the timing of mineralization was distinct from the timing of the intrusive activity and that the gold event was probably unrelated to the molybdenum event. I have recently begun looking at porphyry-epithermal deposits in the Aleutian arc for comparison to other arc-related porphyry and epithermal systems around the world.
The top photo shows high-grade banded ore from the Colorado Grande epithermal vein, Ken Snyder Mine, Midas District, NV. Quartz-adularia-calcite bands (white) with naumannite-rich bands containing electrum (black).
Bottom is a location map of the Adanac porphyry molybdenum property, studied by Jessica Smith.
Microanalytical techniques are becoming ever more important in documenting zoning and equilibrium/disequilibrium conditions in ore deposits, particularly at trace levels of elemental and isotopic concentration. The first quantitative analysis of gold in pyrite from Carlin-type deposits was made possible using the ion microprobe; continuing work is required to document the location of gold in these important deposits. In addition to elemental data, isotopic data can now be determined using microprobe techniques. Particularly for fine-grained ores such as Carlin-types, such fine spatial resolution is required to elucidate the geochemical history of mineral grain growth.
The figure at right shows ablation holes in a crystal of galkhaite from the Getchell deposit. Analysis of trace and ultra-trace elements shows distinctive zoning patterns.
H, C, and O isotopes in minerals and inclusions may provide constraints on paleoclimate (Arehart and Poulson, 2001). Alunite in weathering zones is one of very few stable isotopic indicators of paleoclimate which can be dated by direct means (K/Ar). If alunite growth is controlled by mechanisms such as Ostwald ripening, it may be possible to extract more than one datum per sample, and gain insights into shorter-scale climatological changes. Isotopic analysis of fluid inclusions from hydrothermal systems also can provide information on the composition of local meteoric water; multiple episodes of vein formation may allow a more detailed examination of shorter-term climatological changes.
Shown at right is a secular curve for hydrogen isotopic composition of water in the northern Great Basin for the last 30 Ma, based on analysis of alunite.