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Collaborative Research: Deep crustal structure of the Carlin trend: Crustal-scale controls on the location of large-scale mass transfer in hydrothermal systems Introduction The origin of linear arrays of fossil hydrothermal systems around the world has been the subject of much speculation. Over the past 50 years, as our understanding of plate tectonics has expanded, some of those arrays are explainable. For example, the Abitibi gold belt in Canada is widely considered to have formed in an ancient crustal suture, a zone of crustal weakness that allowed hydrothermal systems to develop and redistribute metals in the crust. In the case of the Abitibi belt, there are sufficient exposures at and near the surface to allow clear delineation of the structure. For other linear trends of mineral belts, the deep crustal structure is not so obvious. One of the best examples of such a linear belt that is poorly understood at depth is the Carlin Trend, in northeastern Nevada. The Carlin trend, located near the center of the Great Basin (Figure 1) and had pre-mining resources in excess of 150 million ounces of gold. Current production from this group of mineral deposits comprises approximately 8% of the global production of gold, making it by far the richest plot of ground in the world. The Carlin trend is a striking linear array of deposits, with intervening ground having significant mineralization present (the only difference between the deposits and intervening ground is one of economics, not geology). In these deposits, gold is present primarily at the atomic scale within the structure of pyrite that is disseminated within the host rocks. These host rocks comprise lower Paleozoic sedimentary rocks that have been intruded by plutons and dikes of Mesozoic to Tertiary age. Additional controls on the distribution of gold include structures and permeable stratigraphic units, both of which provided pathways for mineralizing fluids. The temporal and spatial coincidence of igneous rocks with gold mineralization is striking, leading many researchers to conclude that magmatism is a critical component in their genesis. It is still debatable whether magmas contributed metals and fluids to the hydrothermal systems. An alternative hypothesis is that the deposits represent deeply-sourced fluids that have moved into the upper crust along a large-scale structural Figure 1. Terrane map of part of the western US showing sources of igneous components. The SrI = 0.706 is approximately at the boundary between volcanic (green) and sedimentary (light yellow) terranes in central Nevada. Large dashed lines discontinuity in the crust, the nature of which is still under debate. The question to be addressed by the research proposed herein is: Why is the Carlin trend such a striking linear array of mineral deposits? Our hypothesis is that it represents a long-lived crustal-scale structure that focused igneous and hydrothermal activity. Prior to ~45 Ma, this structure was in a generally compressional tectonic regime, but at 45 Ma (onset of extension in the Great Basin) because it was a weakness in the crust, it opened up to facilitate intrusion and associated hydrothermal activity and metal mobilization. This hypothesis will be tested by a combination of geological, geochemical and geophysical approaches. Crustal Architecture of the Great Basin The Great Basin is underlain by several major crustal blocks, each of which has distinctive geological characteristics as elucidated by geophysical, geochemical, and geological data (Figure 2). Although there are some uncertainties in the exact location of their boundaries, these blocks include the Wyoming craton (>2.5 Ga), Proterozoic accreted terranes (<2.0 Ga); and the Mojavia block (2.0 - 2.3 Ga) (Zartman, 1974; Condie, 1981; Bennett and DePaolo, 1987). Based on Nd isotopic mapping, the Mojavia block may be a part of the Archean craton shifted in a left-lateral sense in mid- to late-Proterozoic time, thus opening up a rift basin which was filled with late Proterozoic sedimentary rocks (terrigenous detrital sequence [TDS] of Stewart, 1980). During early- to mid-Paleozoic time, much of the western margin of North America was a passive margin, with continued accumulation of a thick section of dominantly fine-grained clastic and carbonate “eastern assemblage” rocks in the region that is currently the east-central Great Basin. Further offshore in the ocean basin, time-equivalent “western assemblage” rocks comprise deep-water sediments, predominantly cherty rocks with minor amounts of mafic volcanic rocks. In late Paleozoic and Mesozoic times, compressional tectonics associated with the Antler and Golconda orogenies resulted in the development of east-vergent thrust plates such as the Roberts Mountains and Golconda thrusts. Deep-water “western assemblage” Paleozoic rocks were displaced eastward over the time-equivalent “eastern assemblage” rocks. Following the Antler and Golconda orogenies, the craton margin changed from passive to active at the end of the Triassic and the earliest pulse (Nevadan-Elko) of the Cordilleran orogeny began (Speed et al., 1988). Continued compression in the Cretaceous was expressed by a second pulse of contraction in the Great Basin region, the Sevier orogeny (Thorman et al., 1991). The third pulse of contraction, the Laramide orogeny, is poorly represented in the Great Basin, but was well-developed in areas outside of the Great Basin. As the result of tectonic reorganization, decreased convergence rates between the North American and Pacific plates caused foundering of the subducting slab in the Eocene. This resulted in the development of an extensional environment in the Great Basin beginning at ~45 Ma. Since that time, the Great Basin has been undergoing extension, although at different rates in different locations within the region. Geological data Surface and near-surface geology at local and regional scales is well-known because of mapping in three dimensions including pits, underground exposures, and drilling (to 2 km depth). The recent compilation edited by Thompson et al. (2002) summarizes the detailed geology of the Carlin Trend, including stratigraphy, structure, geochronology, with limited geochemical and geophysical data. Maps and cross-sections of the area are summarized from detailed maps of the mining industry. These data provide an important surface base that will ground truth any hypotheses developed during the proposed research. The surface geology of necessity will have to be compatible with the inferred crustal structure. Should we summarize the regional geology in a paragraph here? Geochemical data Compressional tectonism of the active margin was accompanied by plutonic igneous activity in the Great Basin, beginning in the Jurassic. Associated with the Nevadan-Elko phase of this protracted event was the emplacement of a suite of plutonic rocks. Following a decrease in convergence between ~145 and ~120 Ma, plutonic igneous activity again became abundant in the Great Basin during the Sevier phase of this orogenic event in the Cretaceous. The third major period of igneous activity commenced in the Eocene, at the same time as the tectonic regime changed from compressional to extensional. Although igneous activity in the Great Basin continues today, the youngest plutons of any significant size that are exposed at the surface are Eocene in age. These igneous rocks (both plutonic and volcanic) currently exposed at the surface provide a mechanism for sampling of the crust (upper and lower) and possibly the mantle. Insights into crustal structure are available through the use of major and trace elements, and radiogenic and stable isotope measurements on plutons across the Great Basin as well as complementary geophysical data. In particular, the isotope systematics of Rb/Sr, Sm/Nd, U/Pb and O have provided important constraints on the broad structure of the crust. Kistler and Peterman (1973) and Kistler (1978) were among the earliest to recognize and utilize Sr isotope ratios in western North America to examine the crust. Magmas derived from older crustal material (i.e., having a relatively longer residence time in the crust) should generally have higher initial Sr ratios. Kistler utilized initial strontium isotope ratios for plutons to define what he considered a major crustal boundary, where the 87Sr/86Srinitial = 0.706 line was considered the western margin of older continental crust (Figure 1). Similarly, Nd isotopic data were utilized to infer older vs. younger crustal material. Farmer and DePaolo (1983) and Bennett and DePaolo (1987) constructed isotopic maps of western North America from Nd and Sr data. They observed a similar boundary between cratonic North America and accreted terranes but, in addition, could distinguish some of the older cratonic structures, i.e., the Archean (N) province from the Proterozoic (S) province. Similar refinements to the Precambrian crustal architecture are provided by Pb isotope data (e.g., Doe and Stacey, 1974; Doe and Zartman, 1979; Wright and Wooden, 1991, Fleck and Wooden, 1997, Tosdal et al., 2000, among others). In the Sierra Nevada and Great Basin, oxygen isotopes have been used to assess the degree of interaction between magmas and crust. Where significant magma-crust interaction has taken place, oxygen values often are generally higher, reflecting the input of 18O from sedimentary (or metasedimentary) rocks, either as partial melts or as assimilated materials. This was clearly documented in the southern Sierra Nevada by Taylor and Silver (1978) and Taylor (1980). In a traverse across the Great Basin, Solomon and Taylor (1989) and King et al. (2004) documented the change in whole-rock and zircon oxygen isotope signatures from west to east. They showed Figure 4. Bouger gravity map of a portion of northern Nevada, with basin effects removed. Carlin-type deposits shown by purple dots. From Grauch et al., 2003. that plutonic rocks in the western part of the crust are dominated by mantle-derived materials, having oxygen isotope values below +9‰, whereas in the central and eastern portion of the Great Basin, magmas have been influenced by crustal material. King et al. also presented evidence suggesting that plutons of different ages within the central-eastern zone had interacted with crustal material to different degrees. Geophysical Data There are a number of geophysical data that suggest that the location of major mineral belts containing Carlin-type deposits overlie large-scale crustal structures. Gravity data presented by Grauch et al. (2003; Figure 4) indicate that the Carlin trend lies on the eastern boundary of a horst block, and the Battle Mountain-Eureka trend lies on the western margin. Supporting data for this interpretation are present in the magnetic model, although those data are less robust. Need more input here from geophysics community. Proposed Research The question to be addressed by the research proposed herein is: Why is the Carlin trend such a striking linear array of mineral deposits? Our hypothesis is that it represents a long-lived crustal-scale structure that focused igneous and hydrothermal activity. Geological. The proposed research will focus on integrated studies of surface geology, moderate and deep geophysical techniques, and geochem. The geological portion of the research is requisite because the surface geology undoubtedly reflects the underlying crustal structure, which reflects the geological evolution of this part of the world. Any understanding developed from geophysical and geochemical techniques must be consistent with the observed geological relations. Many man-years of effort have been expended in mapping the Carlin trend by Barrick and Newmont geologists. There is an extensive database of geological maps and sections that cover the entire trend. Although we anticipate that little additional geological mapping will be required as part of this research, we will make extensive use of the extant data set in understanding the structural evolution of the Carlin trend, as each event over the history of the trend will have left an imprint on the region. Our task will be primarily one of compilation of the data and integration of those data into a larger picture. Geophysical. The proposed geophysical research will focus on imaging of the Carlin trend, from the surface into the mantle. This will be accomplished by utilizing a number of techniques that will generate complementary datasets. Seismic imaging.... etc. Need geophysics group input here.... Geochemical. The proposed geochemical research will focus on utilizing stable and radiogenic isotopes to assess the changes in magmatic events in both space (on the Carlin trend and laterally from it) and time (intrusions in the same space at different times). The primary vehicle for this sampling will be igneous rocks (mostly plutons), which will have recorded information about their origin and magma-crustal interaction. Spatial imaging will comprise sampling and analysis of plutons of different ages within the Carlin trend, and lateral to the trend. Figure 3 shows proposed sample locations in and around the Carlin trend; at least 100 potential sample locations. In samples of the same age, it is likely that igneous rocks will have a similar region of origin vertical position in the crust/mantle), and will differ only in their post-origin history, that is, their interaction with the crust during their ascent. Therefore, if the Carlin trend represents a crustal-scale structure that guided emplacement of magmas, it is likely that igneous rocks in the Carlin trend will have passed through the crust will have interacted differently with the crust than magmas of similar origin that were not emplaced within that structure. These differences are expected to be evident in the isotopic signatures (Sr, Nd, Pb, O, S) of the plutons. The extant data set for the Great Basin is sufficiently diffuse that the Carlin trend cannot be identified using these criteria, that is, the Carlin trend is a smaller-scale feature than can be recognized with the existing density of data. Therefore, our proposal is to increase the density of data in and around the Carlin trend sufficiently to be able to detect differences in the isotopic signature of plutons within the crustal structure vs. lateral to that structure. Our interpretation will be guided in part by the results of geophysical investigations that will help to more clearly image the Carlin trend below the surface (see below). In samples in the same location but of different ages, isotopic data should provide important information on the origin of the magmas, although such data may be somewhat more difficult to interpret. If the Carlin trend is a long-lived crustal feature as we suggest, then magmas coming through the crustal column should have interacted similarly with that column, but may differ in the isotopic signature of their origin, compounded by their degree of interaction with the crust. Preliminary data from the CT (M. Ressel, unpublished data) suggest significant differences in Pb isotope signatures of Eocene vs. Mesozoic plutons. Oxygen stable isotope analyses will be done for whole-rock, zircon, and quartz. Analyzing these three sample types will allow assessment of the effects of hydrothermal alteration and post-crystallization tectonic processes on the signal of the pluton; typically, whole-rock samples will reflect alteration before quartz, and zircon appears extremely resistant to changes from magmatic values (e.g., King et al., 1997). The oxygen isotope signature of the plutons should reflect in part their origin (as mantle or crustal melts) and in part their subsequent history of interaction with the crust during ascent. Sulfur stable isotope analyses will be done for whole-rock samples and for apatite mineral separates. Although the sulfur isotope signature of plutons may be complex, preliminary data for a NS traverse through eastern Nevada (including the Carlin trend) suggest systematic variations in the sulfur isotope signature with latitude, likely reflecting interaction of the magmas with crustal sulfur. An obvious problem with sulfur isotopes in altered, mineralized plutons is the introduction of sulfur that is post-crystallization. Efforts will be made to avoid such (whole-rock) samples, and analyses done of trace sulfate from apatite where possible; it is likely that apatite sulfate will be more representative of magmatic sulfur than bulk rock analyses of altered rocks. Stable isotope measurements will be carried out at the Nevada Stable Isotope Laboratory (http://www.mines.unr.edu/isotope/) under the direction of the PI Arehart and Co-I Poulson. Radiogenic isotope analyses will comprise Sr, Nd, and Pb of whole-rock samples. Radiogenic isotope data will provide an important complement to the stable isotope data, and can be compared to extant data for the region. As shown in Figure 1, large crustal blocks have been outlined in western North America using reconnaissance radiogenic isotope data. The proposed research will focus more closely on elucidating the details of the area around the Carlin trend. Multiple samples of the same plutonic suite will be analyzed as necessary to obtain the most useful data. Neodymium has been chosen because of the relatively minimal effect of hydrothermal alteration on the Nd isotope composition of plutonic rocks; this resistance to alteration will be critical in obtaining useful data from samples of rocks in altered areas of the Carlin trend. The ages of the rocks to be analyzed are generally well-known so that appropriate age corrections to the isotope ratios can be made; where necessary, additional age determinations will be made. Radiogenic isotope analyses, coupled with concentration measurements of the parent and daughters, will be conducted in the Radiogenic Isotopes Laboratory, Ohio State University (http://www.geology.ohio-state.edu/ril/) under the direction of Co-I Foland as outlined in Foland and Allen (1991). Logistics and timeline A preliminary timeline is outlined in Table 1. We expect to begin the research with a strategy meeting in June, 2005, to be followed on an annual basis by a research meeting in Elko, NV. Because of the academic and geographic diversity of researchers, such a meeting is requisite to keep all participants abreast of progress and plans. Broader impacts of the proposed research If trends of the past two decades continue (Einaudi, 1994; J. Dilles, personal communication, 2002), the number of geoscientists in the US who are working on mineral deposits research will soon be vanishingly small. In a society critically dependent on minerals (per capita consumption of minerals in the US is approximately 25 tons annually; minerals contribute directly to about $125 billion of US GDP: US Census Bureau, 2001), this decline represents the likely loss of critical intellectual mass necessary for the understanding of commodities vital to our national security. The research proposed here will help to counter these trends by providing opportunities for several PhD students focused on developing careers in or related to minerals research. In addition, the research effort will involve multiple undergraduate hourly students (at least 3) who will get exposed to the opportunities in mineral deposits research through participation in the project. This combination of applied science supported in part by industry and in part by governmental agencies such as NSF provides students exposure to both aspects of mineral deposit research, and is critical in training the upcoming generation of mineral geoscientists for positions in industry, academia, or government. Students involved in this research will gain an appreciation for a wide variety of geoscience subdisciplines and .... The results of the proposed research will be broadly disseminated in the scientific literature, as have previous studies of the PIs and their students. In addition, because of our close links to the mineral industry, our research is generally applied to mineral problems shortly following publication or presentation, thereby providing an important linkage between discovery and societal benefit. For example, the recent publications by the PI regarding the age of Carlin-type deposits has been widely utilized to guide mineral exploration in Nevada. This interaction will be fostered formally by our proposed annual meetings in Elko, NV to exchange data and ideas. In addition, we are in a special position to disseminate the results of our research through meetings of the Geological Society of Nevada (GSN), a very active bridge organization between academia and industry. Such presentations often precede formal publication by 12 to 24 months, and thus are a critical early communication to the stakeholders in the mining industry Support for this research will help to maintain and enhance the infrastructure for research and education in several locations, as well as foster inter-institutional cooperation and collaboration. It is expected that students involved in the research will visit other institutions to present and discuss research results. Results of Prior NSF Support (past 5 years) Greg B. Arehart EAR-9805384 - Age of Formation of Carlin-Type Gold Deposits and Relationship to Tectonic Environments (1998-2000). New radioisotopic dates were generated, including the first successful dating of a clearly ore-related sulfide mineral (Tretbar et al., 2000), which indicate an Eocene (~40 Ma) age for magmatism and gold deposits of the Carlin Trend during the transition from a compressional to extensional tectonic environment (Arehart et al., 2000, 2003). Funds supported two MSc theses at UNR (Chakurian, 2001, Tretbar, 2004). EAR-9977209 Acquisition of a Stable Isotope Facility to Study Earth and Environmental Systems and Processes (1999-2001). Funds from this grant were used to install two mass spectrometers and associated preparation systems, including a laser fluorination line. This equipment has been in use since December, 2001, and is being extensively used by visiting researchers, faculty, and graduate students in the Department of Geological Sciences. EAR 0309935 Stable Isotope Constraints on Source, Contamination, and Alteration of Great Basin Igneous Rocks, and Implications for Metallogeny (2003-2006). This research focuses on stable isotope signatures (primarily S) of plutons across the Great Basin that have interacted with the upper crust during emplacement. Preliminary data suggest systematic changes in S signatures in a NS transect in east-central Nevada, reflecting changes in sedimentary rock geochemistry. One MS and one PhD student are being supported. EAR 0412883 Mass Redistribution in Continental Magmatic-Hydrothermal Systems (2004-2005). Partial support for a GSA Penrose Conference to be held in September, 2004. Simon R. Poulson EAR-9977209 and EAR 0309935 (see above) EAR- (with Noble)