Western Region Gold Deposits

Staff

Tasks

Project summary:

The Great Basin is one of the world's great metallogenic provinces and about 11% of the total world production of gold (approximately 74% of the United States production) is produced annually from this region, as well as additional silver and copper. Included within this region are some of the largest Au deposits in the world with a resource potential that exceeds 3,200 metric tonnes (100,000,000 ounces) of gold. The impact of these mines on local economies, on the nation's economic sustainability, and on our national balance of trade are profound, and will continue to be so well into the 21st century. This project focuses on critical questions regarding the genesis of precious-metal deposits in this region in order to develop state-of-the-art mineral and geoenvironmental models used in crucial land-use planning decisions affecting federal lands in the west and in designing exploration strategies by the mining industry. This project also is the cornerstone of the USGS's participation in a consortium with academia (CREG at Mackay School of Mines, Reno, NV), the mining industry, and the Nevada Bureau of Mines and Geology (NBMG).

Figure: Gold ore from the Olinghouse epithermal deposit, Nevada.

This project focuses research efforts on fundamental questions still seeking resolution regarding the genesis of gold-sliver deposits in the Great Basin. These deposits include: (1) world-class sedimentary rock-hosted gold-silver deposits, such as those found along the Carlin trend; (2) pluton-related gold deposits that are peripheral to porphyry copper-gold deposits; and (3) hot-spring gold-silver deposits that are the shallowest deposits in the region. Additional resources have been recognized in (4) sedimentary exhalative deposits, which locally reach economic ore grades and in some cases are precursor auriferous concentrations upon which younger hydrothermal systems are superposed. The thread that ties together the tasks is the known or potential geologic linkages between deposit types. The multidisciplinary scientific investigations undertaken herein in these major gold-silver environments will result in a comprehensive overview that then will be linked into a synoptic major treatise by all participants documenting crustal life cycles of gold in the northern Great Basin. This aim is accomplished through five integrated tasks:

• Task 1: Crustal structure and the spatial distribution of ore deposits
• Task 2: Framework of Carlin-type gold deposits
• Task 3: Fluid evolution and age of Carlin-type gold deposits
• Task 4: Pluton-related gold deposits
• Task 5 Hot-spring gold deposits

Task 1: Crustal Structure and the Spatial Distribution of Ore Deposits (Tien Grauch, task leader, tien@usgs.gov)

The linear arrays of gold deposits in northern Nevada suggest fundamental deep crustal and tectonic controls on their development. This task seeks to constrain any such controls through integrated geophysical and radiogenic isotopic investigations that probe the crustal structure of northern Nevada, especially in the vicinity of the Carlin and Battle Mountain-Eureka trends. Geophysical investigations are integrating gravity, magnetic, magnetotelluric (MT), and seismic-reflection data to model the physical aspects of the deep crust that are related to major structures, crustal composition, and fluid migration. Radiogenic isotopic mapping using initial Pb and Sr isotopic compositions of feldspars in rocks provides independent information about variations in crustal composition through time that may be related to the crustal structure. Integration of these diverse studies, combined with the results from the other tasks, will ultimately provide a better understanding of the tectonic evolution of the area and the role of crustal structure in controlling fluid migration, source of ore, and the spatial distribution of mineral deposits at the surface.

Figure: Sedimentary-rock hosted gold deposits in Nevada.

Task 2: Framework of Carlin-type gold deposits (Ted Theodore, task leader, theodore@usgs.gov)

Sedimentary-rock hosted (Carlin) deposits are the major gold deposits in northern Nevada. Characterization of the range of geologic environments of the deposits, their relationship to structure and stratigraphy, their ages, and metal sources are critical to developing a model for the deposits. Studies will document impact of Paleozoic, Mesozoic, and Tertiary stratigraphy and structures on Carlin-type deposits. Additional studies focus on the role, if any, of syngenetic (sedex) gold occurrences in Devonian rocks in the Carlin trend. Integrated investigations will delineate geologic phenomena important to Carlin-type systems, their relationship to regional geologic events, the relationship if any to tectonomagmatic systems, metal sources for the deposits, and relation of deposits to the tectonic evolution of the region.

Specific subtasks:

1. Bald Mountain-Alligator Ridge Subtask: To determine roles of Eocene deformation and Jurassic plutonism on gold distribution in the southern extension of the Carlin trend in the Bald Mountain-Alligator Ridge mining areas.
2. Structural Subtask: To document and interpret structural framework and hydrothermal ore-fluid conduit history and geometry for Carlin-type gold deposits in the Carlin and Independence Mining Districts.
3. Northern Carlin Trend Subtask: To determine (1) structural history of the presently known northern terminus of the Carlin Trend and relation of geologic structures to distribution of known orebodies, as well as (2) baseline geochemical concentrations and distributions in areas undisturbed by previous mining activities.
4. Eocene tectonic history: (1) To reconstruct an Eocene crustal section in the Elko-Carlin area using the Ruby Mountains as a guide to the deep crust during the time of Carlin mineralization; (2) to compare rates, magnitudes, and styles of Eocene-versus-Miocene extension.

Figure: Goldstrike (Post-Betze) Mine, Carlin Trend, Nevada, the largest Carlin-type deposit in the world, containing more than 35,000,000 ounces gold.

Task 3: Fluid evolution and age of Carlin-type gold deposits (Al Hofstra, task leader, ahofstra@usgs.gov)

Knowing the source and evolutionary path(s) of hydrothermal fluids during ore deposition are essential to improved descriptive and genetic models for Carlin-type gold deposits. Site-specific and reconnaissance studies will constrain the composition and source of ore fluids (meteoric, metamorphic, magmatic), age of the deposits where possible, and any climatic role in genesis, preservation, and destruction. The resultant descriptive and genetic information will improve life-cycle models for Carlin-type gold deposits. Additional study will be devoted to the newly recognized sedex gold occurrences on the Carlin Trend. The resulting sedex gold deposit model should have general applicability and will improve assessments of gold potential in sedimentary basins world-wide and especially in the western United States.

Task 4: Framework of Pluton-related gold deposits (Tom Moore, task leader, tmoore@usgs.gov)

Pluton-related gold deposits are the second most important type of gold deposits in northern Nevada. Major pluton-related gold deposits in the Battle Mining District and at Bingham, Utah are the focus of this task. In the Battle Mountain Mining District, research centers on understanding the relation between the pluton-related gold deposits, flanking peripheral gold-silver deposits, and nearby Carlin-type deposits that may represent a halo zone genetically linked to the Battle Mountain deposits. Work involves (1) 3-D modeling of metal zoning and fluid-flow paths in gold-rich porphyry and peripheral environments using industry supplied geochemical data at Copper Canyon in the Battle Mountain Mining District, and (2) characterization of sediment-hosted gold deposits and their host rocks south of the district in the Shoshone Range (The Cedars quadrangle) with the intention of distinguishing whether this environment is genetically linked to the pluton-related gold deposits at Battle Mountain. In the Bingham, Utah deposits, research focuses on examination of paleothermal anomalies that mark the locations of auriferous magmatic-hydrothermal plumes associated with the ore-forming system.

Specific subtasks:

1. Battle Mountain: To determine (1) three dimensional distribution of metals in the Copper Canyon porphyry copper system and (2) prograde and retrograde fluid-flow paths that contributed to metal distribution.
2. Southern Shoshone Range: To determine the nature of gold mineralization in The Cedars quadrangle and its relation to the stratigraphic and structural framework of the area.
3. Utah Gold Deposits Thermal History: A pilot study to determine the extent of the paleothermal anomaly around the zoned auriferous magmatic-hydrothermal system at the giant Bingham porphyry Cu deposit, using time-temperature data including fission-track analysis, vitrinite reflectance, and conodont alteration indices (CAI).

Figure: Photograph of gold and copper deposits in the Battle Mountain Mining District, Nevada.

Task 5: Hot-Spring Gold (David John, task leader, djohn@usgs.gov)

Hot-spring gold-silver deposits represent the third major deposit type in northern Nevada. These near-surface deposits, the shallowest setting of ore genesis in northern Nevada, provide a link between the magmatic and hydrologic environments. Of critical importance in defining their genesis and role in the metallogeny of the region is understanding their environments, age, and relationship to tectonomagmatic events along the Miocene Northern Nevada Rift and during Eocene volcanism and extensional strain thought to be contemporaneous and important to the formation of the Carlin-type systems.

Specific subtasks:

1. Sheep Creek and northern Shoshone Ranges: Geologic and geophysical investigations of Miocene magmatism and related hydrothermal mineral deposits along the northern Nevada rift in Sheep Creek Range and northern Shoshone Range, including detailed studies of the Mule Canyon Au-Ag deposit.
2. Ivanhoe district: Field, geochronologic, and geochemical studies of Eocene and Oligocene volcanic and sedimentary rocks in the Ivanhoe District.
3. Ar geochronologic studies.

Figure: Esmeralda vein, discovery outcrop of the Aurora Mining District, Nevada.