Chapter 1: Nevada’s Carlin-Type Gold Deposits: What We’ve Learned During the Past 10 to 15 Years
This contribution provides brief introductions to research on Carlin-type gold deposits completed since publication of the 2005 review paper on the deposits in the Economic Geology 100th Anniversary Volume (Cline et al., 2005). Major advances in our understanding of the deposits have resulted from these studies that cover a broad range of topics, from the geology of deposits to recent discoveries and current geologic models.
Studies of host rocks include expanded application of sequence stratigraphy that is refining our understanding of favorable host rocks, now known to have formed on shallow carbonate platforms during lowstands as well as in deep-water slope to basin environments. Sparse igneous dikes at the surface that were emplaced coincident with formation of deposits of the Carlin trend indicate that a batholith of about 1,000 km2 underlies the trend. Reactivated and inverted normal Neoproterozoic faults formed anticlines and fed ore fluids into structurally prepared reactive rock types. Collaborative district studies determined that structural preparation of host rocks along the Carlin trend occurred during three discrete contractional events followed by Eocene extension and coincident mineralization.
Ore and alteration studies identified systematic trace element and sulfur isotope zoning in ore-stage pyrite rims that formed from temporally discrete ore fluids fed by separate structures. Deposit-scale studies determined that ore minerals in shallowly formed deposits are similar to late ore-stage minerals of typical, more deeply formed Carlin-type gold deposits. Breccias containing high-grade ore formed both by replacement and by calcite dissolution and collapse processes. Halos useful in vectoring toward mineralization include rock quality designation values, trace elements above mineralization in premineral rock and in postmineral clay, oxygen isotope ratios, and soil, soil gas, vegetation, and groundwater chemistry. Isotopic studies have indicated relative timing of ore fluid movement through discrete structures.
Deposit ages coincide with spatially related intrusion ages, from about 42 to 35 Ma, and both young from northeast to southwest. Magmatism and deposit formation are interpreted as related to Eocene delamination of subcontinental lithospheric mantle. Apatite fission track data indicate that the Betze-Post deposit, which contained >1, 240 tonnes (40 Moz) of gold, formed in <15,000 to 45,000 years.
New geologic maps illustrate structural and stratigraphic relationships that will contribute to exploration efforts and potential new discoveries. Recent Nevada discoveries include South Arturo on the northern Carlin trend, the Long Canyon deposit in Cambrian-Ordovician rocks in the newly recognized Pequop district in northeastern Nevada, the giant Goldrush deposit on the Battle Mountain-Eureka trend, and the North Bullion deposit at the southern end of the Carlin trend. Two potential new districts of deposits are being actively explored in the Yukon Territory, Canada, and the Golden Triangle, southern China. Deposits in the Golden Triangle and prospects in the Yukon are currently much smaller than deposits in Nevada, and the presence of proximal coeval magmatism, now recognized in Nevada, is unclear. Studies of some of the Chinese deposits indicate that they formed at conditions intermediate to Carlin-type and orogenic deposits.
Recently published geologic models propose that either shallow, basin-related processes or deep magmatic processes provided gold for the Nevada deposits. Studies evaluating the Harrison Pass pluton and the Emigrant Pass volcanic rocks, both the same age as the Carlin deposits, addressed the magmatic model and provide information about potential magmatic ore fluids and systems that may have formed the deposits.