Van Tuyl Lecture: John Muntean, University of Nevada Reno
March 7 @ 4:00 pm - 5:00 pm
John Muntean, Nevada Bureau of Mines and Geology, University of Nevada Reno
241 Berthoud Hall, 4PM
Topic: The Continua between Carlin-type Gold Deposits and Distal Disseminated and Epithermal Gold-Silver Deposits in Nevada
Abstract: Most research on Carlin-type deposits in Nevada has focused on four main camps that account for 95% of the production from Carlin-type deposits in Nevada – the Carlin Trend, Cortez, Getchell, and Jerritt Canyon. Though their local geologic settings differ slightly, they share many common characteristics, including lithologic and structural controls to fluid flow and ore deposition, an As-Hg-Sb-Tl geochemical signature and low Ag:Au ratios in the ores, hydrothermal alteration and ore paragenesis, relatively low temperatures and salinities of ore fluids, and lack of lack of mineral and elemental zoning. The shared characteristics suggest a shared origin, yet there is no consensus on their genesis. Future research should focus on Carlin-style deposits, such as distal-disseminated deposits that have a clear genetic association with magmatic-hydrothermal systems associated with intrusions and deposits that have characteristics of epithermal deposits.
There has been limited research on these Carlin-style deposits. An exception is the Cove distal disseminated deposit where Johnston et al.’s (2008) detailed study demonstrated both polymetallic and proximal Carlin-style mineralization, which led them to conclude that Cove represents a continuum between magmatic-hydrothermal systems and Carlin-type gold deposits in Nevada. New research utilizing recent deep drilling (Muntean et al., 2017) clearly shows Carlin-style mineralization overprints earlier polymetallic mineralization. The recent drilling also shows the Carlin-style mineralization is strongly zoned. The Ag/Au ratios of zones of Carlin-style mineralization and the Ag concentrations of pyrite decrease from the CSD zone underneath the open pit towards the Helen zone located 3 km to the northwest. Arsenian pyrite associated with Carlin-style mineralization in the CSD is coarser grained, euhedral, and becomes much finer grained with narrow arsenian rims in the Helen zone, very similar to the pyrite textures seen in the large Carlin-type deposits. The Kinsley deposit in eastern Nevada, shows very similar zoning features away from an Eocene intrusion. Over a distance of ~3 km, proximal W-(Mo) skarn adjacent to the intrusion zones outward to polymetallic quartz veins/mantos and distal Carlin-style mineralization that locally overprints polymetallic mineralization (Hill, 2016). The overprinting of polymetallic mineralization by Carlin-style mineralization at Cove and Kinsley could be the result of a separate later hydrothermal system. Alternatively, the zoning at Cove and Kinsley represent telescoped systems, where exhumation by erosion or by faulting led to the overprint.
Other Carlin-style deposits, mainly in eastern Nevada, have epithermal characteristics and appear to have formed at much shallower depths than the large Carlin-type deposits. Nutt and Hofstra (2003) pointed out many features at Alligator Ridge deposit consistent with formation at depths of <300 to 800 m, as did Ressel et al. (2015) for deposits in the Northern Pinon Range. The features include higher Ag/Au ratios, jigsaw mosaic quartz and feathery chalcedonic jasperoid suggestive of <180˚C, hydrothermal breccias, and Eocene lacustrine sediments with elevated As, Sb, Tl, and Hg. Quartz after lattice-textured calcite is present at the Gold Point deposit, where Castor and Hulen (1996) reported electrum occurring in banded quartz that filled matrices of brecciated stratiform jasperoid. The absence of the Roberts Mountain thrust in eastern Nevada is likely the main factor for the increase of epithermal characteristics. Where present the Roberts Mountains thrust and contractional deformation in the lower plate carbonates played a major role in forming the large Carlin-type deposits, by diverting upwelling hydrothermal fluids out of high-angle faults and into reactive carbonate rocks. In the absence of such a thrust in the upper crust, as in eastern Nevada, hydrothermal fluids rose toward the surface and interacting with increasing amounts of groundwater, resulting in extensive jasperoid formation.