title: The Effects of contact metamorphism on the host rocks for Carlin-type mineralization at the Getchell Development, Nevada, USA
Creators: name: Eck, Nathan C.
Publisher: name: University of Nevada, Las Vegas
Year: 2010

description: Carlin-type gold deposits (CTGDs) result from low to moderate temperature hydrothermal systems which form replacement bodies in carbonate or calcareous host rocks. The Getchell CTGD is located on the Getchell trend in north central Nevada. The Eocene age mineralization is locally hosted within the metamorphic aureole of the Cretaceous age Osgood stock. Previous studies have noted that the effects of the contact metamorphism can be heterogeneous, with strongly calc-silicate altered carbonates transitioning to relatively pristine limestone over short distances. The main finding of this study was that the variability in calc-silicate alteration is largely dependent on the differing host lithologies at the deposit. Strong calc-silicate alteration from contact metamorphism commonly exists in rocks composed of argillaceous mudstone inter-bedded with limestone while the rocks composed of siliceous carbonaceous mudstones and limestones just a few meters away show no evidence of calc-silicate alteration. 
The different ways in which these rock types respond to the contact metamorphism plays an important role in ore control at the deposit. The strongly calc-silicate altered rocks are poor hosts for Carlin-type gold deposition. The ore fluids which were moderately acidic, pH 3-4, required carbonates to generate porosity and permeability, as well as to liberate iron for the ore fluids to sulfidize to form the Au-bearing pyrite. As such the strongly calc-silicate altered lithologies are not receptive to the Carlin ore fluids, due to most of the carbonate being replaced by calc-silicate minerals. The limestones inter-bedded with siliceous carbonaceous mudstones which have not been altered to calc-silicates, are the best hosts for Carlin-type mineralization. This study observed several locations where strong mineralization is present in this rock type up to the contact with the strongly calc-silicate altered argillaceous mudstones inter-bedded with limestones. 
Additionally mineralization is restricted to areas adjacent to conduits for ore fluids, most commonly faults, but also along dikes. Where conduits transporting ore fluids contact the receptive areas host rock mineralization permeates outward away from the conduit. The best hosts, those which are dominantly limestone, contain grades >1.000 ounce Au per ton (oz/t). Sections composed dominantly of siliceous carbonaceous mudstones have little to no calcareous material for the ore fluids to replace aside from calcite filled fractures. Gold grade in mineralized sections of siliceous carbonaceous mudstone are <0.100 oz/t Au. Faults which cut the strongly calc-silicate altered rocks typically have mineralization which is confined to the fault gouge and does not permeate into the surrounding rock, although in heavily faulted areas there can be significant intervals of fault gouge and breccia with ore grade mineralization (>0.300 oz/t Au). 
The secondary focus of this study examined the distribution of iron-bearing carbonate proximal to the Osgood stock to determine if the stock was responsible for adding iron to the surrounding carbonates. Iron-bearing carbonates have been proposed to be better hosts for Carlin mineralization, providing a source of iron for the ore fluids to sulfidize and precipitate Au-bearing pyrite. This study revealed no systematic distribution of ferroan carbonate that would indicate the iron was sourced from the stock. Rather, the distribution of ferroan carbonate is found to be largely controlled by lithology. The limestones with a silty component were found to be iron-rich while pure limestones associated with the siliceous carbonaceous mudstones were iron-poor.