2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 20
Presentation Time: 1:30 PM-5:30 PM


MARINO, Fredy Alfonso, Department of Geological Sciences, Univ of Nevada, Ralph J. Roberts Center for Research in Economic Geology (CREG), Reno, NV 89557, fredymarino@hotmail.com

The Deep Post underground mine operated by Newmont Mining Corporation, and one of the highest grade gold mines in the northern part of the Carlin trend, Nevada, USA, has a mineable reserve of 2.8 million tons at 26 g/t Au. Gold ore is hosted in planar-laminated silty limestones of the Devonian Popovich Formation and wispy-laminated silty limestone of the Silurian-Devonian Roberts Mountains Formation. Deep Post is a complex orebody containing multiple stages of structurally controlled breccia hosted in carbonate rocks and the adjacent Goldstrike dioritic pluton and related dike swarm. Gold mineralization is spatially and temporally related to Eocene (~38 Ma) biotite-feldspar porphyry dikes, which locally occupy ore-controlling fault and fracture zones.

Pre-mineralization structures include ENE- and N-striking faults, which are filled by framboidal and cubic (brassy) pyrite, clay gouge and quartz. Fault and breccia zones acted as conduits for gold mineralizing fluids, especially the intersections of syn-mineralization NW- and NNW-striking faults, which contain gold-bearing arsenian pyrite and gold grades in excess of 150 g/t Au. These structural zones were reactivated and post-ore minerals, such as realgar, orpiment, and barite, were deposited along them. Post-mineralization structures include WNW-, NE- and E-striking faults, which typically contain clay gouge, stibnite, realgar, orpiment and barite. Decalcification and silification are the main hydrothermal alteration types at Deep Post, particularly in carbonate rocks, where dissolution-related volume loss has enhanced breccia development.

Deep Post is localized in the zone of convergence of several major structures, including the NNW-trending Post fault and Post anticline, the WNW-trending Betze anticline, and the irregular contact of the Goldstrike pluton. Deformation localized along these structural elements formed zones of high fracture density, which provided conduits for hydrothermal fluid flow during gold mineralization. Understanding the inter-relationship between structural systems, hydrothermal alteration, and gold mineralization at Deep Post is important for future exploration efforts here and in other Carlin-type deposits.