Geology and Minerlization
Keystone is a large, complex, early-Tertiary (34.1+/-0.7 Ma) intrusive-centered, structural dome with extensive mesothermal to epithermal zones of alteration developed within exposed upper-plate rock units, and permissive lower-plate units of Devonian-age Horse Canyon calcareous siltstone, Wenban limestone Formations, and Silurian-age Roberts Mountain silty limestone Formation. Detailed geological studies, including age-dating, conducted by the U.S. Gold Corp. geological team at Keystone have recognized very close similarities to the prospective host rocks of the Cortez area with those at Keystone. Further, it appears evident that the Keystone project is comprised of a very expansive gold-bearing hydrothermal system within a complex lower-plate dome cored by a very complex early Tertiary intrusive system of fine-grained to coarse-grained felsic to mafic intrusives as stocks, dikes and sills. Of primary importance at Keystone are the now evident very thick altered and drill-indicated gold mineralized zones and very permissive host characteristics of the prospective Horse Canyon (Carlin Trend Rodeo Creek Formation equivalent) and Wenban limestone host units. Both these units are being encountered in our drill holes at the surface and, or at shallow exploration depths. In addition, large zones of epithermal-type alteration and mineralization, including gold, are being encountered within dike and sill intruded collapse-style breccia bodies at Keystone.
Field reconnaissance has identified several additional textural and composition plug and dikes, indicating the presence of a complex, likely long-lived, late-Eocene intrusive event and associated mineral system. Permissive host rocks comprised of the Devonian calcareous siltstone Horse Canyon and varietal Wenban limestones, and also permissive-looking Roberts Mountain limestones are exposed at the surface in a broad domal uplift (window) of the prospective host units. Broad areas of non-permissive upper plate mapped as Vinini cover much of the outer portions of the district. The geological specifics of the upper-plate rock units that have been thrust over (Roberts Mountain thrust) and broadly cover much of the permissive and prospective lower-plate rocks are being better defined by ongoing geological mapping in the district. Drilling to date has encountered large bodies of dissolution related collapse breccias with associated and hydrothermal styles of alteration, including silicification, argillization, and sulfidation, multiple types and styles of system-related dikes and sills, local very strong pathfinder elements, including locally abundant arsenopyrite, realgar and orpiment, and locally, strong, deep oxidation.