RECOGNITION OF A REGIONAL ORDOVICIAN VALMY FORMATION THRUST SHEET: IMPLICATIONS FOR ASSEMBLY OF THE ROBERTS MOUNTAINS ALLOCHTHON AND EXPLORATION FOR CONCEALED CARLIN-TYPE GOLD DEPOSITS

Detailed geologic mapping and supporting fossil ages in the Independence Mountains and Double Mountain, Nevada show that a thick (100's of meters) sheet of Middle-Upper Ordovician Valmy Formation quartzite (Ovq) structurally overlies other folded and imbricated Ordovician-Devonian units (Vinini Fm., Elder Sandstone, Slaven Chert) of the Roberts Mountains allochthon (RMA). The Ovq is comprised of shallowly dipping beds of moderately sorted, massive quartzite, and occasional chert beds. The Ovq is time equivalent to the shelfal Eureka Quartzite, but is thought to have accumulated as coalescing submarine fans below bypass channels in a deep slope or rise environment. Mapped exposures of Ovq stand out on a regional ASTER-based map of quartz and led us to investigate other areas of high quartz abundance in order to constrain the age of emplacement and distribution of allochthonous Ovq.

Field examination of quartzite exposures combined with previous mapping confirms the existence of thrust faulted Ovq above deformed units of the RMA at several locations. The results suggest the Ovq thrust sheet is a regional feature that is discontinuously exposed along the strike of the RMA for 220 km; from at least as far south as the Shoshone Mountains, Nevada to the Snake River Plain. It has an exposed width of about 60 km that has been exaggerated by Miocene extension. Age of emplacement of the Ovq thrust sheet is constrained by deformed Upper Devonian Slaven Chert below it and unconformable Pennsylvanian-Permian Strathearn Formation above it. At one locale, rocks tentatively identified as Lower Mississippian may overlie the Ovq.

The Ovq thrust sheet was either emplaced as a mechanically rigid roof thrust to other units of the RMA during the Early Mississippian Antler orogeny or during a subsequent Carboniferous event. The latter interpretation is favored on the basis of disharmony between fold axes within and below the Ovq thrust sheet.

Overall thicknesses of deformed RMA between the Ovq and underlying carbonate rocks (Carlin-type gold deposit host rocks) ranges from < 100 meters in the Independence Mountains to > 600 meters in the Shoshone Mountains. Recognition of windows through and klippe of the Ovq as well as deformation in the other RMA units is needed to identify areas where concealed Carlin-type gold deposits may be in reach of drilling.