STRATIGRAPHY AND STRUCTURAL GEOLOGY OF THE ROBERTS MOUNTAINS ALLOCHTHON, CARLIN/BATTLE MOUNTAINS EUREKA TRENDS, NEVADA, USA, CLARIFIED THROUGH BIOSTRATIGRAPHY

Radiolarian biostratigraphy of Paleozoic chert in the Roberts Mountains allochthon (RMA), Nevada, provides new age control that clarifies stratigraphic relationships, reveals structural features, and allows for reasonable cross sections to be drawn. Radiolarians are used with graptolites and conodonts to date chert, shale, and limestone, respectively. Stratigraphic units from the Roberts Mountains allochthon can now be correlated from studies at six distinct locations throughout northeastern Nevada covering over 150 km laterally. Correlated stratigraphy over six mountain ranges not only show contiguous stratigraphic packages but also, facies changes associated with the Ordovician strata along the western parts of the RMA.

All lithologies must be dated because structural boundaries have been observed to follow rheologic changes, such as shale-chert contacts. The allochthon displays a thin stratigraphy that has been tightly folded or imbricately thrusted. The increased stratigraphic resolution of areas previous mapped as undifferentiated Paleozoic strata has a gross tripartite stratigraphic division that occurs on a regional scale consisting of 1) Ordovician shale with lesser quartzite, chert, and limestone (Vinini/Valmy Fms.), 2) Silurian ­Devonian siltstone with chert and minor limestone (Cherry Spring Chert, Elder Fm.), and 3) and Devonian-Mississippian chert and siltstone locally rich in barite and/ or carbonate beds (Slaven Chert, Waterpipe Canyon Fm.). Locally, each of these divisions has been further subdivided and mapped on a scale of 1:6000, allowing for more detailed correlation. Many of the cherts are thin; ranging from 3 to 20 m in thickness, yet may be traced for 10's to 100's of km. Larger structural features, such as discrete nappes, are recognized on the basis of differing stratigraphy, such as the presence/absence of thick quartzite, barite deposits, or distinctive chert units.

The RMA structurally overlies autochthonous carbonates that host Carlin-type gold deposits. Radiolarian, conodonts and graptolite biostratigraphy have been used to define multiple stages of deformation that are recognized and correlated to regional events, such structural reconstruction and analysis have been used to further develop gold exploration projects, and aid in the discovery of new gold deposits along the Carlin and Battle Mtn Eureka Trends.