STRATIGRAPHY AND ANTLER AGE DEFORMATION OF THE ROBERTS MOUNTAINS ALLOCHTHON AND THRUST ZONE AND THE RELATIONSHIP TO CARLIN-TYPE GOLD DEPOSITS IN THE INDEPENDENCE MOUNTAINS OF NORTHERN NEVADA

The Roberts Mountains allochthon (RMA) was emplaced during the L. Dev.-E. Miss. Antler orogeny and extends from southern Idaho, through central Nevada, to southeastern California. The RMA is comprised of Cam.-Dev. siliciclastic outer continental-margin sequences that were thrust eastward over coeval miogeoclinal calcareous assemblages.

The fault zone between RMA and lower-plate (LP) assemblages is an important ore-fluid boundary as most Carlin-type Au deposits are concentrated in chemically favorable LP calcareous rocks proximal to the fault zone. Within the Independence Mountains (IM), Eocene Carlin-type deposits are hosted in altered calcareous miogeoclinal rocks, and are exposed in erosional windows through antiforms that are highly oblique to the regional trend of the RMA.

In the IM, the Snow Canyon Fm., Elder Sandstone, and Slaven Chert combined are ≥1 km thick, and contain L. Cam.-Dev. turbidites (plus minor greenstone, chert, and carbonate rocks). Exposed only at the highest elevations and along the flanks of the range is approximately 200 m of U. Ord. quartzite and chert.

Folding of the RMA here consists of a broad, range-scale N-S-trending antiform and internal, non-coaxial (to the regional trend), E-W-trending antiforms cored by lower-plate assemblages. These likely formed as structural duplexes where the basal thrust of the RMA cut into the lower plate and were translated over inherited structural features including frontal and lateral/oblique ramps, resulting in fault-related folding parallel and oblique to the regional structural trend. The Roberts Mountains thrust (RMT) steps through several different lithologies (including the LP) leading to a broad zone of fault features including cataclasis, block-in-matrix structures, and discrete bands of high strain. Imbricate faults within the upper plate (structurally above the RMT) are cryptic, and often only identifiable by out-of-sequence stratigraphy.

Timing of deformation is constrained to the Antler orogeny by involvement of Miss. foredeep deposits and intrusion of relatively undeformed U. Miss. basaltic dikes. The interplay of stratigraphy and structural architecture of the RMA and associated rocks in the IM and subsequent tectonism and Cenozoic heat anomalies set the stage for the concentration of Eocene Carlin-type gold deposits.