Structural History of the Bald Mountain Mining District, Nevada, USA

The Bald Mountain Mining district, like much of central Nevada, has experienced a complex deformation history. The mining district is considered here as a southern extension of the well-studied Carlin trend of gold deposits to the north. In order to gain a better understanding of how the Bald Mountain Mining District structural style relates to other structural models developed to the north in the Carlin trend, I completed a mapping and structural domain analysis in the district, with a focus on the North Mooney basin area where a high spatial concentration of Carlin-type gold deposits occur. The identification of structural domain boundaries and classes utilized a compiled database of historical mapping and structural measurements along with new mapping and cross section work. The domain analysis revealed four different classes of structural domains, of which monoclinal folds were determined to play the strongest role in gold mineralization. Using the structural domain designations in addition to an analysis of the geometry of commonly occurring structures, I identified five structural styles at Bald Mountain: 1) kilometerscale NNE- to NE-striking reverse faults and NNE- to NE-trending open to overturned folds; 2) kilometer-scale WNW- to NW-striking normal faults with oblique separation; 3) decameter- to kilometer-scale NNW-trending open folds, NNW-striking reverse faults, and N-striking thrusts; 4) meter- to decameter-scale NW-trending tight to close folds; 5) meter- to kilometer-scale N- to NE-striking normal faults. Evidence for structural reactivation and observations of the crosscutting relationships between the five structural styles inform a relative sequence of deformational events at Bald Mountain. These five structural styles were then considered with respect to regional tectonic events and their kinematic and timing relationships in order to assign deformational phases: 1) NNE- to NE-striking reverse faults and NNE- to NE-trending open to overturned folds associated with episodic ESE- to SE-directed shortening through the Paleozoic to early Mesozoic; 2) WNW- to NW-striking strike-slip and tear faulting in the Late Jurassic to Cretaceous; 3) NNW-trending open folds, NNW-striking reverse faults, and N-striking thrusts with ENE-directed shortening through the Late Cretaceous to Eocene; 4) NW-trending tight to close folds with sinistral tranpression on WNW- to NW-striking faults during a mid-Eocene event associated with exhumation of the Ruby Mountain core complex; 5) N- to NE-striking normal faulting with NWSE directed Basin and Range extension from the Miocene. The five structural styles were then compared to previous work including structural models developed in north-central Nevada. Deformation events are similar between the northern Carlin trend and Bald Mountain. Primary differences between the Bald Mountain structural history and the structural models in the northern Carlin trend include the documented Eocene folding event, and a major WNW-striking basement-seated feature with parallel structures that have a significant strike-slip component and partition Bald Mountain into discrete kilometer scale structural domains. Further work to add value to targeting and exploration should place an emphasis on identifying major WNW- to NW-striking faults and understanding their kinematics. Knowledge of these structural boundaries along with how structural styles change across them will help to constrain targeting and lend insight to the significant role that strike-slip faulting played in the Bald Mountain Mining District's complex deformational history.