CONTRASTING STYLES OF SILICIFICATION IN TWO COALESCING, DUCTILE-ROOTED DETACHMENT FAULTS, WHITE PINE RANGE, EAST-CENTRAL NEVADA; IMPLICATIONS FOR ORE GENESIS

Two ductile-rooted detachments coalesce in the southern White Pine Range (Francis et al., 2021). Both fault zones are silicified, but micro and macro textures, and temperatures of formation, are different. One detachment contains a small jasperoid-hosted gold mine, long abandoned but reportedly with significant economic potential. The other detachment appears to be barren.

The White Pine detachment (WPD) has been mapped over most of the White Pine and Grant Ranges, and in adjacent Railroad Valley (>3,000 km²; Francis and Walker, 2001). The WPD is rooted in the Mississippian Chainman Shale, which cuts section in the lower plate as far down as Cambrian; it is associated with breccia and widespread isolated ledges of jasperoid. The jasperoid is mostly microcrystalline or “jigsaw” quartz, with accessory chalcedony. Carbonate features such as fossils and stylolites are preserved. The jasperoid is brecciated locally, with interclast late milky or drusy quartz. The fabric is massive; veining is unimportant. The Gold Point Mine shows visible flakes of gold in jasperoid.

The Currant Gap detachment (CGD) is rooted in the argillaceous Cambrian Lincoln Peak formation, which is exposed only in a small 19 km² area in the southern White Pine Range. The fault is exposed on a hillside as a wall of almost pure quartz 40 m thick and 1.7 km long, called by us the “spine.” This is in contrast to the jasperoid ledges in the WPD, which are separated by other lithologies in the fault zone. Moreover, veining appears to be the dominant mode of emplacement in the CGD. Jigsaw quartz texture in the CGD is subordinate to reticulated and granular texture; fossils are not preserved. We have not observed chalcedony in the CGD.

A cross section from the Gold Point Mine to the spine shows that the two detachments coalesce here. Elsewhere up to 4,000 feet of Cambrian to Devonian strata separate the WPD from the presumed CGD at depth. The disparate textures, field relations, mineral occurrences, and differing oxygen isotopic signatures, suggest that silica in the WPD was emplaced along gel fronts, while quartz in the CGD crystallized from fluid in carbonate primary pore spaces and in fractures to form veins (Holk et al., in press). We consider that the disparate modes of emplacement, and presence of mineralization only in the WPD, is due to disparate sources of hydrothermal fluids, and different migration paths along the detachments.