REINTERPRETATION OF THE STRUCTURAL SETTING AND RELATIVE AGE OF EPITHERMAL SILVER-BARITE MINERALIZATION AT THE WATERMAN MINE NEAR BARSTOW, CALIFORNIA

The relation between epithermal silver-barite mineralization and detachment faulting in the central Mojave Desert, California, is poorly understood because barite is found in upper- and lower-plate positions and in strata interpreted as syn- and post-faulting. At the Waterman mine in the Waterman Hills, silver-bearing minerals and barite occur mainly in steeply dipping veins in fractures that cut steeply tilted mudstones of the Early Miocene Pickhandle Formation. Most mineralized fractures parallel bedding, giving the deposits a distinctly stratabound appearance, but many veins cross cut bedding at low to moderate angles and some cut strata above and below the mudstone. The most conspicuous mineralization occurs as barite in cockade structures and banded, comb-structured veins typical of epithermal conditions. More than 0.5 km of semi-continuous, along-strike workings reveal that 1) previously steeply tilted mudstone was flexed around a southwest-plunging axis, 2) during flexing, shear on numerous fault surfaces produced striae that plunge south on the southerly limb and north on the northerly limb of the flexure consistent with flexure during extension-normal contraction, and 3) during flexing, brittle strain was guided into mechanically susceptible mudstones forming anomalous fracture permeability. Mineralization accompanied faulting and flexing as indicated by a) barite veins along fractures that offset earlier barite veins, b) angular masses of dilatant breccia and mineralized rock at fault intersections, and c) iron-oxide-cemented breccia in the core zones of veins. We conclude that mineralization occurred late in the structural history of the area during regional transpression rather than during sedimentation in a supra-detachment basin as previously reported.