High-Salinity Overpressured Fluids Associated with MVT Mineralization, Shady Dolomite (Cambrian), Virginia

R. J. Barnaby, R. J. Bodnar

In Virginia, MVT Pb-Zn deposits are present in platform margin carbonates of the Lower Cambrian Shady Dolomite. Ore deposition occurred during late Paleozoic thrust faulting and was preceded by dissolution and brecciation of host carbonates, creating a porous and permeable network for later mineralizing fluids. Ore mineralization was followed by precipitation of saddle dolomite, authigenic quartz, and calcite.

Large pseudosecondary fluid inclusions (up to 30-40 µm) in authigenic quartz contain a liquid and vapor phase and halite daughter minerals. The L-V (L) occurred between 110° and 140°C; total homogenization (by halite disappearance) occurred at 176°-225°C. Salinities, based on Tm (halite), are estimated to be 31-33 wt. % NaCl equivalent, assuming a pure NaCl-H₂O system. However, Te (-26° to -25°C) indicates the presence of divalent cations (Ca²⁺ and/or Mg²⁺).

Minimum trapping pressures, estimated from the intersections of the inclusion isochores with the liquids curves corresponding to inclusion salinities, range from 430 to 1,370 bars. The range of pressures and temperatures thus obtained exceeds that defined by hydrostatic load (100 bars/km), using the likely range of geothermal gradients (20°-35°C/km). The fluids forming the inclusions, therefore, were overpressured, which is consistent with other evidence indicating the former presence of overpressured fluids.

A minimum depth of formation calculated from the lowest temperature of total homogenization (176°C) using a geothermal gradient of 35°C/km is about 4-5 km, consistent with the estimated 5-km thick overlying Paleozoic section. Maximum burial depth estimated by projecting the isochores delimiting the P-T conditions of fluid inclusion formation up to a maximum temperature of 250°C is about 6-7 km. This additional pressure could be generated by tectonic thickening during thrusting.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.