Origin of the McLaughlin Mine sheeted vein complex; metal zoning, fluid inclusion, and isotopic evidence

Abstract

The McLaughlin deposit is a large hot spring-type gold deposit located in the northern Coast Ranges of California at the faulted lithologic contact between the Coast Range ophiolite and the Great Valley sequence. The McLaughlin deposit is centered around the sheeted vein complex, a large multistage vein swarm, localized in a dilation zone formed by rheologic contrasts in the footwall polymictic melange. The surface expression of the sheeted vein complex is a subaerial sinter terrace, which merges into the sheeted vein complex. Metal zoning in the sheeted vein complex is highly telescoped. The sinter is enriched in mercury, whereas gold and silver are restricted to the upper 350 m. with the proportion of gold to silver decreasing with depth. Gold is typically present as electrum and is associated with silver and base metal-bearing sulfosalts. Below 350 m mineralization is dominated by small quartz veins with base metal sulfides. Fluid inclusion studies indicate that NaCl is the main dissolved salt, in concentrations averaging 2.4 wt percent NaCl equiv. The bulk of the salinity data can be explained by variable amounts of adiabatic boiling and mixing with steam-heated ground water. Fluid inclusion homogenization data ranges from 121 degrees to 263 degrees C with the deepest portion of the system examined having the highest temperatures and the shallowest samples having the lowest temperatures. Fluid inclusion gas concentrations vary with depth below the sinter; CO 2 , and to a lesser extent CH 4 , shows a systematic increase with depth. Fluid pressures, calculated from fluid inclusion homogenization temperatures and gas analysis, are in excess of the hydrostatic pressure and approach, or exceed, lithostatic pressure. Self-sealing of the system and tectonic dilation most likely resulted in catastrophic phase separation and hydrothermal eruption breccias. Geothermometry results, based on fluid inclusion gas ratios, show a reasonable correspondence with the highest fluid inclusion homogenization temperatures. There is a strong trend for decreasing delta 18 O quartz values with depth due to the effect of temperature. The calculated delta 18 O (sub H 2 values are fairly constant at about 9 to 10 per mil. Oxygen and hydrogen isotope values, as well as geologic relationships, are consistent with those of hydrothermal fluids originating as meteoric water; these evolved to increasingly high ones by interaction with Great Valley sequence sedimentary rocks. Lead isotope compositions, of sulfides and potential source rocks, indicate that lead, and by inference other metals, was derived from a crustal source. The lead source may have been detritus from the Sierra Nevada batholith that comprises the Great Valley sequence, or alternatively, it may have been derived from a thorough mixture of radiogenic lead and nonradiogenic lead that occurred prior to mineralization.