ORIGIN OF THE MCLAUGHLIN MINE SHEETED VEIN COMPLEX - METAL ZONING, FLUID INCLUSION, AND ISOTOPIC EVIDENCE

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 sequenc e. The McLaughlin deposit is centered around the sheeted vein complex, a large multistage vein swarm, localized in a dilation zone formed by theologic contrasts in the footwall polymictic melange. The surface e xpression of tile sheeted vein complex is a subaerial sinter terrace. Metal zoning in the sheeted vein complex is highly telescoped. The sin ter 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 s ilver and base metal-bearing sulfosalts. Below 350 m mineralization is dominated by small quartz veins with base metal sulfides. Fluid inclu sion studies indicate that NaCl is the main dissolved salt, in concent rations averaging 2.4 wt percent NaCl equiv. The bulk of the salinity data can be explained by variable amounts of adiabatic boiling and mix ing with steam-heated ground water. Fluid inclusion homogenization dat a ranges from 121 degrees to 263 degrees C with the deepest portion of the system examined having the highest temperatures and the shallowes t samples having the lowest temperatures. Fluid inclusion gas concentr ations vary with depth below the sinter; CO2, and to a lesser extent C H4, shows a systematic increase with depth. Fluid pressures, calculate d from fluid inclusion homogenization temperatures and gas analysis, a re in excess of the hydrostatic pressure and approach, or exceed, lith ostatic pressure. Self-sealing of the system and tectonic dilation mos t likely resulted in catastrophic phase separation and hydrothermal er uption breccias. Geothermometry results, based on fluid inclusion gas ratios, show a reasonable correspondence with the highest fluid inclus ion homogenization temperatures. There is a strong trend for decreasin g delta(18)O(quartz) values with depth due to the effect of temperatur e. The calculated delta(18)O(H2O) values are fairly constant at about 9 to 10 per mil. Oxygen and hydrogen isotope values, as well as geolog ic relationships, are consistent with those of hydrothermal fluids ori ginating as meteoric water; these evolved to increasingly high ones by interaction with Great Valley sequence sedimentary rocks. Lead isotop e 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 bathol ith that comprises the Great Valley sequence, or alternatively, it may have been derived from a thorough mixture of radiogenic lead and nonr adiogenic lead that occurred prior to mineralization.