Geologic setting, paragenesis, and physicochemistry of gold quartz veins hosted by plutonic rocks in the Pataz region

Abstract

The Pataz region is comprised of different low-grade metamorphic Proterozoic to early Paleozoic basement series overlain by folded late Paleozoic to early Tertiary sediments and almost undeformed volcaniclastic rocks of late Tertiary age. The basement is intruded by the Pataz batholith along a north-northwest-trending fracture zone. This is the most important intrusive body of the region and consists of a geochemically varied calc-alkaline magma suite.The Pataz batholith hosts numerous auriferous quartz veins with a simple two-stage sulfide paragenesis. Microthermometric measurements indicate that stage I is characterized by medium chlorine, K-rich fluids probably related to magmatic dewatering. Stage II displays medium to high chlorine solutions with significant amounts of Na, K, Ca, and/or Mg. This fluid composition is consistent with the strong interaction with the host rock, as evidenced by the observed alteration features. The initial chloride contents of stage II are similar to those of stage I. Fluid inclusions further show a wide range of homogenization temperatures that probably reflect tectonically induced changes of pressure. Native gold with bimodal Au/Ag ratios was found to be related chronologically to the second stage and spatially to pyrite that formed in the first stage as did most of the quartz and some arsenopyrite. Fractured pyrite was an important control on gold deposition, frequently accompanied by galena, sphalerite, and chalcopyrite.There is much evidence that vein formation occurs at considerable depth and is geometrically as well as temporally and genetically linked with the cooling stage of the Pataz batholith. Local physicochemical factors account for quantitative mineralogic differences and for the deposition of either electrum or fine gold with less than 15 wt percent Ag. The early K-bearing chloride solutions appear to be essentially magmatogenic. Fluids of the gold stage represent either new magmatogenic pulses or solutions recycled in a convective circulation system that included the nearby basement series. Gold leaching from Precambrian to Paleozoic country rocks appears appropriate in order to explain the regional distribution of the metal.