A petrographic and fluid inclusion study of the Purple Vein and Post/Betze orebodies, Carlin, Nevada
Abstract
The Purple Vein (Meikle Mine) and Post/Betze orebodies, two Carlin-type, gold deposits, display important differences in size, alteration, silicification, pyritization, and gold grade. A core hole was examined from each deposit to gather petrographic and microthermometric data to determine the mineral paragenesis, to determine fluid pressure, temperature, and composition to compare of the two orebodies, and to determine depths of formation; The parageneses determined were divided into three stages. (1) The Early Mineralization Stage contains quartz, pyrobitumen, barite, sphalerite, sulfosalts, quartz, and pyrite in the Purple Vein deposit and consists of calcite in the Post/Betze deposit. High salinity basinal brines, {dollar}>{dollar}10 wt.% NaCl equivalent, were present. (2) The Gold Ore Stages consist of quartz, pyrite, and marcasite that precipitated from fluids with salinities of {dollar}\leq{dollar}10 wt.% NaCl equivalent and minimum trapping temperature modes of 220{dollar}\sp\circ{dollar} to 225{dollar}\sp\circ{dollar}C in the Purple Vein deposit and 150{dollar}\sp\circ{dollar} to 180{dollar}\sp\circ{dollar}C and 200{dollar}\sp\circ{dollar} to 210{dollar}\sp\circ{dollar}C in the Post/Betze deposit. Fluids were CO{dollar}\sb2{dollar}-bearing and immiscible. Gold is present in rims of As-rich, fine-grained pyrite on earlier pyrite. Other minerals in the Gold Ore Stage in the Post/Betze deposit are arsenopyrite, stibnite, micas, and clays. Stibnite, calcite, and barite appear late in both Gold Ore Stages. (3) The Post-Ore Stages are sparsely represented by oxides; Minimum pressures of 750 bars and 800 bars for ore deposition of the Post/Betze and Purple Vein deposits suggest that the orebodies were formed at minimum depths of 2.8 kilometers and 3.0 kilometers, respectively, assuming lithostatic pressure conditions.