Geology of skarn and high-grade gold in the Carr Fork Mine, Utah

Abstract

High-grade gold mineralization occurs as an overprint to copper-gold skarns in the Carr Fork mine. Geological and geochemical studies and geostatistical comparison of the distribution of gold in the copper-gold skarns and in the high-grade gold mineralization show that gold was deposited in two separate and distinguishable events: deposition with copper in skarn ores, under multiple controls, and high-grade gold mineralization, which formed a shoot in skarns hosted by the Parnell limestone. The gold content of material produced at this stage is much higher than that of the skarn ores.High-grade gold accompanies distinct alteration characterized by clay pyrite and quartz-pyrite(-tennantite), anomalous Au, As, Sb, Hg, Si, S, and Tl, and depletion of Pd, Ca, and Se. The high-grade gold mineralization comprises at least three episodes determined from petrographic and microprobe studies; it preceded the quartz molybdenum veins.The statistical characteristics of the high-grade gold are separable from the copper-gold mineralization in histograms, scattergrams, and variograms. The mean of the high-grade gold composites is 0.3 troy oz/short ton. Directional variograms of skarn ore show that a pervasive N 20 degrees E fault set and the distance to the Bingham stock contact controlled the copper and gold grades. Elevation seems to have been an additional control on gold grades in skarn.The shoot of high-grade gold mineralization in the Parnell limestone comprises a diluted drilled resource of 1.2 million tons. Evidence suggests that it continues some distance beyond present drilling.