Geology of the Chukar Footwall mine, Maggie Creek District, Carlin trend, Nevada

The Chukar Footwall mine forms part of the NW striking Carlin trend in northern Nevada, and lies beneath the southwest highwall of the Gold Quarry, a world-class deposit with total 1999 reserves, resources, and mineral inventories in excess of 24M oz gold. The Chukar Footwall orebodies are hosted in the planar to wispy silty limestone and calc-silicates of the Silurian-Devonian Roberts Mountains Formation (SDrm). The Devonian Popovich Formation (Dp), a micritic package locally hosting economic gold mineralization, structurally overlies the SDrm. The Raven dike intrudes the DSrm at several mine levels along northwest-trending structures. The dike is composed of abundant dark green, millimeter sized subhedral-anhedral phenocrysts in a light colored aphanitic groundmass. It is altered to clay and pyrite, but does not appear to be an ore fluid feeder structure. The time of the Raven dike emplacement into NNW-trending structures has been determined by U-Pb zircon geochronology, producing an age of 200.3+/-5.1 Ma (early Jurassic). Apatite separates from the Raven dike yielded a fission track pooled age of 17.7+/-3.7 Ma for a thermal event that may have been associated with mineralization at Chukar Footwall; however, initial cooling began at 26.2+/-5.5 Ma.; These rocks were probably deformed during the Antler orogeny, generating the Chukar anticline, a northeast trending open structure with a subhorizontal plunge. High gold grades are commonly situated along the hinge and the southeast limb and in small parasitic folds. A conjugate system of structures is represented by northwest and northeast striking faults, where the former cuts the latter. Kinematic indicators denote predominantly oblique normal slips for both fault sets.; The Chukar Footwall mine exhibits the hydrothermal alteration assemblages typical of Carlin-type gold deposits: (a) decalcification, (b) dolomitization, (c) silicification, (d) argillization, and (e) baritization. The uniqueness of this deposit relative to the more typical Carlin-type is the (1) sharp boundaries between fresh and altered rocks, and (2) presence of abundant visible gold. Gold mineralization is spatially related with strong decalcification in the vicinity of intersections of northeasterly structures with the Chukar anticline. Coarse, visible gold occurs in decarbonatized silty limestone along fractures of all orientations as well as along bedding planes. Also, visible gold is present in late barite veinlets, coprecipitating with the latter phase.; Stable isotope transects reveal systematic delta18O and delta13C shifts in wall rocks where approaching structures and/or changes of intensity of hydrothermal alteration. At a mine scale, these shifts define a generalized trend toward lighter oxygen and heavier carbon values for the wallrocks from the higher to the deeper mine levels, with a negative correlation between oxygen and carbon. Late stage calcite veins delta 13C values lie within the values of the nearby altered limestone, suggesting the carbon could have been derived from the wallrocks. Isotope and fluid inclusion data point to gangue precipitation from interaction between meteoric water and the wallrocks. delta34S from sulfides and sulfates are consistent with a sulfur derivation from sedimentary sources. A correlation has been found between the presence of visible gold and the isotopic signature of barite. Barite precipitated from meteoric water as it descended into open structures.; Microthermometric data from late stage barite+/-gold veins indicate significant variation in both homogenization temperatures and salinities with depth. Mean salinities and mean homogenization temperatures from the deeper mine levels range from 3.03 to 3.18 wt % NaCl equiv and between 183.2° and 179.7° C. In contrast, lower salinities (∼1.16 wt % NaCl equiv) and homogenization temperatures (177.3°C) were recorded in samples from shallower levels. Similarly, recorded data from late calcite yield...