Geology of the Chukar Footwall Mine, Maggie Creek District, Carlin Trend, Nevada: A progress report

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 DSrm. 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 a
feeder structure. These rocks were 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. Aconjugate 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.
Geochemical analyses show that As, Fe, and Sb have a high correlation with Au,
whereas Ag and Ba have a weak correlation with this element. Finally, pyrite and stibnite
are the most common sulfides. Arsenic minerals are scarce, occurring in the
deeper mine levels replacing decalcified rock. Calcite occurs as multistage veins,
stockworks, and in vugs.
The Chukar Footwall mine exhibits the hydrothermal alteration assemblages
typical of Carlin-type gold deposits: (a) decalcification, (b) dolomitization, (c) silicification,
and (d) 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 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.