61 Recent Advances in Economic Geology

Sunday, 19 October 2014
1:00 PM - 5:00 PM
207

ORE AND BRECCIA CLASSIFICATION OF THE 144 ZONE GOLD DEPOSIT, BARE MOUNTAIN RANGE, NEVADA

Paper number: 61-1
Program: Technical Sessions
Time: 1:00 PM – 1:15 PM

Authors

Abstract

The 144 Zone is an oxidized, breccia-hosted, disseminated gold deposit that formed along the contact of the Early Cambrian Carrara Formation limestone and the overlying Bonanza King Formation dolomite within the Bare Mountain range in southern Nevada. Gold mineralization occurs within rock that exhibits breccia textures. Research goals include classifying clay, oxide and other minerals as well as breccia types to identify the habits of gold mineralization and the minerals associated with gold. Research is also aimed at determining the paragenesis of Au mineralization and brecciation in the 144 Zone. Underground mapping provided spatial relationships between breccia types, host rocks and alteration assemblages. Clay minerals and a post-ore oxidation event have made it difficult to identify primary mineral assemblages; however, samples of different breccia types collected along transects from low/no Au to high grade were analyzed using transmitted and reflected light petrography, applied reflectance spectroscopy, secondary electron microscopy, and electron probe microanalysis to characterize mineral assemblages, identify clay and oxide minerals, and locate gold.

Two breccia types have been identified. Breccia type 1 (BT1) has clasts of dolomite, phengitic dolomite, and quartz with goethite cemented by a quartz-rich matrix; breccia type 2 (BT2) has clasts of dolomite, phengitic dolomite, and quartz with goethite, cemented by a phengite- and goethite-dominant matrix with minor quartz. Neither breccia type has a preferred association gold.

Clast and matrix compositions and textures show that both breccia types formed by selective replacement of lower-most Bonanza King dolomite. Fluid-rock reactions partially altered dolomite producing “apparent” clasts. Partial replacement of dolomite by quartz and precipitation of pyrite produced BT1. Near complete decarbonatization of dolomite, concentration of phengite and precipitation of pyrite and Au produced BT2.

Gold is most closely associated with cubic goethite that replaced pyrite, but is also closely associated with amorphous goethite. Gold is located in pitted zones of goethite and along the contact of goethite with quartz. Quartz is typically associated with and encloses Au-bearing goethite.