Hypogene Alteration and Ore Characteristics at the Cove Gold-Silver Deposit, Lander County, Nevada

ABSTRACT
Cove is a world-class gold-silver mine, with pre-mining
reserves totaling 3.6 million ounces Au and 164.3 million ounces
Ag. Ore is hosted by the middle to early-late Triassic Augusta
Mountain Formation, a post-Sonoma orogeny passive margin
sequence. Three sedimentary host units have been exposed by openpit
mining: 1) the early Ladinian Home Station Member, consisting
of massively bedded silty to sandy dolostone; 2) the late Ladinian
Panther Canyon Member, consisting of a lower primary dolostone
submember and an upper transitional submember that grades from
basal microcrystalline limestone, through middle silty limestone and
calcite-cemented sandstone, to upper conglomerate; and 3) the late
Ladinian to early Karnian Smelser Pass Member, consisting of
medium to thickly bedded limestone with lesser carbonate shale
interbeds.
Hypogene alteration assemblages in Eocene porphyritic intrusives
are grouped as early weak biotitization, middle propylitization,
later quartz-sericite (QS) alteration, and latest calcite veining.
Alteration in the sedimentary units consists of decarbonatization,
dolomitization, bleaching, sulfidation, silicification, sericitization,
and C-enrichment.
Four geologic cross sections were compared to compatible
block model cross sections developed from core and assay data
using Vulcanª software to determine spatial relations between host
rock characteristics and precious metals distributions. The deposit is
centered on the N44¡W-trending, 18.1¡ SE-plunging asymmetrical
Cove anticline, which is segmented by extensional faulting.
The strongest ore intervals are stratabound within the Panther
Canyon transitional submember, in a horst block between two Nand
NE-striking faults. A large sill in the upper clastic part of the
transitional unit focused the flow of the mineralizing fluids, producing
in its footwall the largest high-grade ore zone in the deposit. The
Smelser Pass limestone formed a physiochemical barrier that also
focused fluids and Au-Ag deposition in the underlying transitional
unit. Another relatively large stratabound ore zone occurs in the
strongly decarbonatized Home Station Member. Hypogene Au and
Ag ores typically coincide in space.
Hypogene ore consists of Carlin-style and base metal veintypes
(BMVT), both associated with QS alteration. BMVT minerals
comprise disseminated, crustiform, and vein sulfides, sulfosalts,
oxides, and native metals, and were deposited in a clear paragenetic
sequence that can be generalized as early quartz and galena, middle
sphalerite, late pyrite, latest Ag sulfosalts, and post-ore carbonates.
Carlin-style ore consists dominantly of disseminated Fe ± As sulfides.
The relations between the two ore types are not fully understood,
but preliminary evidence suggests that the BMVT system
overprints the Carlin-style system.