Regional- and District-Scale Dissolution, Deformation and Fluid Flow in Sedimentary Rock-Hosted Au Deposits of Northern Nevada

ABSTRACT
Most sedimentary rock-hosted Au deposits in northern Nevada
have similar mineralogical and geochemical characteristics. This
suggests that these deposits may have formed from a common
regional process. Most deposits share an effect from the abundance
of hydrothermal fluid that has passed near or through the host rocks.
Several geologic features may have served as regional-scale conduits
during fluid flow such as: (1) permeable lower-plate rocks to the
Roberts Mountains thrust, (2) the Roberts Mountains thrust zone
itself and parallel thrusts, (3) district-scale shear zones and faults, and
(4) the regional-scale northeast-trending Crescent Valley-
Independence lineament and associated faults and the Getchell trend.
Although satisfactory ages of sedimentary rock-hosted Au
deposits in northern Nevada have not been resolved or are enigmatic,
deformation along regional- and district-scale crosscutting
deformation zones, may have been synchronous with Au transport
and deposition. Many regional-scale lineaments and district-scale
faults and shear zones appear to have been active before, during, and
after Au mineralization. In fact, syn-deformational ore deposition
has been proposed for some sedimentary rock-hosted Au deposits.
Tectonic activity along these structural features produced fault rocks
and deformation features with distinct petrologic features. These
tectonized zones provided conduits that served as major zones of
fluid flow.
The main features in these zones are collapse and dissolution
breccias in Paleozoic platform edge rocks, as well as fault rocksÑ
including tectonic breccia and phyllonite, hydrothermal breccia,
polygenetic breccia, and mŽlange. In addition, hydrothermal alteration
has significantly affected the physical and chemical properties
of regional- and district-scale conduits. Alteration types associated
with Au mineralized rocks include: carbonation (carbon introduction),
decarbonatization, argillization, and silicification. These alteration
processes played a direct role in fluidÐconduit interaction during
tectonism and fluid flow. Although local PÐTÐX conditions are
important, preparation for Au deposition mainly resulted from
hydrothermal fluid reactions with the host lithologies, starting with
decarbonatization, which resulted in volume reduction and strain
that also resulted in increases in porosity and permeability in the
host conduits. Argillization accompanied deposition of Au and
directly contributed to ductile deformation, mesoscopic shear folding,
and phyllonite development. Late syn-gold silicification
decreased permeability in the conduits, but allowed enhanced brittle
deformation and brecciation before the conduits sealed. The main
Au deposition stage involved FeÐ and AsÐsulfide deposition, a transition
from illite to kaolinite deposition, and pulses of quartz deposition.
These events were accompanied by phyllonite development,
fault gouge, dissolution-collapse, and tectonic brecciation.