U.S. Geological Survey
Open-File Report 98-620
Abstract Field and
laboratory investigations of the giant Betze gold orebody, the largest
Carlin-type deposit known, in the north-central Carlin trend, Nevada
document that the orebody is composed of individual high-grade oreshoots
that contain different geologic, mineralogic, and textural characteristics.
The orebody is typical of many structurally controlled Carlin-type deposits,
and is hosted in thin-bedded, impure carbonate or limy siltstone, breccia
bodies, and intrusive or calc-silicate rock. Most ores in the Betze
orebody are highly sheared or brecciated and show evidence of syndeformational
hydrothermal deposition. The interplay between rock types and pre- and
syn-structural events accounts for most of the distribution and zoning
of the oreshoots. Hydrothermal alteration is scale dependent, either
in broad, pervasive alteration patterns, or in areas related to various
oreshoots. Alteration includes decarbonatization (~decalcification)
of carbonate units, argillization (illite-clay), and silicification.
Patterns of alteration zoning in and surrounding the Betze orebody define
a large porous, dilated volume of rock where high fluid flow predominated.
Local restriction of alteration to narrow illite- and clay-rich selvages
around unaltered marble or calc-silicate rock phacoids implies that
fluid flow favored permeable structures and deformed zones. Gold mainly
is present as disseminated sub-micron-sized particles, commonly associated
with Asñrich pyrite, although one type of oreshoot contains micron-size
free gold. |
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