Implications of Stream-Sediment Geochemistry in the Northern Carlin Trend, Nevada

ABSTRACT
An expansive west-to-east increase of metal concentrations has
been found in stream sediments in the Santa Renia Fields and
Beaver Peak 71/2-minute quadrangles, which straddle the northern
Carlin-trend Au deposits. The width of the Carlin trend is well outlined
by As concentrations, maximum 54 ppm near the deposits, as
well as lesser elevated concentrations of Sb and Zn and even less
elevated concentrations of Ba, Ca, Cd, Cu, and Pb. However, broad
increases and corresponding changes in background levels of many
metals and magnetite abundances east of the Carlin trend coincide
with a significant change in physiography that reflects much higher
elevations and steep slopes to the east. Robust erosion and high
stream flow emanating from high ridges in the east probably contributed
significantly to increased metal concentrations in background
levels of stream sediments due to an enhanced presence of
minerals with high specific gravities and a correspondingly diminished
presence of those with low specific gravities. The latter probably
have been preferentially flushed down stream because of high
transporting capacities for sediment by streams in the east. Further,
background concentrations of metals in rocks apparently are not
higher in the eastern half of the area.
In addition, a 10 km2 area of high polymetal concentrations
well above local backgroundÑincluding sharply elevated abundances
of Ag, As, Au, Cd, Co, Cu, Mn, Ni, P, Sb, Sc, Te, V, and especially
ZnÑis present south-southwest of Beaver Peak, approximately
10 km east of the Carlin trend. This area is underlain by
siliceous rocks in the upper plate of the Roberts Mountains thrust,
and includes the small Coyote bedded barite deposit hosted by
Devonian Slaven Chert. High metal contents in stream sedimentsÑ
one sample contains as much as 5,000 ppm Zn at the mouth of a
catchment basin below the Coyote depositÑare inferred to result
from a number of geochemically anomalous rocks, which include, in
order of probable declining importance: (1) <1Ðm-wide gossanous sequences of lower Paleozoic stratiform-mineralized rock present in Slaven Chert at the Coyote deposit; (2) 20Ð to 30Ðm-wide intensely brecciated middle or late Paleozoic thrust zones in Slaven Chert; (3) widespread mmÐscale accumulations of stratiform sulfide concentrations along parting surfaces in rhythmically bedded chert of the Slaven Chert; and (4) contamination from dumps at the Coyote deposit. Stratabound gossan in Slaven Chert at the Coyote deposit contains as much as 6,500 ppm Zn, 75 ppm Cd, and 200 ppm Cu. Furthermore, Ordovician Vinini Formation and Silurian Elder SandstoneÑthe latter, in places, including a basal Cherry Spring chert unitÑsouth-southwest of Beaver Peak contain highly anomalous concentrations of several base metals and appear to have provided separate sources for the anomaly.