Geology of the Redline Gold Skarn Deposit, Converse Project, Humboldt county, Nevada

ABSTRACT
The Redline low-grade Au skarn deposits are located on the
Converse property, 18 miles (29 km) northwest of Battle Mountain,
NV, and nine miles (14.5 km) south of the Lone Tree mine. The project
is a joint venture between Newmont Gold Company and the Nike
Joint Venture (50% each). The Nike JV consists of UUS Inc. (a subsidiary
of Cameco Corp.) and Romarco Nevada (50% each).
Geophysical work and drilling by UUS resulted in discovery of significant
mineralization in 1995. Drilling to date consists of 102
holes, generally on 200- to 400-ft (61- to 122-m) centers. As at year
end 1998, in situ geologic resources at a cutoff grade of 0.015 oz
Au/ton (0.51 g Au/t), without engineering or economic implications,
are: Indicated 48 million tons at 0.024 oz Au/ton for 1.2 million
ounces Au (44 M tonnes at 0.83 g Au/t for 36 tonnes Au) plus
Inferred 35 million tons at 0.023 oz Au/ton for 0.8 million ounces
(31 M tonnes at 0.80 g Au/t for 25 tonnes Au).
Two main zones of Au mineralization, North and South
Redline, underlie two bedrock hills which are entirely covered by
alluvium. Depth to bedrock is as little as 28 ft (8.5 m) at North
Redline and 121 ft (37 m) at South Redline, and ranges to over 800
ft (240 m) in the paleovalley between the hills. The Au deposits
occur in a calc-silicate alteration system which surrounds an inferred
stock of quartz monzodiorite to granodiorite porphyry, the Redline
porphyry. The stock is 1,500 to 2,000 ft (450 to 600 m) in diameter
and occurs at the intersection of a northwest-trending aeromagnetic
high and a north-south aeromagnetic linear. A dike-like arm of the
stock extends north-northeast from the northeast corner of the pluton.
The calc-silicate alteration halo ranges from 800 to 1,400 ft (240
to 430 m) in width with a vertical extent of over 950 ft (290 m). Host
rocks comprise a receptive unit of calcareous sandstone and siltstone
within the Mississippian, Pennsylvanian, and Permian Havallah
sequence. Alteration assemblages include pyroxene hornfels, pyroxene>
garnet skarn, biotite hornfels, and quartz hornfels, with a retrograde
overprint of amphibole, epidote, chlorite, calcite, quartz and
sulfide minerals. Biotite and quartz hornfels are early in the alteration
sequence and are overprinted by the much more abundant
pyroxene hornfels. Light green to grass green diopsidic pyroxene is
the dominant hornfels mineral. Pyroxene-garnet skarn replaces
hornfels but is greatly subordinate to hornfels in volume; skarn
pyroxene is dark green and hedenbergitic. The garnet:pyroxene ratio
in skarn is <1:10. Local variations in the garnet:pyroxene ratio are present in relation to the stock contact and certain dikes, but a broad consistent zonation in the ratio has not been recognized. Sulfide minerals include pyrrhotite, chalcopyrite, pyrite, molybdenite, and others. Sulfide content is low, generally <0.5%. A Re-Os age date of 41.0±0.2 Ma obtained from a sample of molybdenite from South Redline is believed to date the age of Au mineralization. Gold commonly occurs as free particles. Gold is presumed to be part of the retrograde assemblage and seems to be most strongly associated with chalcopyrite and chlorite. The southwest margin of the stock is affected by silica-secondary biotite alteration and contains low-grade Au values. A late sericite-chlorite-calcite alteration-brecciation event occurs on the west side of South Redline. Gold mineralization is strongest in the middle to outer portions of the calc-silicate alteration halo. The mineralized bodies generally dip west at 15¡ to 35¡, subparallel with bedding in the sandstone. The distribution of mineralized rocks in map view suggests partial control of Au deposition by a north-south pre-mineral structure on the east side of the Redline porphyry. The Redline system shows a somewhat telescoped horizontal geochemical zonation pattern. Gold is associated with elevated Cu. Concentrations of Ag, Zn, Pb, Sb, and As occur progressively more distal from the stock, with considerable overlap between elements. One drill hole intersects a sulfideenriched zone outside the calc-silicate halo which is enriched in Au, Pb, Zn, and As. Avertical geochemical zonation pattern is evident in many drill holes.