Redline Gold Skarn Deposits, Humboldt County, Nevada, USAi

Regional Level Types
Redline Gold Skarn Deposits	Deposit
Humboldt County	County
Nevada	State
USA	Country

Structure: The deposit area is underlain by the regionally extensive Mississippian, Pennsylvanian, and Permian Havallah sequence, which comprises the upper plate of the Golconda thrust, a regional structure dating from the late Permian-early Triassic Sonoma orogeny.The Golconda thrust is exposed intermittently along the east side of the Havallah Hills and west flank of Battle Mountain 4.5 miles east of Converse/Redline. Several thrusts separate various time-stratigraphic rock units within the Havallah sequence. The deposit has also been affected by a set of high angle northwest-striking faults of Mesozoic age; a prominent set of north-striking early Tertiary faults; and east-striking and northeast-striking faults, some with movement as recent as late Miocene and younger.

Alteration: 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

Tectonics: Tertiary basin

Commodity: Ore Materials: gold Gangue Materials: pyrrhotite, chalcopyrite, pyrite, molybdenite Sulfide contentof the ore is low, generally <0.5%.

Deposit: Skarn deposit. 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 at North Redline and 121 ft at South Redline up to more than 800 ft in the paleovalley between the hills. The Au deposits occur in a calc-silicate alteration system that surrounds the Redline porphyry, a stock of quartz monzodiorite to granodiorite composition, 1,500 to 2,000 ft in diameter. Gold commonly occurs as free particles, most strongly associated with chalcopyrite and chlorite. Gold is presumed to be part of the retrograde metamorphic assemblage. 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 degreess, subparallel with bedding in the sandstone.

Deposit type: Distal disseminated Ag-Au

Development: In the late 1980s and 1990s the current Redline property was explored by three former lessees, Chevron, Cyprus, and Independence, on three sections of public land in the Converse project, which then comprised a portion of the larger Nike property. From 1989 to 1990, Chevron completed a reconnaissance gravity, an IP survey, and drilled three RC holes, all of which bottomed in alluvium. In 1991 and 1992 Cyprus obtained ground magnetometer data and drilled six RC holes, four of which intersected low level anomalous Au in shallow bedrock below alluvial cover near a small outcrop at the north edge of the property. From 1993 to 1994, Independence Mining Co. drilled nine rotary holes mainly to test faults interpreted from the Chevron reconnaissance gravity map. Two of these holes intersected gold-mineralized, contact-metamorphosed, sedimentary rocks beneath 340 and 755 ft of alluvial cover. One of these holes is located off the northeast margin of the South Redline deposit as it is now known, and the other is located in the western part of the North Redline deposit. Uranerz U.S.A. Inc. (now UUS Inc.) acquired the Nike property in late 1994 and completed a joint venture with Romarco Nevada, Inc. in 1995. On what is now the Converse portion of the property, they conducted an aeromagnetic survey and a detailed gravity survey to map the high-relief bedrock surface, followed by a seven-hole mud rotary drill program with selected holes placed on gravity high features (bedrock ridges) near the Independence holes that had encountered gold mineralization. Hole NKM-14 cut a Au intercept of 60 ft grading 0.038 oz Au/ton (1.29 g Au/t) in hornfelsed sandstone bedrock . This hole is considered to be the discovery hole for the South Redline Au deposit. Three other holes over the North Redline deposit also intersected hornfels and skarn with anomalous to lowgrade gold concentrations. These three holes were located directly over the downdip portion of some of the strongest North Redline mineralized skarn, but the mud rotary system was unable to effectively penetrate the hornfels, and the holes terminated above well-mineralized rocks. Since subsequent drilling was concentrated at South Redline, the discovery of significant mineralization at North Redline did not take place until almost two years later. A second round 11-hole RC drilling program at South Redline in early 1996 confirmed the presence of a large alteration system with thick intervals of low-grade Au-mineralized hornfels and skarn. Because the skarn system is located along the boundary with Santa Fe-controlled private land, the Converse joint venture was formed with Santa Fe in mid-1996 followed by a third round 11-hole RC drill program late in the year. In 1997 a 25-hole RC and core program was completed, and in 1998 a 43-hole RC program was completed. Five of the 1997 holes were at North Redline, and significant intercepts of mineralized rock were obtained from these holes, including a 430 ft interval grading 0.023 oz Au/ton, and 385 ft at 0.033 oz Au/ton beginning at a depth of only 45 ft (14 m). Because of these results in 1997, much of the 1998 drill program was focused at North Redline. Five RC holes were drilled in 1999 to the north of North Redline. Total drilling by UUS from 1995 through 1999 on the Converse property stands at 80,981 ft (24,683 m) in 102 holes, including 5,017 ft (1,529 m) of core. Most recent holes in the deposit area have been completed to depths of 800 to 1,000 ft (240 to 305 m), with the deepest being core hole NKC-43 at 1,518 ft (463 m). Uranerz did the drilling on behalf of joint-venture partners Romarco Minerals and Santa Fe Pacific Gold, a unit of Newmont Gold . The drilling has outlined 39 million metric tonnes grading 0.7 gram gold per tonne (43 million short tons grading 0.02 oz. per ton) in the South Redline mineralized area. An additional 6.4 million metric tonnes (7.1 million tons) at the same grade was delineated in the North Redline deposit, about a half mile to the north. Newmont Gold (50 percent) intended to take over the operatorship of the project once earn-in by the Nike Joint Venture (Romarco / Uranerz) was completed by the end of 1998. Uranerz was in the process of being acquired by Cameco in 1998.

Geology: 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 sulfide zone outside the calc-silicate halo which is enriched in Au, Pb, Zn, and As. A vertical geochemical zonation pattern is evident in many drill holes.

Ore(s): The distribution of mineralized rocks suggests partial control of Au deposition by a north-south pre-mineral structure on the east side of the Redline porphyry stock.

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Link to USGS MRDS:	10310472

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