Eldorado Gold Property (WBG deposit; Wall Street; Black Hawk; Gracey), Eldorado Mining District (Colorado Mining District), Eldorado Mountains, Clark Co., Nevada, USAi

Regional Level Types
Eldorado Gold Property (WBG deposit; Wall Street; Black Hawk; Gracey)	- not defined -
Eldorado Mining District (Colorado Mining District)	Mining District
Eldorado Mountains	Mountain Range
Clark Co.	County
Nevada	State
USA	Country

Structure: The northern Eldorado Mountains are characterized by many west-dipping listric normal faults which sole into a west-dipping detachment, all formed about between 15 and 10 Ma due to regional extension. The structural style changes markedly in the southern Eldorado Mountains, where Tertiary volcanic strata dip west and are cut by east-dipping listric faults. The Eldorado Canyon and mining district are located between the two structurally distinct areas and represent an ?accommodation zone?. The deposit is bounded on all sides by post-mineral, high-angle faults.

Alteration: Propylitic alteration is widespread throughout the host rocks. Anomalous gold also occurs in zones of local silicification, argillic alteration, and quartz-alunite veining.

Tectonics: The deposit is situated within a probable structural ?accommodation zone? marking the boundary between different styles of faulting in the northern El Dorado mountains and the southern portion of the range.

Commodity: Ore Materials: gold Gangue Materials: quartz, calcite, hematite, manganese oxides, clays

Deposit: Anomalous gold occurs primarily in the Nelson monzonite, a heterogeneous, sill-like stock of Miocene age. Mineralization also occurs in Precambrian mafic crystalline rocks and in a Miocene volcanic sequence of andesitic and dacitic tuffs, flows and volcaniclastic rocks more than 3000 feet thick. The deposit consists of three roughly elliptical, shallow, sub-horizontal pods. Pervasive disseminated gold occurs in flat-lying lenses within host rocks of varying composition and alteration. Mineralization is associated with locally anastomosing shear zones which formed during a period of regional extensional faulting that affected the ElDorado Mountains between 10 and 15 Ma, contemporaneously with the time of mineralization.Propylitic alteration is widespread throughout the host rocks. Gold mineralization is tied to a quartz-calcite-veining event that post-dates the propylitic alteration. Anomalous gold also occurs in zones of local silicification, argillic alteration, and quartz-alunite veining. The WBG mineralized zone trends roughly east-west and is underlain by a seemingly chaoctic distribution of altered volcanic rocks, varied plutonic rocks, and breccia. Structural and lithologic controls on the shape of the WBG deposit are subtle. In the Black Hawk mine area, the Nelson monzonite hosts most mineralization. The host rock is brecciated with strong quartz-calcite veining and pervasive hematite-stained clay alteration. In the Gracey Pit, mineralization is localized along the volcanic/monzonite contact, and to a lesser extent along subhorizontal shears. In the Wall Street Pit area, mineralization is both subhorizontal and along steeply-dipping (50 degrees) faults. The deposit is truncated on all sides by post-mineral, moderate- to steeply dipping faults. Mineralization post-dates the intrusion of the Nelson monzonite, and precedes the bounding faults, bracketing it between 14 and 11 Ma.

Deposit type: Detachment-fault-related polymetallic Cu-Au-Ag-Pb-Zn deposits

Development: Gold was first discovered in the area in the 1700s by Spanish explorers who named Eldorado Canyon. Gold was later discovered in 1857 on the Honest Miner claim about 2 miles west of Nelson, after which the district was organized in 1861. Between 1863 and 1907, several mines in the district sporadically produced an unrecorded total amount of gold and silver. Between 1907 and 1954, the mines produced 100,600 ounces of gold and 2,360,000 ounces of silver. A small heap leach operation on the Wall Street claim produced a small amount in the late 1970s, which was the last significant production for the district. Numerous companies have examined the bulk-mineable potential of the district since the 1970s, including Intermountain Exploration, Amselco, Exxon, Weaco, Homestake, Zephyr Resources, and Alta Gold. The combined exploration efforts of these companies resulted in the identification of this laterally-continuous disseminated gold deposit encompassing the historic Wall Street and Black Hawk mines. The deposit was held by Nevada Pacific Mining Company in 1995 and was listed among the properties of Nelloro Corp. in 1997.

Geology: A thick sequence of Tertiary volcanic rocks overlies Precambrian granodioritic gneiss and schist in the Eldorado district. The Nelson Monzonite, a sill-like stock, intrudes both volcanic and Precambrian rocks.

Ore(s): Ore is associated with shear zones dating from a period of regional extensional faulting about 15-10 Ma.

References

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USGS MRDS Record:	10310510

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