Deep Post orebody, Betze-Post pit, Lynn Mining District, Eureka County, Nevada, USAi

Regional Level Types
Deep Post orebody	Ore Body
Betze-Post pit	Pit
Lynn Mining District	Mining District
Eureka County	County
Nevada	State
USA	Country

Structure: Roberts mountain thrust n-nnw-trending zone considered main hydrothermal conduit; nn W to n, and ne-striking faults

Alteration: Decarbonization; argillic and silicic hydrothermal; oxidation.

Tectonics: Basin and range; originally, w. Edge of n. American craton

Commodity: Commodity Info: Au occurs as micron to submicron-sized particles associated with quartz and clays in the oxide zones and with pyrite and marcasite in the sulfide zones. Simple refractory ore in the lower portions of the lower Post/Betze orebody is characterized by silicification with kaolinite & 5-10% disseminated pyrite. Carbon found distal and probably unrelated to the orebodies. Increasing Au grades associated with increasing amounts of pyrite/marcasite. Ore Materials: GOLD, AURIFEROUS PYRITE, ARSENIAN PYRITE, ARSENOPYRITE, MARCASITE Gangue Materials: quartz, pyrite, orpiment, realgar, kaolinite, dickite, sericite, illite, alunite, barite, stibnite, goethite, hematite, jarosite, gypsum, calcite

Deposit: The higher grade ore is generally found adjacent to faults and in crests of folds, below impermeable units. The deposit contains both sulfide and oxide ore. Lower Post and Betze are horizontally and vertically adjacent parts of the same orebody, discovered and then named separately. Betze is NW of and below Post; the Lower Post is directly below Post. Supergene alunite from Post has been K/Ar dated at 8.48 to 9.58 Ma (Arehart, cited in Lauha & Bettles). The Post orebody is oxide ore, hosted by Vinini Fm. The Lower Post/Betze ores are refractory, hosted by Popovich and to a lesser extent Roberts Mountains Formation. The secondary mineralization trend at Lower Post/Betze is N40-70E.

Deposit type: Sediment-hosted Au

Development: Anomalous gold detected in outcrops; orebody encountered in first drill hole. Western States Minerals commenced mining in 1986, and sold the Post deposit to Barrick Goldstrike in January 1987. Lower Post was discovered by deep drilling in 1987. Further drilling in 1987 defined the Betze orebody, "the largest deposit discovered to date in the Carlin trend" (Lauha & Bettles, 1993). The Post orebody is mined by Barrick Goldstrike as a major ore source for the Goldstrike mine processing facilities. Most reserves are mined from Betze pit, which was expected to reach depth of approx. 1,600 feet, with a stripping ratio of 1:7. Refractory ore is processed by autoclave , which is expected to reach a daily capacity of 15,000 tons/day in 1993. The Newmont portion of Lower Post will be mined sometime after 1993. ; econ.com: cyanidation relatively unsuccessful on refratory sufide ore, so barrick processes such ores by roasting or pressure oxidation, then cyanidation.

Geology: Arehart, foland, et al. (1993, latest pub. On this deposit) gives age of mineralization As 117 ma. Other possible ages of mineralization are: 36-37 ma, 39 ma, or 158 ma. The lower post, betze, and deep post deposits lie beneath the post deposit and spacially distinct from post. Post lies on the upper plate of the roberts mountain thrust; betze & lower post are in the lower plate of this thrust. Lower post and betze are a single orebody (arehart et al). Associated igneous rocks, shown above, also host ore in the post deposit. Nn W to N faults and some NE faults sho W movement after ore formation. Faulting caused brecciation and preparation for higher grade mineralization. Collapse & hydrothermal breccias of above-named host rocks also host ore. Minor Au mineralization found in goldstrike intrusive, usually along faults or joints. American barrick and newmont use different names for the same units (such As roberts mountain fm.).

Ores(s): Structurally controlled near southern terminus; penetrative farther north. High angle faults control higher grade au. Imbricate thrusts related to roberts mt. Thrust or later event influenced ore localization. Oxide ore generally occurs in vinini fm.(or newmont's rodeo creek unit); refractory sulfide ore typically occurs in popovich & roberts mt. Formations. Plutonic rocks often acted As dams to mineralizing fluids.

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Link to USGS MRDS:	10310437

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References