Borealis mine, Borealis Mining District, Mineral County, Nevada, USAi

Regional Level Types
Borealis mine	Mine
Borealis Mining District	Mining District
Mineral County	County
Nevada	State
USA	Country

Structure: Major regional structural trends affecting the mine area are the Walker Lane shear zone and the Borealis Trend. NE-trending faults with subordinate N-S and E-W- striking structures.

Alteration: Host rock alteration consists mainly of pervasive silicification, alunitization, kaolinization, oxidation, argillization, and propylitization. The conduit for hydrothermal fluids was a 100-200 ft. wide core zone of silica breccia with abundant iron oxide minerals. The silica breccia zone narrows downward and grades downward into oxidized quartz-sulfide and unoxidized quartz-pyrite alteration. Argillic and propylitic alteration zones develop outward from core. Barren opal and chalcedonic silica overlie deposit.

Commodity: Ore Materials: gold, cinnabar, chalcopyrite, chalcocite, sphalerite, covellite, tetrahedrite-tennantite, enargite, matildite, nagyagite, niccolite. Gangue Materials: quartz, alunite, k-feldspar, barite, pyrite, hematite, montmorillonite, kaolinite, marcasite, arsenopyrite, pyrrhotite

Deposit: The original Borealis orebody was localized on the south side of a sporadically mineralized NE-trending fault zone. The pyroclastic breccia host rock is pervasively silicified by hydrothermal quartz and potassium feldspar with alunite and barite. Gold mineralization terminates abruptly along the gently NW-dipping contact with underlying andesitic flow rocks. A strongly acid-leached zone high in silver values is developed below this contact. The intensely brecciated acid-leached zone contains pods, stringers and stockwork veinlets of oxidized quartz-sulfide in a powdery bleached, siliceous matrix. Intense acid leaching (characterized by quartz, TiO2, clinochlore, barite, jarosite/alunite) grades outward to moderate argillic alteration (kaolin, montmorillonite). NE-trending small mineralized pods associated with intense acid leaching occur as structurally controlled replacements underneath and to the south of the main ore body. The deposit may be characterized as a hot spring vent ore. The current Borealis deposit is an epithermal gold system marked by large areas of silicification, hydrothermal brecciation, and argillic alteration. The gold deposits are structurally controlled along a series of northeasterly-trending faults that dip steeply to the northwest. The Borealis Zone has three separate targets: the Borealis Mine proper, Freedom Flats mine, and the Graben deposit. Gold occurs as micron-sized particles in highly altered andesite and tuff. The average gold-to-silver ratio is one to five. The Borealis district contains multiple coalescing hydrothermal centers having alteration and mineralization characteristic of high-sulfidation systems. Gold deposits of the district typically have high-grade gold mineralization centrally located along steeply dipping structures and have lower-grade gold mineralization both surrounding the high-grade and commonly occurring in more permeable volcanic rocks in relatively flat-lying zones. The gold deposits with minor amounts of silver mineralization are hosted by Miocene andesitic flows, laharic breccias and volcaniclastic tuffs, which generally strike northeasterly and dip shallowly to the northwest. Pediment gravels cover the altered-mineralized volcanic rocks at lower elevations along the mountain front and there is potential for discovery of more blind deposits, similar to the Graben and the Freedom Flats deposits. Parts of the greater Borealis project area with known occurrences of gold mineralization defined by historical exploration drilling and mine production include: Borealis, East Ridge and Gold View, Northeast Ridge, Freedom Flats, Deep Ore Flats (also known as Polaris), Cerro Duro, and Jaimes Ridge. All of these deposits still have gold mineralization remaining in place, contiguous with the portions of each individual deposit that previously has been mined. Alteration consists mainly of silicification, oxidation, argillization, and propylitization. The conduit for hydrothermal fluids was a 100-200 ft. wide core zone of silica breccia with abundant iron oxide minerals. The silica breccia zone narrows downward and grades downward into oxidized quartz-sulfide and unoxidized quartz-pyrite alteration. Argillic and propylitic alteration zones develop outward from core. Barren opal and chalcedonic silica overlie deposit.

Deposit type: Hot-spring Au-Ag

Development: The Borealis deposit was mined in the early 1900s and intermittently through the 1930s. The modern-day Borealis deposit was discovered in 1978 by Houston Oil and Minerals Co. (later bought out by Tenneco). Houston announced plans for the open-pit operation in 1980 and began production in 1981, which continued through 1986, when Echo Bay Minerals purchased Borealis along with the Manhattan and McCoy deposits from Tenneco. Production continued through mine closure in 1990. During this time, 635,000 ounces of gold were produced from the deposit and the high-grade sulphide Graben zone was discovered. The Freedom Flats orebody was discovered during a pediment drilling program southwest of the main Borealis deposit. The third drill hole of the program intersected the main high-grade ore. Freedom Flats lies only 2000 ft. southwest of the original Borealis deposit and remained undiscovered despite seven years of exploration in the district. Drilling prior to the 1986 program had drilled to within 50 ft. of the southeastern edge of Freedom Flats, intersecting thick altered intervals but only scattered intervals of detectable gold. From 1990 to 1996 Santa Fe Gold held the property and drilled 32 deep holes, expanding the high-grade sulphide gold mineralization They terminated operations prior to merger with Newmont after completing site reclamation. From 1996 to 2003 the property was held by first Cambior and then by Golden Phoenix, who compiled all previous data, drilled 10 holes, and identified additional sulfide mineralization. The property was optioned to Gryphon Gold in 2003.. By the end of 2004, Gryphon had earned a 70% position in the property, with Golden Phoenix retaining 30%. In January of 2005, Golden Phoenix elected to sell its remaining 30% interest in the mine to Gryphon for $1.4 million, to be paid in installments through January of 2006. Gryphon acquired a 100% working interest in Borealis property in February 2005 and developed an accredited gold resource (Central Borealis) of 1.25 million ounces of measured and indicated mineral resources and 0.7 million ounces of inferred mineral resources to date.

Geology: Alunite veining occurs in the mine area at depth. Detailed geology of the Freedom Flats orebody is provided in Eng, 1990 (see reference below).

Ore(s): Ore-controlling features include NE-trending structures, fracturing, paleo-hot springs activity; and the intersection of 3 fault systems.

References

Other Databases

Link to USGS MRDS:	10310350
Link to USGS MRDS:	10198539

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