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Borealis

Nevada, USA

Main commodities: Au Ag
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The Borealis group of deposits are located in the Walker Lane Gold Belt in Mineral County, south-western Nevada, USA, less than 20 km from the California border. Historically, 6 orebodies have been exploited, distributed over a distance of 4.5 km in a general north-easterly direction (Eng, 1990).

The Walker Lane Belt parallels the Carlin and Battle Mountain Eureka Trends immediately adjacent and parallel to the Nevada-California state borders.

Published production and resource figures are:

    9.7 Mt @ 2 g/t Au = 19.5 t Au (Production from 6 pits of trend to 1990, Eng, 1990).
    42 Mt @ 0.92 g/t Au (Measured + indicated Resources, 2007, Gryphon Gold, Jan. 2007)

The Borealis district contains multiple coalescing hydrothermal centres with high-sulphidation alteration and mineralisation characteristics. The gold deposits of the district comprise high grade mineralisation concentrated along steeply dipping structures with lower-grade gold both enveloping the high grade structures and commonly occurring in more permeable volcanic rocks in relatively flat-lying zones.

Mineralisation is hosted by an up to 1000 m thick sequence of Oligocene and Miocene andesitic volcanics which overlie a basement of Cretaceous granite. The host volcanics consist principally of andesitic flows laharic breccias and volcaniclastic tuffs which have been dated at 19 to 13.5 Ma. These volcanics generally strike NE and dip shallowly to the NW. They have been divided into three members, a Basal Andesite and a Middle Andesite, overlain by the Upper Tuff composed of interbedded volcaniclastics, intermediate to mafic lava flows and less abundant tuffs, and is as much as 60 m thick. Pediment gravels cover the altered and mineralised volcanic rocks at lower elevations along the mountain front. At least one of the deposits, the Freedom Flat orebody, is associated with a north-east trending fault which has a steep north-west dip. This fault is largely post mineralisation with both a sinistral strike slip displacement, and a normal component (Eng, 1990; Tenneco. 1987).

The main Borealis deposit, which has plan dimensions of 400 x 125 m, consists of micron sized disseminated gold in a pyritic, silicified pyroclastic unit within the predominantly flow sequence. Peripheral to and below the ore, the flow units are successively argillised and propylitised. The highest gold grades are associated with open space and barite mineralisation. Small high grade deposits exploited by early prospectors occur in the transgressive structures with sponge like silica, marginal to the main deposit (Tenneco, 1987).

At the Freedom Flat deposit, alteration is strongly zoned, both vertically and laterally. The zone of intense alteration is 90 to 150 m wide, and extends for more than 180 m vertically, narrowing with depth. The core of this intense alteration consists of a 30 to 60 m wide zone of hypogene silica breccia with abundant iron oxide minerals. It grades progressively downwards into oxidised quartz-sulphide and un-oxidised quartz-pyrite alteration. Silica breccia grades upwards and outwards into oxidised, powdery, advanced argillic alteration with kaolinite, alunite, quartz and iron oxides. This may be a supergene assemblage. Barren chalcedonic silica and opal overlie the deposit. Un-oxidised argillic alteration is developed adjacent to the zone of intense silicification and grades outwards into regional propylitic alteration. Siliceous sinter and fissure type quartz veining are absent from the orebody. Goethite, limonite and hematite are abundant and appear to have formed from the oxidation of pyrite. Gold occurs as sub-micron sized particles in sulphide and iron oxide minerals (Eng, 1990).

The Freedom Flats deposit has plan projection dimensions of 365 x 60 to 75 m. More than 75% of the mined gold was from a steeply plunging shoot of high grade ore that measured 120 x 45 to 60 m in plan. Radiometric ages on the probable supergene alunite were from 11.6 to 6.5 Ma, while the host is 17.6 to 16.9 Ma (Eng, 1990).

Other exploited pits include: Northeast Ridge, Goldview, Deep Ore Flats, Jame's Ridge, Cerro Duro. Current resource areas include: Graben, North Graben.

The most recent source geological information used to prepare this summary was dated: 2006.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
   Selected References:
Tenneco Minerals  1987 - The Borealis Project: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada    pp 243-244


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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