Bornite

bornite

mawsonite

chalcocite

chalcopyrite

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Formula: Cu5FeS4
Sulphide, copper-bearing mineral
Specific gravity:4.9 to 5.3
Hardness: 3
Streak: Dark grey
Colour: Reddish silver grey on fresh break
Solubility: Moderately soluble in nitric acid
Common impurities: Ag,Ge,Bi,In,Pb
Environments:

Pegmatites
Carbonatites
Metamorphic environments
Hydrothermal environments

Bornite is a widely occurring copper ore usually found associated with chalcocite, chalcopyrite, covellite, pyrrhotite, pyrite and other sulphides, especially in the enriched zone of mesothermal (moderate temperature) and hypothermal (high temperature) veins. It is less frequently found as secondary deposits in the oxidation zone of copper veins. It occurs disseminated in ultramafic rocks, in contact and regional metamorphic deposits and in pegmatites.

Localities

The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia, are skarn deposits at the contact between granodiorite and volcanic rocks. Bornite is a primary sulphide that has been observed as an exsolution intergrowth with chalcocite, associated with hematite, at the Two Mile deposit (AJM 22.1.36).

At the Mount Kelly deposit, Gunpowder District, Queensland, Australia, the deposit has been mined for oxide and supergene copper ores, predominantly malachite, azurite and chrysocolla. The ores overlie primary zone mineralisation consisting of quartz-dolomite-sulphide veins hosted in dolomite-bearing siltstone and graphitic schist.
Bornite occurs rarely as a primary and supergene mineral, as dark, golden brown grains often rimmed by covellite and replacing chalcopyrite in veins or disseminated in the host sedimentary rock. Paragenesis for the primary zone is dolomite followed by pyrite, then chalcopyrite and sphalerite, and lastly bornite (AJM 22.1.19).

At the Mount Lyell Mines, Queenstown district, West Coast municipality, Tasmania, Australia, bornite is commonly associated with chalcocite, and contains small inclusions of mawsonite. Crystals to 6 cm associated with fluorite have been reported. Gold is a rare associate (AJM 21.2.21-22).

At the Judkins quarry, Nuneaton, Warwickshire, England, UK, bornite has been found as a replacement of chalcocite, with chalcopyrite and calcite (RES p323).

Alteration

chalcopyrite CuFeS2 (primary) readily alters to the secondary minerals bornite, covellite and brochantite.

chalcopyrite and chalcocite to bornite
CuFe3+S2 + 2Cu2S = Cu5FeS4
This reaction occurs in the supergene enrichment zone (JRS 18.14).

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