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Formula: Cu5FeS4
Sulphide, copper-bearing mineral
Crystal System: Orthorhombic
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 Shengou lead-zinc mine, Xide County, Liangshan Yi, Sichuan, China, irridescent bornite occurs on a
dark grey matrix
(AESS).
Bornite from Shengou - Image
At Charcas, Charcas Municipality, San Luis Potosí, Mexico,
the primary minerals are
sphalerite, galena,
chalcopyrite, bornite,
tetrahedrite,
arsenopyrite, pyrite and
silver minerals such as jalpaite,
diaphorite and acanthite. In
the host rock, as metamorphic or alteration minerals, danburite,
datolite, hedenbergite,
epidote, chlorite,
andradite, actinolite
and wollastonite have been reported.
Quartz, calcite and
danburite crystallised during the entire life of the systems, throughout
the intrusive emplacement, metamorphism, and mineralising events. With depth, both
sphalerite and galena decrease
while chalcopyrite increases.
Secondary sulphides formed include
bornite, covellite,
digenite and chalcocite.
Native silver, native gold,
hematite and goethite were
deposited after the sulphides
(Minrec 55.6.727-728).
Bornite is moderately abundant at Charcas, but macroscopic crystals are very rare. It occurs as a thin
reaction rim on primary
chalcopyrite, making it a
secondary mineral. This replacement ranges in thickness from
a thin film to a layer 0.2 mm thick; such coatings also occur on
tetrahedrite crystals. Rare, individual, well formed bornite
crystals to 1 cm, occasionally twinned, have been found. Associated minerals include
chalcopyrite,
tetrahedrite, danburite and
calcite
(Minrec 55.6.736-737).
Bornite from Charcas - Image
At the Judkins quarry, Nuneaton, Warwickshire, England, UK, bornite has been found as a replacement of
chalcocite, with chalcopyrite and
calcite
(RES p323).
Bornite from the Judkins Quarry - Image
At the Luck Fairfax Plant, Loudoun county, Virginia, USA, (surely should be Fairfax Quarry, Centreville,
Basin, Fairfax County, Virginia, USA) bornite has been found
embedded in prehnite as
pseudomorphs after
chalcopyrite
(R&M 98.2.124).
Bornite from the Fairfax Quarry - Image
At the Mufulira Mine, Mufulira, Mufulira District, Copperbelt Province, Zambia, bornite is a primary economic
constituent of the orebodies, occurring primarily as finely disseminated grains along bedding planes in the orebody
quartzite. In high-grade areas in the orebody at Mufulira East,
solid masses of bornite up to several kilograms in mass were common, often in association with amorphous
graphite. Despite the abundance of bornite, crystallised specimens
were encountered only rarely
(MinRec 55.4.456)
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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