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Formula: Ag(Fe,Ni)8S8
Sulphide, pentlandite group,
silver- and nickel- bearing mineral
Crystal System: Isometric
Specific gravity: 4.66 calculated
Hardness: 3½
Colour: Bronze-brown
Common impurities: Cu
Environments
Argentopentlandite is a scarce mineral, described in no more than ten deposits worldwide, occurring in one of
four associations:
(a) Cobalt-nickel
skarn-type mineralisation
(b) Nickel-copper deposits associated
with ultrabasic rocks, within or along the contacts of the mineralised
rocks with the host rock, or near, but not in, ultrabasic sills
(c) Metamorphosed ultramafic sequences, in which case the hydrothermal
origin is related to dyke emplacement or to metamorphic remobilisation of
tourmalinite
(d) As hydrothermal mineralisation within paragneissic and
metamorphosed basic rocks during their retrogression
(CM 34.939–947).
Associated minerals include pyrite,
pyrrhotite, mackinawite,
cubanite, chalcopyrite,
stannite, galena,
sphalerite, calcite and
quartz
(HOM).
Localities
At Ranua, Lapland, Finland, argentopentlandite has been identified in a
nickel ore. The ore body has a
gabbroic host rock and is cut by a sulphide vein of extraneous origin
containing abundant sphalerite,
galena, chalcopyrite, and
pyrrhotite. Mixing of the two sulphide assemblages in the vein obviously
gave rise to the formation of argentopentlandite, which occurs there not only in
chalcopyrite but also in association with
sphalerite and galena
(AM 57.137–145).
At four localities in Finland, Outokumpu, Vuonos and Miihkali in North Karelia, and Hietajärvi in South Savo,
argentopentlandite occurs as exsolution bodies in chalcopyrite,
either in the contacts of the orebodies or in the mineralised zones parallel to them. In addition to
chalcopyrite, the major paragenetic minerals are
pyrrhotite and
cobaltpentlandite. Other associated minerals include
pyrite, siegenite,
sphalerite, stannite,
mackinawite and
cobaltite-gersdorffite, as
well as occasional melonite. The oxides are
eskolaite and zincochromite.
Pentlandite is often observed in association with
argentopentlandite either as inclusions or exsolution bodies.
The sulphide minerals apparently crystallised in the following order:
pyrite → siegenite and
cobaltite-gersdorffite →
pyrrhotite and pentlandite
→ chalcopyrite,
sphalerite and stannite →
argentopentlandite → mackinawite
(AM 57.137–145).
At the Hitura Nickel Mine, Nivala, North Ostrobothnia, Finland, the nickel
deposit is hosted in ultramafic rocks. Argentopentlandite has been
found in the mica gneiss wall rock
of the deposit. The paragenetic minerals are chalcopyrite,
pyrrhotite, pentlandite,
mackinawite, and rare cubanite
(AM 57.137–145).
At Kerimäki, Savonlinna sub-region, Southern Savonia, Finland, in a
nickel-copper mineralisation in a
noritic body, argentopentlandite occurs as inclusions in
chalcopyrite in the copper-rich
contacts of the mineralised rock with its host rock. The associated minerals are
pentlandite and pyrrhotite
(AM 57.137–145).
There are two co-type localities, the Oktyabrsky Mine, Talnakh Cu-Ni Deposit, Noril'sk, Putoran Plateau, Taimyr
Peninsula, Taymyrskiy Autonomous Okrug, Krasnoyarsk Krai, Russia, and the Khovu-Aksy Ni-Co deposit, Chedi-Kholsky
District, Tuva, Russia.
At El Charcon, Aguilas, Murcia, Spain, the mineralisation where the argentopentlandite was found consists of a
system of veins hosted within a metamorphic complex of quartzite and
quartz schists.
The mineralisation consists of massive ore with minor breccia
containing chalcopyrite,
sphalerite, pyrite,
galena, arsenopyrite,
cubanite and argentopentlandite, together with
quartz and minor carbonates as gangue
minerals. The paragenetic sequence is divided into three stages of deposition:
(I) Chalcopyrite, sphalerite I,
argentopentlandite, cubanite and
quartz
(II) Pyrite, sphalerite II,
chalcopyrite,
arsenopyrite and quartz
(III) Galena and minor carbonate
(CM 34.939–947).
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