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Formula: PbCu2Bi4(Se,S)8
Sulphosalt, bismuth- and selenium-
bearing mineral
Crystal System: Monoclinic
Specific gravity: 7.82 calculated
Hardness: 3½
Colour: Black
Environments
Sedimentary environments
Hydrothermal environments
Watkinsonite occurs in vein-type uranium deposits with other
tellurides and
selenides
(HOM).
Localities
At the type locality, the Otish Mountains U deposit, Otish Mountains, Baie-James, Jamésie, Nord-du-Québec, Québec,
Canada, the sedimentary rocks were essentially of volcano-detrital origin, and subsequently were intruded by thick
doleritic dykes and sills. The mineralisation is mainly associated
with these doleritic bodies.
Two major hydrothermal metallogenic cycles of uranium are inferred.
The first led to the deposition of two distinct mineral assemblages: albite
with uranium minerals, and copper-,
lead-, bismuth-
selenides, tellurides and
sulphides in a carbonate gangue
(dolomite and calcite), with
chlorite as a product of hydrothermal alteration of
amphibole and biotite.
The second cycle produced pitchblende and
coffinite mineralisation in a
calcite gangue.
Sulphides associated with a quartz
gangue were observed rarely, emplaced between the two major metallogenic
cycles.
Mineralised zones are surrounded by zones of hydrothermal alteration. The most external zone is rich in
epidote in microfractures. Toward the centre,
chloritisation of amphibole and
biotite is evident. The central zone exhibits extensive
albitisation. The main uranium
minerals are uraninite and
brannerite.
In the second metallogenic cycle carrying uranium,
pitchblende, associated with
coffinite, occurs in a calcite
gangue, rich in hematite inclusions.
Pyrite, marcasite and
galena rarely occur in this type of mineralisation.
The following mineral associations were observed (arrow indicates proposed sequence of crystallisation):
1. Bi2(Se,Te,S) → skippenite →
watkinsonite → clausthalite →
chalcopyrite → electrum
2. poubaite → watkinsonite →
součekite
3. součekite →
wittichenite
4 aikinite →
clausthalite-galena solid
solution.
Aggregates of skippenite commonly enclose irregular grains of
watkinsonite, and watkinsonite partly replaces skippenite
crystals. Locally, clausthalite associated with
Bi2(Se,S)2 outlines interstices of watkinsonite grains and generates an intergrowth at
skippenite-watkinsonite contacts. On a fresh fracture, the
aggregates of watkinsonite are black, and exhibit a moderate metallic lustre and conchoidal fracture. The
size of individual grains varies from 0.01 to about 3 mm
(CM 25.4.625-638).
At the Zálesí uranium deposit, Zálesí, Javorník, Jeseník District, Olomouc Region, Czech Republic, the scarce samples
containing watkinsonite were collected from the mine dump. A relatively abundant
selenide mineralisation is associated with the older, uraninite stage of
mineralisation. Watkinsonite occurs as irregular aggregates from 0.01 to about 4 mm. On a fresh fracture, it
is yellowish to brownish grey with a metallic lustre and a characteristic conchoidal fracture. Watkinsonite
aggregates in a quartz gangue are
usually intergrown with practically sulphur-free
clausthalite. Uraninite,
chalcopyrite, bornite,
covellite, löllingite and
sulphur-rich umangite were also
observed in close association
(CM 48.1109-1118).
Watkinsonite from the Zálesí uranium deposit
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At Alberoda, Aue-Bad Schlema, Erzgebirgskreis, Saxony, Germany, the uranium
deposit contains a rich variety of bismuth minerals, including the rare
bismuth selenides
watkinsonite, nevskite and
copper-bearing bohdanowiczite.
Watkinsonite and nevskite only occur intergrown with
clausthalite;
bohdanowiczite is more widespread and also is associated with
copper selenides.
The bismuth selenides were
deposited at temperatures of about 100°C. Other bismuth minerals from this
locality comprise members of the
bismuthinite–aikinite
series and native bismuth and
bismuth sulphides, such as
matildite, bismuthinite and
wittichenite, intergrown with
clausthalite. In
clausthalite, watkinsonite may be associated with
bohdanowiczite, nevskite or
both
(CM 43.3.899-908).
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