Abstract
Platinum-group elements (PGE) are typically associated with mafic
and ultramafic intrusive rocks and the main exploration targets
are layers and zones rich in PGE-bearing sulphides. Some PGE
occurences, however, are in sulphide-poor situations and this
raises the possibility that PGE deposits may be present in parts
of mafic and ultramafic intrusives courrently considered to have
low exploration potential.A multidiciplinary study was undertaken
on four subeconomic deposits of platimum-group metals to develop a
model of formation for PGE-deposits lacking significant base-metal
sulphides. Two of the deposits occur in Albania, in the Tropoja
and Bulqiza massifs, and are part of an ophiolitic belt created in
an oceanic environment during the Upper Jurassic. The other two
deposits occur in Madagascar, in the Andohankiranomena and
Lavotrafo ultramafic massifs, and are within a Pan-African rifted
zone.A Pt-rich chromitite style of mineralization was identified
in the Andohankiranomena and Tropoja deposits, where the PGE are
mostly included in chromite. A Pt- and Pd-rich silicate (dunite)
style of mineralization was identified in the Lavatrafo and
Bulqiza massifs, where PGE mineralization is assocciated with
interstitial material between olivine grains. The four deposits
have contrasting patterns of PGE distribution and individual
elememnt ratios, suggesting that different mineral species
(alloys, arsenides and sulphides) host the PGE. No primary
geochemical halos were detected around any of the deposits and
weathering has littel effect on the distribution of the PGE.The
study showed that alloys and arsenides are the main carriers for
platinum in all the deposits. Pt-Fe alloys, in particular, are
often present in PGE deposits poor in base-metal sulphides and two
phase systems were investigated experimentally: Pt-Ir-Fe-S (and
the related subsystem Ir-Fe-S) and Fe(Cu)-Pt-Rh-S. A comparison of
experimental results with natural phases in the depoists suggests
that fluid-assisted exsolution of Pt, Ir and other elements from
original higher-temperature solid solutions could be wide-spread.
This supports the fluid-driven multistage mineralization concept
suggested by field data. The experimental work also indicated that
Pt-Rh-Fe alloys can coexist with two types of immiscible sulphide
melt (one low in Rh and high in Cu, the other Rh-rich); both types
of sulphide melt may have been present in the Tropoja deposit.
Original language | English |
---|---|
Journal | Trans. Instn. Min. Metall. (Sect. B: Appl. earch sci.). |
Volume | 108 |
Pages (from-to) | B119-B150 |
Publication status | Published - 1999 |