OCCURRENCES OF PLATINUM-PALLADIUM-ENRICHED CHROMITITES IN THE THETFORD MINES OPHIOLITE, SOUTHERN QUÉBEC

Chromitites in the Thetford Mines ophiolite occur in four contexts, based on their position within the ophiolite pseudostratigraphy, and on diagnostic structures, textures, and mineral compositions. 1) Intra-mantle discordant. These are probably intra-conduit cumulates formed through differential settling of chromite and olivine, with additional Cr being provided by assimilation of wallrock pyroxene. Nodular varieties have been observed. 2) Crust-mantle interface concordant. These appear to represent true cumulates from underplating sills or ponded magma replenishments trapped at this structural/rheological barrier. 3) Reactional intra-crustal (North Arm type). These form through the incongruent dissolution into primitive intrusive/replenishing magmas of feldspar or pyroxene from the host rocks. In the Finneth deposit, 1-3cm chromitite reaction rims separate wehrlite and dunite. 4) Intra-crustal discordant (Hall type). These PGE (platinum group element)-rich chromitites fill breccias within dunite pipes that cross-cut the "cumulate stratigraphy". A link with fluid-rich boninites is inferred, since a high Cl-H₂O-content delays the appearance of chromite on the liquidus, and may also play a role in the development of the dunitic halo. The Starchrome and César bodies may also be related. The Starchrome body is characterized by a clinopyroxene-rich websterite host that is crosscut and replaced by dunitic to orthopyroxenitic veins. Massive to anti-nodular PGE-rich chromitites (0.5-10 cm) may occupy the cores of orthopyroxenitic veins. César occurs near a brecciated contact between sub-ophiolitic margin sandstones and obducted harzburgite. The brecciated peridotite appears to be veined by PGE-rich chromitite (~ 4cm). Preliminary whole-rock PGE determinations from the Hall and Starchrome chromitites yield anomalous mantle-normalized profiles, with Pd/Ir of 0.25 and 49.3, respectively. Pyroxenitic and dunitic hosts to the chromitites have similar profile shapes, suggesting a genetic link. Chromite grains at Hall contain euhedral inclusions of laurite-erlichmanite [RuS₂-OsS₂], base metal (Ni-Cu-Fe) sulphides, isoferroplatinum [Pt₃Fe], and prassoite [(Rh,Ir,Pt,Cu,Fe)S]. At Starchrome, PGE-bearing phases include complex alloys of Cu-Ni-Fe-Pt-Pd, and potarite [PdHg], principally in intergranular sites and in serpentine-filled fractures and veins.