22 Geologic Evolution of the Northern Appalachians: The Quebec-Vermont Connection (Posters)

Tuesday, 13 March 2001
8:30 AM - 12:00 PM

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

Paper number: 22-2
Program: THEME SESSIONS
Time: 8:30 AM – 12:00 PM

Authors

Abstract

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.