Abstract

The Salt Chuck intrusion, Alaska, is a Paleozoic, Alaskan-type, ultramafic-mafic rock complex that contains a Cu-Au-Pd-Ag deposit from which about 0.3 million metric tons of ore at an estimated grade of 0.95 percent Cu, 1 ppm Au, 2 ppm Pd, and 6 ppm Ag were recovered. Near the mine, layered gabbroic rocks are intruded by a massive clinopyroxenitic unit with a few thin magnetite layers. Clinopyroxenite contains cumulus clinopyroxene, magnetite (with ilmenite lamellae), and apatite; postcumulus minerals are plagioclase, magnesiohastingsite, and phlogopite-eastonite. Alteration and metamorphism have produced assemblages of epidote, actinolite, chlorite, sericite, titanite, and calcite, especially in contact with sulfides. Most of the sulfide ore occurs in clinopyroxenite near its contact with layered gabbro; sulfides commonly consist of chalcopyrite, bornite, digenite, chalcocite, and covellite. The sulfides are disseminated in clinopyroxenite, but some are in pyrite-pyrrhotite veins, epidote veinlets, and calcite-rich veins.Platinum-group minerals (PGM) are predominantly kotulskite (PdTe) as single grains or with sperrylite (PtAs 2 ), palladium antimony minerals, and gold; the PGM occur mainly free in epidote, at sulfide grain boundaries, and in decreasing quantities in bornitc, chalcopyrite, digenite, chalcocite, and covellite. Kotulskite is variably intergrown with or rimmed and successively altered by temagamite (Pd 3 HgTe 3 ), sopcheite (Pd 3 Ag 4 Te 4 ), and hessitc (Ag 2 Te).The high Cu and precious metal content is interpreted to have resulted when a calcium-bearing fluid remobilized elements from primary magmatic sulfide assemblages. This fluid was either deuteric, or external fluid that had become enriched in Cl by reaction with magmatic amphibole, mica, and apatite (remnants of which have Cl contents in excess of 0.1 wt %). The fluid migrated along or to the clinopyroxenite-gabbro contact where it reacted with the primary igneous mineral assemblage, replacing magmatic sulfides by more Cu-rich assemblages and precipitating platinum-group minerals. This process continued to very low temperature where digenite, chalcocite, and covellite were stable and mercury- and silver-bearing PGM partly replaced earlier kotulskite.

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