RHODIUM-BEARING SPERRYLITE FROM THE NOMGAN PLUTON (MONGOLIA)

Abstract

Several zones of disseminated chalcopyrite-bornite mineralization have been recorded in the Nomgon differentiated pluton of troctolite-anorthosite-gabbro composition, located in the Orkhon-Selenga basin and surrounded by the basement rocks of Riphean-early Paleozoic age. The present study is aimed to find and study minerals of platinum-group elements from this pluton. The minerals of the platinum group elements were flushed from a crushed sample by bromoform. The search, identification and qualitative analysis of the minerals was carried out in polished section prepared from the heavy fraction, using a JSM electron microscope with a KEVEX attachment. Rhodium-bearing sperrylite also is the main platinum mineral, while palladium is represented exclusively by compounds of palladium with bismuth, tellurium and less often, lead. Silver also was found as a compound with bismuth and tellurium. No compounds of palladium with antimony and arsenic were found in this zone. Thus a distinct mineral zoning was observed, reflecting a drop in temperature of mineral deposition. Our data on the high contents of rhodium and sulfur in sperrylite of the Nomgon pluton suggests that natural sperrylite and hollingworthite are completely miscible. This complete miscibility apparently related to the relatively high temperature of crystallization.