COMPOSITION AND OCCURRENCE OF ELECTRUM AT THE MORNING-STAR DEPOSIT, SAN-BERNARDINO COUNTY, CALIFORNIA - EVIDENCE FOR REMOBILIZATION OF GOLDAND SILVER

Electrum, acanthite and uytenbogaardtite have been examined from six d epths within the tabular quartz +/- calcite stockwork and breccia-fill ed veins in the fault-zone-hosted Morning Star deposit of the northeas tern Mojave Desert, California. Six distinct types of electrum have be en identified on the basis of mineral association, grain morphology an d composition. Two types, (1) pyrite-hosted and (2) quartz-hosted elec trum, occur with acanthite after argentite and base-metal-sulfide mine rals in unoxidized portions of the orebody; the remaining four types, (3) goethite-hosted electrum, (4) electrum cores, (5) electrum rims an d (6) wire electrum, are associated with assemblages of supergene mine rals in its oxidized portions. Pyrite-hosted, quartz-hosted and goethi te-hosted electrum range in composition from 61 to 75 wt.% Au and have uniform textures and no zoning. In lower portions of the oxidized ore zone, electrum seems to replace goethite and occurs as small grains o n surfaces of the goethite. Textural evidence favors supergene remobil ization of Au and Ag, which were deposited as electrum on or replacing goethite. This type of electrum is identical in appearance and compos ition to primary electrum. In the upper portions of the oxidized zone, secondary electrum occurs as a gold-rich rim on a core of electrum an d as wire-like grains, both with acanthite and uytenbogaardtite. Such secondary electrum contains from 78 to 93 wt.% Au. Textural relations and associated minerals suggest that the primary electrum was hydrothe rmally deposited and partially remobilized by supergene processes.