Abstract

Electrum, acanthite and uytenbogaardtite have been examined from six depths within the tabular quartz+ or -calcite stockwork and breccia-filled veins in the fault-zone-hosted Morning Star deposit of the northeastern Mojave Desert, California. Six distinct types of electrum have been identified on the basis of mineral association, grain morphology and composition. Two types, (1) pyrite-hosted and (2) quartz-hosted electrum, occur with acanthite after argentite and base-metal sulfide minerals in unoxidized portions of the orebody; the remaining four types, (3) goethite-hosted electrum, (4) electrum cores, (5) electrum rims and (6) wire electrum, are associated with assemblages of supergene minerals in its oxidized portions. Pyrite-hosted, quartz-hosted and goethite-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 on surfaces of the goethite. Textural evidence favors supergene remobilization of Au and Ag, which were deposited as electrum on or replacing goethite. This type of electrum is identical in appearance and composition to primary electrum. In the upper portions of the oxidized zone, secondary electrum occurs as a gold-rich rim on a core of electrum and 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 hydrothermally deposited and partially remobilized by supergene processes.

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