INTERNAL STRUCTURE OF AU-PD-PT GRAINS FROM SOUTH DEVON, ENGLAND, IN RELATION TO LOW-TEMPERATURE TRANSPORT AND DEPOSITION

Gold is widespread and sometimes abundant in drainage and overburden derived from Lower Devonian sediments and minor volcanics in the South Hams district of Devon, southwest England. It is frequently enriched in palladium and is accompanied at some localities by potarite (PdHg) and native platinum. Much of the gold is quite coarse-grained with dendritic or other intricate forms that cannot survive any mechanical dispersion. Some of these grains are identical to gold that occurs in carbonate veins at Hope's Nose near Torquay. Microchemical mapping of the gold and platinum-group element-rich grains has revealed a wealth of internal structure that reflects the history of grain growth. The first of four main stages produced dendritic and zoned grains of gold enriched in Pd (less-than-or-equal-to 10 wt% Pd), gold-potarite (16 wt% Pd) or gold-bearing potarite (34 wt% Pd). A second phase comprised deposition of gold with 8 wt% Ag in thin, intragranular films and 'crack fills', together, in some places, with patches of a Cu-Au intermetallic compound (two varieties with 9 wt% and 21 wt% Cu, respectively). This occurred after grains were stressed and subjected to a corrosive environment, possibly induced by the deposition of metallic mercury. The third phase produced overgrowths of Ag-enriched gold, which truncated previous growth zones and intergranular films. Some grains are also mantled with one to five distinct rims of native Pt that contains varying amounts of Hg, Pd, Au, Ag and Cu. Minute grains of selenide minerals (Bi-, Pb-, Hg-, Cu- and more complex Pd-bearing varieties) occur within Ag-enriched gold in overgrowths and intragranular zones. At one locality overgrowths of iron oxide have partially or completely replaced Ag-enriched gold during a final phase of activity. Precious-metal grains are thought to have grown from oxidizing saline fluids at about 100-degrees-C. Metals carried as chloride complexes were deposited where solutions came into contact with more reducing fluids or reactive rocks. Replacement of gold by iron oxide may have occurred in a subsequent weathering environment.