SUPERGENE ORE AND HYPOGENE NONORE MINERALIZATION AT THE NAGAMBIE SEDIMENT-HOSTED GOLD DEPOSIT, VICTORIA, AUSTRALIA

The Nagambie gold deposit, central Victoria, Australia, consists of ap proximately 7 million metric tons of ore averaging 1.2 g/t gold. Miner alization is confined largely to east-west- and N 50 degrees E-oriente d shears and faults contained in a doubly plunging anticlinal dome of undifferentiated Silurian and Early Devonian siltstones and shales. Or e is associated with an east-west-trending anticlinal axis and is part icularly concentrated in two east-northeast-trending structural zones: the Central and 303 shear zones. Five stages of protore vein genesis have been identified. The first two (A, B) are barren crenulated massi ve quartz. Primary gold mineralization occurs in stibnite-bearing quar tz stockwork stringers which belong to the third (C) and fourth (D) qu artz veining stages, whereas the fifth consists of unmineralized small veins (E). Fluid inclusion studies on the two gold mineralization sta ges C and D yield trapping temperatures and pressures of 280 degrees /- 25 degrees C and 1,000 to 1,200 bars, and 200 degrees +/- 30 degree s C and 700 to 1,000 bars, respectively. The mineralization fluids con tain H2O, CO2 (0-69 mole %), CH4 (0-21 mole %), and trace N-2, with wt percent NaCl equiv values in the range of 0 to 10.2 wt percent. Calcu lated delta(18)O values (15.0-6.8 parts per thousand) and measured del ta D data (-88 to -99 parts per thousand) of the hypogene fluids indic ate a meteoric origin. These fluids underwent extreme O-18 enrichment by reacting with country rocks (metamorphic or igneous). Genesis of th e ore, as opposed to the protore, is as secondary supergene enrichment accompanied by varying degrees of surficial limonite alteration. Ore mineralization occurs almost exclusively in limonitized fractures up t o 60 m below the surface in and adjacent to major shears. Based on fea tures such as the spatial distribution, high purity (999.26 fine), and the form of the gold and its close association with limonite, it is p roposed that the gold is mobilized by surficial leaching and concentra ted Dear the water table. Vertical movement of the water table, perhap s due to seasonal changes as well as erosion, produces rapid dissoluti on of hypogene gold and/or aurostibite from above or at the water tabl e.