Abstract

The Nagambie gold deposit, central Victoria, Australia, consists of approximately 7 million metric tons of ore averaging 1.2 g/t gold. Mineralization is confined largely to east-west- and N 50 degrees E-oriented shears and faults contained in a doubly plunging anticlinal dome of undifferentiated Silurian and Early Devonian siltstones and shales. Ore is associated with an east-west-trending anticlinal axis and is particularly 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 massive quartz. Primary gold mineralization occurs in stibnite-bearing quartz stockwork stringers which belong to the third (C) and fourth (D) quartz veining stages, whereas the fifth consists of unmineralized small veins (E). Fluid inclusion studies on the two gold mineralization stages C and D yield trapping temperatures and pressures of 280 degrees + or - 25 degrees C and 1,000 to 1,200 bars, and 200 degrees + or - 30 degrees C and 700 to 1,000 bars, respectively. The mineralization fluids contain H 2 O, CO 2 (0-69 mole %), CH 4 (0-21 mole %), and trace N 2 , with wt percent NaCl equiv values in the range of 0 to 10.2 wt percent. Calculated delta 18 O values (15.0-6.8%) and measured delta D data (-88 to -99%) of the hypogene fluids indicate a meteoric origin. These fluids underwent extreme 18 O enrichment by reacting with country rocks (metamorphic or igneous). Genesis of the 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 to 60 m below the surface in and adjacent to major shears. Based on features such as the spatial distribution, high purity (999.26 fine), and the form of the gold and its close association with limonite, it is proposed that the gold is mobilized by surficial leaching and concentrated near the water table. Vertical movement of the water table, perhaps due to seasonal changes as well as erosion, produces rapid dissolution of hypogene gold and/or aurostibite from above or at the water table.

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