DEPLETION AND ENRICHMENT ZONES IN THE BENDIGO GOLD FIELD: A POSSIBLE SOURCE OF GOLD AND IMPLICATIONS FOR EXPLORATION

Abstract

A regional lithogeochemical survey in the Bendigo region of central Victoria, Australia, covering an area of 3,750 km2 and including the giant Bendigo gold field and other large regional gold fields such as Maldon, Castlemaine, and Fosterville, was completed in 2004 by Ionex Pty Ltd. A total of 142 rock samples were collected and analyzed for gold and associated elements. Extensive areas having an anomalously low gold content (termed “depletion zones”) were detected in the vicinity of known gold fields and constitute a large proportion of the surveyed area. The results show that the gold content in the depletion zones is 70 percent below regional background levels. The Bendigo gold field itself is located within a positive gold anomaly (enrichment zone) with an average value of 14 ppb in all rock types and an extent of approx. 100 km2. To the north is a depletion zone on the order of 700 to 800 km2, with an average gold content of less than 0.5 ppb in all rock types. In total, the gold enrichment and depletion zones occupy an area of more than 1,000 km2 against a regional background of 0.7 to 4.7 ppb Au (avg 1.29 ppb). If this depletion zone extends to a depth of 2 km, the volume of depleted rock would be at least 1,400 km3 and the extent of apparent depletion would be 3,000 to 4,000 tonnes (t) of gold. The quantum of apparent gold depletion at Bendigo is on the same order of magnitude as the gold enrichment in the Bendigo gold field and associated dispersion haloes (3,000 t Au). Substantial depletion zones are also present adjacent to the other important gold deposits within the survey area. The presence of extensive gold depletion zones adjacent to the major gold deposits of central Victoria raises the possibility that these rocks were a source of gold in the deposits. These findings provide possible new criteria for the appraisal of prospective regions.