Download files
Abstract
Orogenic Au deposits hosted by Woods Point Dyke Swarm (WPDS) have been a significant contributor to Au-production in the Victorian Gold Province since production began in 1861. However, the coarse nature of Au-mineralisation coupled with the heterogeneous distribution of high Au-grade zones renders exploratory drilling ineffectual and dictates exploration focus on delineating a probable host structure rather than investigating the vein-hosted mineralisation directly. This study investigates the occurrence and distribution of Au-mineralisation within the veins to improve the understanding of the genesis of Au. The application of these findings is demonstrated via the proposal of geochemical and textural vectors to Au-mineralisation in the WPDS
This study investigates the geology, geochemistry and nature of Au-mineralisation within the Morning Star and Rose of Denmark Mines. The work involved integration of alteration geochemistry, textural and microstructural investigation of veins, baro-acoustic decrepitation, microthermometry, Laser Raman Spectroscopy, ore mineral petrography and ore mineral chemistry, in order to describe the nature of ore fluids and the processes that have modified them in this system. The veins selected for analysis are based on historical observations of potential high Au-grade zones and were assessed based on their differing morphologies in order to delineate changes in the physico-chemical processes inducing Au-precipitation.
The fineness of electrum is relatively low and variable, in the context of orogenic Au, reflecting transport of Au via bisulfide complexes. The elemental association of Au and Bi points to a magmatic source of Au and implicates Bi in the scavenging and transport of Au in solution.
The auriferous fluids are metamorphic in composition, with magmatic affinities. Fluid inclusion analysis reveals the presence of low salinity, CO2-rich fluids that homogenise within a range of 190oC-320oC. Petrographic observations fluid inclusion assemblages along with CO2 profiles of
veins documented by baro-acoustic decrepitation of inclusions and microthermometry preserve evidence for phase separation within fluids during quartz formation. Laser Raman Spectroscopy reveals the presence of abundances of N2 and CH4 within volatile phases of inclusions, indicative of interaction with carbonaceous slates. It follows that Au-mineralisation in the WPDS is largely controlled by the rheology of the ore fluids, specifically, via phase separation or structurally controlled mixing of co-genetic fluids.
This study proposes that improved evaluation of LILE systematics within alteration halos, fluid inclusion characterisation and textural analysis of quartz veins presents a robust measure of the potential of Au-mineralisation within dyke hosted veins in the WPDS.