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2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 4
Presentation Time: 8:45 AM

MODELLING MELTS IN HYDROTHERMAL SYSTEMS - THE LIQUID BISMUTH COLLECTOR MODEL


TOOTH, Blake Alfred1, BRUGGER, Joël2, LIU, Weihua3 and CIOBANU, Cristiana1, (1)School of Earth and Environmental Sciences, University of Adelaide, North Terrace, Adelaide, 5000, Australia, (2)South Australian Museum and Adelaide Univ, (previously: Earth Sciences Department, Monash Univ, Melbourne), North Terrace, Adelaide, 5000, Australia, (3)Exploration and Mining, CSIRO, c/o Building 28 School of Geosciences, Monash University, Clayton, 3168, Australia, blake.tooth@adelaide.edu.au

The Liquid Bismuth Collector Model (LBCM) was first outlined by Douglas et al. (2000). In this model Au is concentrated from undersaturated solutions by Bi melt. The system comprising an aqueous fluid and a Bi-Au binary melt is important because its study represents the first step in further understanding Bi rich polymetallic melts in hydrothermal environments. Bi-Te systems are prime candidates for deeper study and have shown even greater potential as scavengers of Au than Bi melts alone.

Bi has a low melting point (271.4°C) and the Au-Bi system has a eutectic temperature of 241.1°C. Bi also has a negative dP/dT relationship between solid and liquid phases resulting in melting point depression of 7.6°C per kbar increase in pressure. Thus Bi is a crucial component of polymetallic melts because it has a potentially large effect in keeping these systems molten down to low temperatures. It also makes the Bi-Au-fluid system within reach of laboratory studies without excessive technical difficulties. Therefore, we are also developing an experimental setup to test the results of this modelling study.

The importance of melts in ore deposit genesis is currently being re-appraised by some researchers, and the importance of the LBCM is that Bi melts are able to concentrate Au from fluids below levels currently considered the minimum for mineralisation. This model is also significant for the potential of the melts themselves to be derived from hydrothermal fluids, extending the model beyond remobilisation of pre-existing assemblages.

The results of a thermodynamic modelling study utilising the currently available properties for Au and Bi species and the HydroChemistry (HCh) package developed by Geoscience Australia is presented here. A non-random two liquids model, is used to simulate the interaction of an Au-Bi melt and hydrothermal fluids of varying compositions. Working retrospectively from the currently accepted binary phase diagram, thermodynamic properties for maldonite (Au2Bi) are fitted and concentrations of an equilibrated fluid are calculated. Depending on fluid composition, an Au-Bi melt of 5-10 Wt % Au can be expected from fluid concentrations of 0.04 – 0.5 ppb.

References Douglas N., Mavrogenes J., Hack A., England R., 2000, Geological Society of Australia, Abstracts No. 59, 15th Australian Geological Convention

Session No. 72

T70. Au-Ag-Te-Se Deposits and Other Precious Metal Deposits
Monday, 29 October 2007: 8:00 AM-12:00 PM
Geological Society of America Abstracts with Programs. Vol. 39, No. 6, p.196
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