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Origin of mixed brine-sulphide inclusion trails from broken hill new south wales investigated by LA-ICP-MS
Goldschmidt Conference Abstracts 2006
A703
Origin of mixed brine-sulphide inclusion trails from broken hill new south wales investigated by LA-ICP-MS
An experimental study of the solubility of gold in crude oil: Implications for ore genesis
P.J. WILLIAMS1, L.A. FISHER2, B.W.D. YARDLEY2, L. FORBES2
A.E. WILLIAMS-JONES, A.A. MIGDISOV
1
School of Earth Sciences, James Cook University, Townsville 4811, Australia ([email protected]) 2 School of Earth Sciences, University of Leeds, Leeds LS2 9JT, United Kingdom Complex assemblages of secondary inclusions occur in quartz from Western A Lode which is part of the giant Pb–Zn–Ag deposit at Broken Hill. They are of interest in the light of recent speculation that sulphide melts and aqueous fluids evolved from them could have been generated during the transitional amphibolite– granulite conditions of peak metamorphism. Most inclusion trails consist exclusively of either sulphides galena ± pyrrhotite or moderate to high salinity aqueous brine. However, composite trails are also present containing both aqueous and sulphide types, along with mixed inclusions in which sulphides are enveloped by aqueous liquid ± salts in varying proportions. The brine inclusions have variable salinities (28– 48 wt% NaClequiv.) and homogenisation temperatures (>150 °C). Those with salinities > 33 wt% NaClequiv. typically decrepitate without total homogenisation at temperatures up to 480 °C. LA-ICP-MS data indicate that the aqueous brine inclusions have K P Na, Mn Fe, high Pb and Zn, and Pb/Zn ratios approximately an order of magnitude higher than published data for high temperature crustal brines. Sulphide inclusions have Pb/ Zn and Pb/Fe ratios much higher than those of the brines, and Pb:Zn:Cu:Fe ratios that are much more variable than their Pb:Ag:Sb ratios. Most of the sulphide inclusions are strongly Fe- and Zn-depleted compared to published experimental sulphide eutectic compositions. The best explanation of these data is that the sulphide inclusions do not represent melts but were precipitated from aqueous solution, possibly through a mixing process involving brines with different salinities and temperatures. Final sulphide emplacement in this particular quartz-rich lode type is interpreted to have been a retrograde hydrothermal process. The distinctive Pb- and Znrich and very high Pb/Zn aqueous brine signature could ultimately be related to synmetamorphic sulphide melting. However, such a fluid would have had to have been retained in the system until it were able to interact with cool external fluids at temperatures well below those of the metamorphic peak. doi:10.1016/j.gca.2006.06.1525
Department of Earth and Planetary Sciences, McGill University, Montreal, Canada ([email protected]) Hydrocarbons occur with gold in several ore deposit types. For example, in the Witwaterstrand basin, over 40% of gold production has come from carbon seams, which are now widely interpreted to represent migration paths of liquid hydrocarbons. Moreover, liquid hydrocarbons are preserved as fluid inclusions in overgrowths on detrital quartz grains in the siliciclastic rocks, suggesting that oil migration was an essential part of the early history of the basin. In Nevada, many of the Carlin gold deposits contain bitumen and in some cases liquid hydrocarbons. As in the Witwaterstrand, there is commonly a close spatial association between the gold and the hydrocarbons, with both occupying the same structures. In some cases, there is even a demonstrable link between the gold deposits and producing oil fields. Where a genetic association between gold and hydrocarbons has been proposed, the role generally attributed to the hydrocarbons is in the deposition of the gold. However, it is attractive to speculate, as some others have done, that liquid hydrocarbons may also play a role in the transport of this metal. In order to investigate the possibility that liquid hydrocarbons could constitute a gold-depositing ore fluid, we conducted experiments aimed at determining the solubility of gold in crude oil at temperatures of 150 and 200 °C. The experiments were performed in titanium autoclaves with the metal being introduced as gold wire. The autoclaves were heated in an IsoTemp Fisher oven for 3 to 5 days, depending on the temperature of the experiment. After equilibration, the autoclaves were quenched to room temperature and a sample of crude oil was collected for analysis. Gold precipitated on the walls of the autoclaves during quenching was dissolved by aqua regia after removal of the crude oil from the autoclave. Concentrations of gold in the oils and the washing solutions were determined using neutron activation analysis. Experiments were conducted on crude oil from two localities. Both oils dissolved similar concentrations of gold but, whereas one of the oils deposited most of its gold on quenching, a substantial proportion of the gold remained dissolved in the other oil. The solubility of gold in the crude oil averaged 9 ppm at 150 °C and 14 ppm at 200 °C. The results of these preliminary experiments show that liquid hydrocarbons are capable of transporting ore level and higher concentrations of gold. While this does not necessarily mean that they played this role in the deposits containing them, it leaves open this possibility and also the possibility that hydrocarbons could remobilize existing gold deposits. doi:10.1016/j.gca.2006.06.1526
A703
Origin of mixed brine-sulphide inclusion trails from broken hill new south wales investigated by LA-ICP-MS
An experimental study of the solubility of gold in crude oil: Implications for ore genesis
P.J. WILLIAMS1, L.A. FISHER2, B.W.D. YARDLEY2, L. FORBES2
A.E. WILLIAMS-JONES, A.A. MIGDISOV
1
School of Earth Sciences, James Cook University, Townsville 4811, Australia ([email protected]) 2 School of Earth Sciences, University of Leeds, Leeds LS2 9JT, United Kingdom Complex assemblages of secondary inclusions occur in quartz from Western A Lode which is part of the giant Pb–Zn–Ag deposit at Broken Hill. They are of interest in the light of recent speculation that sulphide melts and aqueous fluids evolved from them could have been generated during the transitional amphibolite– granulite conditions of peak metamorphism. Most inclusion trails consist exclusively of either sulphides galena ± pyrrhotite or moderate to high salinity aqueous brine. However, composite trails are also present containing both aqueous and sulphide types, along with mixed inclusions in which sulphides are enveloped by aqueous liquid ± salts in varying proportions. The brine inclusions have variable salinities (28– 48 wt% NaClequiv.) and homogenisation temperatures (>150 °C). Those with salinities > 33 wt% NaClequiv. typically decrepitate without total homogenisation at temperatures up to 480 °C. LA-ICP-MS data indicate that the aqueous brine inclusions have K P Na, Mn Fe, high Pb and Zn, and Pb/Zn ratios approximately an order of magnitude higher than published data for high temperature crustal brines. Sulphide inclusions have Pb/ Zn and Pb/Fe ratios much higher than those of the brines, and Pb:Zn:Cu:Fe ratios that are much more variable than their Pb:Ag:Sb ratios. Most of the sulphide inclusions are strongly Fe- and Zn-depleted compared to published experimental sulphide eutectic compositions. The best explanation of these data is that the sulphide inclusions do not represent melts but were precipitated from aqueous solution, possibly through a mixing process involving brines with different salinities and temperatures. Final sulphide emplacement in this particular quartz-rich lode type is interpreted to have been a retrograde hydrothermal process. The distinctive Pb- and Znrich and very high Pb/Zn aqueous brine signature could ultimately be related to synmetamorphic sulphide melting. However, such a fluid would have had to have been retained in the system until it were able to interact with cool external fluids at temperatures well below those of the metamorphic peak. doi:10.1016/j.gca.2006.06.1525
Department of Earth and Planetary Sciences, McGill University, Montreal, Canada ([email protected]) Hydrocarbons occur with gold in several ore deposit types. For example, in the Witwaterstrand basin, over 40% of gold production has come from carbon seams, which are now widely interpreted to represent migration paths of liquid hydrocarbons. Moreover, liquid hydrocarbons are preserved as fluid inclusions in overgrowths on detrital quartz grains in the siliciclastic rocks, suggesting that oil migration was an essential part of the early history of the basin. In Nevada, many of the Carlin gold deposits contain bitumen and in some cases liquid hydrocarbons. As in the Witwaterstrand, there is commonly a close spatial association between the gold and the hydrocarbons, with both occupying the same structures. In some cases, there is even a demonstrable link between the gold deposits and producing oil fields. Where a genetic association between gold and hydrocarbons has been proposed, the role generally attributed to the hydrocarbons is in the deposition of the gold. However, it is attractive to speculate, as some others have done, that liquid hydrocarbons may also play a role in the transport of this metal. In order to investigate the possibility that liquid hydrocarbons could constitute a gold-depositing ore fluid, we conducted experiments aimed at determining the solubility of gold in crude oil at temperatures of 150 and 200 °C. The experiments were performed in titanium autoclaves with the metal being introduced as gold wire. The autoclaves were heated in an IsoTemp Fisher oven for 3 to 5 days, depending on the temperature of the experiment. After equilibration, the autoclaves were quenched to room temperature and a sample of crude oil was collected for analysis. Gold precipitated on the walls of the autoclaves during quenching was dissolved by aqua regia after removal of the crude oil from the autoclave. Concentrations of gold in the oils and the washing solutions were determined using neutron activation analysis. Experiments were conducted on crude oil from two localities. Both oils dissolved similar concentrations of gold but, whereas one of the oils deposited most of its gold on quenching, a substantial proportion of the gold remained dissolved in the other oil. The solubility of gold in the crude oil averaged 9 ppm at 150 °C and 14 ppm at 200 °C. The results of these preliminary experiments show that liquid hydrocarbons are capable of transporting ore level and higher concentrations of gold. While this does not necessarily mean that they played this role in the deposits containing them, it leaves open this possibility and also the possibility that hydrocarbons could remobilize existing gold deposits. doi:10.1016/j.gca.2006.06.1526