Abstract
Analysis of fluid and melt inclusions from arc-related intrusions and porphyry copper deposits (PCD) reveal that many fluid inclusions from PCD are typically characterized by XCO2 < 0.10, which is lower than that found in volatile phases exsolved from shallow (e.g., 5 to 10 km), arc magmas, in general (XCO2 up to order ~ 0.45). This disparity remains to be resolved.
The efficiency with which copper can be removed from arc magmas into exsolving volatile phases is a function of the competition between crystalline phases (± liquid sulphides), and the exsolving vapor ± brine. Experiments in melt-vapor-brine systems permit the investigation of the partitioning of copper between silicate melts and volatile phases under magmatic conditions. However, the effect of CO2 on melt-volatile phase equilibria relevant to the formation of PCD has remained unconstrained. In this study, the partitioning of copper in CO2-bearing, sulfur-free and sulfur-bearing, experiments may provide additional insights into copper partitioning and the generation of PCD.
We present results from experiments performed at 800 oC and 100 MPa in CO2-bearing, sulfur-free and sulfur-bearing melt-vapor-brine systems with XCO2 (bulk vapor ± brine) = 0.10 and 0.38. The compositions of vapor and brine inclusions and run-product glasses were used as proxies for the compositions of the magmatic phases. The salinities of vapor inclusions that nucleated clathrate (CO2 ± H2S clathrate) upon cooling were determined via Raman analysis and microthermometry [1]. The partitioning of copper between brine and vapor (Db/vCu(±2σ)) increases from 25(±6) to 100 (±30) for sulfur-free experiments and from 11(±3) to 95(±23) for sulfur-bearing experiments, as XCO2 is increased from 0.10 to 0.38. The partitioning of copper between vapor and melt increases with the addition of sulfur at XCO2 = 0.10: (Dv/mCu (±2σ)) = 9.6(±3.3) (sulfur-free, metaluminous melt); 18(±8) (sulfur-bearing, peralkaline melt); and 30(±11) (sulfur-bearing, metaluminous melt). These values are to be contrasted with (Dv/mCu (±2σ)) = 2(±0.8) at XCO2 = 0.38 (the effect of sulfur cannot be distinguished at this mole fraction of CO2). These data demonstrate that changes in the salinity of the vapor and brine, which are controlled by changes in XCO2, play a major role in controlling copper partitioning in sulfur-free, CO2-bearing systems. Sulfur-bearing experiments demonstrate that magmatic vapors are enriched in copper in the presence of sulfur at low XCO2. However, the enrichment of copper in the magmatic vapor is suppressed for sulfur-bearing systems at high XCO2. These data indicate that the efficient removal of copper from silicate melts into vapor ± brine is mitigated by high concentrations of CO2. Furthermore, the poisoning effect of CO2 is more pronounced for sulfur-bearing volatile phases. As a result, high concentrations of CO2 may play a negative role in the formation of PCD.
The efficiency with which copper can be removed from arc magmas into exsolving volatile phases is a function of the competition between crystalline phases (± liquid sulphides), and the exsolving vapor ± brine. Experiments in melt-vapor-brine systems permit the investigation of the partitioning of copper between silicate melts and volatile phases under magmatic conditions. However, the effect of CO2 on melt-volatile phase equilibria relevant to the formation of PCD has remained unconstrained. In this study, the partitioning of copper in CO2-bearing, sulfur-free and sulfur-bearing, experiments may provide additional insights into copper partitioning and the generation of PCD.
We present results from experiments performed at 800 oC and 100 MPa in CO2-bearing, sulfur-free and sulfur-bearing melt-vapor-brine systems with XCO2 (bulk vapor ± brine) = 0.10 and 0.38. The compositions of vapor and brine inclusions and run-product glasses were used as proxies for the compositions of the magmatic phases. The salinities of vapor inclusions that nucleated clathrate (CO2 ± H2S clathrate) upon cooling were determined via Raman analysis and microthermometry [1]. The partitioning of copper between brine and vapor (Db/vCu(±2σ)) increases from 25(±6) to 100 (±30) for sulfur-free experiments and from 11(±3) to 95(±23) for sulfur-bearing experiments, as XCO2 is increased from 0.10 to 0.38. The partitioning of copper between vapor and melt increases with the addition of sulfur at XCO2 = 0.10: (Dv/mCu (±2σ)) = 9.6(±3.3) (sulfur-free, metaluminous melt); 18(±8) (sulfur-bearing, peralkaline melt); and 30(±11) (sulfur-bearing, metaluminous melt). These values are to be contrasted with (Dv/mCu (±2σ)) = 2(±0.8) at XCO2 = 0.38 (the effect of sulfur cannot be distinguished at this mole fraction of CO2). These data demonstrate that changes in the salinity of the vapor and brine, which are controlled by changes in XCO2, play a major role in controlling copper partitioning in sulfur-free, CO2-bearing systems. Sulfur-bearing experiments demonstrate that magmatic vapors are enriched in copper in the presence of sulfur at low XCO2. However, the enrichment of copper in the magmatic vapor is suppressed for sulfur-bearing systems at high XCO2. These data indicate that the efficient removal of copper from silicate melts into vapor ± brine is mitigated by high concentrations of CO2. Furthermore, the poisoning effect of CO2 is more pronounced for sulfur-bearing volatile phases. As a result, high concentrations of CO2 may play a negative role in the formation of PCD.
| Original language | English |
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| Publication status | Published - 2012 |
| Event | Goldschmidt - Montreal, Canada Duration: 24 Jun 2012 → 29 Jun 2012 |
Conference
| Conference | Goldschmidt |
|---|---|
| Country/Territory | Canada |
| City | Montreal |
| Period | 24/06/12 → 29/06/12 |