The importance of CO 2 on freezing point measurements of fluid inclusions; evidence from active geothermal systems and implications for epithermal ore deposition

Abstract

In studies of epithermal precious and base metal ore deposits, estimates of salinity (total dissolved salts) are frequently in error when based on fluid inclusion ice melting measurements in the absence of an independent determination of the CO2 content of the inclusion fluid. For a fluid of known composition, the melting point of ice (Tin) may be calculated from Tm= --• Kimi where Ki is the molal freezing (or melting) point depression constant and mi is the molality of a component i (i = Na +, K +, CI-, CO2, etc.). For fully dissociated solute species such as CI-, in the salinity range considered here, K = 1.72 Kelvin/molal, and for undissociated, nonpolar species such as CO•, K --1.86 Kelvin/molal. Fluid inclusion ice-melting data from New Zealand geothermal fields correlate well with values calculated using the above equation and the measured compositions of discharges from wells from which the inclusion samples were obtained. Loss of the dominant dissolved gas, CO•, during boiling at depth results in large, systematic decreases in apparent salinity (in terms of Tm) in the Broadlands field. Misinterpretation of fluid inclusion freezing data may lead to substantial errors in the reconstruction of the physico-chemical environment of ore formation in fossil systems. For example, in the absence of CO• analyses, inclusion fluids similar in gas content to the Broadlands geothermal fluid (NaC1 = 0.2 wt %, CO2 up to 4.4 wt %) may be interpreted to have salinities of 0.85 wt percent NaC1, leading to errors of 23 percent in the estimated depth of formation at 280øC, -0.5 unit in the estimated pH of the ore fluid, and on the order of 200 times in the estimated solubility of an ore component such as lead. Such errors may be transmitted into subsequent estimation of fluid flux or duration of ore formation.A review of published fluid inclusion studies shows that true salinities of inclusion fluids from epithermal precious metal deposits are coincident with those from the majority of explored geothermal systems but that the salinities of inclusion fluids from epithermal base metal deposits are substantially higher, indicating the presence of an evolved, high-salinity fluid during ore metal transport. In the case of low-salinity epithermal deposits, the extreme range in gold to silver ratios between deposits can be accounted for by variations in the H•S to chloride ratio in the original hydrothermal fluid. These variations (primarily in the total gas) occur between active geothermal systems and can be recognized in epithermal deposits through fluid inclusion freezing and crushing studies.

Mixing diagrams for hydrothermal solutions: Applications to interpretation of fluid inclusion data of hydrothermal ore deposits in Japan.

“…It should be noted that equations (7) and (8) implicitly exclude boiling in an open system, and that the homogenization tempera ture is equal to temperature of the solution. Simultaneous cooling and dilution during boil ing in an open system has been discussed by Hedenquist and Henley (1985), and is not treated in this paper.…”

Shallow-level, late-stage gold mineralisation in Sawyers Creek, Shotover valley, northwest Otago, New Zealand

“…The CO 2 liquid and vapour homogenise in these inclusions near 26°C, but the small size precludes accurate determination of homogenisation temperature. Freezing-point depressions in the aqueous inclusions are almost certainly due to dissolved CO 2 (Hedenquist & Henley 1985a). Co-existence of inclusions with widely varying fluid composition suggests that the fluids were immiscible during entrapment Such immiscibility allows estimates of both temperature and pressure to be obtained from fluid-inclusion data.…”

Processos metalogenéticos em ambientes de arco magmático tipo andino, caso de estudo: mineralizações da região dos Andes Patagônicos setentrionais do Chile

“…This relation could represent the immiscibility of an originally aqueous carbonic fluid with consequent gas loss, that is, with CO 2 partitioning to the gas phase. As discussed by Hedenquist and Henley (1985a), the immiscibility and preferential loss of the gas phase to the bubble, in aqueous system with some quantities of CO 2 result in a small decrease of temperature accompanied by a great decrease in the apparent salinities. In this case the boiling trend seems to be strongly masked by the mixing trend.…”

“…Hedenquist and Henley (1985a) showed that in most epithermal deposits CO 2 is present as a dissolved gas in fluid inclusions, sometimes in small not visible proportions, and also discussed the effect of these discrete CO 2 concentrations on melting point depression. On the basis of the low-temperature phase relations in the H 2 O-CO 2 system Hedenquist and Henley (1985a) and Diamond (2003),…”

“…Thus, hydrostatic pressure conditions are assumed based on the presence of hydrothermal fracturing and in the fact that in the upper one to two kilometers of the crust, the pressure of hydrothermal fluids is generally hydrostatic (Hedenquist and Henley, 1985a). Hass (1971) calculated the effect of salinity on the temperature-depth relations of a brine of constant composition at hydrostatic pressure.…”