Hydrothermal fluids dissolve and precipitate vast quantities of carbonate during the formation of Carlin-type gold deposits. This mass transfer results in extensive volumes of decarbonatized rock and hydrothermal calcite with distinct δ18O and δ13C signatures, occurring as both veins and wall-rock pseudomorphs. However, the processes which cause the fluids to switch between carbonate-dissolving and carbonate-precipitating are not well understood. Here, we present a model for the formation of ore-stage calcite through the pseudomorphic replacement of pre-existing calcite based on a detailed study of UV-fluorescent (UVF) calcite veins from the Nadaleen trend Carlin-type gold deposits in Yukon. The UV fluorescence in the veins is a visual indicator of their high-Mn, low-Fe chemistry, which we use to develop our model.
The proximity of UVF calcite veins to ore-stage alteration and the realgar within the veins indicate that they formed during Au mineralization. Vein orientation and crosscutting relationships with faults and dikes suggest they formed by replacing pre-existing calcite veins that developed during folding. Cathodoluminescence responses, showing the selective replacement of pre-ore calcite by Mn-rich calcite, support this hypothesis. We propose that differences in Mn, Sr, and Mg content between pre-existing calcite and UVF calcite drive this replacement reaction as the fluids approach calcite saturation.
UVF calcite veins have fluid-dominated δ13C and δ18O compositions, suggesting that the fluids dissolved fluid-buffered hydrothermal carbonate upstream along the flow path. In our model, unreactive and high-permeability decarbonatized rock allows upstream fluids to reach reactive calcite at the edge of the dissolution zone, where they replace the calcite with ore-stage calcite pseudomorphs. Over several iterations of this process, the fluids repeatedly precipitate and dissolve ore-stage calcite, moving it downstream as a roll front, which concentrates Mn at the edge of the dissolution zone. We suggest that this mass transfer also occurs at the deposit scale to form zoned carbonate alteration along the flow path.
Additionally, we present carbonate clumped isotope thermometry results that indicate UVF calcite veins formed at temperatures of ~140°C, although higher temperatures of up to 300°C are possible if the veins exhumed slowly after their formation. Calculated fluid δ13C and δ18O values at these temperatures suggest a meteoric fluid source, although the isotopic data are not inconsistent with a magmatic source if fluid temperatures were ≥200°C because less isotopic fractionation occurs at higher temperatures.