Zinc-lead skarn mineralization at Tin Creek, Alaska: Fluid inclusions and skarn-forming reactions

Abstract

The Tin Creek Zn-Pb skarn in Farewell District, Alaska, is zoned from predominantly a garnet skarn with abundant chalcopyrite and stockwork quartz veining in the northern part (MW zone) to a pyroxene skarn dominated by sphalerite in the southern part (TC1 zone). Microthermometric analysis of primary, 2-phase (liquid + vapor), NaCl-H 2O fluid inclusions in quartz and calcite from the skarn indicates a corresponding variation in homogenization temperature (Th) and fluid salinity: higher Th (mostly >280°C) and salinity (4.2–6.7 wt% NaCl equiv.) in the MW zone, and lower Th (mostly <320°C) and salinity (2.5–5.0 wt% NaCl equiv.) in the TC1 zone. The MW zone is interpreted as proximal to the heat source, probably a hidden igneous intrusive, and the TC1 zone as a relatively distal facies of the same skarn-forming event. Development of the skarn zonation likely resulted from thermal gradient, progressive fluid-rock reactions, and continuous interaction and dilution of hydrothermal fluids along fluid flow path. Numerical modeling of skarn-forming reactions, by titrating an impure limestone (90% calcite and 5% each of quartz and kaolinite) into an initial fluid composition constrained by fluid inclusion and mineralogie data, produces a final mineral assemblage comparable to that at Tin Creek. The calculation demonstrates that a complete sequence of skarn assemblages forms during progressive equilibration of a very small amount of limestone with the infiltrating fluid. The model also predicts an extremely small concentration of CO 2 in the skarn-forming fluid, in agreement with the fluid inclusion data from Tin Creek and other skarn deposits. This suggests that CO 2 escape is not a significant factor facilitating sulfide precipitation during skarn formation.