Fluid and chemical constraints on sphalerite precipitation in the Boundary Zone Zn-Pb prospect, Yukon, Canada

Sphalerite (ZnS) is the main ore mineral in clastic-dominant (CD-type) massive sulfide deposits. However, the precise physicochemical conditions of ore formation are often poorly constrained. This study uses sphalerite mineral chemistry and fluid inclusion microthermometry to constrain conditions of Zn mineralization at the Boundary Zone prospect of Macmillan Pass district, Canada.

The sulfide mineralization comprises pyrite, sphalerite, galena, and minor chalcopyrite. Sphalerite with contrasting trace element compositions is hosted in Late Ordovician-Early Silurian (Duo Lake Formation) and Late Devonian (Portrait Lake Formation) black mudstones. Different paragenetic stages of sphalerite formation preserve distinct trace element patterns within and between host rock intervals, and overall, Ge, Ga, Cu, and Cd, are relatively enriched compared to In. Trace element incorporation mechanisms vary, with both direct and coupled substitution (e.g., 3Zn²⁺ ↔ (Ge)⁴⁺ + 2Cu⁺) pathways suggestive of compositional and fluid temperature differences during sphalerite precipitation.

Homogenization temperatures (T_h) of CO₂-N₂-bearing, 2-phase primary aqueous fluid inclusions in Portrait Lake sphalerite range between 154 – 249°C (median= 179°C). The T_h values of quartz-hosted CO₂-N₂-H₂S-CH₄-bearing aqueous inclusions from late veins are in the range of 207 – 236°C (median= 223°C). These temperatures are consistent with sphalerite trace element geothermometry (GGIMFis; 163 – 279°C) and are comparable to nearby Tom and Jason CD-type deposits in the Macmillan Pass district.