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Sedimentary exhalative (SEDEX) deposits formed in the Selwyn Basin coeval with 3 magmatic episodes in the Cambrian, Silurian and Late Devonian. At Macmillan Pass, the Tom, Jason and Nidd deposits are hosted by Late Devonian strata and have well preserved vent complexes beneath overlying stratabound sulphide and sulphate facies. Drill core has been sampled from the vent complexes at Nidd and Tom. Detailed petrography (reflected and transmitted light, cathodoluminescence, back-scatter electron imaging) has developed a paragenesis and, at Tom, critically assessed textural relationships between the vent facies and the overlying stratabound sulphide and sulphate facies. The mineral chemistry of major phases has been determined from quantitative electron microprobe analyses in the vent and stratabound ore from Tom. The geochemical signatures of the hydrothermal vents (Tom, Nidd) are comprehensively characterized through bulk rock geochemical analysis of vent facies material for major, minor and rare earth elements. Detailed petrography has highlighted subtle feeder veins that preferentially replaced laminations leading to the formation of pseudo-bedded sulphide-rich layers in the grey facies rocks at Tom. This provides an important link between the vent and the overlying stratabound facies. Within the vent, main stage sulphide minerals succeeded destructive carbonate alteration, and were overprinted by pulses of sulphide minerals coeval with, and replacing, siderite, Ba-carbonate and barite. Aside from the ore components (Pb, Zn and Ba), vent facies samples are enriched above upper crustal values in Au, Ag, As, Cd, Cu, Ge, In, Mo, Sb, Sn and Ta. Notably, Au in two samples from Tom exceeds 100 ppb. Rare-earth element profiles broadly track the composition of unmineralised shale in the area but have a positive Eu anomaly, likely attributable to the presence of Ba feldspar in the samples. This highlights the potential importance of the compositions of underlying strata in controlling the geochemistry of the hydrothermal fluids.

SEDEX mineralisation, Macmillan Pass (Yukon): petrography, mineralogy and bulk geochemistry of the Tom and Nidd deposits