Geochemical Constraints on the Genesis of the Scheelite Dome Intrusion-Related Gold Deposit, Tombstone Gold Belt, Yukon, Canada

Abstract

The Scheelite dome intrusion-related gold deposit, western Selwyn basin, Yukon, is hosted in hornfelsed metasedimentary strata that lie adjacent to the exposed apices of a monzogranite to quartz monzonite plutonic complex of the mid-Cretaceous Tombstone-Tungsten magmatic belt, Tintina gold province, Alaska and Yukon. A variety of mineralization styles occur throughout a 10- × 3-km east-trending corridor and include reduced Au- and W-rich skarns, Au-, W- and Ag-Pb-Zn-Sb–rich quartz tension-vein arrays, and multiphase fault veins and isolated zones of Au-rich sericite-carbonate altered rock. Integrated U-Pb SHRIMP data for magmatic zircon and Ar-Ar data for magmatic and hydrothermal biotite indicate that gold mineralization occurred within 1 to 2 m.y. of magma emplacement. Fluid inclusion, oxygen isotope, and arsenopyrite geothermometry data indicate that hydrothermal minerals formed at depths of 6 to 9 km over a temperature range from 550°C. High-temperature Au-rich skarns formed at >400°C, whereas vein-hosted mineralization formed at 280° to 380°C. In skarns, Au is strongly associated with enrichments of Bi, Te, W, and As, whereas a variety of Au-rich veins occur, with As-rich (type 1), and Te- and W-rich (type 2) end members. Silver-Pb-Zn-Sb veins are typically Au poor and represent the latest and lowest temperature phase in the hydrothermal paragenesis. The fluid inclusion data indicate that all mineralization styles were formed from low-salinity (≤4 wt % NaCl equiv) aqueous-carbonic fluids, consistent with the composition of fluid inclusions within infilled miarolitic cavities in the intrusive rocks. However, the nonaqueous fluid was predominantly CH 4 in skarn, CO 2 in Au-Te and Au-W veins, and a fluid with roughly equal amounts of CO 2 , CH 4 , and N 2 in Au-As and Ag-Pb-Zn-Sb veins. Oxygen isotope data are consistent with a mineralizing fluid of predominantly magmatic origin that was variably modified to more positive δ 18 O values during interaction with 18 O-enriched metasedimentary strata. Sulfur isotope data suggest two possible sources of sulfur, a magmatic source characterized by δ 34 S values of approximately −5 to 0 per mil and sulfur from the metasedimentary country rocks characterized by more negative δ 34 S values of approximately −15 to −10 per mil. Collectively the data indicate that gold at Scheelite Dome was deposited from a magmatic-hydrothermal system. Interaction of magmatic fluids with graphitic hornfels rocks resulted in reduction of the ore fluids, higher CH 4 /CO 2 ratios, and modification of the oxygen and sulfur isotope values of the ore fluids toward those of the metasedimentary hornfels. Progressive reduction and cooling of hydrothermal fluids, in addition to phase separation in vein-hosted mineralization, were the mechanisms for gold deposition. Compared to other intrusion-related gold deposits associated with the Tombstone-Tungsten magmatic belt magmatism, exposed mineralization at Scheelite Dome is predominantly hosted by hornfelsed metasedimentary rocks. This results in more diverse mineralization styles and a greater spread of isotope and fluid inclusion data.