GEOCHEMISTRY OF FLUID INCLUSIONS FROM OROGENIC AND INTRUSION-RELATED GOLD DEPOSITS IN THE TINTINA GOLD PROVINCE, ALASKA AND THE YUKON TERRITORY

Microthermometry, noble gas, ion chromatography, and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of fluid inclusions (FI) in quartz from orogenic and intrusion-related (IR) gold deposits of the Tintina Gold Province (TGP), Alaska and the Yukon Territory have defined the temperature, pressure and composition of the fluids responsible for the precious metal mineralization. Ore forming fluids are generally gas-rich, low to moderate salinity, and formed at temperatures of 290-380oC, and pressures of 1-2 kilobars. Helium-isotope ratios have R/Ra values that varied from <0.1 for Pogo to >15 for Clear Creek. Preliminary He data suggest that deposits in the Yukon (Clear Creek, Scheelite Dome) have a definite mantle contribution, whereas ore-forming fluids in the TGP deposits sampled in Alaska (Fort Knox, Pogo, Donlin Creek) are dominated by He with a crustal origin. The 40Ar/36Ar ratios from Donlin Creek are close to those for air (295.5) and indicate contamination by air-saturated groundwaters in this epizonal ore deposit. The other gold deposits have typical ratios of approximately 500, suggesting a consistent gas source added from the crust or mantle. Most of the deposits have magmatic Na-Cl-Br values whereas Donlin Creek is nonmagmatic with a Na/Cl ratio >7. Despite the small (5-10 micron) size and dilute nature of the FIs several major and minor elements were detected by LA-ICP-MS. Fluid trapped in the inclusions from the Fort Knox IR quartz veins vary in concentration from 11-3426 ppm As, 10-5056 ppm Sb, 579-2220 ppm Cu, 927-2228 ppm Zn, 258-350 ppm Ag, and 61-497 ppm Pb. Similar ranges of values characterize the FIs from sheeted veins hosted by stocks at Scheelite Dome and Clear Creek. Analysis of FIs from the orogenic Pogo deposit and IR Shotgun prospect showed no obvious distinction from those that typified the IR deposits. At Donlin Creek, FIs exhibited highly anomalous concentrations of 4.1% As and 1.6% Fe, as well as 944-3378 ppm Sb, though samples from the adjacent sub-economic Dome property were 103 to 372 ppm As, 0.7-1.7% Fe, and 24-100 ppm Sb. The noble gas isotope, Na-Cl-Br, As, Fe, and Sb data show that the orogenic Donlin Creek deposit is distinct from the IR deposits in the TGP and suggest it formed at a shallower depth, from nonmagmatic fluid stemming from different crustal source rocks.