GEOCHRONOLOGY AND MINERALIZATION OF THE LIESE ZONES, POGO DEPOSIT, ALASKA

The Pogo deposit, located 145 km east of Fairbanks in east-central Alaska, consists of Au-As-Bi-Te mineralization in four or more stacked, sub-parallel, low-angle, shear zone-hosted, quartz veins (termed Liese zones) that cut Paleozoic and older gneiss and schist of the Yukon-Tanana terrane. The current resource estimate is 10.7 Mt at an average grade of 0.52 oz/t, for a resource of 5.6 Moz Au. Liese quartz veins contain ~3% sulfide minerals (arsenopyrite-pyrite-pyrrhotite), with the majority as linear bands that are sub-parallel to the vein-wallrock contact and define multiple fluid pulses. The bismuth-gold mineralogy includes (in order of abundance): joseite-B (Bi₄Te₂S), gold (fineness 850-1000), tetradymite (Bi₂Te₂S), pilsenite (Bi₄Te₃), native bismuth, bismuthinite (Bi₂S₃), Ingodite (BiTeS), hedleyite (Bi₇Te₃), sulphotsumoite (Bi₃Te₂S), joseite-A (Bi₄TeS₂), maldonite (Au₂Bi), tsumoite (BiTe), and baksanite (Bi₆Te₂S₃). Measured arsenic compositions of arsenopyrite vary from 30.4-37.2 at%; most are 35-37. The high-arsenic arsenopyrites occur with loellingite and pyrrhotite (T= 450-600^oC). The low-arsenic arsenopyrites occur with pyrite (T= <450^oC). Along with evidence for some arsenopyrites in disequilibrium with pyrite, and alteration of pyrrhotite to pyrite and marcasite, these data indicate a change from very low fS₂ and/or high T (loellingite-pyrrhotite stable) to higher fS₂ and/or lower T (pyrite stable) over time. Fluid inclusions in quartz from the main Liese veins are low salinity, liquid- and vapor-rich aqueous (T_h= 174-247^oC; 2-5 wt% NaCl eq.), saline aqueous (T_h= 188-348^oC; 14-18 wt% NaCl eq.), and aqueous-carbonic (T_h= 140-340^oC; 3-46 mol% CO₂>>CH₄). Several extension veins that splay off of the Liese veins contain more saline inclusions (T_h= 175-328^oC; 30-34 wt% NaCl eq.), with one or two daughter minerals and coexisting inclusions that exhibit properties of pure CH₄. (Note=T_h is uncorrected for pressure). MoS₂ from the Liese vein has a Re/Os age of 104 Ma, which is 5-12 Ma younger than the age of last metamorphism (U-Pb, zircon) and constrained by Ar/Ar (hornblende) and U-Pb (zircon) ages of nearby intrusions (~107 Ma and 95-92 Ma). Biotites from intrusions, gneiss, and wall rock in quartz yield Ar/Ar ages of ~92 Ma, presumably due to thermal reset from the former intrusions.