STRATABOUND SRHDG RESOURCE IN MIDDLE PROTEROZOIC ROCKS OF THE BIG BELT MOUNTAINS NEAR YORK, MONTANA

The York prospect contains small load Au deposits (2t), Au placers (8 t) and a SRHDG resource containing ~200t Au at grades > 0.3 g/t. It covers a 13x1 km area along strike of the middle Proterozoic Greyson Fm. (Prichard equiv., ~1470 Ma) and is near the northern boundary of the Helena embayment of the Belt Basin where growth faults, mafic dikes and sills, and basemetal sedex deposits are known. Regionally, the rocks are very low metamorphic grade (1md illite) and are cut by L. Cret.-E. Tert. thrust faults and granitic intrusions. Au is restricted to K-spar rich beds (~7 wt % K20), termed "reefs", that transect bedding at small angles and differ from the dolomitic silty mudstones that comprise the Greyson Fm. Ten reefs 1 to 30m thick are known over a stratigraphic interval of 550m. The interval thickens to the west, contains fining upward sequences, mudchip debris flows, soft sediment slumps, and small scale normal faults that suggest sedimentation in a half graben. Silty mudstones are zoned laterally from proximal hard K-spar reefs, to tan soft porous beds, to hard greenish beds, to distal gray beds. Silty mudstones are replaced by orthoclase (non-luminescent), albite, quartz, Fe-dolo, and pyrite, contain 1-5 mm zoned dolomite crystals, and trace cpy, sph, gal, tenn, and aspy. Au is inferred to reside in disseminated pyrite, because native Au is only found in oxide ores. Brittle reefs are cut by 1-5mm extensional Fe-dolo, quartz, pyrite, +/-K-spar, +/-barite veinlets that are concordant and discordant to bedding and by later milky quartz veinlets with secondary 3-phase H20-CO2 inclusions. Pyrite in the dolo-quartz veinlets contains inclusions of galena and Au–Ag tellurides. Reefs have Au/Ag<1, are enriched in K, Na, Si, S, Au, Ag, Pb, Te, Cr, but lack As, Sb, Hg. Reef K-spar has uniform d18O of ~10‰ over 11 km suggesting lateral T gradients were small. Dolo crystals, veinlet dolo and galena, and reef K-spar have radiogenic 87Sr/86Sr >=0.719 and lie along an array suggesting that Pb was derived from a single source similar to Belt rocks. U-Pb and Rb/Sr data on these samples are suggestive of a Mesozoic age. Impure reef K-spar yielded a K-Ar age of 89±3.2 Ma. Apparently, the K-spar reefs formed from Au- and H2S-rich fluids of uncertain origin that moved in response to Cretaceous deformation and/or magmatism. The dolomite-quartz-pyrite veinlets formed during subsequent extension.