TIMING OF MIDDLE PROTEROZOIC MINERALIZATION AND MAGMATISM IN THE HELENA EMBAYMENT OF THE BELT BASIN

The Middle Proterozoic Newland Formation in the Helena Embayment (Lower Belt, Belt Supergroup) consists of dolomitic and calcareous micro-turbidites and turbidites, with subordinate sub-wavebase deposits of limestone, dolomite, non-calcareous shale, conglomerate and calcarenite. The Newland Fm. contains many stratabound sulfide deposits which contain a measured and indicated resource of 10.9 Mt containing 2.9% Cu and an inferred resource of 11 Mt containing 2.5% Cu. The most numerous deposits cluster near the northern margin of the Helena Embayment. A Re-Os date on chalcopyrite (Saintilan, et. al. 2021), which overprints beds of pyrite in the deposits, provides a crystallization date of 1488 Ma +/- 34 Ma. This date, together with a cassiterite date of 1475 +/- 4 Ma from the Sullivan Pb-Zn-Ag deposit (Slack, 2020) suggests a prolonged period of widespread subaqueous hydrothermal activity in the Belt basin during Lower Belt time. Detrital zircons from Newland Fm. and earlier Lower Belt sediments show a probable separate sediment source area from that for detrital zircons from the Greyson Fm. and the middle Prichard Fm. Fresh zircon from a group of volcanic tuff(?) beds in the Newland Fm. give a U-Pb date of 1485 +/- 3.3 Ma supported by a 1483 +/- 5 Ma date on an andesite dike that cuts the Newland Fm. in the same area. Taken together, the above-described features suggest a complex depocenter configuration for the Belt Basin in Lower Belt time with synchronous volcanic and hydrothermal activity. Such features record a basin wide common magmatic event during initial basin formation that provided heat energy to drive hydrothermal waters to the subaqueous hotsprings fields at Sullivan, Black Butte, and other locations which resulted in several giant sulfide deposits.