MESOPROTEROZOIC SYNSEDIMENTARY SULFIDES IN THE NORTHERN HELENA EMBAYMENT OF THE BELT BASIN, MONTANA

Stratabound pyritic zones in shale of the Newland Formation within the lower Belt Supergroup contain important copper deposits near Black Butte, Meagher County, Montana. Investigations of the copper occurrences with more recent work by Tintina Resources Inc. have enhanced our understanding of an unusual style of sediment-hosted copper deposit formed in a deep water hot springs sedimentary exhalative setting during Middle Proterozoic time. This work has provided additional insight into the setting of the Helena embayment, an eastern extension of the Belt basin, and its northern shelf-to-basin transition zone. An extensive synsedimentary hydrothermal field occupied an area of at least 15 miles by 5 miles, and persisted during deposition of at least one kilometer of Newland shale and carbonate. Hydrothermal cells, capped by seafloor hotsprings, introduced large volumes of iron and silica and smaller volumes of zinc, copper, cobalt, lead, silver, gold, barium, and strontium into basin muds. Locally, these components comprise large percentages of the Newland Formation rock mass. Higher and more widespread sulfide concentrations at specific stratigraphic levels which show evidence of increased syndepositional slumping of local sedimentary units suggest increased hydrothermal activity accompanied episodic major subsidence events. Recent work has outlined at least 1.18 billion pounds of copper resource and 35.6 million pounds of cobalt resource. Hydrothermal activity appears focused at the interplay between an east-west trending basin margin fault zone and a northeast trending structural zone recognized as part of the Great Falls Tectonic Zone.