Geology, geochemistry, and ore deposits of the Oligocene Hog Heaven volcanic field, northwestern Montana

Abstract

The Oligocene Hog Heaven high potassium dacite-rhyolite volcanic field covers 50 km 2 of northwestern Montana. Its composite flow domes and dikes, ash-flow deposits, volcaniclastic rocks, and sparse alkali basalt dikes intrude or unconformably overlie the Late Proterozoic Belt Supergroup near the crest of the Purcell anticlinorium. North-northwest-, northeast-, and northwest-trending extensional faults and fractures controlled dome and dike emplacement and subsequent Ag-Au-Pb-Cu-Zn-As high sulfidation mineralization.The felsic volcanic rocks are similar mineralogically, have silica contents between 65.2 and 72.9 wt percent, and are meta- to peraluminous. They are anomalously high in Sr and Ba and have low initial 87 Sr/ 86 Sr ratios of approximately 0.704. Both rock and ore leads have identical and relatively unradiogenic lead isotope compositions.Petrography, whole-rock geochemistry, and Sr and Pb isotopy of the felsic volcanic rocks are most compatible with an origin by partial melting of ancient, perhaps Late Archcan, high alumina, lower crustal rocks of intermediate composition. Lead was apparently derived from the same lower crustal source; the other ore elements could have been derived from one or more sources.Known mineral deposits are restricted to the east dome complex. They are numerous, small except for the Hog Heaven mine, and occupy several stratigraphic and structural settings. Deposit types include open-space fill, replacement, vein and stockwork, and breccia hosted. Precious metal deposition accompanied silicic and subsequent advanced argillic alteration. The Ag-rich Hog Heaven mineralized system is typical of other high sulfidation deposits with respect to (1) the close temporal and spatial relationship to dome emplacement, (2) wall-rock alteration assemblages, (3) the relatively small deposit size compared with some adularia-sericite systems, and (4) high ore Pb/Cu ratios. The deposit has the highest known Ag/Au ratio of any high sulfidation system (2,233/1) and is unusual in that Pb + Zn + Cu halos surround Ag + Au-enriched zones. Deposit types include open-space fill, replacement, vein and stock-work, and diatreme hosted. Precious metal deposition accompanied silicic and subsequent advanced argillic alteration.