GEOLOGY, GEOCHEMISTRY, AND ORE-DEPOSITS OF THE OLIGOCENE HOG-HEAVEN VOLCANIC FIELD, NORTHWESTERN MONTANA

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 bas alt dikes intrude or unconformably overlie the Late Proterozoic Belt S upergroup 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 simila r mineralogically, have silica contents between 65.2 and 72.9 wt perce nt, and are meta- to peraluminous. They are anomalously high in Sr and Ba and have low initial Sr-87/Sr-86 ratios of approximately 0.704. Bo th 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 o rigin by partial melting of ancient, perhaps Late Archean, high alumin a, lower crustal rocks of intermediate composition. Lead was apparentl y derived from the same lower crustal source; the other ore elements c ould have been derived from one or more sources. Known mineral deposit s are restricted to the east dome complex. They are numerous, small ex cept for the Hog Heaven mine, and occupy several stratigraphic and str uctural settings. Deposit types include open-space fill, replacement, vein and stockwork, and breccia hosted. Precious metal deposition acco mpanied silicic and subsequent advanced argillic alteration. The Ag-ri ch Hog Heaven mineralized system is typical of other high sulfidation deposits with respect to (1) the close temporal and spatial relationsh ip to dome emplacement, (2) wall-rock alteration assemblages, (3) the relatively small deposit size compared with some adularia-sericite sys tems, 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 i n that Pb + Zn + Cu halos surround Ag + Au-enriched zones. Deposit typ es include open-space fill, replacement, vein and stockwork, and diatr eme hosted. Precious metal deposition accompanied silicic and subseque nt advanced argillic alteration.