GEOCHRONOLOGY AND GEOCHEMISTRY OF THE IDAHO-MONTANA PORPHYRY BELT

Most molybdenum and copper resources of the Headwaters Province occur in a northeast-trending “belt” of granitic porphyry-related deposits that extends from the Boise Basin of central Idaho, near the suture zone, to the Little Belt Mountains of central Montana, in the interior of the North American craton (Armstrong et al., 1978; Rostad, 1978). Host intrusions were emplaced during two major periods of silicic magmatism, in late Cretaceous time (the Idaho, Pioneer, and Boulder Batholiths), and in Tertiary time. These host rocks were emplaced into Archean-Paleoproterozoic basement blocks and supracrustal sequences along the Great Falls Tectonic Zone, a region of structurally complex basement thought to represent an orogenic zone produced during assembly of the craton (O’Neill et al., 2002). We report the results of 40Ar/39Ar, geochemical and isotopic tracer studies designed to elucidate the temporal and geochemical evolution of the porphyry belt.

We present 40Ar/39Ar analyses from 17 of 25 sites visited. Samples from Cretaceous plutons become younger toward the northeast. Samples from Tertiary plutons show a tighter range in age that is independent of their position relative to the continental margin. Geochemically, Cretaceous and Tertiary plutons exhibit distinct rare-earth-element (REE) patterns. Cretaceous rocks exhibit steep light REE profiles, which start at abundances 60-250 times chondrite and decrease in slope smoothly to heavy REE abundances of 2-20 times chondrite. REE profiles from Tertiary plutons in the southwestern and central portions of the porphyry belt are slightly less steep and display a prominent negative Eu anomaly. REE patterns for Tertiary plutons and dikes in the northeastern portion of the porphyry belt are markedly different and are similar to the patterns displayed by the Cretaceous rocks. Two major features are displayed in the Sr-Nd-Pb data: 1) a northeastward trend towards more highly radiogenic compositions, typical of oceanic to continental interior transects, and 2) discrete clusters of data that suggest distinct differences in the source regions of the major Cretaceous batholiths as well as of the Tertiary plutons.