AN EXPLORATION MODEL FOR WRANGELLIA'S HIGH GRADE COPPER DEPOSITS AT KENNECOTT, ALASKA

The high grade copper deposits at Kennecott, Alaska in the Wrangellia terrane are among the richest known copper occurrences on earth. The orebodies are stratabound replacement-enlarged veins that are controlled by high-angle faults hosted within the structurally deformed Triassic Chitistone Limestone. The Chitistone Limestone overlies the anomalously copper-rich, prehnite-pumpellyite metamorphosed Nikolai Greenstone (basalt). The vast majority of the known mineralization in the Kennecott district occurs within 100m of this basalt-limestone contact. Field, petrographic, and stable isotope data from the Kennecott mine area and surrounding prospects suggest that the deposits formed in the lowermost Chitistone Limestone when a copper-bearing, oxic fluid originating from the Nikolai Greenstone mixed in favorable structural traps with a sulfur-bearing reduced, basinal fluid originating from the Chitistone Limestone. While the original copper-bearing Nikolai fluid was produced by dehydration reactions during prehnite-pumpellyite metamorphism at 200-250ºC, the vast majority of the orthorhombic chalcocite+djurleite mineralization in the Chitistone Limestone did not occur until fluid temperatures were ~90ºC. The driving forces for the fluid mixing are envisaged to be a mixture of gravity and tectonic pumping generated by structural imbrication during upper Jurassic to lower Cretaceous orogeny coincident with the accretion of Wrangellia (~110 Ma). Exploration criteria for Kennecott-type deposits in Wrangellia or elsewhere in the world include (in order of importance): proximity to a source for the copper, especially intermediate to mafic igneous rocks or, potentially, redbeds; structurally prepared basinal, carbonate rocks with in-situ or migrated reductants; the presence of hydrothermal dolomite veins or breccias with dedolomitized haloes indicating multiple fluid flow events; strong gravity, resistivity, and/or CSAMT geophysical anomalies.