17 GSA: Wrangellia—Tectonics and Metallogeny, 30 Years of Progress

Tuesday, 9 May 2006
7:55 AM - 11:30 AM

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

Paper number: 17-10
Program: THEME SESSIONS
Time: 11:00 AM – 11:20 AM

Authors

Abstract

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.