2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 7
Presentation Time: 3:00 PM


PRICE, Jason B.1, HITZMAN, Murray W.2, NELSON, Eric P.1 and HUMPHREY, John1, (1)Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401, (2)Chair, National Research Council Committ on Induced Seismicity Potential and Dept. Geology & Geological Engineering, Colorado School of Mines, Golden, CO 80401, jprice@mines.edu

The Kennecott copper deposits represent some of the richest copper occurrences known. The orebodies are stratabound veins that occur within 100 m of a metabasalt-limestone contact and have been described as “overturned canoes with exaggerated keels”. The major orebodies, along with minor veins, are controlled by high-angle, NE-striking, sinistral-normal faults and floored by low-angle, bedding-parallel faults, both of which exhibit high-angle shortening and low-angle NW-SE extension strain axes. Open-space fill and wallrock replacement by sulfides along these two fault sets caused the archetypal triangular shape of the major veins.

Copper, mobilized from the underlying Triassic Nikolai Greenstone by a warm (~200ºC), oxic fluid produced by dehydration reactions during prehnite-pumpellyite grade metamorphism that also carried sulfur, precipitated as both native copper and copper sulfides in the Nikolai (δ34S ~0‰). These copper- and sulfur-bearing fluids from the greenstone mixed with anoxic, basinal, fluids containing reduced sulfur that originated within the Triassic Chitistone Limestone. Mixing of the two fluids occurred in the lowest 100 m of the Chitistone Limestone in favorable structural traps. The fluids, localized in a circulation cell established between the limestone and greenstone, reacted with original sedimentary pyrite and precipitated extremely depleted copper sulfides (δ34S as low as -36.7‰) in the uppermost Nikolai Greenstone. The bulk of the hypogene mineralization occurred at ~90ºC with the precipitation of massive orthorhombic chalcocite-djurleite in the Chitistone Limestone.

The mineralization event, including the development of high-angle structural traps and the mixing of the two fluids, occurred during the waning stages of the late Jurassic to Cretaceous orogeny coincident with the accretion of the Wrangellia terrane (~110 Ma).