COPPER MINERALIZATION AT KENNECOTT, ALASKA: A FLUID MIXING EVENT CONTROLLED BY HIGH-ANGLE STRUCTURES

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).