Abstract: Relations Between Triassic Carbonate Sabkha and Kennecott-Type Copper Deposits, Wrangell Mountains, Alaska

A. K. Armstrong, E. M. MacKevett, Jr.

Recent investigations show that sabkha deposits in the Wrangell Mountains, Alaska, were important in the genesis of Kennecott-type copper core. Massive chalcocite-rich lodes at Kennecott and nearby deposits formed in the lower 110 m of Upper Triassic Chitistone Limestone. The Chitistone and superposed Upper Triassic and Jurassic sedimentary rocks formed in a marine basin on and surrounded by Nikolai Greenstone, a thick, extensive, largely subaerial deposit of tholeiitic basalt with intrinsically high copper content. Lowermost 110 m of the Chitistone contains three cyclic sequences each consisting of shallow subtidal limestone grading upward to intertidal stromatolitic microdolomite. The youngest cycle contains well-developed sabkha features and dolomitic pisolitic and lam nate-crust caliches and underlies shallow-marine limestone. The ore deposits are related to the youngest supratidal sequence. This carbonate sequence is a regional sabkha facies that developed between 90 to 110 m above the Nikolai Greenstone. The facies, which contained abundant gypsum-anhydrite, was exposed to vadose weathering that leached much gypsum-anhydrite and developed a vuggy zone interbedded with porous caliche zones. Subsequent marine deposition capped the porous zone with an impermeable seal.

We hypothesize that copper-bearing solutions derived from surrounding Nikolai terrane during protracted weathering in pre-latest Jurassic time migrated downdip into the subsiding and deeply buried porous sabkha and caliche bed aquifer where copper was concentrated and deposited in strongly reducing environments deep in the basin. The deposits were affected by subsequent thermotectonic events. Late Jurassic and Early Cretaceous regional orogeny probably mobilized and redistributed previous deposits and induced additional ore deposition; Cenozoic volcanism, the Wrangell Lava, and shallow plutonism possibly modified some ore bodies.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC