The Triassic Chitistone Limestone, Wrangell Mountains, Alaska: stressing detailed descriptions of sabkha facies and other rocks in lower parts of the Chitistone and their relations to Kennecott-type copper deposits

Abstract

Recent investigations show that sabkha deposits were important in the genesis of Kennecott-type copper ore. Massive chalcocite-rich lodes at Kennecott and nearby deposits formed in the lower 110 meters of the Upper Triassic Chitistone Limestone. The Chitistone and superimposed Upper Triassic and Jurassic sedimentary rocks formed in a marine basin on and surrounded by the Nikolai Greenstone, a thick, extensive, largely subaerial succession of tholeiitic basalt with intrinsically high copper content. Lowermost 110 meters of the Chitistone contains three incomplete upward-shoaling lime mud cyclic sequences that each consist of shallow subtidal limestone grading upward to intertidal stromatolitic fine-grained dolomite. The youngest cycle contains well-developed sabkha features and dolomitic pisolitic and laminate crust caliches and underlies shallow-marine limestone. The ore deposits are related to the youngest supratidal cycle. This carbonate cycle represents a regional sabkha facies that developed between 90-110 meters above the Nikolai Greenstone. This facies, which contained abundant gypsum-anhydrite, was exposed to vadose weathering that leached much gypsum-anhydrite and developed a vuggy zone interbedded with porous dolomitic caliche zones. Subsequent marine deposition capped the porous zone with an impermeable seal. The youngest sabkha horizon served as a permeable conduit for the ore-forming solution and was instrumental in localizing the major Kennecott-type ores.