Stratabound Copper Deposits in East South-Central Alaska: Their Characteristics and Origin
Six stratabound Kennecott-type copper deposits are being studied to evaluate the characteristics of the ore-host rock system. Three (Mountain Grill, Radovan, and Clear-Porcupine) are in the Middle to Upper Triassic Nikolai Greenstone — a thick sequence of subaerial amygdaloidal tholeiite flows — and the other three (Binocular, Nelson, and Peavine) are in the Upper Triassic Chitistone Limestone, a platformal limestone and dolomite unit.
X-ray diffraction, petrographic and chemical studies have led to the identification of ten ore minerals in the deposits, the most abundant being digenite, chalcocite, djurleite, covellite, bornite, and chalcopyrite. Minor arsenic and antimony sulfide phases are also present. Mineral deposits in greenstones have both Cu and Cu - Fe sulfides in contrast to the essentially Cu-sulfides in limestones.
The mineralization is proposed to have formed in two stages. Hydration-dehydration reactions associated with alteration-metamorphism of the Nikolai Greenstone liberated copper from the metal oxides and the mafic minerals. Copper was carried by the circulating fluids along the existing fracture-fluid pathway system, reacting and precipitating ore minerals in them. The weak and widely distributed mineralization in the belt is probably of this type. Greenstone alteration was probably coincidental with Cretaceous accretionary orogeny as shown by K- Ar systematics (Silberman et al. 1981).
Cenozoic deformation induced a steep fracture system that enhanced the porosity and permeability of the rocks, thus focusing the meteoric water circulatory system for effective and efficient solution and transport of metals. Activity in such a system at this stage was probably driven by a thermal charge from the Tertiary intrusives. Lack of extensive alteration of the Chitistone Limestone and of recrystallization at the ore vein-limestone contacts suggests that the ore solutions were at relatively low temperatures. The mineral associations and textures suggest that the upper temperature that prevailed in the Triassic ore-host rock system was about 200 °C.
KeywordsSulfide Zeolite Dehydration Magnetite Lime
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