Abstract

The Ambler district is located along the southern margin of the Brooks Range, Alaska. Mineral exploration since the late 1950s in this remote area has led to the discovery of a number of volcanogenic massive sulfide deposits and several carbonate-hosted copper deposits. The district is composed of two geologic terranes. The Ambler terrane consists of Devonian and older schistose metasedimentary rocks overlain by Middle Devonian to Lower Mississippian phyllitic shales and siltstones. Late Devonian carbonate sediments (Bornite sequence) and mixed carbonate and volcanic rocks (Ambler sequence) are interbedded within this thick metasedimentary succession. A low-angle fault separates the Ambler terrane from the overlying, southerly derived Angayucham terrane which is composed of Devonian to middle -Mesozoic mafic and ultramafic rocks. Both terranes were deformed and metamorphosed during the Jurassic (?) to Cretaceous Brooks Range orogeny. Significant mineralization is restricted to the Ambler and Bornite lithologic successions. The Ambler sequence, which contains a number of volcanogenic massive sulfide deposits, averages 1.5 km in thickness and is composed of approximately 60 percent volcanic and volcaniclastic rocks, 25 percent carbonate sediments, and 15 percent pelitic sediments. The volcanic rocks are bimodal with rhyolite predominating over basalt. Much of the rhyolite was apparently deposited as submarine ash-flow tuff. Two major types of mineralization are recognized: massive sulfide adjacent to major hydrothermal vent areas, and laterally extensive massive sulfide lacking well-defined vent areas. The Bornite carbonate sequence contains significant copper mineralization within large, structurally controlled zones of hydrothermal dolomitization.The bimodal volcanic sequence is suggestive of a rifting-related tectonic environment. This interpretation is consistent with the distribution of carbonate and volcanic sequences on what may be high structural blocks and with possible coeval ocean-floor spreading to the south suggested by the Angayucham mafic volcanic rocks.

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