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Terry, D.A., 1997

Lithotectonic setting and metallogenic significance of barite-manganese occurrences in the Tetlin district, eastern Alaska Range

Bibliographic Reference

Terry, D.A., 1997, Lithotectonic setting and metallogenic significance of barite-manganese occurrences in the Tetlin district, eastern Alaska Range: London, Ontario, Canada, University of Western Ontario, Ph.D. dissertation, 362 p., illust., 2 maps.

Abstract

Several stratiform barite and manganese occurrences were discovered during recent base metal exploration in the Tetlin district of the eastern Alaska Range. The Tetlin district, occurring on the southern margin of the composite Yukon-Tanana terrane, is underlain by lithotectonic assemblages coextensive with subterranes defined to the northwest in the Delta massive sulphide district. The southernmost Tetlin assemblages, that border on the Denali transform fault and which host the manganese-barite mineralization, comprise sequences of chert, phyllite, limestone, chert-pebble conglomerate, rhyolite and basalt flows, intruded by gabbro sills. Fossil dating in conjunction with U/Pb dates of felsic volcanic rocks identify a Middle to Late Devonian age. The stratiform manganese concentrations are hosted by laminated cherts containing phyllitic interbeds. They comprise millimeter to centimeter scale layers of complex minerals including spessartine, pyroxmangite/rhodonite, braunite, pyrolusite, and rhodochrosite. The thickest manganese bed is capped by chert containing magnetite and asbestiform riebeckite. The manganese units have high Mn/Fe ratios, low concentrations of Co, Cu, Ni, Zn, and Th and high U. They have low ΣREE and display a slight Ce anomaly when normalized to seawater. The massive to thin-bedded barite units are also hosted in chert-phyllite sequences. Up to 15% disseminated to banded fine-grained pyrite is locally associated with the barite. The barite contains low concentrations of trace elements and very low ΣREE. Δ34S determinations of barite range from +16.9/thousand to +43.7/thousand. Δ34S values for associated pyrite ranges from -11.2/thousand to -25.2/thousand. Based on field relationships and geochemical data from the bimodal volcanic suite and related mafic sills the lithotectonic assemblages hosting the barite and manganese mineralization are interpreted to have been deposited in a late Paleozoic back-arc marginal basin setting. Morphology, mineralogy, and geochemical signature indicate that the barite and manganese occurrences are of sedimentary-exhalative origin. The lack of associated base metals may result from either: (1) significant transport distance from a hydrothermal vent site, providing necessary time and physiochemical gradients for metal fractionation to occur, or (2) exhalation from low temperature vents associated with relatively shallow hydrothermal circulation cells.

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