Cornwallis Lead–Zinc District; Mississippi Valley-type deposits controlled by stratigraphy and tectonics

Abstract

Cornwallis Lead-Zinc District in the central Canadian Arctic includes occurrences of galena and sphalerite with similar geological settings and controls on mineralization. It includes the Polaris deposit, with 25 million tons of about 19% grade ore, representing 4.7 million tons of metal.The district occurs in and was controlled by the Cornwallis Fold Belt, a steep-sided anticlinorium of Proterozoic to Devonian formations, that overlies a basement horst.Four controls on mineralization are: (A) deposits are stratabound within the Ordovician Thumb Mountain Formation; (B) ore occurs in breciated dolomite, in contrast to usual limestone of the host formation; (C) deposits are located close to shale of the Cape Phillips Formation; and (D) the host formation was subject to erosion and karstification in Early Devonian time during Pulse 3 of the Cornwallis Disturbance.Mineralization is of Mississippi Valley-type, having formed in carbonate rock by epigenetic processes. The sequence of stratigraphic and tectonic events leading to mineral formation was as follows:(1) An Ordovician to Lower Devonian geosynclinal sequence was deposited, containing formations that could be a source of Zn, Pb, Fe, and S, as well as a potential host formation.(2) The sequence was folded by three pulses of the Cornwallis Disturbance.(3) Uplift in Early Devonian time allowed deep erosion that exposed the host Thumb Mountain Formation in anticlinal culminations.(4) Caverns and pores developed in the upper part of that formation, by karst-type solution.(5) During subsidence an unconformable sedimentary cover buried the host formation and its caverns to a considerable depth.(6) Two formation fluids developed, with metal ions in one and sulphur ions in the other.(7) These two brines migrated laterally and upward, and met in cavities in the Thumb Mountain Formation.(8) In these cavities temperatures and other conditions were suitable and the brines precipitated the sulphides, galena, sphalerite, and pyrite.Deposition probably occurred between temperatures of 52 °C and 102 °C, suggesting that the caverns were at depths of at least 1280 m (4200 ft). This probably occurred in Late Devonian time, prior to Pulse 4 of the Cornwallis Disturbance. Major structural events following mineral emplacement raised the occurrences to higher topographic levels where they became exposed.Metal deposition in this model resembles the accumulation of petroleum in that the components were carried upward and laterally from source formations to structural culminations, and deposited there in open spaces that served as traps.