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Geology and genesis of zinc-lead deposits within a late proterozoic dolomite, Northern Baffin Island, N. W. T.

University of British Columbia

Economically important Mississippi Valley type zinc-lead deposits exist in a late Proterozoic dolomite, the Society Cliffs Formation, at north Baffin Island, District of Franklin, N.W.T., Canada. The Society Cliffs Formation ranges from 1,000 to 2,000 feet in thickness and is underlain by up to 2,000 feet of black, organic-rich shale, the Arctic Bay Formation, and overlain by either black shale and limestone of the Victor Bay Formation or by red, fine- to coarse-grained clastic rocks of the Strathcona Sound Formation. Disconformities exist between each of the formations. Society Cliffs Formation has undergone at least four temporally distinct episodes of karstification since its deposition. The most important karst episode, with respect to the formation of the zinc-lead deposits, occurred during the hiatal interval between the deposition of the Victor Bay Formation and the deposition of the Strathcona Sound Formation. During this hiatal interval a holokarst developed in Society Cliffs Formation and a large integrated cave system of the Mammoth Cave-Flint Ridge Cave System type was formed; i.e. long, nearly horizontal, tubular passages were formed during initial periods of base-level stabilization, followed by the development of sub-vertical canyons beneath the tubes when the base-level dropped. After this karst episode the Society Cliffs Formation was deeply buried and the cave system was partially or completely filled with sulphide and carbonate minerals. The zinc-lead deposits are characterized by banded structure which comprises pyrite, relict marcasite, sphalerite and galena interlayered with sparry dolomite. The zinc-lead deposits contain several sedimentary structures that were formed by a chemical deposition-chemical corrosion process. These include cross-stratification, cut-and-fill and onlap. Onlap indicates the paleocaves were filled from the floor up. The meteoric waters which formed the caves did not form the zinc-lead deposits because the temperature of ore deposition was between 200°C and 150°C, the calculated oxygen isotope composition of the ore fluid is +12.8 per mil, and mineral stability and isotopic data indicate the oxygen fugacity decreased during ore deposition. The sulphide sulphur isotope composition of the zinc-lead deposits has a relatively narrow range about +26 per mil, similar to that of sulphate evaporite (+23.7 per mil) which exists locally within the Society Cliffs Formation. Lead isotope data indicate the lead in the deposits was derived by at least a two-stage process from a source with a uniform uranium-thorium ratio. The ore fluid and contained metals are postulated to have been derived from the Arctic Bay Formation during a late-stage dewatering of the shale. Sulphide deposition may have been caused by the chemical reduction of sulphate which existed in the ore fluid when the ore fluid entered hydrocarbon-filled caves. The hydrocarbons were probably expelled from the Arctic Bay Formation shale during an earlier stage of thermal metamorphism and dewatering.

Text

2010-02-25

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http://hdl.handle.net/2429/20910

Geological Sciences

University of British Columbia

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