The stratigraphy, structure, geochemistry, and metallogeny of the Moran Lake Group, Central Mineral Belt, Labrador

North, Jon W. (1988) The stratigraphy, structure, geochemistry, and metallogeny of the Moran Lake Group, Central Mineral Belt, Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

The lower Proterozoic (Aphebian) Moran Lake Group was deposited in a marine shelf environment unconformably on granitoid rocks of the Archean Nain province. The Moran Lake Group is unconformably overlain by the continental (Helikian) Bruce River Group. The Moran Lake Group consists of two provisional formations viz.; the Warren Creek Formation which has been informally subdivided here, into a Lower Member composed of siltstone, sandstone, and shale, with occassional red beds and calcareous beds, and an Upper Member consisting of pyritic black shale, and siltstone, with minor vesicular basalt, arkose, and banded ironstone; and the Joe Pond Formation consisting of pillowed and massive basalt. The Moran Lake Group underwent polyphase deformation during the Makkovikian-Ketilidian event ca. 1810-1790 Ma, prior to the deposition of the overlying Helikian Bruce River Group which was affected by Grenvillian deformation ca. 1000 Ma. -- The geology and geochemistry of Fe-Zn-Cu-bearing sulphide beds in the Upper Member of the Warren Creek Formation indicate that basinal heat flow was low to moderate during the deposition of the Lower Member of the Warren Creek Formation, and high during the late stages of Upper Member deposition. Circulation of metal chloride-bearing formation brines occurred by slow seepage and upward advective flow along pressure gradients and normal faults in the lower part of the Upper Member. These brines were expelled, depositing pyritic Fe-Zn-Cu-rich sulphide beds in stable, reduced brine pools downslope from their discharge vents. An increase in the geothermal gradient near the end of Upper Member deposition, caused uplift and downward penetration of seawater in shallow ephemeral convection cells which ultimately deposited stratabound Zn-rich, Cu-poor, disseminated sulphide mineralization in a porous host lithology. The change in geothermal gradient and character of sulphide mineralization, reflects the development of a stable magma chamber below the basin, from which the Joe Pond Formation basalts were ultimately derived. The paucity of Pb and Ag and overall low base metal content of both the Fe-Cu-Zn sulphide beds and the Zn-rich, Cu-poor stratabound sulphide mineralization, indicate that the Moran Lake Group has limited to nil potential for economic sedimentary exhalative Zn-Pb-Cu (Ag) deposits. The mechanisms by which the mineral occurrences in the Warren Creek Formation were formed however, may represent incipient stages of classic "sedex" mineralization which did not reach maturity.

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