Dolomitization, brecciation and zinc mineralization and their paragenetic, stratigraphic and structural relationships in the Upper St.George Group (Ordovician) at Daniel's Harbour, western Newfoundland

Lane, Thomas E. (Thomas Edward) (1990) Dolomitization, brecciation and zinc mineralization and their paragenetic, stratigraphic and structural relationships in the Upper St.George Group (Ordovician) at Daniel's Harbour, western Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The sphalerite deposit at Newfoundland Zinc Mines near Daniels Harbour, western Newfoundland is situated in the upper part of the Lower Ordovician St. George Group, a complex of dolostones, limestones and breccias in the middle of a Lower Paleozoic shallow-water, carbonate platform sequence. It is a zinc-dominated Mississippi Valley Type (MVT) deposit, a subtype of MVTs that is characteristic of the Appalachians. This study shows that zinc mineralization occurred during one phase of a complex history of repeated dolomitization and fracturing of the host carbonates along northeast-trending lineaments. This history is interpreted through an integrated analysis of the sedimentologic, stratigraphic and structural framework, petrography, cathodoluminescence and geochemistry. -- The upper St. George Group carbonates were deposited along the edge of the tropical Iapetus Ocean during Early to early Middle Ordovician time when the passive continental margin began to experience the initial effects of plate convergence. Shallow subtidal muddy carbonates (Catoche Formation) shallowed upwards into restricted-water, rhythmically interbedded peloidal grainstones and mudstones (Peloidal or Costa Bay Member) and peritidal laminites and burrow-mottled mudstones (Aguathuna Formation). Deformation and fragmentation of the platform and marine regression during early Middle Ordovician time resulted in the regional St. George Unconformity and the formation of rock-matrix breccias from subsurface karst. The mine stratigraphy records a minimum of 5 stages of faulting, subsurface dissolution and erosion of the platform at this time. The platform was gradually flooded during Middle Ordovician time as the upper member of the Aguathuna Formation and limestones of the Table Point Formation were deposited over the St. George Unconformity. Continued convergence of the continental margin caused collapse of the platform and generation of a foreland basin in which a thick sequence of siliciclastic flysch was deposited and eventually overridden by Taconic thrust sheets. -- Seven dolomite crystal types or generations (I through VII) crystallized in four major settings. Microcrystalline, syngenetic dolostones (I) with enriched ¹HO replaced subtidal to peritidal mudstones at or near the surface. Some of these dolostones were then incorporated as clasts in conglomerates and solution breccias. Composite diagenetic crystals (II) grew during burial forming turbid, replacive cores near surface and clear rims at depths where pressure solution (>300 m) was active. Pore-filling, clear, zoned dolomite cements (III) sealed most remaining porosity in early dolostones. -- Epigenetic coarse dolostone/sphalerite (D/S) complexes overprinted the earlier dolomites. They formed around fracture systems as extensive stratabound bodies within, and local discordant bodies across the Catoche Formation. Their development occurred in 5 main stages: (1) Regional compression generated linear, stratabound fracture zones along faults and around rock-matrix breccias, (2) Xenotopic pre-ore dolomites (IV) replaced dolostone-mottled limestones and formed zebra fabrics along fractures. (3) Hydrothermal (140ﾟC mode) ore fluids caused extensive dissolution of carbonates. (4) Sulphides precipitated in two stages along fracture zones as fracturing, faulting and dissolution continued. (5) Post-ore hydrothermal dolomites (V, VI), dominated by replacive and pore-filling saddle dolomites with depleted ¹HO, crystallized around widespread, dilatant fractures forming spar breccias, pseudobreccias and coarse sparry dolostones. Faults associated with regional uplift displaced the D/S complexes and formed fluid conduits creating a fourth and final environment for late fault-related, turbid dolomites (VII) which replaced limestones. -- The genesis of epigenetic dolostones and sulphides is interpreted to be the result of regional fracturing during the initial stages of the Siluro-Devonian Acadian Orogeny. Rising geothermal gradients generated by structural thickening of the crust probably caused warming of basinal fluids, the release of metals and sulphur from minerals in the sediments and basement and the formation of hydrothermal convection in the sedimentary pile. Steep fractures penetrated the base of the sedimentary pile and enabled fluids to rise directly from basement depths of 4 to 6 km. At the deposit site the warm, buoyant fluids rose to the top of fracture aquifers where they displaced cooler formation waters. Cooling, loss of CO₂, sulphate reduction and/or increased pH probably accounted for sulphide precipitation.

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