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

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. 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.; 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.; Seven dolomite crystal types or generations (I through VII) crystallized in four major settings. Microcrystalline, syngenetic dolostones (I) with enriched {dollar}sp{lcub}18{rcub}{dollar}O replaced subtidal to peritidal mud-stones 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. 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. (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{dollar}spcirc{dollar}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 {dollar}sp{lcub}18{rcub}{dollar}O, 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 replace limestones. (Abstract shortened by UMI.)...