Zinc Deposits Related to Diagenesis and Intrakarstic Sedimentation in the Lower Ordovician St. George Formation, Western Newfoundland

Abstract

ABSTRACT Sphalerite mineralization in the Lower Ordovician St. George Formation occurs in a white dolomite pseudobreccia confined to a unit of dolomite-mottled limestone and dolomite. This section is interbedded dolomite-mottled biointrapelsparite and dolomitized micrite deposited in two cyclically alternating epeiric environments. A transitional shallowing phase produced the overlying 50 ft (15 m) thick dolomitized biomicrite. An alternation between very shallow marine and sabkha depositional environments produced the next 200 ft (61 m) of preserved section -- a succession of incomplete cyclic units, bounded by diastems. A typical cyclic unit proceeds through breccias, dololaminite or argillaminite, and mottled bioturbated dolomite, to massive, fine-grained dolomite. This over-all progression of depositional environments, from the marine to the subaerial, culminated near the end of Early Ordovician time with the sedimentary pile being uplifted at least 300 ft (92 m) above baselevel. Because of this uplift, caves developed in the limestone into which carbonate residues and detrital, dolomite-rich pisolitic sediment overlain by collapse breccias were deposited. Adjacent to caves, the calcite among the interdolomite mottle in the dolomitic limestone was dissolved, leaving an open network of undissolved dolomite network. In these areas of greatest porosity, sphalerite was deposited in colloform fashion about the mottle nuclei. A return to over-all subsidence caused marine inundation of the karst drainage system and the remaining open spaces were filled with white, coarsely crystalline dolomite. When marine flooding reached the uppermost erosion surface, deposition of the earliest Middle Ordovician Table Head sediments began. Two models for Zn precipitation can be suggested for this paleohydrologic system: 1) mobilization of 10 to 50 ppm Zn from host-rock carbonates by meteoric waters, and precipitation downstream at the interface with formation fluids; 2) precipitation of Zn by meteoric water dilution of upward-welling, Zn-rich formational chloride brines. End_Page 393------------------------ Newfoundland Zinc Mines location map. End_Page 394------------------------