The relationships between dolomitization and sphalerite mineralization in the Lower Ordovician Upper Knox carbonate rocks of the Copper Ridge District, East Tennessee

The relationships of dolomitization to sphalerite mineralization were investigated in the Lower Ordovician Upper Knox carbonate rocks, of the Copper Ridge District, East Tennessee. Part of the Upper Knox Group consists of the Kingsport and Mascot formations, which consist of three easily recognizable lithofacies: limestone, fine-grained dolostones (Fd) and coarser than fine-grained dolostones (CFd).

Textural and trace element data suggest that the neomorphism of the limestones and the formation of the CFd occurred in the presence of fresh water. . Theoretical considerations involving mixing of fresh and marine water show that dolomitization is feasible for mixtures between 10-40 wt. % of marine water, and neomorphism of limestone between 40-60 wt. % marine water. The presence of high-Mg calcite inclusions in the CFd indicates that the dolomitization occurred quite early. Presence of ghost fossils, crystal zoning, poikilotopic textures, as well as lack of mudcracks suggest that the CFd formed from dolomitization of subtidal to intertidal limestones. . This imples that the preserved limestones and the CFd are probably facies equivalents. In contrast, both the absence of some features and the presence of others suggest that the Fd are of the type that form in Holocene peritidal environments. These criteria include presence of mudcracks, birdseyes, planar to wavy cryptalgal laminae, indirect evidence of the former presence of gypsum, rarity of fauna, and the fine-grained nature of the constituent dolomite crystals. Low Na and Sr concentrations in

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iv the Fd suggest that their formation also involved presence of fresh water.

A fence diagram, based on sections measured during this study in conjunction with those reported by Harris (1969), shows that most of the Mascot represents a broad, tidal flat environment, adjacent to the Kingsport shallow marine environment. The Kingsport was dotted with islands. 7 These islands served as catchment areas of fresh water which has the potential to cause brecciation as well as dolomitization depending on the amount of marine water mixed with it. The fence diagram also shows that ore-grade mineralization is related to facies boundaries that resulted from the record of islands and adjacent shallow marine environments.

Textural relationships between sphalerite and various types of dolomite crystals indicate that the mineralization is later than the formation of the dolostones. However, textural evidences could be misleading since fluctuations in the amount of introduced reduced sulfur to an ore solution could result in equilibrium or disequilibrium boundary relationships. That introduction of reduced sulfur was the mechanism of precipitation of sphalerite is suggested by equilibrium boundary relationships between sphalerite and some vug-fill dolomite crystals. Nonetheless, some limestone beds in close proximity to abundantly mineralized breccia bodies are not dolomitized, but farther away, the same beds are dolomitized implying that ore-forming solutions did not form dolostones. . However, in some bedded-ore occurrences sphalerite and dolomite crystals of the CFd variety are intimately

associated, implying that mineraliation and dolomitization were coeval. The association of sphalerite with CFd in bedded-ore deposits, the need for sufficient sulfur to produce ore-grade mineralization, and the relationship of ore-grade mineralization to facies boundaries is used to speculate that the mineralization was probably earlier than the post-Knox unconformity.