Problems on the origin of ore deposits in the lower Ordovician formations of east Tennessee

Most recent workers in the East Tennessee zinc and barite districts are in general agreement that the host breccias were formed by solution-collapse processes in early Middle Ordovician time, probably in an ancient carbonate aquifer system of regional extent. There is little agreement, however, on the source and nature of the ore fluids from which the epigenetic minerals were formed. In this symposium Hanshaw and his colleagues have presented evidence in support of the concept that much secondary dolomite may have been formed by ground water. On the other hand, Roedder has shown that fluid inclusion studies indicate hot saline brines (75 degrees -150 degrees C) as the source fluids for the ore minerals, and he suggests that they may have been deeply circulating connate brines. Other investigators in the past have proposed a variety of source fluids, including juvenile waters, meteoric waters that have become enriched by the leaching of particular elements from the surrounding rock, and sea water.

In the zinc deposits, sphalerite is accompanied locally by minor amounts of galena and pyrite, and occasionally, traces of chalcopyrite. Dolomite is the chief gangue mineral and is associated with lesser amounts of quartz, calcite, barite, fluorite, anhydrite, gypsum, and bitumen. Some deposits containing barite as the dominant ore mineral have various amounts of fluorite and pyrite; sphalerite and galena may be present locally along with the other gangue minerals typical of the zinc deposits. The paragenetic sequences of the minerals have been determined for most of the mines and many of the prospects, and, except for minor refinements, will be little changed by additional studies. The real problem, though, is the correlation of these depositional sequences from mine to mine and from district to district. Perhaps the major difficulty in the correlation is the seemingly uncomplicated mineralogy that disguises a rather complex regional paragenesis.

Questions regarding the routes the ore fluids travelled to reach the sites of deposition, the reasons why the ores are concentrated in some areas more than others, and the reasons why the mineralogy varies from area to area are questions that are unanswered or at best only partly answered. Studies to date have produced convincing evidence concerning the nature of the structures and the physical conditions responsible for the localization of ore in the Kingsport Formation (Lower Ordovician). On the other hand, the relation of the Kingsport ores to the smaller deposits in the underlying Lower Ordovician and Cambrian rocks is but little understood, and no final answer to the origin of these deposits can be reached until this relationship has been satisfactorily explained.

Fluid inclusion studies, trace element analysis, and isotopic studies of all sorts are needed to further our understanding of these seemingly simple ores. Continued detailed studies of field relations, both regional and local, are of major importance, not only for what they will reveal in themselves, but also to perfect the frames of reference in which to place the growing mass of laboratory data.