POROSITY AND ORE DEPOSITION AT EDWARDS AND BALMAT, NEW YORK
Many ore deposits are associated with channelways such as fault fissures, breccia zones, or porous formations, which afforded conduits obviously more favorable than the surrounding rocks. However, the zinc deposits of the Edwards-Balmat district, New York, replaced selectively certain bands or zones in an impure crystalline limestone unassisted by visible channels. Since the ore shoots extend more than 3000 feet without any great change in character, it is clear that the ore carrier operated for long distances in restricted channels with surprising uniformity, guided by features which, in many details, elude the human eye. The illumination of these controls by microscopic and other laboratory methods, in the hope that the results might shed some light on the nature of the ore fluid, has been the object of this investigation. With respect to the nature of the controls, the author believes it is established that: (1) Porosity was an important factor in determining the channels of deposition. (2) The necessary degree and continuity of porosity were conditioned by microbrecciation which shortly preceded ore emplacement. (3) This microbrecciation was concentrated, for physical reasons, in zones of impure limestone, roughly half carbonate and half silicate. The openings utilized by the ore fluids were of microscopic dimensions, and probably nearer the lower than the upper limits usually regarded as capillary. The process of ore emplacement did not enlarge these openings but probably reduced them. Paradoxically, porosity of the purer limestone types often found adjacent to ore bodies is consistently higher than that of the ore zones themselves and openings between grains are larger than those utilized by the ore fluids. The suggestion is advanced that under the temperature-pressure conditions of ore deposition this situation was reversed. Much more research would be necessary to establish any definite conclusion as to the nature of the ore fluid. To the author, the suggestion of a gaseous medium seems attractive, but an inquiry into its possibilities is felt to be beyond the scope of this investigation.