Environment of ore deposition in the Creede mining district, San Juan Mountains, Colorado; Part III, Progress toward interpretation of the chemistry of the ore-forming fluid for the OH Vein

Abstract

Mineral assemblages involving pyrite, hematite, chlorite, quartz, K-feldspar, sericite, sphalerite, galena, and chalcopyrite are interpreted with the aid of data from fluid-inclusion studies to show that the environment most typical of ore deposition for the OH vein had the following characteristics: (1) temperature of 250 degrees C, (2) pressure of about 50 bars (the fluids were boiling near the top of the ore zone), (3) pH of 5.4, (4) salinity of (Na (sub 0.9) K (sub 0.1) )Cl fluid of about 1 molal, and (5) total concentration of sulfur in solution of 10 (super -1.7) molal. Consideration of the iron content of sphalerite shows that the activities of oxygen and sulfur varied considerably during ore deposition. Reactions between hematite, iron-rich chlorite, pyrite, quartz, and water controlled redox reactions and prevented the chemical environment from varying sufficiently to form bornite, anglesite, or magnetite. The bulk of the ore was deposited from solutions that were clearly sulfate rather than sulfide rich, yet at times the chemical conditions became so reducing that H 2 S would have been the dominant sulfur species in aqueous solution had equilibrium prevailed. The chlorite-bearing buffer is incompatible with a chemistry in which equilibrium is maintained between oxidized and reduced sulfur species, unless large changes in the amount of total sulfur in solution are permitted. Since the mineralogical evidence does not support large changes in sulfur concentration, we conclude that there were recurrent departures from redox equilibrium among the aqueous sulfur species.The ores were deposited from a freely convecting hydrothermal system that probably was initially charged by meteoric solutions, although the salts, metals, and sulfur may well have been derived from deeper sources. The circulating solutions deposited gangue and ore minerals near the top of the convecting cell in a hypogene enrichment process that extracted metals and sulfur from whatever sources were available at depth and swept them toward the surface. Boiling, with the loss of acid components (H 2 S and CO 2 ) which recondensed in the cooler overlying rocks, led to the formation of an intensely altered, sericitic capping above the ore. Precipitation of ore is attributed to cooling and perhaps to a slight pH rise complementary to the loss of acid constituents through boiling.Clusters of finely banded, iron-rich zones in otherwise iron-poor sphalerite are a consequence of the introduction of small quantities of more reduced (magmatic?) fluid that imposed a local, temporary, low-redox chemical signature upon the circulating system. Each successive pass of the same low-redox pulse produced an iron-rich band in the sphalerite. The mass ratio of fluid to sphalerite deposited requires only a few parts per million of zinc to be deposited in each cycle. Combined with a previous estimate of flow rate, this results in the geologically uncomfortable, but quantitatively tenable, estimate of length of time for mineralization of from a few hundred to a few thousand years.A circulating-fluid model for ore deposition has important implications for mineral exploration. Minerals having retrograde solubilities (e.g., anhydrite or molybdenite) will be concentrated in the hottest part of the system. In contrast to the conventional once-through-and-out model, the circulating model predicts a wide, barren gap between the shallow and deep facies of mineralization.