ENVIRONMENT OF ORE DEPOSITION IN THE CREEDE MINING DISTRICT, SAN-JUANMOUNTAINS, COLORADO .5. EPITHERMAL MINERALIZATION FROM FLUID MIXING IN THE OH VEIN

Detailed fluid inclusion studies on coarse-grained sphalerite from the OH vein, Creede, Colordado, have shown that the abrupt color changes between growth zones correspond to abrupt changes in the nature of die ore fluids. Within each growth zone, however, the composition of the fluids remained constant. The base of a distinctive orange-brown growt h zone marks a sharp increase in both temperature and salinity relativ e to the preceding yellow-white zone. The orange-brown growth zone can be correlated along much of the vein and is believed to represent a t ime stratigraphic interval. Along the vein, temperatures and salinitie s of fluid inclusions within this interval show a systematic decrease from about 285 degrees C and 11.5 wt percent NaCl equiv near the base of the vein to about 250 degrees C and 8 wt percent NaCl equiv, respec tively, near the top of the vein. The iron concentration of this sphal erite growth zone shows a similar pattern, decreasing from about 2.8 t o 1.2 mole percent FeS. When plotted on an enthalpy-salinity diagram, the fluid inclusion data define a spatial trend indicating die progres sive mixing of deeply circulating hydrothermal brines with overlying, dilute ground waters. The hydrothermal brines entered the OH vein from below at a temperature, salinity, and density of approximately 285 de grees C, 11.5 wt percent NaCl equiv, and 860 kg/m(3), respectively, wh ereas the overlying ground waters appear to have been preheated to rou ghly 150 degrees C and had an assumed salinity of 0 wt percent and a d ensity of 920 kg/m(3). The greater density of the heated ground water promoted mixing with the hydrothermal brine within the open fractures, causing sphalerite deposition. Although there were also episodes of b oiling during Vein mineralization, boiling appears unimportant for thi s sphalerite. Isotopic evidence and geochemical modeling studies also indicate that mixing was the depositional mechanism for sphalerite. An important aspect of the mixing hydrology of the Creede system involve s an aquitard overlying the OH vein. This low permeability zone restri cted the flow of ground water into tile vein from above and forced the upwelling hydrothermal fluids to flow laterally along the vein. The m ixing environment thus occurred along the interface between a deeply c irculating hydrothermal convection cell and a topographically driven s hallow ground-water system.