Geochemistry and mineralogy of the carbonates of the Creede mining district, Colorado

Abstract

This paper describes the results of a study of the carbonate gangue in the silver-base metal ore deposit of the Creede mining district, located in Tertiary volcanic rocks of the central San Juan Mountains, southwestern Colorado. Two carbonate stages can be distinguished and defined on the basis of mineral association, compositional range and position in the sequence of ore deposition. Electron microprobe analyses show that the early pre-ore carbonate stage is comprised of rhodochrosite with a compositional range of 68 to 93 mole percent MnCO3, 7 to 32 mole percent FeCO3, 2 to 15 mole percent CaCO3, and 0.5 to 4 mole percent MgCO3. The later intra-ore carbonate stage is made up of siderite-manganosiderite with a broad range of iron content (33 to 94 mole percent FeCO3; 6 to 67 mole percent MnCO3; 1 to 13 mole percent CaCO3; and 1 to 10 mole percent MgCO3). Both carbonate stages are spatially restricted within the vein system, the rhodochrosite limited to the southern third and the siderite limited to the northern two-thirds. The two stages have not been found at the same sample locality. The two stages of carbonate define two of the five main depositional stages in the history of the Creede hydrothermal system and thereby document broad scale variations within the depositing, and continually evolving fluids. However, the complexity of the textural and mineralogical variations within each carbonate stage limits their usefulness in unraveling the finer details of the character of the ore fluids. The frequently massive rhodochroslte has undergone several periods of leaching and regrowth which, except at one locality, cannot be resolved by compositional, textural or color variations. Although some compositionally zoned siderite rhombs have been documented, the siderite, too, has had a complicated history of leaching and regrowth. The zoning indicates (i) an early low-calcium siderite + hematite growth followed by (2) relatively high-calcium siderite depositlon (without hematite) followed by (3) low-calcium siderite deposition without hematite. The composition of the high-calcium siderite lies well outside of the recognized stability field for siderite at 250?C and, therefore, is considered either a metastable phase or possibly a new mineral. Carbon and oxygen isotopic compositions of the carbonates together with the hydrogen isotopic composition of inclusion fluids suggest that the fluids depositing the carbonates either were deep-seated in origin, or, less likely, were meteoric waters which came in contact with a large volume of magmatic rock with which it exchanged isotopes. The oxygen and hydrogen isotopic compositions of the fluids that deposited the carbonates are significantly different from those that yielded the other vein minerals (sphalerite, chlorite, sericite, quartz, adularla). The latter minerals appear to-have been deposited from two distinctive fluids of meteoric origin. The stability fields of minerals present during the carbonate stages have been calculated for 250?Co The results indicate that conditions during carbonate deposition were not drastically different from those suggested by Barton and others (1977) for main-stage ore deposition, although little ore, if any, was deposited during the carbonate stages. A new value for the free energy of formation of siderite was calculated since all of the most recently published values predict too small a stability field for the siderite based on the occurrence of siderite at Creede, The change to the more iron-rich composition of the later stage carbonate was probably controlled by the pyrite-chlorite buffer which was not present during early-stage carbonate deposition.