Fluid inclusion and metal ratio analysis of Cordilleran Pb-Zn-Cu-(Ag-Au) veins of the Montezuma district : Summit County Colorado, USA

Abstract

Evidence from fluid inclusion microthermometry in Pb-Zn-Cu-(Ag-Au) veins and district scale metal ratio zonation analysis indicate that the Cordilleran veins of the Montezuma mining district Summit County, Colorado, USA are indicative of subepithermal setting about a central hydrothermal source. Cordilleran-type polymetallic mineralization is a class of ore deposits that are spatially and temporally related to felsic igneous centers and can also be genetically related to porphyry mineralization (Fontboté and Bendezú, 2009). At Montezuma, the Teritary-aged Montezuma Stock is cross cut by several Cordilleran-type veins and is spatially related to over 80 additional veins hosted in Precambrian country rock. Five stages of mineralization in veins are identified: Stage 1. early quartz-pyrite, Stage 2. barite-incipient base metals Stage 3. base metals, Stage 4. carbonates and Stage 5. late quartz-lead-silver. There is a systematic decline in precipitation temperatures from 341 to 156°C along the progression of the paragenetic sequence, which suggests the waning of a source pluton. District scale metal ratio zonation maps from historical production data support the interpretation of a central magmatic source and that thermal decline is the primary control on ore deposition. Two district scale zones are identified: a copper rich zone (CRZ) in the center of the district, which is surrounded by a copper poor zone (CPZ). With distance from the inferred center of the district, there is a general decline in copper abundance relative to lead and silver. Thermal gradients accompanied by a decrease in metal solubilities are the mechanism for this zonation pattern, but developed late in the paragenesis. Other chemical and physical controls of phase separation, ligand removal, dilution and pH increase are likely present during vein mineralization as well. Approximate salinities ranged from 11.69 to 3.70 wt.% equivalent NaCl and showed less systematic patterns, and may reflect these additional processes. Temperature decline and variable additional depositional processes are consistent with analogous Cordilleran-type vein fields, which have proven links to a magmatic source and possible underlying stockwork porphyry base metal mineralization. Therefore, data from this study indicates that there is most likely porphyry Mo mineralization under the copper rich zone, but this may be sub economic in nature.