CHARACTERIZATION OF SULFIDE MINERALIZATION WITHIN A HYDROTHERMALLY ALTERED PHASE OF THE BISMARK QUARTZ MONZONITE, BISMARK ZN-SKARN DEPOSIT, CHIHUAHUA, MEXICO

The Bismark Quartz Monzonite (45.2 + 1.1 Ma) is generally coarse-grained but finer-grained porphyry phases are observed underground near contacts with surrounding sedimentary rocks, which have been altered to garnet skarn. The relationship between the main Bismark intrusion and ore-mineralization is unclear. Sulfide mineralization observed in an altered phase of the main Quartz Monzonite may cast light on this problem. Samples of this phase were surveyed by optical microscopy, SEM-BSE, and SEM-EDS analysis. The rock is light gray in color and porphyritic with partially resorbed quartz, pervasively altered feldspar, sulfides, and accessory phases in a fine-grained silicified matrix. Sericitic alteration, overprinted by calcite veining and carbonatization of feldspar, is observed. Sulfides include sphalerite, galena, bournonite (PbCuSbS3), chalcopyrite, pyrite, and arsenopyrite. The sulfides occur as single and composite grains within the rock matrix and in calcite veins. Two sphalerite subgroups are observed based on textural association and chemistry: a low-Fe sphalerite group [sphalerite (Fe < 1wt.%) + galena], and a high-Fe sphalerite group [sphalerite (Fe=7-11 wt. %) + galena]. In the high-Fe group sphalerite occurs as large, homogenous grains and galena is less prevalent. Both the high-Fe sphalerite and galena occur in the rock matrix and within calcite veins. In the low-Fe sphalerite group, galena is a major phase. Sphalerite and galena occur as isolated grains, and as complex intergrowths that form rims (galena followed by sphalerite) and irregular overgrowths on chalcopyrite, bournonite and the high-Fe sphalerite. Preliminary conclusions about mineralization within this phase of the Bismark Quartz Monzonite are: (1) The presence of two distinct sphalerite chemistries indicates that Zn and Pb sulfide mineralization was a two-stage process, (2) The appearance of the high-Fe sphalerite within calcite veins suggests that this stage was associated with the adjacent formation of mineralized exoskarn by a high-temperature Zn-bearing ore fluid, and (3) The high-Fe sphalerite is chemically similar to the sphalerite that comprises the main Bismark ore body.