FLUID INCLUSION STUDY OF SELECTED CALCITE ASSOCIATED WITH NATIVE COPPER, QUINCY MINE, KEWEENAW PENINSULA, MICHIGAN

Abstract

The Keweenaw Peninsula is famous for hosting the largest accumulation of native copper anywhere in the world. Previous studies have looked for the fluids responsible for creating the native copper deposits but were unable to conclusively demonstrate that fluid inclusions can provide valuable insight into the hydrothermal/metamorphogenic fluids responsible for these unique deposits. This report focused on fluid inclusions from calcite precipitated during the native copper mineralizing event. Calcite crystals with inclusions of native copper growing into vugs from a single location, the Quincy Mine, were hypothesized to have a higher chance to be undisturbed by seismic events that would have caused fracturing and leakage of fluid inclusions. Detailed petrographic analysis of the calcite crystals were completed in optical light, SEM, and cathodoluminescence. The calcite was subdivided into three stages consistent with published mineral paragenesis for the Keweenaw native copper deposits: Stage I mostly predated native copper; Stage II was synchronous with native copper formation; and Stage III was a distinct stage after the precipitation of native copper. Fluid inclusions within these 3 stages of calcite were classified as primary, pseudosecondary or secondary and when possible grouped as fluid inclusion families. Homogenization temperatures ranged from liquid only inclusions (<50°C) to 166°C (uncorrected for pressure), but FIFs of all the stages were found to have inconsistent homogenization temperatures within FIFs. The variability of homogenization temperatures of the fluid inclusions within given FIFs found in this report indicate that they have been affected by post entrapment modification. However, the median homogenization temperatures are believed to approximate the original trapping temperatures. Using median temperature from a Stage I FIF a pressure correction yielded geologically reasonable results. Salinity within individual FIFs are more consistent than homogenization temperatures suggesting less effect of post-emplacement modification. Within and between families, Stage I ranged from 5.9-18.4 CaCl2 equiv. wt. % (5.0-19.5 NaCl equiv. wt. %). Stage II ranged from 8.9-30.7 CaCl2 equiv. wt. % (7.9-54.9 NaCl equiv. wt. %). Stage III ranged from 0.0-30.5 CaCl2 equiv. wt. % (0.0-49.7 NaCl equiv. wt. %). It is likely that Ca ions were the dominant ions in solution as many of the higher salinity inclusions had final melting temperatures well below what was possible for the NaCl system and had first melting temperatures below -40°C. The large variation in salinity can be explained by the entrapment of variable proportions of high salinity copper-bearing hydrothermal/metamorphogenic fluids with low salinity meteoric waters. This