Spatial and Temporal Distribution of Hydrothermal Minerals and Sources of Hydrothermal Fluids Inferred from Light Stable Isotopes, Keweenaw Peninsula Native Copper District, Michigan

Abstract

Hydrothermal native copper deposits are hosted by Mesoproterozoic Midcontinent Rift-filling volcanic and sedimentary rocks in Michigan’s Keweenaw Peninsula. The genesis of the native copper deposits has been a point of interest since their discovery. Native copper and associated mineral assemblages vary temporally and spatially. A refined mineral paragenesis is presented and used as the basis to spatially compare mineral assemblages as it is essential that spatial comparison involve only minerals that are temporally/genetically, related to each other. The main-stage minerals associated with precipitation of native copper are spatially zoned. The higher-grade zones correspond to the area of native copper deposits and cross-cut stratigraphy. Late-stage minerals are superimposed on main-stage minerals and are not spatially zoned. The mineral assemblages can be equated to temperature of precipitation through previously published experimental metamorphic petrology, mineral chemistry, and stable isotope pairs.

Synthesis of previously published and new light stable isotopic data on hydrothermal minerals are used to draw inferences about the sources of the hydrothermal fluids. The equated temperatures of precipitation with isotopic fractionation equations are used to calculate the isotopic composition of the hydrothermal fluids. The oxygen isotopic composition of main-stage hydrothermal fluids based on isotopic composition of calcite, quartz, and chlorite, when combined with limited hydrogen isotope data for chlorite, epidote, and pumpellyite infer that the fluids were generated by metamorphogenic processes. These copper-bearing hydrothermal/metamorphogenic fluids rose from the deep source zone and mixed with meteoric waters in the zone of precipitation of native copper and associated minerals. Prior to mixing, the relatively shallow meteoric waters may have evolved in the rift-filling clastic sedimentary rocks overlying rift-filling basalts. Main-stage calcite can be distinguished from late-stage calcite by oxygen and carbon isotopes suggesting a different source of the late-stage hydrothermal fluids. The late-stage hydrothermal fluids are primarily meteoric waters although the meteoric waters may also have evolved in the rift-filling sedimentary rocks. Mixing of late-stage fluids with metamorphogenic fluids cannot be precluded. This study confirms the long-held hypothesis that the native copper precipitating hydrothermal fluids were generated by burial metamorphism. The hypothesis that fluid mixing was a mechanism promoting precipitation of native copper is supported by this study. In contrast, post-native copper late-stage fluids are dominantly meteoric water.