Characterization of barren and mineralized hydrothermal systems in an impact structure: the granophyre unit and footwall of the Sudbury Igneous Complex, Ontario, Canada

Abstract

The Sudbury Igneous Complex (SIC) is the product of a differentiated impact melt sheet and one of the best preserved impact sites globally. The top 1.5 km of the 2.5 km-thick crystallized melt sheet, referred to as the granophyre unit (GR), exhibits strong textural, mineralogical, geochemical, fluid inclusion, and oxygen isotopic evidence for a long and complex hydrothermal history. This hydrothermal system has not previously been documented, and is characterized by an early magmatic-hydrothermal stage that has been overprinted by a later hydrothermal stage. The magmatic-hydrothermal stage was dominated by fluids exsolved from the GR and resulted in pervasive saussuritization of plagioclase, alteration of alkali feldspar and plagioclase to endmember compositions, and precipitation of epidote and muscovite in pits and void spaces. The overprinting hydrothermal stage was dominated by modified seawater sourced from the hydrothermal system in the overlying Onaping Formation and produced a pervasive alteration assemblage characterized by epidote, ferro-actinolite, stilpnomelane, biotite, and titanite with lesser muscovite, chlorite, and ferrotschermakite. The results from this study record a previously unidentified complex and protracted post-crystallization history of fluid-rock interaction for the GR that is related to the ore-forming hydrothermal system in the overlying Onaping Formation. The relationship between the hydrothermal system in the GR and the mineralized systems in the footwall to the Sudbury Structure (SS) has not previously been established. The Nickel Rim South (NRS) Cu-Ni-PGE deposit, located in the east range of the Sudbury Structure, Ontario, records textural and geochemical evidence of a complex, multi-stage hydrothermal history and provides an excellent comparison to the GR. Results from fluid inclusion petrography and microthermometry are used in conjunction in order to characterize the hydrothermal system at NRS. The combined results of fluid inclusion petrography and microthermometry, evaporate mound analysis, oxygen isotopic values of quartz, and trace element chemistry of alteration minerals reveal a complex hydrothermal history at NRS related to the cooling SIC, which contributed to hydrothermal remobilization of metals into the footwall. The hydrothermal fluids are notably different from fluids documented in other mineralized footwall systems that are represented by high temperature, multi-solid fluid inclusions inferred to be critical for ore remobilization. Instead, the hydrothermal system at NRS shares more similarities with the GR hydrothermal system, bringing into question the role of fluids represented by high temperature, multi-solid inclusions in ore remobilization.