Geochemical Evolution of Fracture Filling Minerals from the Chalk River Laboratory Site, Ontario, Canada

Abstract

The isotope geochemistry combined with fluid inclusion studies of several generations of fracture minerals from the Chalk River Laboratory site (CRL) has been applied to investigate the past fluid evolution including hydrothermal processes and hydrogeochemical evolution of the rock mass. Typical fracture minerals found at the CRL site include chlorite, quartz, dolomite, and calcite. Fracture mineral investigations use oxygen and carbon isotopes from calcites combined with fluid inclusion information such as homogenization temperatures (Th), and melting temperatures (Tm) to calculate temperature and salinity of calcite forming fluids. By combining Th with oxygen isotopic data, we were able to use δ18O geothermometry calculations to estimate past isotopic characteristics and composition of the fluids responsible for calcite precipitation. From petrologic evidence, calcite from the CRL site mainly includes four varieties: fibrous calcite, metasomatic calcite, crystal calcite, and vuggy calcite. Fibrous calcite precipitated at a temperature of 78 to 128 oC with a δ13C signature of -4.91 to -7.88 ‰ (VPDB) and a δ18O signature of -9.36 to -17.34 ‰ (VPDB). These calcites were formed at an elevated temperature, in a low salinity, Na-Cl fluid that could have been a mixture of hydrothermal water derived from meteoric fluids or seawater. Metasomatic calcite precipitated at 62.1 to 90.0 oC with a δ13C signature of -4.64 to -8.59 ‰ (VPDB) and a δ18O signature of -11.98 to -15.08 ‰ (VPDB). These fluids were elevated in temperature, had higher salinity and a Ca-Na-Cl composition similar to a sedimentary basinal brine. Crystal calcite separated into three groups according to fluid inclusion analyses and results, which are (a) elevated-temperature (67 to 113 oC) low-salinity calcite (lower than 15.14 wt. %), (b) elevated-temperature (73.7 to 91.7 oC) high-salinity (30 to 40 wt. %) calcite, and (c) higher-temperature (179.6 to 199 oC) low-salinity (lower than 7.33 wt. %) calcite. Group (a) has a δ13C isotopic signature of -5.61 to -10.42 ‰ (VPDB) and a δ18O signature of -8.35 to -16.04 ‰ (VPDB), Group (b) has a δ13C isotopic signature of -4.64 to -8.60 ‰ (VPDB) and a δ18O signature of -12.34 to -15.04 ‰ (VPDB), and Group (c) has δ13C signature of -5.59 to -8.06 ‰ (VPDB) and a δ18O signature of -10.03 to -16.17 ‰ (VPDB). Group (a) most likely formed from a hydrothermal fluid with a meteoric water origin, Group (b) could have formed during hydrothermal fluid mixing with an evaporated seawater or basinal brine, and group (c) seems to have formed as a result of a mixture of meteoric and lower salinity metamorphic or crystalline rock fluids. Vuggy calcite precipitated at 85 to 89 oC with a δ13C signature of -7.47 to -9.04 ‰ (VPDB) and a δ18O signature of -9.32 to -10.59 ‰ (VPDB). This case is from a high temperature, high Ca-Na-Cl salinity fluid which is hydrothermal fluids mixed with basinal brines. Strontium isotopic ratios, thorium-uranium ratios and REE data associated with the fracture calcites show that they have a limited water/rock interaction with the host bedrock. Some elevated thorium or uranium concentration were sourced from specific rock types such as pegmatite intrusions in the site.