Fluid Temperature and Salinity Characteristics of the Matagami Volcanogenic Massive Sulfide District, Quebec

Abstract

Fluid inclusions hosted within various lithologic units of the >40 million metric ton (Mt) Matagami district, Abitibi greenstone belt, preserve samples of Archean volcanogenic massive sulfide (VMS) and postvolcanogenic massive sulfide hydrothermal fluid. Microthermometric measurements on ore-hosted primary two-phase liquid-vapor inclusions from Matagami’s south limb deposits indicate that the VMS hydrothermal fluid was highly saline (16.2 ± 4.7 wt % NaCl-CaCl2 equiv, 1σ, n = 230) and of moderate temperature (trapping temperature, Tt = 208° ± 32°C, 1σ, n = 230). A fluid with these characteristics is capable of transporting ~5 × 10−4 m (30 ppm) Zn as ZnCl3 − and ZnCl4 2− chloride complexes. However, the low temperature of this fluid precluded efficient Cu transport (≤ 3 ppm), which may in part explain the relatively Cu poor nature of the Matagami deposits. Calculated densities of this ore fluid as high as 1.10 g/cm3 are consistent with a bottom-hugging brine model. However, a subset of the data indicate a fluid less dense than ambient seawater, suggesting that buoyant hydrothermal plumes were also present. A microthermometrically determined high CaCl2 content (XNaCl <0.55, molar Na/Ca = 2.3/1) for the VMS ore-hosted primary fluid is consistent with an Archean hydrothermal fluid more Ca-rich than modern-day seawater. Quartz-epidote veins located in the hydrothermal cracking zone of the Bell River Complex host primary liquid-vapor-halite inclusions. These inclusions are interpreted to be samples of the deep-seated equivalent to the VMS ore-hosted hydrothermal fluid described above. Microthermometry indicates that these inclusions trapped a high-temperature brine (Tt = 373° ± 44°C, 1σ , n = 92; 38.2 ± 1.9 wt % NaCl equiv, 1σ , n = 92). We interpret this brine to be a phase-separated product of (modified) model seawater (3.2 wt % NaCl), an exsolved magmatic fluid, or a combination thereof, deep within the hydrothermal system at 650° to 670°C and a near-lithostatic pressure of 90 MPa. Phase separation and subsequent convection lowered the temperature of the brine prior to its entrapment within the hydrothermal cracking zone. The occurrence of high-temperature brine overlain by lower temperature/salinity fluid suggests a two-cell convection model, consistent with metal mass-balance calculations for the south limb. The high salinity of the ore-hosted fluid inclusions indicates two possibilities: (1) a significant amount of brine was incorporated into the upper cell and mixed with heated seawater during convection; (2) Archean seawater itself was very saline and of variable salinity. With the cooling of the Bell River Complex, lower temperature fluids, dominantly of seawater origin, circulated deep within the hydrothermal system. Modified by water-rock interaction, yet not phase separated, these fluids sealed off the remnant permeability of the fracture network of the hydrothermal cracking zone and were locally trapped as secondary liquid-vapor fluid inclusions (homogenization temperature, Th = 242° ± 17°C; 9.1 ± 1.6 wt % NaCl equiv, 1σ , n = 14) hosted within the Bell River Complex quartz-epidote vein material. Post- and/or waning-stage VMS hydrothermal activity is evident from the presence of quartz-epidote veins crosscutting Wabassee Group hanging-wall rocks. Microthermometry on quartz-hosted primary liquid-vapor fluid inclusions suggests that this activity occurred at relatively low temperatures (Th = 76°–177°C, n = 212), over a wide range of salinity (6.0–32.4 wt % NaCl-CaCl2 equiv, n = 212), and with a high apparent CaCl2 content (XNaCl <0.06). These fluid inclusion data illustrate the importance of subsea-floor chemical and physical processes directly related to metal transport and deposition in VMS systems. In particular, phase separation deep within the hydrothermal system is interpreted as a key process for generating saline brines capable of forming significant ore deposits.