Hydrothermal fluid chemistry and flow paths associated with the Archean Matagami VMS system, Abitibi subprovince, Canada

Cathodoluminescence reveals that volcanogenic massive sulfide (VMS)-related quartz from the ∼2.7 Ga Abitibi greenstone belt, Canada, is of preserved hydrothermal origin. Fluid inclusions hosted within the quartz preserve samples of Archean VMS-related hydrothermal fluid. Microthermometry on ore-hosted primary liquid-vapor inclusions from the Matagami deposits indicates that the VMS hydrothermal fluid was highly saline (16.2 +/- 4.7 eq.wt.% NaCl-CaCl 2 [1sigma; n = 230]) and of moderate temperature (Tt = 208 +/- 32°C [1sigma; n = 230]).; Quartz-epidote veins located in the hydrothermal cracking horizon (HCH) of the Bell River Complex (BRC) at Matagami host primary liquid-vapor-halite inclusions. These inclusions are samples of the deep-seated equivalent to the VMS ore-hosted hydrothermal fluid and were trapped as high-temperature brines (Tt = 373 +/- 44°C [1sigma; n = 92]; 38.2 +/- 1.9 eq.wt.% NaCl [1sigma; n = 92]) interpreted as phase separated products of (modified) seawater, an exsolved magmatic fluid, or a combination thereof deep within the hydrothermal system at ∼650°C and a near-lithostatic pressure of 90 MPa.; The HCH represents a discrete 350 m thick zone of high original permeability (10-10 to 10-8m2; maximum model values) located within a strongly layered gabbro-anorthosite horizon of the Layered Zone, upper BRC. The base of this interval is 1,000 m below the top of the BRC. Mass balance modeling suggests that the HCH could have been a primary source from which hydrothermal fluids leached metals.; X-ray diffraction (XRD) has identified large VMS-related hydrothermal alteration signatures, which extend well beyond (∼1,000+ m) the limits of the Matagami deposits. These signatures include hanging wall alteration assemblages probably produced during the waning stages of VMS hydrothermal activity, which are characterized by relatively low temperature fluids (Th = 76° to 177°C [n = 212]) of variable salinity (6.0 eq.wt.% to 32.4 eq.wt.% NaCl-CaCl2 [n = 212]) and high apparent CaCl2 content (XNaCl < 0.06). Bulk whole-rock and chlorite microprobe analyses have identified trends associated with footwall alteration. Progressive alteration is characterized by a decrease (up to 35%) in silicification (SiO 2), as well as by increases in MgO and Fe2O3 (up to 6500%) as chlorite +/- talc is formed.