Comparative stable isotope and temperature investigation of minerals and associated fluids in two regionally metamorphosed (Kuroko-type) volcanogenic massive sulfide deposits

Abstract

The B.M.&S. No. #12 deposit in New Brunswick, Canada, and the Woodlawn deposit in N.S.W., Australia, are examples of Lower Palaeozoic volcanogenic massive sulfide deposits that have been extensively deformed and metamorphosed at moderate and low grades of greenschist facies regional metamorphism. These deposits yield similar stable isotope signatures for carbonates and quartz deposited during metamorphism, and also for fluid inclusion waters contained in these minerals. In both deposits the metamorphic quartz and carbonates haveδ 18O values of ∼ 8–13.6‰ and 11–12.6‰, respectively. The carbonates haveδ 13C values of -0.2 to -12‰ and 87Sr/ 86Sr ratios between 0.7073 and 0.7115. Bulk fluid inclusion waters extracted from the quartz and carbonates have δD values of − 87 to −169‰ andδ 18O values of −14 to −32‰ The B.M.&S and Woodlawn fluids are mainly water-NaCl liquids with salinities averaging around 8 and 6.5 wt.%, respectively., but range up to ∼ 38 wt.% in a small population of three phase inclusions in one B.M.&S sample. Regional metamorphism is suggested to have generated these salinites through water losses occurring in response to periodic boiling of fluids of mainly marine derivation, combined with possible contributions from waters liberated through thermal cracking of organic matter contained in the associated black shales. Exchange between fluids and organic matter may account for the negativeδ 13C and δD values shown by the metamorphic carbonates and waters, respectively. Strontium isotope ratios in the B.M.&S. carbonates span the full range of variation. This implies appreciable fluid mobility and exchange with other sources of strontium during moderate grade greenschist facies metamorphism. Selected sulfur isotope data for massive sulfides in the New Brunswick and Woodlawn deposits suggest attainment of isotopic equilibrium within the metamorphic temperature ranges ∼ 370-270°C and ∼ 320-200°C, respectively. Fluid inclusion microthermometry yields respective homogenization (minimum) temperatures of ∼ 440-120°C and ∼ 350-120°C for fluids contained in the metamorphic quartz and carbonates. The appreciable overlap between these geothermometers is attributed to deformation-induced rupture-and-seal events which involved the fluid inclusions, and to accompanying recovery and recrystallization processes which locally affected both fluid inclusions and the massive sulfides during retrograde cooling from peak regional metamorphic T P conditions. For the B.M.&S. deposit, estimated pressures of metamorphic fluid entrapment range from around 2000 to 200 bars, and are therefore well below a maximum metamorphic pressure of ∼ 4 kbars determined from mineralogical barometry.