Rhenium–osmium abundance and isotopic compositions of massive sulfides from modern deep-sea hydrothermal systems: Implications for vent associated ore forming processes

Zhigang Zeng,Shuai Chen,David Selby,Xuebo Yin,Xiaoyuan Wang

doi:10.1016/j.epsl.2014.04.017

Abstract

Studies of rhenium (Re) and osmium (Os) concentrations and isotopic compositions in seafloor hydrothermal sulfides are an important tool for understanding the evolution of hydrothermal systems, allowing the determination of both metal sources and reconstructing the physicochemical conditions of their deposition. The Re–Os concentrations and isotopic compositions of 38 massive sulfide samples have been studied in different hydrothermal fields from the East Pacific Rise (EPR), Mid-Atlantic Ridge (MAR), Central Indian Ridge (CIR), Southwest Indian Ridge (SWIR), and Back-Arc Basin (BAB). The majority of the sulfides possess 187Os/188Os that span a narrow range (1.004 to 1.209), which is most easily explained as a seawater-derived component. This may suggest that those initial 187Os/188Os isotope compositions of ancient seafloor hydrothermal sulfide deposits which were formed by the mixing process between seawater and hydrothermal fluid, are possible for analysing ancient seawater Os components. Only two of samples (MAR05-TVG1-10-2 and MAR05-TVG1-21 from the Logatchev hydrothermal field (LHF), MAR) possess moderately less radiogenic 187Os/188Os (0.645 to 0.730), which may reflect the lesser extent of hydrothermal fluid–seawater mixing during hydrothermal ore-forming process.The Re and Os concentrations and 187Re/188Os ratios of pyrite and Fe–Cu sulfide mineral aggregates (avg Re 11.46 ppb; avg Os 17.76 ppt; avg 187Re/188Os 11,980.4; n=24) are usually higher than those of sphalerite or Zn-enriched sulfide mineral aggregate samples (avg Re 0.31 ppb; avg Os 7.09 ppt; avg 187Re/188Os 206.99; n=9), suggesting that Re and Os are more likely to be incorporated into Fe- and Fe–Cu sulfide mineral facies. A significant positive correlation is observed between Os/Re ratios and Pb concentrations in the sulfide samples from the Edmond hydrothermal field (EHF), whereas Pb-enriched sulfide is characteristic of low-temperature paragenetic association, suggesting that Os enriched under low-temperature (<200 °C) condition. In addition, our Re–Os data are used to estimate that modern seafloor sulfide deposits contain roughly 0.6–44 t (avg 4 t, n=38) of Re, and 1 to 48 kg (avg 8 kg, n=38) of Os. The Os flux of global low-temperature hydrothermal fluids to vents is about 11 kg/yr, and the excess Os (i.e. Os not incorporated in sulfides) may be carried and become bound in metalliferous sediment, Fe–Mn crusts and nodules distal to the hydrothermal vents.

Highlights

E range of the Miocene seawater 187Os/188Os value (0.73 to 0.85; Peucker-Ehrenbrink et al., 1995), suggesting that the Os of the Kuroko ore deposits in the northeast Japan was mainly derived from seawater (Terakado, 2001a)
The radiogenic Os compositions (0.566–1.054) of sulfide samples from the TAG hydrothermal field indicate that there is a variable mixing of ocean crust-derived Os and seawater Os, and this mixing is controlled by the redox conditions (Brügmann et al, 1998; Ravizza et al., 1996)
Analytical techniques developed by Birck et al (1997) have allowed precise measurements of Os at a few tens of parts per trillion levels, enabling the analysis of more common sulfide samples (Gannoun et al, 2003)

Summary

Introduction

E range of the Miocene seawater 187Os/188Os value (0.73 to 0.85; Peucker-Ehrenbrink et al., 1995), suggesting that the Os of the Kuroko ore deposits in the northeast Japan was mainly derived from seawater (Terakado, 2001a). In the late Triassic Gacun volcanogenic massive sulfide deposit, southwestern China, the Re-Os isotopic compositions of sulfide ores yield a 8-point isochron age of 217 ± 28 Ma, with an initial 187Os/188Os of 0.52 ± 0.73, suggesting a mixing of hydrothermal fluid and late Triassic seawater (Hou et al, 2003). The Re-Os isotopic composition of seafloor hydrothermal sulfides can provide important information on metal and fluid sources, mixing between hydrothermal fluid and seawater, and geochemical processes, little is known about the Re-Os isotopic composition of sulfides from seafloor hydrothermal systems in mid-ocean ridge (MOR) and back-arc basin (BAB), due to their low concentrations (Re < 0.2 ppb; Os < 2 ppt) in sulfides and the difficulty in obtaining pure sulfide samples (e.g. fine-grained intergrowth and the removal of other minerals such as sulfates and oxides).

Results

Discussion

Conclusion