Element distribution in the root zone of ultramafic-hosted black smoker-like systems: Constraints from an Alpine analog

Abstract

Fluid-rock interactions at Mid-Oceanic Ridges lead to metal deposition in the so-called seafloor massive sulfides at ultramafic-hosted systems. Due to restricted access to the seafloor and scarcity of 3D exposures, these systems are poorly understood at-depth. A way to access the vertical dimension is to focus on fossil analogs preserved on-land such as the one preserved in serpentinites from the Platta nappe (SE Switzerland). For this example, we document the element distribution in the mineralized rocks at three distinct levels in the rock column using both whole rock chemical analyses and LA-ICPMS analysis of sulfide (chalcopyrite and pyrrhotite) and magnetite. We bring, for the first time, chemical signatures of the mineralization in the root zone of ultramafic-hosted black smokers. At any given depth, the Co/Ni ratio is maximum in the most mineralized samples indicating that this ratio is linked to the intensity of hydrothermal alteration. Additionally, the Co/Ni ratio decreases in mineralized rocks towards the paleosurface, whereas the Se content increases. An episode of carbonation recorded in the highest structural level of the system was responsible for a slight remobilization of the former Cu stock. We propose a model in which the uprising mineralizing fluid mixed with seawater within the host serpentinites, before venting at the paleoseafloor.

Introduction

Mineralized systems have been recognized and studied in many tectonic settings. At spreading centers, the evidence of active hydrothermalism leading to metal deposition is recorded through the formation of the so-called high-temperature (HT; i.e. > 300 °C) black and “low-temperature” (LT; i.e. 250–300 °C) white smokers (Hannington et al. 1995; Tivey 2007). Over the last three decades, the number of mineralized systems reported along mid-oceanic ridges has considerably increased reinforcing the statement that hydrothermalism is a fundamental process at all spreading-rate mid-oceanic ridges (Beaulieu et al. 2013).

HT black smokers have been firstly recognized along the fast-spreading East Pacific Rise (EPR) in late 70's (Cyamex, 1979). The more recent exploration of the slow-spreading Mid-Atlantic Ridge and ultraslow-spreading South West Indian Ridge allowed to identify HT black smoker venting at lower spreading-rate ridges (Rona 1985; Fouquet et al. 2010; Tao et al. 2012). In these settings, black smokers can be either associated with sedimentary rocks (Zierenberg et al. 1993), mafic rocks (Hannington et al. 1995) or ultramafic rocks where mantle exhumation is accommodated by detachment faulting (i.e. along the oceanic core complexes at slow-spreading ridges, Fouquet et al. 2010).

Because of the nature of the host rock, mineralization at mafic-hosted and ultramafic-hosted hydrothermal systems are slightly different. They consist of Fe-Cu-Zn-rich deposits in mafic settings and Cu-Co-Zn-(Ni)-(Au)-rich deposits in ultramafic settings (e.g. Fouquet et al. 2010. Along the Mid-Atlantic Ridge, these systems were extensively studied through mineralogical (Hannington et al. 1995; Marques et al. 2006), geochemical (Charlou et al. 2002; Rouxel et al. 2004a) or tectonic approaches (McCaig et al. 2007). However, due to restricted access to the seafloor on which the vertical dimension is rarely accessible, ultramafic-hosted mineralized systems have been mainly studied in two dimensions preventing recognition of elemental distribution below the venting site. Hence, this inhibits the complete understanding of the hydrothermal processes forming these systems.

Fossil mineralized systems outcropping on-land represent good opportunities to decipher these processes. Ultramafic-hosted systems have been recognized worldwide in the Bou Azzer ophiolite in Morocco (Leblanc and Billaud 1982), in Cyprus (Foose et al. 1985; Talhammer et al. 1986), in the Northwestern American Cordillera (Foose, 1986; Candela et al. 1989), in the Eimeshan igneous province in China (Song et al. 2003), in the Outukumpu thrust belt in Finland (Peltonen et al. 2008), in the Urals (Nimis et al. 2008; Maslennikov et al., 2017) and in the Platta nappe (Dietrich 1972; Coltat et al. 2019b). However, for the most part, the primary extensional structures have been disturbed during subsequent deformation and metamorphism and the metallic stock has been locally partially reworked. In some cases, the origin of the mineralization is hard to retrieve (Foose, 1986; Talhammer et al., 1986; Song et al. 2003).

In the Platta nappe, Coltat et al., 2019b recognized a fossil Jurassic ultramafic-hosted black smoker-like system. The overall geometry of this Marmorera-Cotschen hydrothermal system (MCHS) has been preserved from the Alpine overprint. Also, the structural relationships between mineralization, mafic intrusions and detachment fault can be restored. We propose here, based on this geometry, to evaluate the elemental distribution at different structural positions, i.e. at distinct paleo-depths below the detachment, through in-situ LA-ICPMS on metal-bearing phases (chalcopyrite, pyrrhotite, magnetite) and whole rock geochemistry. The geochemical signatures of the MCHS are compared with the ones reported at present-day seafloor massive sulfides (SMS) to better decipher the processes that occur during the ascent of hydrothermal fluids. On this basis, we propose that the mineralizing fluids mixed with seawater within the host serpentinites on their way up to the seafloor.