Unravelling the root zone of ultramafic‐hosted black smokers‐like hydrothermalism from an Alpine analog

Abstract

Mid‐Ocean Ridges host various types of hydrothermal systems including high‐T black‐smokers found in ultramafic rocks exhumed along slow spreading ridges. These systems are mostly described in two dimensions as their exposure on the present‐day seafloor lacks the vertical dimension. One way to understand these systems at depth is to study their fossilized equivalents preserved on‐land. Such observation can be done in the Platta nappe, Switzerland, where a Jurassic‐aged mineralized system is exposed in 3D. Serpentinites host a Cu‐Fe‐Ni‐Co‐Zn‐rich mineralization made of sulphides, magnetite and Fe‐Ca‐silicates either replacing serpentinites or within stockwork. Fe‐Ca‐silicates, abundant at the deepest levels, vanish in the mineralization close to the palaeo‐detachment. Fluids were channelized along mafic dykes and sills acting as preferential drains. Warm carbonation (~130°C) is the latest hydrothermal record. We propose that this system is an analog to the root zone of present‐day serpentinite‐hosted hydrothermal systems such as those found along the Mid‐Atlantic Ridge.

The syn‐rift tectono‐stratigraphic record of rifted margins (Part I): Insights from the Alpine Tethys

“…It also preserves remnants of a proto‐Oceanic Core Complex (OCC; Epin et al., 2019). The entire exhumed mantle domain is characterised by intense hydrothermal fluid activity and precipitation of metalliferous mineralisation (Coltat, Boulvais, et al., 2019; Coltat, Branquet, et al., 2019). Extensional structures found in the lower Platta Unit are characterised by polyphase extensional detachment faults, truncated by late, high‐angle normal faults (Figure 8c; Coltat et al., 2020; Epin et al., 2019).…”

The nature of the interface between basalts and serpentinized mantle in oceanic domains: Insights from a geological section in the Alps

“…In line with these findings, the mafic extrusive rocks show typical pumpellyite-bearing assemblages (Dietrich, 1969;Dietrich et al, 1974). In this low-grade northern part of the nappe, syn-kinematic greenschist facies (albite-epidote-chlorite) assemblages are also found at the base of some of the extrusive bodies, within deformed basalts that overlie serpentinites, like at the Marmorera outcrop (Epin et al, 2019) and on the site of this study (Coltat et al, 2019b) (Fig. 2b).…”

“…Red stars point out the location of established Jurassic detachment plane juxtaposing basalts onto serpentinite where carbonation has been observed (Epin et al, 2019, Coltat et al, 2019b. (c) Roughly E-W oriented section of the Falotta area corresponding to a ~1500m in length of almost continuous crest.…”

Origin of Fe‐Ca‐Metasomatism in Exhumed Mantle Rocks at the MARK Area (23°N, ODP Leg 153) and Implications on the Formation of Ultramafic‐Hosted Seafloor Massive Sulfide Deposits

“…Fe-Ca metasomatism occurs either along peridotite-magmatic dykelets contacts or in replacement of the former pyroxenite veins, and is associated with little mineralization (magnetite, pyrite, pyrrhotite, and chalcopyrite; Gaggero et al, 1997). Petrographic and structural features at MARK thus show strong analogies with fossil Alpine systems, suggesting somewhat similar hydrothermal processes and geological history, even if the latter are more mineralized (Coltat et al, 2019;Toffolo et al, 2017). Hence, the MARK area represents an undisturbed and in situ oceanic system without metamorphism related to late orogenic overprint from Alpine systems.…”