Abstract
Marly limestones from the Lower Silurian sedimentary units of the Tunguska basin (East Siberia, Russia) underwent metamorphism along the contact with the Early Triassic Kochumdek trap intrusion. At ≤ 2.5 m from the contact, the limestones were converted into ultrahigh-temperature marbles composed of pure calcite and sulfide-bearing calcsilicate layers. The sulfide assemblages in the gabbro and marbles were studied as potential tracers of spurrite-merwinite facies alteration. The gabbro-hosted sulfides show Fe-Ni-Cu-Co speciation (pyrrhotite and lesser amounts of chalcopyrite, pentlandite, and cobaltite) and positive δ34S values (+2.7 to +13.1‰). Both matrix and inclusion sulfide assemblages of prograde melilite, spurrite, and merwinite marbles consist dominantly of pyrrhotite and minor amounts of troilite, sphalerite, wurtzite, alabandite, acanthite, and galena. In contrast to its magmatic counterpart, metamorphic pyrrhotite is depleted in Cu (3–2000 times), Ni (7–800 times), Se (20–40 times), Co (12 times), and is isotopically light (about –25‰ δ34S). Broad solid solution series of (Zn,Fe,Mn)Scub, (Zn,Mn,Fe)Shex, and (Mn,Fe)Scub indicate that the temperature of contact metamorphism exceeded 850–900 °C. No metasomatism or S isotope resetting signatures were detected in the prograde mineral assemblages, but small-scale penetration of magma-derived K- and Cl-rich fluids through more permeable calcsilicate layers was documented based on the distribution of crack-filling Fe-K sulfides (rasvumite, djerfisherite, and bartonite).
Highlights
Petrological analysis of metamorphic rocks commonly aims to reconstruct peak metamorphic conditions and P-T paths from data on rock-forming minerals [1,2,3,4,5]
This study addresses several issues related to the diverse sulfide mineralization in marbles from the Kochumdek River and in the Kochumdek trap intrusion:
We focused the study on sulfides in the aureole rocks, largely due to their diversity where in addition to ubiquitous pyrrhotite, the sulfide assemblage includes numerous rare phases, each being a potential proxy of growth conditions
Summary
Petrological analysis of metamorphic rocks commonly aims to reconstruct peak metamorphic conditions and P-T paths from data on rock-forming minerals [1,2,3,4,5]. The metamorphic conditions of the spurrite-merwinite facies are useful for examining the formation of high-temperature phases that accumulate trace elements [8,9,10]. Large variations in the major- and trace-element compositions and isotopic δ34S values of sulfides have implications for the source of material involved in the formation of metamorphichosted sulfides [28]. Sulfides re-equilibrate and re-crystallize rapidly in response to changes in their growth medium and are sensitive to material transport across chemical/geological interfaces. In this respect, sulfides may be independent sources of information on mass transfer between sedimentary and igneous rocks during contact metamorphism [3,8], but to our knowledge have never been used for such purpose
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