Sulfide Minerals as Potential Tracers of Isochemical Processes in Contact Metamorphism: Case Study of the Kochumdek Aureole, East Siberia

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).