Oxidation and decomposition of stratiform SedEx sulfidic mineralization in the epidote-amphibolite facies producing cassiterite, V-rich micas, In-Sn-Ag-Sb-Pb-Bi-Zn-Fe-As-Cu-Ni-Co sulfides and Fe-Ca-Pb carbonates in situ (Bystrý potok locality, Gemeric unit, W. Carpathians)
Authors:   State Geological Institute of Dionýz Štúr, Mlynská dolina 1, Sk-817 04 Bratislava, Slovakia, Radvanec Martin , Holický Ivan ,   State Geological Institute of Dionýz Štúr, Mlynská dolina 1, Sk-817 04 Bratislava, Slovakia, Gonda Stanislav ,   State Geological Institute of Dionýz Štúr, Mlynská dolina 1, Sk-817 04 Bratislava, Slovakia
Published: January 2023

The studied area of Bystrý potok locality (Gemeric unit, W. Carpathians) is built of lenses of lydites and limestones, being a part of the graphite bearing phyllites of Silurian Holec Beds. The Holec Beds represent the bedrock of the albite-quartz bearing keratophyre (trachyte sensu IUGS classif.) sequence with stratiform SedEx (sedimentary-exhalation) sulfidic mineralization in the Gemeric Lower Paleozoic stratigraphy. This SedEx mineralization was contemporaneous with Late Silurian–Devonian keratophyre/basalt magmatism and originated at 280–340 °C from the seafloor exhalates in Lower Paleozoic rift magmatism. The Silurian-Devonian rock sequences were metamorphosed in Permian. The limestone lenses were altered to Permian skarn in the epidote-amphibolite facies (526–546 °C, 3–6 kbar) and in related chlorite-apatite zone (420–540 °C) they consisted of garnet Grs41.4-60.2Sps19.8-32.8Alm16.7-22.2Adr0-8.5, hedenbergite Wo44.5-50.1Fs31.6-38.2En12.7-20.7, epidote, actinolite, fluorapatite, titanite, chlorite, ankerite and siderite. The disseminated original millerite was partially replaced by hauchecornite, hauchecornite-(Sb) and cobaltite was formed in the silicate matrix of the skarn. The Permian metamorphism of the epidote-amphibolite facies and the chlorite-apatite zone has also released a fluid phase rich in O2, H2O, CO2, H3PO4, H2S, HF and V from the organic matter-bearing rock (Holec Beds) into the overlying bed with keratophyres and primary stratiform SedEx sulfidic mineralization where V-rich micas, V-rich chlorite and schreyerite formed. The primary stratiform SedEx sulfidic mineralization composed predominantly of pyrite 1 less pyrrhotite, chalcopyrite, sphalerite 1, galena 1, arsenopyrite, ferrokesterite, stephanite, gudmundite, bismuthinite 1, PbBiSb-rich sulphide (A2B2S5-type) and kobellite has been oxidized and decomposed by this fluid phase to form a new metamorphic minerals in situ. The pyrrhotite decomposed to form szomolnokite, pyrite 2, goethite and siderite. The galena 1 was partly oxidized and decomposed to anglesite and cerussite. The In-rich ferrokesterite, Sn-rich sakuraiite and In-rich sphalerite 1 oxidized, decomposed, and reacted with the fluid phase to form cassiterite, chalcopyrite, In-rich sakuraiite and sphalerite 2. The PbSbBi-rich sulfide (A2B3X6-type), wittite, bismuthinite 2, native Bi and Se-rich galena 2 are new minerals, formed by the decomposition of original PbBiSb-rich sufhide (A2B2X5-type), kobellite and bismuthinite 1. Tetrahedrites 1–3 ranging from tetrahedrite-(Fe), kenoargentotetrahedrite-(Fe) to rozhdestvenskayaite-(Fe) show a gradual ordering in three separate zones controlled by immiscibility gaps in the Cu-Ag substitution of tetrahedrite group. These tetrahedrites were formed by the decomposition of the original SedEx ferrokesterite, stephanite and gudmundite. The gudmundite was also commonly oxidized to form valentinite and the decomposition of ferrokesterite, galena 1 and gudmundite also produced bournonite and plumosite-like. Altogether 22 reactions illustrate the relationship between the source SedEx mineralization and the new metamorphic minerals, which have formed at the expense of the original stratiform SedEx sulfidic mineralization in situ. The metamorphogenic fluid phase, released from the Holec Beds, has been enriched with additional elements from the stratigraphic horizon of the stratiform SedEx sulfidic mineralization and is genetically part of the Permian metamorphic-magmatic-hydrothermal (MMH) cycle (281–256 Ma).