Composition and genesis of accessory mineralization in manganese silicate rocks of the Triassic Sikhote-Alin chert formation

Manganese silicate rocks together with silicate-magnetite ores and jaspers (Late Anisian-Ladinian) form lenticular or tabular bodies in the Triassic Sikhote-Alin chert formation. The lower part of the formation (Olenekian-Early Anisian) is enriched in clayey and organic matter. Nickel and cobalt compounds and other ore minerals in the Sikhote-Alin manganese silicate rocks belong to two genetic groups including minerals of the valence and ultimately reduced Ni, Co, and other metals. Minerals of the valence species of Ni, Co (sulfoantimonides, sulfoarsenides, sulfides, antimonides, arsenides, tellurides, and silicates), and other metals (galena, sphalerite, chalcopyrite, arsenopyrite, wolframite, scheelite, molybdenite, cassiterite, stannite, cinnabar, stibnite, boulangerite, jamesonite, bournonite, lollengite, bismuthite, fahlore, altaite, native Sb and Bi, etc.) formed from the protolith material during metamorphism under the same conditions as the rock-forming minerals. The presence of minerals of ultimately reduced Ni, Co (maucherite, native Ni, Ni phosphide, Ni and Co chromides, disordered solid solutions, and intermetallic compounds of Ni with Cu, Zn, Sn, and Pb), and other metals (native Pb, Zn, Fe, Sn, Se, Au, Pt, "cupriferous gold", and intermetallic compounds of Cu, Sn, Pb, Sb, Al, and Zn) in the manganese silicate rocks is due to the influence of the organic matter of the underlying carbonaceous silicites. During metamorphism, the most volatile components (first of all, poorly bound water and hydrocarbons) were released from the heated carbonaceous rocks; as a result, a metal-enriched fluid with highly or ultrahighly reducing properties appeared, which migrated along fractures into other rocks. The manganese silicate rocks are the products of contact metamorphism of siliceous rhodochrosite rocks formed through the diagenesis of biogenic siliceous muds enriched in Mn oxides and organic matter. Erosion of the weathering crust of islands composed of gabbroids of the Kalinovka, Vladimiro-Aleksandrovskoe, and Sergeevka complexes (in the late Middle Triassic-Late Triassic) played the leading role in the formation of metalliferous sediments. Manganese silicate rocks localized in the stratigraphic column above the carbonaceous silicites of the Triassic chert formation are enriched in Au (up to 35.38 ppm), Pt (11.27 ppm), and Pd (5.33 ppm). They contain noble-metal minerals and a wide spectrum of native elements and intermetallic compounds. The presence of Au-Pd-Pt-Ni-Co association (typomorphic for basic and ultrabasic rocks) in the Triassic protoliths of the manganese silicate rocks and carbonaceous silicites is probably due to the sorption of these elements by Mn and Fe hydroxides and organic matter during the exogenous weathering of the ancient Sikhote-Alin gabbroids.