Geochemistry and metal potential of triassic carbonaceous silicites in Sikhote Alin

Abstract

In the Triassic siliceous formation of Sikhote Alin, carbonaceous silicites occur in the late Olenekian-middle Anisian member (4–20 m) of alternating cherts and clayey cherts (“phtanite member”) near the section base. The silicites are represented by radiolarian and spicule-radiolarian cherts alternating with clayey cherts. They contain up to 8.5% Corg. In the majority of sections, the rocks underwent structural and mineral transformation at the mesocatagenetic stage. The slightly oxidized organic (primarily, marine sapropelic) matter contains quinones, methyl, methylene, and ether groups. The content of neutral bitumens in rocks shows a wide variation range. The carbon isotopic composition in phtanites and clayey phtanites (δ13C from −27.3 to −30.2‰) is identical to that in many Paleozoic-Mesozoic bitumens and oils. As compared with other Mesozoic sedimentary rocks of Sikhote Alin, the carbonaceous silicites are enriched in V, B, Mo, Ni, Cu, and Ag. Anomalously high concentrations of Ba are recorded in phtanite rock sections at the Gornaya and Khor rivers and in the vicinity of Khabarovsk. Modal value of the Au content in phtanites and clayey phtanites is three or four times higher than the clarke value in carbonaceous silicites and reaches anomalous values in some sections (e.g., Ogorodnaya River section). Carbonaceous silicites of this section are also enriched in Pt. Positive Au-Corg correlation is recorded in clayey phtanites of the Ogorodnaya River section containing more than 0.5% Corg. In organic fractions, Au and Ag are concentrated in alcohol and alcohol-benzene bitumens, asphalt acids, and asphaltenes. Migration of bitumens from high-carbonaceous clayey phtanites to the pore-fissure space of cherts and phtanites also fostered the concentration of these metals in some low-carbonaceous layers of the member.