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Geochemistry of ordovician black shales from the Leningrad region

Proskurnina A. V.

Geology, Geophysics and Environment

2012

Vol. 38, no. 4

519--520

The black shales zone is located in southern periphery of the Baltic Shield and forms part of the Vendian-Paleozoic platform cover. This zone, together with the Sablino, Koporye, Mozhaiskoe, Popovka, Izhora, Ust-Luga, represents a potentially large noble metal and rhenium resource in the Leningrad region. This paper presents mineralogy and geochemistry of black shales that are important estimating the potential for possible future mining. Sampling was carried out at seven points in the Leningrad region, where samples were collected from different parts of the strata. X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), inductively-coupled mass spectrometry (ICP-MS) and scanning electron microprobe (SEM) analyses were mainly used to identify the mineral species and estimate metal contents. Illite, kaolinite, chlorite, quartz, and feldspar were identified as main silicates with minor amount of iron oxides such as jarosite and goethite. Pyrite in black shale typically occurs in framboidal form and as euhedral crystals. Framboidal pyrite was found in the black shale samples from Sablino, Koporye and Ust-Luga. Such form of occurrence is typical for pyrite that is formed during early diagenesis. The content of organic matter in the studied black shale samples is 1.3-16.5 wt. %. The total rare earth elements (ΣREE) content of black shale samples ranges from 65.67 ppm to 262.95 ppm. The average REE content of 15 samples from different parts of the Leningrad region is higher than 134.19 ppm for the world black shale calculated by Ketris and Yudovich (2009). The content of rhenium in the samples is 0.14-0.25 ppm. The concentration of Re is higher in the western part of the studied area. XAS shows that concentration of the palladium group elements is higher in western part of the region, where the Pd content varies from 0.45 ppm to 0.77 ppm. Additionally, the results of this study also yield contents of U (286 ppm), V (2140 ppm), Mo (1260 ppm). Therefore, black shales as a perspective source of critical metals demand further detailed study.