We report on the mineralogy, petrography, and oxygen isotopic compositions of primary olivine and plagioclase/feldspathic mesostases in chondrules and of secondary magnetite and fayalite in chondrules and matrix of an oxidized Bali-like CV3.1 carbonaceous chondrite, Kaba. In this meteorite, compositionally nearly pure fayalite (Fa98–100) associates with hedenbergite (Fs~50Wo~50), magnetite, and Fe,Ni-sulfides. There are several textural occurrences of this mineral paragenesis: (i) coarse-grained intergrowths in interchondrule matrix, (ii) veins starting at the opaque nodules in the peripheries of type I chondrules and crosscutting fine­grained rims around them, and (iii) rims overgrowing olivine of type I and type II chondrule fragments. Oxygen isotopic compositions of fayalite and magnetite are in disequilibrium with chondrule olivines. On a three-isotope oxygen diagram, δ17O vs. δ18O, compositions of olivine plot along primitive chondrule minerals (PCM) line having a slope of ~1.0; deviations from the terrestrial fractionation line, Δ17O = δ17O − 0.52 × δ18O, range from ~−8‰ to ~−5‰. In contrast, fayalite and magnetite plot along mass-dependent fractionation line with a slope of ~0.5; their δ18O values range from −1 to ~+9‰; Δ17O is nearly constant (average ± 2SE = −1.5±1‰). Oxygen isotopic compositions of chondrule plagioclase and feldspathic mesostases are in disequilibrium with chondrule olivines: they deviate to the right from the PCM line by ~12‰ and plot close to the mass-dependent fractionation line defined by fayalite and magnetite.

The paper comprises new analytical data on the nature and occurrence of gold in solid pyrobitumen, closely associated with the main gold-bearing sulfide arsenic ores of the Bakyrchik gold deposit (Kazakhstan), related to post-collisional magmatic-hydrothermal origin. Gold mineralization of the deposit occurs mainly in the form of an “invisible” type of gold in the structures of arsenian pyrite and arsenopyrite, and the form of gold-organic compounds of pyrobitumen in carbonaceous-terrigenous sequences of Carboniferous formation. Microscopic and electron microscopic analysis, Raman and FT-Infrared analysis, mineralogical and three-step sequential extraction analysis (NH₂OH·HCl, H₂O_2,HNO₃ + HCl) has been carried out using 9 ore samples (from 3 different types of ores) for a comprehensive study of pyrobitumen and sulfide arsenic ores focusing mainly on organic matter. The sequentially extracted precious metal content of pyrobitumen reaches up to 7 ppm gold and other metals like Ag 4 ppm, Pt 31 ppb, and Pd 26 ppb, forming metal–organic compounds, while arsenic sulfide minerals incorporate 11 ppm gold, 39 ppm Ag, 0.49 ppm Pt. The enrichment of gold associating with organic matter and sulfide ore minerals was confirmed in this study. Organic matter was active in the migration of gold and the capture of gold by pyrobitumen. Moreover, the reductive organic matter agent released gold, most likely for the sulfide arsenic ore minerals. Pyrobitumen was a decisive factor in the concentration, transportation, and preservation of gold in the deposit.

Detailed mineralogical and geochemical study of red mud samples from Hungary suggests geological and geochemical processes that determine the spatial distribution of certain elements inside the red mud pitfalls. The major processes are the following: (1) Heavy mineral grains (anatase, rutile, titanomagnetite, etc.) tend to subside due to gravitational differentiation and at present accumulate in the deepest horizons of the pitfalls. (2) Kaolinite reacts to cancrinite under hyperalkaline conditions. (3) Due to diagenetic processes, goethite-cancrinite aggregates form in situ. (4) Light mineral grains (e.g., cancrinite) move upward. (5) Cancrinite transforms to calcite at the shallowest horizons, due probably to the reaction with atmospheric CO₂. All these processes have a significant role in accumulation tendencies of different groups of elements inside the pitfalls. The behaviour of chalcophile elements and the HFSE elements follow common geochemical rules and remind features of the host bauxite or even its precursor igneous or metamorphic lithologies. The REEs and Sc are possibly adsorbed on goethite and in the channels of cancrinite. Based on linear mixing model calculations, the major container of these elements is cancrinite. The proportion of the REEs and Sc in the Ti-phases, carbonates, phosphates, zircon, etc. is subordinate relative to the amount accumulated by goethite and cancrinite.