Published online by Cambridge University Press: 02 April 2024
During the epigenetic alteration of boehmitic karstic bauxite in the Vlasenica region of Yugoslavia, which was caused by siliceous water descending through the deposit by means of cracks, fissures, etc., oolites were less altered compared with the coexisting matrix material, which was intensely kaolinized. The following zonal alteration pattern of the oolites was identified: a diaspore enrichment zone, followed by a diaspore-boehmite enrichment zone, and then the original bauxite. Considering this alteration pattern and that determined previously for the matrix material, the following overall pattern of alteration was established: a kaolinite-diaspore zone, followed by a boehmite-diaspore zone, and then the original bauxite. The alteration solutions reacted with the bauxite to form solution-matrix and solution-oolite subsystems, which were characterized by different types of diffusion and geochemistry.
In the kaolinite-diaspore zone Si-metasomatism kaolinized the matrix, and excess Al partly migrated into the oolites, forming diaspore, and outward into the bauxite. The latter Al-remobilization resulted in a transition zone, in which new boehmite was formed in both the matrix and the oolites, and diaspore only in the oolites. Thermodynamic models of oolites and whole bauxite alterations were established, on the basis of stability diagrams calculated for the mineral assemblages of both the oolites and the matrix, in the alteration zones and in the deposit as a whole. Based on these models, the genesis of the diaspore and the relationship of diaspore-boehmite-kaolinite assemblages in natural systems at low temperature and low pressure can be described.
Tokom epigenetske alteracije bemitskih karsnih boksita u području Vlasenice, izazvane perkolacijom voda obogaćenih rastvorenom silicijom koje su ponirale kroz pukotine, rasedne zone i si., ooliti su slabije alterisani u poredjenju sa koegzistentnom osnovom koja je intenzivno kaolinisana. Identiflkovan je sledeći tip zoname alteracije za oolite: zona obogaćenja dijasporom, iza koje siede zona obogaćenja dijasporom i bemitom i zatim originalní boksit. Na osnovu razmatranja ovog tipa alteracije i tipa alteracije za materijal osnove, ustanovljen je sledeći tip alteracije boksita kao celine: kaolinitsko-dijasporska zona, iza koje siede bemitsko-dijasporska zona i zatim originalni boksit. Alteracioni rastvor je reagovao sa boksitom obrazujući kompleksan sistem, koji se sastojao od dva podsistema: rastvor-materijal osnove i rastvor-ooliti, koji su se medju sobom razlikovali kako po modelima difuzije tako i po geohemijskim karakteristikama.
U kaolinsko-dijasporskoj zoni Si-metasomatizam je doveo do kaolinizacije materijala osnove, a višak Al je delom migrirao kako u koegzistentne oolite, obrazujući u njima dijaspor, tako i dublje u boksit. Remobilizacija Al dublje u boksit dovela je do obrazovanja prelazne zone, u kojoj se obrazovao bemit i u osnovi i u oolitima, i dijaspor samo u oolitima. Postavljeni su termodinačički modeli za alteraciju oolita i boksita kao celine, a na osnovu dijagrama stabilnosti proračunatih za mineralne asocijacije i oolita i osnove, u zonama alteracije i u samom ležištu. Na osnovu svega ovoga razmotreni su tačnije i pouzdanije geneza dijaspora i odnos dijaspor-bemit-kaolinit u prirodnim sistemima na niskim temperaturama i niskim pritiscima.