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7851017 
Journal Article 
Mineralogy of ferromanganese weathering crusts from Codru Moma Mountains (Romania) 
Mosonyi, E; Ghergari, L; Marza, I 
2021 
1 
Neues Jahrbuch fuer Mineralogie. Abhandlungen
ISSN: 0077-7757 
197 
1 
85-103 
WOS:000613191000006 
Fe-Mn-enriched alteration deposits of Bohnerz type were developed in paleokarst cavities of Triassic dolomitic limestone from Tauti-Ponoras zone (Moneasa-Dezna area in Codru Moma Mountains). The topic of this study is the petrographically, mineralogically and geochemically characterization of these Fe-Mn rich sial weathering crusts establish its genetic processes and identify possible Fe and Mn sources by applying optical microscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), classical wet chemical and thermal analyses (TG-DTA = thermal gravimetric-Differential Thermal Analysis). Based on geochemical data processing and Mn mineralogy - (braunite, manganosite, pyrolusite, psilomelane) - the Mn source of the fossil weathering crust was established. It appeared to be a heterolithological substrate. During Cretaceous and Pliocene uplift and humid to intermediate climatic conditions, mineral weathering were controlled by supergene and hydrothermal processes. The supergene mineralizations were formed by (i) the dissolution of iron-bearing carbonate rocks and (ii) silicate detrital depositions due to descending meteoric and ascending thermal waters along the fissures and fracture systems. This weathering crust was probably overprinted by Neogene igneous hydrothermal depositions. Beside vertical tectonic movements and geochemical composition of heterolithological substrate, climatic variations (temperature, wet and dry periods) appeared to play an important role for karstification, supergene weathering and covering of the karst surface by Fe-Mn crusts and nodules. Sialitization that included argillization took place under both aliochthonous (non-carbonatic rock weathering) and autochthonous (in karst/paleokarst cavities alteration) conditions. Within the karst/palaeolcarst cavities, pH, Eh and draining conditions were variable leading to: Kaolinization, halloysitization, illitization of aluminosilicate minerals from sedimentary and igneous clasts; kaolinite-halloysite 7 angstrom transformations by progressive hydration and formation of soluble Fe-Mn compounds. These products were overprinted by Neogene hydrothermal waters ascending along fractures and fissures associated with supergene oxidation and hydrolysis of disseminated, hydrothermal iron sulfides (pyrite, marcasite) and repeated precipitation of Fe and Mn oxy-hydroxides. At later stage mainly occurred during extreme oxidation (Eh > 0.7V) under neutral pH conditions owing to different geochemical thresholds. 
paleosoil; paleokarst; sialitization weathering; kaolinization; halloysitization; ferromanganese nodular ore; Mn-source; Codru Moma; Apuseni Mountains