Mineral and chemostratigraphy of a Toarcian black shale hosting Mn-carbonate microbialites (Úrkút, Hungary)

Abstract

is really a grey shale with moderate to low TOC content that accumulated in a starved basin. The 33 organic matter content and anoxic characteristics resulted from rapid accumulation of microbial 34 organic matter from microbial booms, accompanied by a geothermally generated hydrothermal 35 circulation system, and a high rate of authigenic mineral formation (clay minerals and proto-ore 36 minerals). The inferred enzymatic Mn and Fe oxidation blocked carbonate formation by 37 decreasing the pH. The system remained suboxic via syngenetic mineral accumulation (Fe-rich 38 biomats), and became anoxic during diagenesis in conjunction with pyrite generation. The 39 separation of black shale beds and Mn-ore beds is not distinct through the section. Instead, a 40 distal hydrothermally induced clay-rich authigenic assemblage (marlstone) best describes the 41 black shale, in which Mn-oxide proto-ore beds (Mn-rich laminae) formed from the beginning of 42 black shale deposition, when the oxygen supply in the sedimentary basin was insufficient for 43 enzymatic Mn(II) oxidation. Mn-oxide proto-ore turned into Mn-carbonate ore via microbially 44 mediated processes during early diagenesis. The drivers for Mn-bearing organic matter-rich 45 marlstones were most probably a combination of regional and local processes, with generation of 46 a tectonic rift system that promoted geothermally generated hydrothermal fluids, which initiated 47 3 microbial blooms. Black shale mineralogy, geochemistry, and organic matter at Úrkút differ 48 from those of the epicontinental shelf black shales of the Tethyan ocean. 49 50

Microbial metallogenesis of Cryogenian manganese ore deposits in South China

“…The mineralogical composition of the Mn ore is dominated by Mn-carbonate (Ca-rhodochrosite and kutnohorite) along with Fe minerals (goethite, pyrite, celadonite, and Fe-smectite). Mn ore beds are separated by the black shale host (Polgári et al, 2013(Polgári et al, , 2016a. The entire ore bed is composed of millimeter-scale woven structures with widespread microbe fossils, indicating a biogenetic origin for the Mn-carbonate deposit, and both Mn and Fe are initially enriched in the biomats (Polgári et al, 2007(Polgári et al, , 2012a(Polgári et al, ,b, 2013(Polgári et al, , 2016a).…”

“…indeed, Mn enrichment itself serves as an indicator for obligatory oxic conditions in the geological record (Maynard, 2010;Johnson et al, 2016b). Changes in oxygen supply determined whether Mn ores (the enzymatic Mn oxidation engine starts under obligatory oxic conditions) or black shales (formed under slightly decreasing oxygen supply) accumulated in both the post-Sturtian Nanhua Basin (Zhang et al, 2015;Yu et al, 2016) and the Early Jurassic Úrkút Basin (Polgári et al, 2012a(Polgári et al, , 2016a. The oxic and low temperature (<100˚C) aquatic systems would have favored microbially mediated Mn(II) oxidation in both locations (Tebo et al, 2004;Tang et al, 2013).…”

Microbially Mediated Ore-Forming Processes and Cell Mineralization

“…The Chinese study shows more similarities to the manganese ore formation found in Hungary than the Brazilian study does. There are about 480 million years between the Úrkút and the Datangpo formations; however, the processes characterizing their ore enrichment and metallogenetic mechanisms are similar (Polgári et al, 2012a, b, 2013, 2016a,b; Molnár et al, 2017; Yu et al, 2019). Both of them are Mn carbonate (Ca rhodochrosite and kutnohorite) ore in black shale environment, in which only the amount of iron differs.…”

Dapedium sp. from the Toarcian (Lower Jurassic) Úrkút Manganese Ore Formation (Bakony Mts., Hungary) and an overview of diversity of dapediiform fishes

“…A discontinuous chert layer (Cservár Flintstone Member) represents the closes part of the formation (Bíró 2014). For further details on the geology and stratigraphy of the Úrkút Mn ore deposit, see Szabó and Grasselly (1980), Szabó et al (1981), Grasselly and Pantó (1988), Polgári et al (1991Polgári et al ( , 2012Polgári et al ( , 2013Polgári et al ( , 2016a, Pantó et al (1996), Haas (2012) and Bíró (2014).…”