Organotemplate silica deposition in Neoproterozoic deep-marine environments: evidence from the Penganga Group, Adilabad, India

Silica in bedded cherts interstratified with manganese carbonates in a deep‐water carbonate ramp succession of the Neoproterozoic Penganga Group, India, displays film‐like, spherical to rod‐shaped, and tubular branching microstructures. The microstructures resemble mineralized extracellular polysaccharides, biofilms and bacterial morphologies. The microstructures suggest silicification by nucleation of silica on organic‐templates or indirectly as sorbed species accumulating on organic templates. Given that similar microstructures have also been documented in Archean cherts it is proposed that organotemplates might have been an important sink for the deposition of silica in Precambrian deep‐water marine environments.     

Iron isotopes constrain biologic and abiologic processes in banded iron formation genesis

The voluminous 2.5 Ga banded iron formations (BIFs) from the Hamersley Basin (Australia) and Transvaal Craton (South Africa) record an extensive period of Fe redox cycling. The major Fe-bearing minerals in the Hamersley-Transvaal BIFs, magnetite and siderite, did not form in Fe isotope equilibrium, but instead reflect distinct formation pathways. The near-zero average δ⁵⁶Fe values for magnetite record a strong inheritance from Fe³⁺ oxide/hydroxide precursors that formed in the upper water column through complete or near-complete oxidation. Transformation of the Fe³⁺ oxide/hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of δ⁵⁶Fe values: (1) addition of marine hydrothermal , (2) complete reduction by bacterial dissimilatory iron reduction (DIR), and (3) interaction with excess that had low δ⁵⁶Fe values and was produced by DIR. Most siderite has slightly negative δ⁵⁶Fe values of ∼ −0.5‰ that indicate equilibrium with Late Archean seawater, although some very negative δ⁵⁶Fe values may record DIR. Support for an important role of DIR in siderite formation in BIFs comes from previously published C isotope data on siderite, which may be explained as a mixture of C from bacterial and seawater sources.Several factors likely contributed to the important role that DIR played in BIF formation, including high rates of ferric oxide/hydroxide formation in the upper water column, delivery of organic carbon produced by photosynthesis, and low clastic input. We infer that DIR-driven Fe redox cycling was much more important at this time than in modern marine systems. The low pyrite contents of magnetite- and siderite-facies BIFs suggests that bacterial sulfate reduction was minor, at least in the environments of BIF formation, and the absence of sulfide was important in preserving magnetite and siderite in the BIFs, minerals that are poorly preserved in the modern marine record. The paucity of negative δ⁵⁶Fe values in older (Early Archean) and younger (Early Proterozoic) BIFs suggests that the extensive 2.5 Ga Hamersley-Transvaal BIFs may record a period of maximum expansion of DIR in Earth’s history.     

Effects of mass transfer, compaction and secondary porosity on hydrothermal upgrading of Paleoproterozoic sedimentary manganese ore in the Kalahari manganese field, South Africa

In the northwestern part of the Kalahari manganese field low-grade carbonate-rich Mamatwan-type ore is altered to high-grade oxide-rich Wessels-type ore in association with normal faults. Mass balance calculations, based on the assumption that manganese was geochemically immobile, suggest that upgrading of the manganese ore can be attributed to leaching of Mg, Ca, CO₂ and SiO₂ from the sedimentary ore with residual enrichment of Mn. Hydrothermal alteration resulted in development of about 10 to 20% of secondary porosity in the ores and the orebed was compacted to two thirds of its original stratigraphic thickness. Received: 28 May 1996 / Accepted: 22 January 1997     

Primordial non-Gaussianity from LAMOST surveys

The primordial non-Gaussianity (PNG) in the matter density perturbation is a very powerful probe of the physics of the very early Universe. The local PNG can induce a distinct scale-dependent bias on the large scale structure distribution of galaxies and quasars, which could be used for constraining it. We study the detection limits of PNG from the surveys of the LAMOST telescope. The cases of the main galaxy survey, the luminous red galaxy (LRG) survey, and the quasar survey of dif- ferent magnitude limits...     

Intraspecific cleaning by juvenile Cape white seabream Diplodus capensis (Sparidae) off eastern South Africa

Juveniles of the Cape white seabream Diplodus capensis were observed cleaning adult conspecifics in a large tidepool off Sodwana Bay, South Africa. Although nine other tropical fish species were present and interacted with a nearby pair of Labroides cleaner wrasses, only adults of D. capensis posed for and were cleaned by the D. capensis juveniles. Such cleaning interactions have not been reported previously for this species or among marine fishes off South Africa, and thus add to the growing list of facultative cleaners globally.     

Post-Gondwana pedogenic ferromanganese deposits, ancient soil profiles, African land surfaces and palaeoclimatic change on the Highveld of South Africa

Several ferromanganese wad deposits are developed on the Archæan Malmani dolomite succession of the Transvaal Supergroup along the plateau forming the watershed between rivers draining to the Indian and Atlantic Oceans from the Highveld area between Johannesburg and Lichtenburg. The deposits were studied at the Wes Wits Gold Mine and the Ryedale, Houtkoppies and Klipkuil ferromanganese mines. The ferromanganese wad deposits are located in the Waterval saprolite, which formed by deep chemical weathering along the post-Gondwana African surface. The parent rocks for the wad are the Fe- and Mn-rich dolomites of the Malmani Subgroup of the Transvaal Supergroup at Wes Wits Mine and at Klipkuil, and Fe- and Mn-rich blackband Fe ores of the Ecca Group of the Karoo Supergroup at Ryedale and Houtkoppies. This saprolite is unconformably overlain by a ferruginous alluvial succession, informally defined as the Weswits formation. Diamondiferous gravel bars occur in the lower part of the fluvial succession. The unconformity at the base of the succession with incised valleys is thought to correspond to the post-African I event of uplift and erosion. Manganiferous soil nodules, derived from a lateritic weathering profile that originally covered the Waterval saprolite, are concentrated in the lower part of the Weswits formation. Climatic conditions became drier during the deposition of the Weswits formation, and plants with deep taproots vegetated the surface of the alluvium, giving rise to the formation of a ferric poclzol. Further aridification took place and eventually the plants with deep taproots died and a pediment developed along which a stone lag was concentrated. This pediment is thought to represent the post-African II surface of erosion. The pediment is overlain by the Hutton soil, representing Kalahari sand and dust that have been reworked by fluvial and peclogenic processes. Massive ferruginous soil nodules grew in situ in the Hutton soil, indicating more pluvial climatic conditions at times, most probably corresponding to the Quaternary ice age events in the Northern Hemisphere. A thin, modern orthic soil developed on top of the Hutton soil in the present day mild, subhumid climatic conditions.     

Rise in seawater sulphate concentration associated with the Paleoproterozoic positive carbon isotope excursion: evidence from sulphate evaporites in the ∼2.2–2.1 Gyr shallow‐marine Lucknow Formation,South Africa

Abstract Past oceanic sulphate concentration is important for understanding how the oceans’ redox state responded to atmospheric oxygen levels. The absence of extensive marine sulphate evaporites before ～1.2 Gyr probably reflects low seawater sulphate and/or higher carbonate concentrations. Sulphate evaporites formed locally during the 2.22–2.06 Gyr Lomagundi positive δ¹³C excursion. However, the ～2.2–2.1 Gyr Lucknow Formation, South Africa, provides the first direct evidence for seawater sulphate precipitation on a carbonate platform with open ocean access and limited terrestrial input. These marginal marine deposits contain evidence for evaporite molds, pseudomorphs after selenite gypsum, and solid inclusions of Ca‐sulphate in quartz. Carbon and sulphur isotope data match the global record and indicate a marine source of the evaporitic brines. The apparent precipitation of gypsum before halite requires ≥2.5 mm L^?1 sulphate concentration, higher than current estimates for the Paleoproterozoic. During the Lomagundi event, which postdates the 2.32 Gyr initial rise in atmospheric oxygen, seawater sulphate concentration rose from Archean values of ≤200 μm L^?1, but dropped subsequently because of higher pyrite burial rates and a lower oceanic redox state.     

Zoning of platinum group mineral assemblages in the UG2 chromitite determined through in situ SEM-EDS-based image analysis

In situ scanning electron microscopy–energy dispersive X-ray spectrometry analysis of platinum group minerals (PGM) and base metal sulfides in the UG2 chromitite shows that this ore body is zoned along at least ∼6 km of strike. The uppermost part of the UG2 chromitite, referred to as the leader seam, is ∼16 cm thick and has a PGM assemblage that is dominated by PGE arsenides, sulpho-arsenides, and alloys (∼70 vol.% of all PGM), which are typical secondary PGM assemblages in other segments of UG2. This is the first time such laterally persistent secondary assemblages have been identified in the UG2 chromitite, as previously, they were only known to occur adjacent to transgressive fluid-bearing structures (e.g., pipes, faults). The underlying main seam is thicker (one to nine seams totaling ∼130 cm) and has a PGM assemblage that consists mostly of Pt sulfide, Pt–Pd sulfide, Pt–Rh–Cu sulfide, laurite, and Fe–Pt alloys (∼85 vol.% of all PGM), typically regarded as primary magmatic constituents of UG2 chromitite. There are, however, some subtle vertical changes in the PGM assemblages of the main seam that include the occasional presence of secondary assemblages in the top and bottom parts. The origin of these secondary PGM assemblages is related to alteration by hydrothermal fluids and/or fluid-rich melts that infiltrated during crystallization of the UG2 and may possibly have been derived from the UG2 chromitite itself.     

Ancient geochemical cycling in the Earth as inferred from Fe isotope studies of banded iron formations from the Transvaal Craton

Variations in the isotopic composition of Fe in Late Archean to Early Proterozoic Banded Iron Formations (BIFs) from the Transvaal Supergroup, South Africa, span nearly the entire range yet measured on Earth, from –2.5 to +1.0‰ in ⁵⁶Fe/⁵⁴Fe ratios relative to the bulk Earth. With a current state-of-the-art precision of ±0.05‰ for the ⁵⁶Fe/⁵⁴Fe ratio, this range is 70 times analytical error, demonstrating that significant Fe isotope variations can be preserved in ancient rocks. Significant variation in Fe isotope compositions of rocks and minerals appears to be restricted to chemically precipitated sediments, and the range measured for BIFs stands in marked contrast to the isotopic homogeneity of igneous rocks, which have δ⁵⁶Fe=0.00±0.05‰, as well as the majority of modern loess, aerosols, riverine loads, marine sediments, and Proterozoic shales. The Fe isotope compositions of hematite, magnetite, Fe carbonate, and pyrite measured in BIFs appears to reflect a combination of (1) mineral-specific equilibrium isotope fractionation, (2) variations in the isotope compositions of the fluids from which they were precipitated, and (3) the effects of metabolic processing of Fe by bacteria. For minerals that may have been in isotopic equilibrium during initial precipitation or early diagenesis, the relative order of δ⁵⁶Fe values appears to decrease in the order magnetite > siderite > ankerite, similar to that estimated from spectroscopic data, although the measured isotopic differences are much smaller than those predicted at low temperature. In combination with on-going experimental determinations of equilibrium Fe isotope fractionation factors, the data for BIF minerals place additional constraints on the equilibrium Fe isotope fractionation factors for the system Fe(III)–Fe(II)–hematite–magnetite–Fe carbonate. δ⁵⁶Fe values for pyrite are the lowest yet measured for natural minerals, and stand in marked contrast to the high δ⁵⁶Fe values that are predicted from spectroscopic data. Some samples contain hematite and magnetite and have positive δ⁵⁶Fe values; these seem best explained through production of high ⁵⁶Fe/⁵⁴Fe reservoirs by photosynthetic Fe oxidation. It is not yet clear if the low δ⁵⁶Fe values measured for some oxides, as well as Fe carbonates, reflect biologic processes, or inorganic precipitation from low-δ⁵⁶Fe ferrous-Fe-rich fluids. However, the present results demonstrate the great potential for Fe isotopes in tracing the geochemical cycling of Fe, and highlight the need for an extensive experimental program for determining equilibrium Fe isotope fractionation factors for minerals and fluids that are pertinent to sedimentary environments.     

Provenance and reconnaissance study of detrital zircons of the Palaeozoic Cape Supergroup in South Africa: revealing the interaction of the Kalahari and R��o de la Plata cratons

In order to facilitate the understanding of the geological evolution of the Kalahari Craton and its relation to South America, the provenance of the first large-scale cratonic cover sequence of the craton, namely the Ordovician to Carboniferous Cape Supergroup was studied through geochemical analyses of the siliciclastics, and age determinations of detrital zircon. The Cape Supergroup comprises mainly quartz-arenites and a Hirnantian tillite in the basal Table Mountain Group, subgreywackes and mudrocks in the overlying Bokkeveld Group, while siltstones, interbedded shales and quartz-arenites are typical for the Witteberg Group at the top of the Cape Supergroup. Palaeocurrent analyses indicate transport of sediment mainly from northerly directions, off the interior of the Kalahari Craton with subordinate transport from a westerly source in the southwestern part of the basin near Cape Town. Geochemical provenance data suggest mainly sources from passive to active continental margin settings. The reconnaissance study of detrital zircons reveals a major contribution of Mesoproterozoic sources throughout the basin, reflecting the dominance of the Namaqua-Natal Metamorphic Belt, situated immediately north of the preserved strata of Cape Supergroup, as a source with Archaean-aged zircons being extremely rare. We interpret the Namaqua-Natal Metamorphic Belt to have been a large morphological divide at the time of deposition of the Cape Supergroup that prevented input of detrital zircons from the interior early Archaean Kaapvaal cratonic block of the Kalahari Craton. Neoproterozoic and Cambrian zircons are abundant and reflect the basement geology of the outcrops of Cape strata. Exposures close to Cape Town must have received sediment from a cratonic fragment that was situated off the Kalahari Craton to the west and that has subsequently drifted away. This cratonic fragment predominantly supplied Meso- to Neoproterozoic, and Cambrian-aged zircon grains in addition to minor Silurian to Lower Devonian zircons and very rare Archaean (2.5?Ga) and late Palaeoproterozoic (1.8-2.0?Ga) ones. No Siluro-Devonian source has yet been identified on the Kalahari Craton, but there are indications for such a source in southern Patagonia. Palaeozoic successions in eastern Argentina carry a similar detrital zircon population to that found here, including evidence of a Silurian to Lower Devonian magmatic event. The Kalahari and Río de la Plata Cratons were thus in all likelihood in close proximity until at least the Carboniferous.