Phosphatic Chert Research Articles

Chemically-oscillating reactions during the diagenetic oxidation of organic matter and in the formation of granules in late Palaeoproterozoic chert from Lake Superior

Filamentous and coccoidal microfossils have been reported since the 1950's from a range of granular cherts from the late Palaeoproterozoic southwestern Superior Craton, Canada-United States. However, the chemical and mineral compositions of granules, the presence of microfossils in granules, and the common presence of granules in intercolumnar space of stromatolitic chert are poorly documented and explained. Furthermore, the depositional model for the origin of granules in wave-agitated waters does not entirely explain their mineral diversity nor their characteristic morphologies and patterns. We report on the crystallinity of organic matter, mineral diversity, and compositions of microfossils in granules from three different kinds of late Palaeoproterozoic cherts, namely phosphatic, organic, and haematitic. Stromatolitic organic-rich chert from the Gunflint Fm contains granules with euhedral carbonate and equidistant concentric laminations of organic matter, akin to fractal patterns from the Belousov-Zhabotinsky (B-Z) chemically-oscillating reaction. These granules also contain authigenic anatase, ferric-ferrous silicates, and Fe-oxides. Filamentous and coccoidal microfossils similar to those of the Gunflint occur in chert from the Biwabik Formation and share morphology, and co-occur with Mn-siderite and apatite. Granules in phosphatic chert in the Michigamme Formation often contain filamentous and coccoidal microfossils composed of organic matter, sericite, and apatite. Bulk carbonate associated with these Michigamme granular phosphatic chert beds has systematically negative δ13Ccarb values around −3.1±0.9‰ (1σ) and δ18Ocarb-SMOW between +20.8 and +30.7‰, which suggest some contribution from the diagenetic oxidation of organic matter. Notably, residual carboxylic acid is detectable in C-XANES spectra of organic matter from granular phosphatic chert, which is a residual reactant of B-Z type reactions. Along with previously reported observations of pyritised microfossils from the Gunflint Formation, these distinct mineralogies indicate variable modes of preservation for the products of chemically-oscillating reactions that likely relate to the availability of different oxidants in the diagenetic environment. We conclude that the late Palaeoproterozoic shallow-marine environments of the Lake Superior area were populated by morphologically similar micro-organisms, and that the diagenetic oxidation of organic matter through chemically-oscillating reactions contributed to the formation of spheroidal rosettes, granules, and concretions during a late Palaeoproterozoic Great Putrefaction Event. Granules in chert that contain organic matter or microfossils thus provide a reliable petrographic context to search for a record of putrefaction of microbial life on the early Earth and on other ancient planetary surfaces.

Age and implications of the phosphatic Birmania Formation, Rajasthan, India

The Birmania inlier in western Rajasthan, India, contains important phosphate deposits, the depositional age of which is poorly constrained. Here we provide the first direct age constraints for the phosphate-bearing Birmania Formation using a combination of paleontological and detrital zircon U–Pb data. The occurrence of the multicellular algal fossil Wengania exquisita in phosphatic chert of the Birmania Formation suggest that it was deposited during the Ediacaran. Detrital zircon age distributions contain prominent populations of 1.7–1.9Ga grains, with subordinate younger grains that range from 650 to 980Ma. The distribution broadly resembles those from Neoproterozoic strata from both cratonic and Himalayan India but, like zircon age distributions from the Marwar Group, lacks 1.0–1.2Ga grains, which may suggest that both areas shared similar local sources. The lack of zircon grains younger than ∼650Ma is consistent with an Ediacaran depositional age because almost all Cambrian or younger strata from India have yielded Cambrian or latest Neoproterozoic age grains. These findings raise the possibility of a previously unrecognized late Neoproterozoic episode of phosphogenesis on the India craton. The presence of Wengania exquisita further supports strong palaeogeographic affinity between the India and South China during the Neoproterozoic and Cambrian.

Carbon Isotope Features of the Sugetbrak Section in the Aksu-Wushi Area, Northwest China: Implications for the Precambrian/Cambrian Stratigraphic Correlations

The upper Qigeblaq Formation (Fm) dolostones and the Yurtus Fm phosphatic cherts, black shales, limestones, and dolostones are widely distributed in the Precambrian/Cambrian transitional succession of the Aksu-Wushi area. Negative δ13C excursion above the Yurtus Fm/Qigeblaq Fm boundary was determined in this study. The pronounced negative carbon isotope excursion occurs in the phosphatic chert layers at the bottom of the Cambrian Yurtus Fm, below which the first appearance of the Asteridium-Heliosphaeridium-Comasphaeridium (AHC) acritarch assemblage zone. The δ13C curve of the lower part of the Yurtus Fm in the Aksu-Wushi area was found to be correlated with the early Cambrian δ13C curves of the Zhujiaqing Fm (Daibu Member), the lower part of the Yanjiahe Fm on the Yangtze Platform in China, the lower Tal Fm in India, the Sukharikha Fm in Siberia, and the upper part of the Tsagaan Oloom Fm in Mongolia through biostigraphy. The lower part of the Yurtus Fm in the Tarim Basin is at the Nemakit-Daldynian stage, and the Precambrian/Cambrian boundary of the Aksu-Wushi area may be located in the phosphatic chert unit which just below the first appearance AHC acritarch assemblage zone. The negative δ13C excursion (N1) across the Precambrian/Cambrian boundary in the studied section may have resulted from oceanic overturning and sea level rise.

First Record of Metazoan Eggs and Embryos from Early Cambrian Chert Member of Deo ka Tibba Formation, Tal Group, Uttarakhand Lesser Himalaya

Abstract: The phosphatized globular to sub-oval metazoan eggs with distinctively ornamented covering and phosphatized polar lobe forming embryos are recorded for the first time from the black phosphatic chert lenticles and bands of early Cambrian Chert Member of Deo ka Tibba Formation, Tal Group, Uttarakhand Lesser Himalaya. Similar metazoan eggs have been recorded earlier from early Cambrian rocks of Shaanxi Province, China, northwestern Canada, Australia, Mongolia, Siberia and Kazakhstan. These eggs are found in association with Small Shelly Fossils (SSF) and may belong to them. However, polar lobe forming embryos have been earlier recorded from Neoproterozoic rocks of southwestern China only. Similar embryos are common in modern molluscs and bilaterians.

Discovery of animal eggs and embryos from the Ediacaran (Terminal Proterozoic) Chambaghat Formation, Krol Group, Himachal Lesser Himalaya

The globular to suboval microfossils with distinctively ornamented outer coverings interpreted as animal eggs and embryos have been discovered from the black phosphatic chert lentils of the Ediacaran (Terminal Proterozoic) Chambaghat Formation (Krol sandstone), Krol Group, Himachal Lesser Himalaya, India. Similar animal eggs and embryos have earlier been recorded only from the phosphorites of the uppermost Neoproterozoic Doushantuo Formation (Ca. 570±20 Ma) exposed at Weng’an, South China. Present record of eggs and embryos is comparable with extant eggs and embryos of cnidarians and bilaterians like molluscs, annelids and arthropods. The eggs and embryos from the Terminal Proterozoic rocks of India are the only one recorded from the equivalent stratigraphic horizon outside China. This discovery of eggs and embryos adds to the understanding the evolutionary trends in the Proterozoic metazoan life.

Depositional environments of the lower Permian Dwyka diamictite and Prince Albert shale inferred from the geochemistry of early diagenetic concretions, southwest Karoo Basin, South Africa

The upper Dwyka and lower Ecca Groups in the Karoo Basin of South Africa document the climatic and palaeoenvironmental changes associated with the final Permo-Carboniferous deglaciation of the Gondwana supercontinent. The depositional environments of these groups have, until recently, been interpreted on the basis of sedimentological and palaeontological evidence. Here we use the geochemistry of early diagenetic concretions – septarian calcite concretions from the upper Dwyka Group and phosphatic chert concretions and beds from the lower Ecca Group – to infer the depositional environment of these rocks in the southwestern Karoo Basin. δ 18O values (7.8 to 8.9‰ SMOW) suggest that the calcite concretions precipitated from a mixture of meteoric and glacial melt waters rather than Permian seawater. δ 13C values (− 15 to − 3‰ PDB) indicate that the carbon was derived from a mixture of craton-derived calcareous material and organic matter, bacterially degraded in the lower sulphate-reduction to upper methanogenesis zones during early burial diagenesis. The rare-earth element (REE) patterns, Sr concentrations and 87Sr/ 86Sr ratios (0.716–0.737) significantly greater than Permian seawater (0.708), together also support the interpretation that calcite and phosphatic concretions formed in glacial, fresh water sediments.

Significance of lower and middle Paleozoic phosphatic chert in the Toquima Range, central Nevada

Ordovician to Lower Mississippian cherts of the Roberts Mountains allochthon in central Nevada contain nodules, lenses, and streaks of phosphate as apatite. Major marine phosphate deposition is limited to latitudes less than 50° on margins facing large oceans to the east or west, or of Tethyan (equatorial sea) type; the Nevada margin was suitably located in the lower to middle Paleozoic. The Roberts Mountains allochthon overthrust the shelf of western North America very soon after deposition of the youngest phosphatic cherts, implying that they are most likely not far-traveled or exotic to North America. Phosphates of similar age and nature in the northern Sierra Nevada suggest proximity of Nevada and the northern Sierra Nevada in the early to middle Paleozoic.

Zonation of chalcophile elements about the howard's pass (XY) Zn-Pb deposit, Selwyn Basin, Yukon

ABSTRACT Goodfellow, W.D., Jonasson, I.R. and Morganti, J.M., 1983. Zonation of chalcophile elements about the Howard's Pass (XY) Zn-Pb deposit, Selwyn Basin, Yukon. In: G.R. Parslow (Editor), Geochemical Exploration 1982. J. Geochem. Explor., 19: 503–542. The Selwyn Basin comprises those rocks of Paleozoic age that were deposited in a fault-controlled epicratonic basin or aulacogen bounded by the Mackenzie Platform on the east and north, and the Cassiar Platform to the west. Boundaries between sedimentary facies within the Selwyn Basin during Ordovician to Middle Silurian time were parallel to and symetrically disposed about the long axis of that basin, consisting of chert in the central and deeper zone and shales in the flanking zones. Conodont dates indicate that the Howard's Pass (XY) Zn-Pb deposit formed during Llandoverian time in a local third-order basin within the shale facies. Various geological features such as abrupt facies changes, local thickening and slumping of sedimentary units suggest that growth faults played an important role in basin development as well as serving as conduits for the discharge of ore-forming fluids. The only sulphides recognized to date at the XY deposit are sphalerite, galena, chalcopyrite and pyrite. Sphalerite and galena can occur as concordant, fine laminations, or remobilized into sedimentary dewatering structures, or transposed and remobilized into a tectonic (slaty) cleavage. Pyrite most commonly forms diagenetic microframboids which postdate the laminated sulphides but predate the sphalerite and galena emplaced in dewatering structures. Within the main ore-bearing unit, i.e. the Active Member, the Pb/(Pb+Zn) ratio decreases stratigraphically upwards and laterally towards the margins of the deposit. This zonation accompanies a change in host-rock composition from cherty limestone at the base to carbonaceous chert towards the top of the Active Member. In addition to Zn and Pb, the other chalcophile elements concentrated in the Active Member are Cd and Hg, both of which are bound isomorphously in sphalerite. Both ore-forming and ore-associated elements are zoned stratigraphically and laterally in the host rocks of the XY deposit. The footwall carbonaceous mudstones are enriched in Ni, Cu, Co, Mo, As, Sb, Zn, Cd, Hg and Pb relative to rocks of similar age and composition located remote from mineralization. Ni, Cu, Co, Mo, As and Sb now reside in diagenetic pyrite but in whole rock chemical analyses show a strong positive correlation with organic C. On the other hand, Zn, Cd, Hg and Pb are bound in sphalerite or galena. By comparison, the overlying phosphatic chert is enriched in Ni and Cu only, both of which correlate positively with organic C. The zonation of these elements reflects a decrease in pyrite content in the rocks and also a decrease in trace-element contents of pyrite itself away from the XY deposit. On the basis of extremely low S/C ratios, the low pyrite content outside the XY deposit is thought to be due to a lack of sufficient Fe2+ to fix S2- generated by bacterial reduction during diagenesis. The higher content of Fe near the XY deposit probably reflects Fe debouched onto the seafloor during hydrothermal activity. The increased trace-element content of pyrite in rocks hosting the XY deposit is due most likely to elevated levels of these elements accumulated by dead organic matter from ore-forming fluids during sedimentation. A period of Devonian hydrothermal activity in the Howard's Pass area can be recognized by elevated levels of Zn, Hg, Sb, Ag, As, Mo and Ni in highly carbonaceous and calcareous rocks. The spatial association of these Devonian metalliferous mudstones with the underlying XY deposit suggests that structures which served as conduits for the discharge of ore-forming fluids during the Lower Silurian were reactivated in Devonian time.

Origin and metamorphism of peperite and associated rocks in the Devonian Elwell Formation, northern Sierra Nevada, California

The Late Devonian Elwell Formation is part of a late Paleozoic island-arc assemblage exposed in the northern Sierra Nevada, California. In the 10-km interval from Mount Elwell (Plumas County) to Packer Lake (Sierra County), it consists largely of numerous andesitic sills interleaved with phosphatic radiolarian chert and turbidity-current and debris-flow deposits. Peperite, formed by interaction of intrusive andesitic magma and still-moist, incompletely lithified “chert” not far beneath the sea floor, occurs sparingly. The whole assemblage was thoroughly recrystallized (greenschist-facies regional metamorphism) during the Late Jurassic Nevadan orogeny. The phosphatic chert is composed of white, fluorapatite-rich lenses and nodules enclosed in a black matrix consisting of microgranular quartz and carbonaceous material (probably graphite). Radiolaria (family Entactiniidae) are so abundant that the chert is thought to have originated as radiolarian ooze. The phosphatic lenses and nodules probably formed by direct inorganic precipitation of fluorapatite from pore water within decaying radiolarians at the sediment-water interface. The andesitic sills are generally aphyric and aphanitic; original textures varied from hyalopilitic in sill interiors to pilotaxitic at sill margins. The andesite was charged with plagioclase (now albite) microlites, and quench morphologies occur where sills intruded the wettest “chert.” Relatively thick sills commonly display columnar structure, whereas a few originally more viscous sills are flow laminated. The top of a sill emplaced essentially at the sediment-water interface is pillowed. Many sills bear xenoliths of phosphatic chert, and scarce quartz megacrysts (evidently derived from the underlying, dacitic Sierra Buttes Formation) appear nearly everywhere. Concentrations of major-element oxides and immobile trace elements indicate that the andesite originated as part of the tholeiitic rock association of island arcs; rare-earth–abundance patterns are particularly diagnostic of this paleotectonic setting. Most peperite has poorly sorted (coarse ash to coarse block) andesitic fragments uniformly distributed through a voluminous black chert matrix. Where invaded “chert” retained the most pore water, quenched andesitic magma formed irregularly shaped lapilli and small pillows that initially exhibited ductile behavior. Rapid cooling below the softening point of glass resulted in brittle fracture of the lapilli during dispersal through the “chert” by steam explosions. Steam explosions also incorporated into the peperite rigid clasts of locally dewatered chert, and shattered already-formed phosphatic nodules. Where the invaded “chert” was better lithified, freezing sill margins simply granulated quietly in situ by thermal-contraction cracking of microlitic glass; fluid “chert” injected widely spaced contraction joints deep within the sills. In all cases, there was sufficient heated connate water to cause palagonitization and chloritization of andesitic fragments. Greenschist-facies metamorphism produced two mineral assemblages in the interiors of andesitic sills, the second perhaps reflecting locally higher Pco 2 /Ph 2 o during metamorphism: (1) albite + quartz + epidote + actinolite + chlorite + Ti-rich mineral ± calcite ± stilpnomelane ± white mica, and (2) albite + quartz + chlorite + Ti-rich mineral + calcite ± stilpnomelane ± white mica. Epidote-rich specks occurring sporadically in the sills represent metasomatism on a scale of millimetres to centimetres; such specks are mineralogically zoned in andesitic fragments in intensely recrystallized peperite. Additional examples of local meta-somatic activity include syntectonic crystallization of calcite rhombohedra across original vesicle walls in a sill. Cleavage developed locally during regional metamorphism is parallel to axial planes of closely appressed folds that deformed andesitic fragments in peperite and flow laminae in sills.

Permian and Carboniferous Stratigraphy Wapiti Lake Area, Northeastern British Columbia

ABSTRACT The late Paleozoic succession in the vicinity of Wapiti Lake, northeastern British Columbia, 90 mi (150 km) southwest of Grand Prairie, Alberta, consists of strata referable to the Permian Ishbel Group that rest unconformably on the eroded surface of the Mississippian Rundle Group, the contact being marked by a spectacular basal limestone/chert submarine talus breccio-conglomerate. An unusual light grey to white porous dolomite unit which caps the fossiliferous Rundle succession was the primary prospective reservoir rock in an anticlinal trap believed to exist beneath folded and overthrust lower Paleozoic strata near Window Lakes; it was tested by the wildcat well Texas Gulf Sulphur et al. Narraway C-86-G in 93-I-7. The Permian Ishbel group consists of three formations. In ascending order these are: 1) the Belcourt Formation, a cherty dolomite with Schwagerinid foraminifera of Wolfcampian to Lower Leonardian age; 2) the feature-forming Ranger Canyon Formation chert rock, here of Guadalupian age on the basis of brachiopod content, which is found in virtually every Permian section from the Liard and Tetsa Rivers in the north to the Crowsnest area 1500 km to the south, and resting here, as elsewhere, on a thin phosphatic chert conglomerate and regional unconformity within the Permian; and 3) the Mowitch Formation, a porous brown sandstone, also of Late Permian age, which is paraconformably overlain by recessive dark grey siltstones of the Triassic Spray River Group. The Belcourt Formation carbonate facies is unique to this area, being the age equivalent of fine-grained clastic phosphatic sequences elsewhere, such as the Mt. Greene Beds on Peace River, correlative strata in Pine Pass, the Johnston Canyon Formation of the Banff area and further south, and the lower part of the fossiliferous phosphatic succession west of the Elk Valley in southeastern British Columbia.

Lower Cretaceous of Casper Arch, Wyoming

An integrated study of outcrop and subsurface stratigraphy and petrography, of the Cloverly Group of the Casper Arch area, Wyoming, shows that the Lakota Conglomerate was probably deposited by eastward- and northeastward-flowing streams, as evidenced by cut-and-fill structures, trough cross-stratification, and the trend of channel sandstone bodies. The mineralogy of the Fuson Shale suggests that it was dominantly derived from a residual soil cover. The Fall River Sandstone, which overlies the Fuson Shale, is a marginal marine sandstone body which consists of one thick sandstone unit in the southeast portion of the Casper arch. This sandstone body, as traced westward and northwestward, is separated by a thin shale unit, into two distinct sandstone units; the lower sandston unit thins and disappears about half of the way across the Casper arch, and its stratigraphic interval is replaced siltstone; whereas, the upper sand body continues to the western flank of the arch. The Lakota Conglomerate is composed largey of a mixture of well rounded detrital chert grains and multi-cycle quartz grains. The Lakota detritus contains approximately 23 per cent black and 18 per cent white chert grains. The black phosphatic chert was derived from the Rex Chert Member of the Phosphoria Formation west and southwest of the Casper Arch area, possibly in eastern and central Idaho. The Lakota detritus consists predominantly of multi-cycle, subrounded to well rounded grains of plutonic quartz, with subordinate metamorphic and vein quartz. Subangular to angular quartz grains make up a small proportion of the quartz grains. The non-opaque heavy-mineral assemblage of the Lakota Conglomerate is mature; 97 per cent of it consists of numerous varieties of subrounded to well-roun ed zircon and tourmaline. The subrounded to well-rounded ultra-stable quartz, zircon, and tourmaline are multi-cycle grains, derived largely from pre-existing sandstones. Angular hornblende, staurolite, rutile, zircon, and tourmaline form 3 per cent of the non-opaque heavy-mineral assemblage. The angular non-opaque heavy-mineral grains are first-cycle detritus and were apparently derived from crystalline remnants of the Ancestral Rocky Mountains on the south and southwest.