Detrital Pyrite Grains Research Articles

Textural and geochemical investigation of pyrite in Jacobina Basin, São Francisco Craton, Brazil: Implications for paleoenvironmental conditions and formation of pre-GOE metaconglomerate-hosted Au-(U) deposits

The Jacobina Basin has a well-preserved sedimentary record, including continental and marine deposits, and hosts Au-(U)-Py mineralization in metaconglomerate beds similar to the Witwatersrand gold province. Based on petrographic observations, in situ trace-elements, and multiple sulfur isotope analyses (32S, 33S, 34S, and 36S) on pyrite from alluvial and marine facies, several types of pyrite were recognized. The pyrite grains identified in the alluvial metaconglomerates resemble those found in several pre-GOE gold-bearing metaconglomerates, including detrital and epigenetic varieties. Detrital inclusion-bearing pyrite is enriched in several redox-sensitive trace-metals as well as Au, which indicate an association with carbonaceous shales. On the other hand, the sources of detrital massive pyrite are more variable and include igneous, metamorphic, and hydrothermal sources from the Paleoarchean hinterland of Jacobina Basin. Epigenetic pyrite in metaconglomerates formed during metamorphism by the recrystallization of detrital pyrite with negligible contributions from external hydrothermal fluids to the basin. Diagenetic and epigenetic pyrite are found in marine lithologies. In a metapelite sample, the pyrite grains formed near the sediment-water column interface, with S sourced from the photolytic sulfate (SO42−, Δ33S < 0) dissolved in the water column. A quartzite sample, in turn, has detrital pyrite grains that were likely reworked from continental deposits, as well as pyrite formed by the assimilation of elemental sulfur (S8, Δ33S > 0) that accumulated in sediment pore water during diagenesis. Significantly, the pyrite associated with terrestrial metasediments shows a wide range in δ34S values but quite restricted ranges in Δ33S and Δ36S values, whereas pyrite associated with the marine metasedimentary rocks exhibits limited δ34S values but has a wide range in Δ33S values and correlated Δ36S values close to the Archean array. These data suggest distinct preservation routes for MIF-S from atmospheric SO42− and S8 in terrestrial and marine environments. Conditions on the terrestrial surface resulted in re-equilibration of distinct S sources, diminishing the amplitude of the Archean atmospheric signal. In contrast, SO42− dissolved in shallow marine settings while S8 settled to the floor, favoring the preservation of MIF-S isotopic signatures. Such processes may also explain the apparent differences in interpretations of atmospheric conditions derived from uncharacterized pyrites from Archean sources. Our data suggest that the Earth’s atmosphere remained anoxic, and terrestrial conditions were such as to allow the syngenetic accumulation of gold, as recently proposed for the Witwatersrand gold deposits.

Geochemistry and microbial community composition across a range of acid mine drainage impact and implications for the Neoarchean‐Paleoproterozoic transition

AbstractStreams impacted by acid mine drainage (AMD, also known as acid rock drainage) represent local environmental and ecological disasters; however, they may also present an opportunity to study microbial communities in environments analogous to past conditions. Neoarchean continents had streams and rivers replete with detrital pyrites. Following the emergence of oxygenic photosynthesis, Cyanobacteria colonized streams and rivers on continental surfaces. The combination of labile detrital pyrite grains and locally produced O2 generated by Cyanobacteria produced ideal conditions for pyrite oxidation similar to that found at modern AMD‐impacted sites. To explore the connection of modern sites to ancient conditions, we sampled sites that exhibited a range of AMD‐impact (e.g., pH from 2.1 to 7.9 [Fe2+] up to 5.2 mmol/L [SO42−] from 0.3 to 52.4 mmol/L) and found (i) nearly all analytes correlated to sulfate concentration; (ii) all sites exhibited the predominance of a single taxon most closely related to Ferrovum myxofaciens, an Fe‐oxidixing betaproteoabacterium capable of carbon and nitrogen fixation, and (iii) signs of potential inorganic carbon limitation and nitrogen cycling. From these findings and building on the work of others, we present a conceptual model of continental surfaces during the Neoarchean and Paleoproterozoic linking local O2 production to pyrite oxidation on continental surfaces to sulfate production and delivery to nearshore environments. The delivery of sulfate drives sulfate reduction and euxinia—favoring anoxygenic photosynthesis over cyanobacterial O2 generation in near‐continent/shelf marine environments.

Development of in situ sulfur four-isotope analysis with multiple Faraday cup detectors by SIMS and application to pyrite grains in a Paleoproterozoic glaciogenic sandstone

An in situ sulfur four-isotope analysis technique with multiple Faraday cup detectors by ion microprobe was developed and applied to detrital pyrite grains in ~2.4Ga glaciogenic sandstone from the Meteorite Bore Member of the Turee Creek Group, Western Australia. Data are standardized with the UWPy-1 pyrite standard (δ34S=16.04±0.18‰, Δ33S=−0.003±0.009‰, and Δ36S=−0.21±0.24‰, 2 SD) whose sulfur four isotopes were newly determined by gas-source mass spectrometry. Typical reproducibility at two standard deviations (2 SD) of spot-to-spot analyses of standard UWPy-1 pyrite with a primary beam size of ~20μm were ±0.23, ±0.05, and ±0.86‰ for δ34S, Δ33S, and Δ36S, respectively. The measured 36S/32S ratio [1/(6641±27)] is approximately 19‰ lower than the published ratio for VCDT, and we propose a revision of the 36S abundance in VCDT.Pyrite grains in ~2.4Ga glaciogenic sandstone have wide ranging sulfur isotope ratios (−32.7 to 13.5 for δ34S, −3.03 to 11.66 for Δ33S, and −9.7 to 4.6 for Δ36S, respectively). Some pyrite grains are zoned in δ34S values within a grain. Sulfur isotope ratios of most pyrite grains are distributed along a line with slope=−0.9 for Δ33S vs. Δ36S, suggesting that pyrite grains mostly derived from a limited range of source rocks and near-surface sulfur reservoirs. One pyrite aggregate has a distinct texture from other pyrite grains in the same sandstone, and yields a significant mass-independent deficit in 36S with a small excess in 33S (Δ36S/Δ33S~−4‰). This is used to suggest that this grain authigenically formed by biological activity during or after sedimentation. This work demonstrates that the use of multiple Faraday cup detectors provides improved accuracy and precision for in situ sulfur four-isotope analysis with secondary ion mass spectrometry.

Mass-independently fractionated sulfur in Archean paleosols: A large reservoir of negative Δ33S anomaly on the early Earth

Results of multiple sulfur isotope analyses from five Archean paleosols show the widespread presence of mass-independently fractionated sulfur in the regolith developed on the pre-2.5Ga Earth. Analysis of sulfur from a small set of diamictite samples gave similar results. All values of Δ33S are negative, indicating that the Archean surface environments preferentially retained atmospheric S from the SO42− pathway, which carried a negative Δ33S signal, whereas a portion of the S from the S8 pathway, with a positive Δ33S, was transferred to the oceans. The soil SO42− was then converted to sulfide by bacterial sulfate reduction with terrestrial organic matter in the weathering horizon acting as a reductant. Some S from the S8 pathway also included, which resulted in a net soil Δ33S anomaly from atmospheric S in the range −0.3 to −0.6‰, similar to values for pyrites from floodplain sediments. Excess S8-derived S was carried to the oceans, resulting in the negative/positive dichotomy between terrestrial and marine systems. Pyrite that formed in paleosols and pyrite grains that formed in flood-plain deposits, both carrying the terrestrial Δ33S signal, were then recycled into detrital pyrite grains now found in sandstones and conglomerates deposited before the rise of atmospheric oxygen. Therefore the Earth's early regolith constituted a reservoir of S with negative Δ33S values that could be important for balancing the predominantly positive Δ33S signature found in marine sediments.

Evidence for an Intrabasinal Source and Multiple Concentration Processes in the Formation of the Carbon Leader Reef, Witwatersrand Supergroup, South Africa

Laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) trace element maps of pyrite and gold from the Carbon Leader Reef in the Witwatersrand basin provide new evidence that a significant proportion of the pyrite and gold was intrabasinal, derived from the West Rand Group or equivalent shales stratigraphically below this reef. Rounded detrital pyrite grains in the Carbon Leader Reef vary from compact to porous to sooty, with similar textures and composition to diagenetic pyrites reported from sedimentary rocks in the West Rand Group. Detrital porous and sooty pyrites contain 0.4 to 11.3 ppm solid-solution gold, sulfur isotope δ34S values from -17 to +16%, and high levels of As and Te, plus a wide range of other trace elements (in decreasing order: Ni, Co, Sb, Cr, Cu, U, Pb, Bi, Mo, Zn and Ag). The sooty pyrite is the most Au, Te, and Mo rich and is intergrown with alumino-silicates and organic matter. The detrital pyrite textures, S-isotopes and geochemistry resemble diagenetic pyrite developed under suboxic to anoxic bottom water conditions. The LAICP-MS maps also show that the detrital pyrite grains have euhedral hydrothermal pyrite overgrowths containing micro-inclusions of gold, brannerite, and Ni-As sulfides. The pyrite overgrowths are also enriched in solid-solution gold, tellurium, and other trace elements including, in decreasing order, As, Co, Ni, Cr, Cu, Mn, Pb, Bi, and Ag. Fractured and brecciated pyrite associated with brittle bedding-parallel fracture zones in the Carbon Leader Reef are also enriched in these elements due to alteration of pyrite surrounding the fractures. Laser ICP-MS Pb isotope determinations on the cores of detrital pyrite indicate an age between 2750 and 2950 Ma with hydrothermal overgrowths originating between 2100 and 2020 Ma incorporating highly radiogenic Pb. This study demonstrates that both of the two competing theories for the origin of the Witwatersrand gold reefs are likely to be correct. We suggest that the hydrothermal event was widespread (kilometer scale) and involved basinal fluids that scavenged gold, tellurium, arsenic, and trace elements (Co, Ni, Cr, Cu, Mn, Pb, Bi, Ag) from gold-bearing sedimentary units in the Central Rand Group. © 2013 Society of Economic Geologists, Inc.

Microstructural evolution and trace element mobility in Witwatersrand pyrite

Microstructural analysis of pyrite from a single sample of Witwatersrand conglomerate indicates a complex deformation history involving components of both plastic and brittle deformation. Internal deformation associated with dislocation creep is heterogeneously developed within grains, shows no systematic relationship to bulk rock strain or the location of grain boundaries and is interpreted to represent an episode of pyrite deformation that predates the incorporation of detrital pyrite grains into the Central Rand conglomerates. In contrast, brittle deformation, manifest by grain fragmentation that transects dislocation-related microstructures, is spatially related to grain contacts and is interpreted to represent post-depositional deformation of the Central Rand conglomerates. Analysis of the low-angle boundaries associated with the early dislocation creep phase of deformation indicates the operation of {100} slip systems. However, some orientation boundaries have geometrical characteristics that are not consistent with simple {100} deformation. These boundaries may represent the combination of multiple slip systems or the operation of the previously unrecognized {120} slip system. These boundaries are associated with order of magnitude enrichments in As, Ni and Co that indicate a deformation control on the remobilization of trace elements within pyrite and a potential slip system control on the effectiveness of fast-diffusion pathways. The results confirm the importance of grain-scale elemental remobilization within pyrite prior to their incorporation into the Witwatersrand gold-bearing conglomerates. Since the relationship between gold and pyrite is intimately related to the trace element geochemistry of pyrite, the results have implications for the application of minor element geochemistry to ore deposit formation, suggest a reason for heterogeneous conductivity and localized gold precipitation in natural pyrite and provide a framework for improving mineral processing.

STRATIGRAPHY AND PROVENANCE OF THE AURIFEROUS-URANIFEROUS, FLUVIAL TO SHALLOW-MARINE SINQENI FORMATION, MOZAAN GROUP, NORTHERN KWAZULU-NATAL, SOUTH AFRICA

Research Article| September 01, 2012 STRATIGRAPHY AND PROVENANCE OF THE AURIFEROUS-URANIFEROUS, FLUVIAL TO SHALLOW-MARINE SINQENI FORMATION, MOZAAN GROUP, NORTHERN KWAZULU-NATAL, SOUTH AFRICA N. HICKS; N. HICKS School of Geological Sciences, University of KwaZulu-Natal, Private Bag X 54001 Durban 4000, South Africa, Present address: Council for Geoscience. PO Box 900, Pietermaritzburg, 3200, South Africa. e-mail: nhicks@geoscience.org.za Search for other works by this author on: GSW Google Scholar A. HOFMANN A. HOFMANN School of Geological Sciences, University of KwaZulu-Natal, Private Bag X 54001 Durban 4000, South Africa, Present Address: Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa, e-mail: ahofmann@uj.ac.za Search for other works by this author on: GSW Google Scholar Author and Article Information N. HICKS School of Geological Sciences, University of KwaZulu-Natal, Private Bag X 54001 Durban 4000, South Africa, Present address: Council for Geoscience. PO Box 900, Pietermaritzburg, 3200, South Africa. e-mail: nhicks@geoscience.org.za A. HOFMANN School of Geological Sciences, University of KwaZulu-Natal, Private Bag X 54001 Durban 4000, South Africa, Present Address: Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa, e-mail: ahofmann@uj.ac.za Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2012 Geological Society of South Africa South African Journal of Geology (2012) 115 (3): 327–344. https://doi.org/10.2113/gssajg.115.3.327 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation N. HICKS, A. HOFMANN; STRATIGRAPHY AND PROVENANCE OF THE AURIFEROUS-URANIFEROUS, FLUVIAL TO SHALLOW-MARINE SINQENI FORMATION, MOZAAN GROUP, NORTHERN KWAZULU-NATAL, SOUTH AFRICA. South African Journal of Geology 2012;; 115 (3): 327–344. doi: https://doi.org/10.2113/gssajg.115.3.327 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract The Sinqeni Formation of the ~2.98 to 2.87 Ga Pongola Supergroup within the White Umfolozi Inlier in northern KwaZulu-Natal is a fluvial to shallow-marine sequence that has been affected by sea-level changes. Braided alluvial plain conglomerates of the basal Denny Dalton Member are overlain by shallow marine, trough cross-bedded, quartz arenites. A major marine transgression in the Pongola Basin is marked by a conglomerate lag representing a transgressive ravinement surface, which is overlain by banded iron formation, signifying sub-storm wave base deposition on a clastic sediment-starved shelf.Conglomerates of the Sinqeni Formation are locally gold- and uranium-bearing with the ~1 m thick, laterally discontinuous “Mozaan Contact Reef” (MCR) hosting erratic mineralization. Where mineralized, the MCR contains rounded, detrital pyrite grains, derived primarily from erosion of sedimentary pyrites. Three generations of pyrite are recognised, with primary gold hosted as inclusions within compact, rounded detrital pyrite and gersdorffite grains. No uraninite grains are observed in the MCR, with allogenic uraniferous leucoxene and authigenic brannerite forming the major uraniferous component. The uppermost conglomerate of the Sinqeni Formation (CG 4) crops out ~30 m above the Denny Dalton Member conglomerates and hosts uranium precipitated as secondary uraninite and coffinite encrustations around detrital pyrite grains. The conglomerates form a placer deposit which has been subjected to post-depositional alteration. Heavy mineral assemblages as well as bulk geochemical data for conglomerates and quartz arenites suggest a granitoid-greenstone source. Palaeocurrent and mineralogical data suggest a proximal granitoid-greenstone provenance to the west of the White Umfolozi Inlier, whereas equivalent units in the main Pongola Basin have been derived from granitoid-greenstone successions to the north. The reef mineralogy and pyrite geochemistry however, suggest that detritus in the MCR was derived from a variety of sources including granitic basement, Palaeoarchaean greenstones and sulphidic sedimentary rocks of unknown affinity. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

In Situ Trace Element and Sulfur Isotope Analysis of Pyrite in a Paleoproterozoic Gold Placer Deposit, Pardo and Clement Townships, Ontario, Canada

Extensive pyrite-bearing, auriferous cobble and boulder conglomerates are present in the basal 30 in of the Mississagi Formation in Pardo and Clement Townships, Ontario, Canada. The sedimentology of the conglomerates, combined with regional geology, indicates limited fluvial transport in a gravel bed braided river with local hyperconcentrated flows, with material derived from a highly restricted catchment area. Postdepositional overprinting of the conglomerates is related to the Penokean orogeny at 1.85 to 1.5 Ga and alkali metasomatism at similar to 1.7 Ga. Several pyrite varieties, including detrital and postdepositional recrystallized and altered grains, are present in the conglomerates. Detailed in situ laser ablation inductively coupled plasma-mass spectrometry (LA ICPMS) analysis of pyrite revealed that the gold in the deposit is intimately associated to large detrital pyrite grains, as invisible Au, with lesser amounts of free gold. Postdepositional pyrite and pyrite overgrowths have very low Au content. Elements such as Pb, Bi, Sb, Te, Ag, and Hg, together with Au were mobilized during hydrothermal alteration and dissolution of detrital pyrite grains. Small-scale transport and reprecipitation formed part of the postdepositional pyrite and free gold in immediate contact with postdepositional grains. In contrast, Ni, Co, and As were not mobilized. External fluid infiltration is negligible and is indicated only by minor, compositionally distinct, late sulfide veinlets crosscutting the conglomerates. Sulfur isotope analysis of detrital pyrite yields generally positive (delta(34)S values (0.97-9.26%0). The delta(34)S sulfur isotope composition of pyrite overgrowths and postdepositional grains overlaps the isotopic range of the detrital grains, suggesting a near-closed S system during postd.epositional processes. However, the detrital pyrite tends to have slightly negative Delta(33)S while the postdepositional and overgrowth pyrite are either neutral or slightly positive. The S isotope composition of the detrital pyrite is compatible with an origin of S as dissolved sulfate in an ocean under a low oxygen atmosphere. The potential source of the Au-bearing detrital pyrite appears to have been an, as yet, undiscovered Archean deposit located within 1 to 8 kin of the placer deposit.

Earliest microbially mediated pyrite oxidation in ~ 3.4 billion-year-old sediments

Pyrite (FeS2) oxidation in modern sedimentary environments is neither a purely chemical nor purely microbial process, but it is significantly enhanced by the activity of microorganisms that use reduced forms of iron and sulphur in their metabolisms. On the early Earth, where oxygen levels were thought to be <10−5 of the present atmospheric level and chemical oxidants scarce, such biological mediation may have been critical in the redox cycles of iron and sulphur. Here, we show that detrital sedimentary pyrite grains in a ~3.4 billion-year-old sandstone were colonised by microbial communities. The detrital pyrite comes from the basal quartz arenite member of the 3.43–3.35Ga Strelley Pool Formation (SPF) in the East Strelley greenstone belt of the Pilbara Craton, Western Australia. Rock chips and petrographic thin sections of black sandstones occurring on two ridges close to the SPF type locality of Strelley Pool were investigated using optical microscopy, SEM, TEM, laser Raman and NanoSIMS. The detrital pyrite grains exhibit laminated carbonaceous coatings of early Archean age, with localised enrichments of nitrogen that are interpreted as the in situ remains of biofilms growing on these nutrient-rich minerals. Pyrite surfaces contain spherical pits, chains of pits and channels that are morphologically distinct from abiotic alteration features. The pits and channels are widespread, have a clustered distribution typical of microbial colonisation, and are closely comparable to biologically mediated microstructures in the younger rock record and those created by extant Fe- and S-oxidising microbes in the laboratory. They are thus interpreted as trace fossils formed by the attachment of bacteria to the pyrite surfaces. A nano-layer and discreet nano-grains of secondary mineral precipitates, namely Fe-oxides belonging to the magnetite-maghaemite group, attest to pyrite oxidation. These are intimately associated with the biofilms and trace fossils, and are interpreted to represent the fossilised mineral products of biologically mediated pyrite oxidation. These data extend the geological range of microbes capable of metabolising reduced Fe and/or S compounds back to the early Archean and indicate that pyrite-rich sedimentary rocks provide promising targets in the search for extraterrestrial life.

PALEOENVIRONMENTAL CONTROLS ON THE TEXTURE AND CHEMICAL COMPOSITION OF PYRITE FROM NON-CONGLOMERATIC SEDIMENTARY ROCKS OF THE MESOARCHEAN WITWATERSRAND SUPERGROUP, SOUTH AFRICA

A petrographic and mineral chemical study was conducted on pyrite from non-conglomeratic sedimentary rocks of the 3.0 to 2.8 Ga Witwatersrand Supergroup. Detailed petrographic analyses revealed the presence of three paragenetic associations, namely detrital, diagenetic and epigenetic pyrite. Detrital pyrite is commonly observed in sandstone and diamictite from proximal shelf deposits. The mineral chemistry of large (0.1 to 0.5 mm in size) and small (<0.05 mm in size) detrital pyrite grains, based on Co, Ni and As contents, suggests a metamorphic and diagenetic origin, respectively. Significantly, a variety of early diagenetic pyrite forms has survived greenschist facies metamorphism; these occur exclusively in carbonaceous mudstones intercalated with fluvial inner shelf sandstones. The high Ni content of early diagenetic pyrite and the occurrence of early diagenetic Cu-Pb sulfides highlight the importance of organic matter and host rock composition. However, in distal to starved outer shelf depositional settings, detrital pyrite is absent and diagenetic pyrite is scarce. Nevertheless, the composition of diagenetic pyrite in distal magnetite-rich lithologies is similarly affected by host rock composition and possibly Fe-Mn reduction (elevated Co and As concentrations). Epigenetic pyrite, pyrrhotite and base-metal sulfides are prevalent in sandstone intercalations near diabase sills and fault zones. The composition of epigenetic pyrite is influenced by the temperature of the fluids and the interaction of the fluids with the host rock, as well as with precursor and associated iron sulfides. This study finds that local depositional and tectonic settings are instrumental in dictating the distribution, morphology and composition of diagenetic pyrite. In this regard, the formation of metal-rich early diagenetic pyrite is favored in carbonaceous mudstones associated with fluvial regimes in the upper Hospital Hill, Government, Jeppestown, Johannesburg and Turffontein Subgroups, where continental runoff would have supplied nutrients and sulfur species to the shallow water environment. In contrast, the scarcity of diagenetic pyrite in marine-dominated environments of the lower Hospital Hill Subgroup (of which some are carbonaceous) and distal portions of the Government and Jeppestown Subgroups, is consistent with a lack of sulfate in the Archean ocean.

4,500-YEAR-OLD MINING POLLUTION IN SOUTHWESTERN SPAIN:LONG-TERM IMPLICATIONS FOR MODERN MINING POLLUTION

The Tinto river drains the Rio Tinto mining district, which comprises the world’s largest known massive sulfide deposits; these orebodies have been mined from the third millenium BC to the present. The Tinto river is strongly acidic (pH, 1.5–2.5); during flood events, it transports a sandy material, including abundant detrital pyrite grains. A core drilled in the Holocene sediments of the Tinto estuary allows for investigation of recent and historical mining pollution. Two anomalous horizons have been recognized (0–1.3 m; 3–4 m). Both are characterized by very high metal content (100 times over the background) and by the presence of abundant clastic pyrite grains. The metal association (Pb, Ba, As, Cu, Zn, Sn, Tl, Cd, Ag, Hg, Au) is typical of that of the Rio Tinto pyritic ore. The upper horizon corresponds to the modern mining activity; the lower horizon has been dated at 2530 BC (14C AMS calibrated age). We show here that active mining occurred early (Copper Age) in the Rio Tinto area, resulting in a watershed-scale metal contamination. We also show that anthropogenic input of metals may be accumulated and immobilized during thousands of years in estuarine sediments.

Detrital pyrite in Witwatersrand gold reefs: X‐ray diffraction evidence and implications for atmospheric evolution

Pyrite is easily oxidized, and therefore unambiguous evidence of detrital pyrite grains in metasediments is a significant constraint on when an oxygenated atmosphere developed. Compact rounded pyrite in the Witwatersrand gold reefs of South Africa has a detrital habit and is texturally equivalent to and spatially associated with detrital zircon and chromite. X‐ray precession photography reveals that petrographically featureless As‐poor grains are untwinned single crystals of high diffraction quality. This new evidence from crystallography is consistent with mechanically abraded pyrite from primary lode gold deposits, and excludes an origin by replacement of a pre‐existing detrital phase. Further evidence of a detrital origin for the compact rounded pyrite is afforded by isolated grains of arsenian pyrite displaying truncated As‐rich growth bands. The geographically extensive Witwatersrand fluvial conglomerates evidently had a matrix of quartz and pyrite sand and pyritic mud in their unconsolidated state and, thus, the late Archean atmosphere of Earth was likely essentially anoxic.

Probable Life-forms from the Precambrian of the Witwatersrand System (South Africa)

DURING mineralographic work on Witwatersrand gold conglomerates from the Orange Free State goldfield, I encountered several peculiar micro-structures strongly resembling so-called ‘mineralized bacteria’1–4 and, in certain respects, some types of ‘organized elements’ recently under discussion as possible primitive life-forms in meteorites5–7. These structures are preserved in some of the minute detrital pyrite grains of the Elsburg A1 and A3 horizons occurring near the top of the Witwatersrand system, the deposition of which is believed to have been completed about 2,150 million years ago8. As the organized elements are indigenous to allogenic components of the rock, they are certainly considerably older.