Proterozoic Rocks Research Articles

Utilizing compositions of zircon and apatite for prospecting of Cu-Mo-Auporphyry mineralization in the Pekinsky and Tessemsky granitoid massifs of the Taimyr-Severozemelskaya folded area

Research subject. The Tessemsky granite massif is located in the North Taimyr tectonic zone, surrounded by Cambrian rocks. The Pekinsky granite massif is located within the Central Taimyr zone, surrounded by metamorphosed Proterozoic rocks. Aim. To develop a methodology for using the composition of accessory granitoid minerals when prospecting Cu-Mo-Au-porphyry mineralization on the example of the Pekinsky and Tessemsky granitoid massifs of the Taimyr Peninsula. Materials and methods. Accessory zircon and apatite contained in two granitoid samples from the Pekinsky massif (P1, P2) and two granitoid samples from the Tessemsky massif (T2, T3) were studied. Their mineral composition was examined using an EPMA Cameca SX100 instrument. The element content in minerals was determined by LA-ICPMS using an NexION 300S instrument equipped with an NWR 213 attachment. Results. Most of the zircons from the Pekinsky and Tessemsky massifs were formed at T < 738°C in oxidized magma with ΔFMQ of 0.6–2.6, which is a favorable sign for the identification of Cu-Mo-Au-porphyry mineralization. Zircons are characterized by elevated (Eu/Eu*)Y and (Ce/Nd)n/Y ratios, which is also a favorable, though not a strongly reliable, sign for identifying porphyry mineralization. The Eu/Eu* and Sr/Y ratios in the apatites from the Tessemsky massif are higher than those in the apatites from the Pekinsky massif. The rock compositions of both massifs fall within the fields of adakites on the classification diagrams. The estimates of oxygen fugacity (logfO2) calculated from Mn in apatites for four samples agree well within the error limits. Conclusion. Specific features of using the composition of accessory minerals (zircon and apatite) for prospecting the Cu-Mo-Au-porphyry mineralization associated with granitoids were considered. Accessory indicator minerals can be used to rank granitoid massifs in order to assess their ore content. The example of two granite intrusions of the Taimyr Peninsula made it was possible to show that the Tessemsky massif is more promising for the discovery of associated Cu-Mo-Auporphyry mineralization than the Pekinsky massif.

T-t Evolution of the Early Proterozoic Rocks in the Northern Ladoga Region from the Data on U-Pb, Rb-Sr and Sm-Nd Systems in Minerals

This paper presents the results of a study of isotopic systems in minerals and rocks in southern margin of the epi-Archean Karelian craton in the zone of its junction with the Svecofennian mobile belt. U-Pb, Sm-Nd and Rb-Sr mineral ages of metamorphic rocks allowed reconstructing a T-t trend during ~1.88–1.61 Ga, which reflects a wide-ranging cooling history of metamorphic rocks from the peak values of about 650–700 °C at 1.88–1.79 Ga (U-Pb age of monazites and apparent oldest Sm-Nd age of amphiboles) to 300–400 °C at 1.61 Ga (model Rb-Sr age of biotites) in zones of low- and medium-temperature metamorphism. The specificity of removal of deep-seated rocks to the present-day erosion surface and the reconstructed T-t trend comply with the development of thrust-nappe structures during the exhumation of the Svecofennids. It is also assumed that differential vertical block movements played a significant role during the post-orogenic extensional collapse and neorifting.

Geochemical and geochronologic evidence for a contiguous northeastern Wyoming Province

The extent and nature of the Wyoming Province, an Archean craton in southwestern Laurentia, are poorly understood due to limited exposure between spatially isolated basement-cored uplifts. This lack of exposure has led to debate about whether the northeastern Wyoming Province is underlain by contiguous Archean crust or Proterozoic rocks and suture zone associated with the Trans-Hudson orogeny. To assess these models we analyzed samples recovered from drill cores in the buried northeastern Wyoming Province that straddle the proposed Proterozoic suture. Whole-rock geochemical and Nd-Pb-O isotopic data suggest the rocks formed by partial melting of > 3.5 Ga hydrated mafic to tonalitic sources similar to elsewhere in the northern Wyoming Province. Zircon U-Pb dates record Mesoarchean magmatism and metamorphism (3.0–2.8 Ga). Published whole-rock Rb-Sr, hornblende and biotite K-Ar, and apatite fission track dates suggest these rocks have not been heated above 300 °C since 2.5–2.1 Ga. Geophysical potential field data are consistent across northeastern Wyoming contrasting with major discontinuities associated with documented Proterozoic orogens on all other margins of the Wyoming Province. Hence, geochemical, isotopic, and geochronologic data along with geophysical imaging can be most simply interpreted in terms of continuous Archean crust in the northeastern Wyoming Province. A geophysically-imaged reflector east of the Bighorn Mountains may juxtapose Archean terranes with similar ages and sources, similar to the boundary between the Montana metasedimentary terrane and Beartooth-Bighorn magmatic zone in the northwestern Wyoming Province. This work emphasizes the value of integrating geologic and geophysical constraints to constrain Archean provinces and their tectonic evolution.

The missing sedimentological section linking the Neogene and quaternary of the Tandilia system and the Salado basin, Buenos Aires province, Argentina

The Tandilia system and the Salado basin are two adjacent morphostructural units located within the Argentine Pampean plain, each exhibiting distinct and independent geological histories. The Tandilia system preserves outcrops of Argentina's oldest Proterozoic rocks, while the sedimentary record of the Salado basin began during the Jurassic period. However, starting in the Cenozoic era, both units began to display common continental sedimentary fill characteristics. Despite this connection between the Tandilia system and the Salado basin, there is no unified lithostratigraphic framework for the Cenozoic that varies according to the studied localities. This study aims to define the stratigraphy of a sedimentary section that demonstrates the link between these morphostructural units from the Neogene onwards and to correlate it with the formal lithostratigraphic frameworks proposed in the literature. The analysis of well records led to the description of lithofacies and the establishment of facies associations, which were integrated to generate three correlation transects between the wells. Four facies associations were identified, three of which were fluvial and one aeolian. The analysis of these facies associations revealed that the sedimentary fill could be divided into two sedimentary sequences, lower sedimentary sequence and upper sedimentary sequence, separated by a sequence boundary marked by conglomeratic deposits. The definition of a stratigraphic framework for the middle section of the Del Azul creek basin and its correlation with the formal lithostratigraphic frameworks of the region indicate that from the Neogene onwards, both morphostructural units, the Tandilia system and the Salado basin, share sedimentological and tectonic characteristics. This study highlights the significance of subsurface information in understanding the geology of plain areas and its influence on regional geological evolution.

Formation of the Lawn Hill Impact Structure

The Ordovician-aged Lawn Hill Impact Structure is perhaps unique among documented craters, in that it is largely filled with slumped, allochthonous Cambrian carbonates partly fluidised by the impact shock. It is a complex geological feature, with a thin horizon of suevite beneath carbonate sheets. The bulk of the impact ejecta material was either deposited on top of the mobilised carbonates, and not preserved owing to erosion, or is present beneath the base of current drilling. The geometry of the crater in its final form has been modelled in three dimensions using gravity data and dipole–dipole-induced polarisation geophysics, with control from an extensive drill-hole library. Post-impact modification, including terrace formation and carbonate fill, has resulted in an asymmetric ring syncline, which extends to depths >600 m. In the data-rich area of the Century zinc–lead–silver deposit, on the southwest margin of the crater, both intact and remnants of the crater rim and impact ejecta blanket were preserved above the deposit. The Proterozoic ore deposit is located on a detached terrace, which slumped into the crater and was buried by allochthonous carbonates from the Georgina basin. The sequence of events can be shown to comprise: impact with crater formation and injection of carbonate dykes and inter-thrust wedges in the crater walls with concurrent suevite deposits inside and surge deposits outside the crater; this was rapidly followed by slumping of carbonate mega-blocks transported by a fluidised carbonate slurry into the crater, concomitant with collapse of Proterozoic terraces; finally, collapse of the central crater uplift generated imbricate slices of Cambrian carbonate among mainly Proterozoic rocks inside the ring syncline.

Heterogeneous multi-stage accretionary orogenesis — Evidence from the Gunnison block in the Yavapai Province, southwest USA

Proterozoic rocks exposed in the southwestern U.S.A. represent one of the best examples of crustal growth by arc-related magmatism and accretionary orogenesis. Within the Southwest the 1.8–1.7 Ga Yavapai Province is widely regarded as a classic example of juvenile arc crust, however 1.8–2.5 Ga inherited zircon and Nd and Hf model ages have been recognized near Gunnison in central Colorado. These data have led to questions regarding the extent and nature of pre-1.8 Ga crustal material and the genesis of the Yavapai Province. We present evidence for a geochemically distinct, spatially restricted crustal block underlain by pre-1.8 Ga crust material (referred to here as the Gunnison block) in central to western Colorado within the Yavapai Province. The Gunnison block is characterized by 1.8–1.9 and 2.4–2.6 Ga inherited zircon, Pb isotopic systematics (μ = 9.8 ± 0.1, κ = 3.7 ± 0.1) elevated relative to 1.8 Ga depleted mantle values, 1.8–2.5 Ga Nd and Hf model ages, and a distinct pressure-temperature-time history. The geochemical data are consistent with mixing between juvenile 1.8 Ga and pre-1.8 Ga sources. The older crustal component is most similar to the isotopically enriched Mojave Province of eastern California and western Arizona, suggesting greater similarities between these provinces than previously recognized. Monazite and xenotime petrochronology indicate ca. 1.75–1.74, 1.72–1.69, 1.67, and 1.47–1.38 Ga tectono-metamorphic events. These data suggest that the Gunnison block accreted to other components of the Yavapai Province outboard of Laurentia at 1.75–1.74 Ga. The composite Yavapai Province was accreted to the margin of Laurentia during the 1.72–1.69 Ga Yavapai orogeny. Later overprinting is associated with the ∼1.68–1.60 Ga Mazatzal and ∼1.47–1.37 Ga Picuris orogenies. Identification of distinct crustal terranes within the Yavapai Province supports models involving multiple arcs and back-arcs that were progressively assembled prior to their accretion to Laurentia, perhaps akin to the present-day Banda Sea in Indonesia.

Delineation of an exhumed intermediate‐depth crustal fault in a collisional setting: An example from the Himalaya

AbstractThe regionally prominent main boundary thrust (MBT) of the Himalayan fold‐thrust belt in northwest India is typically defined by the presence of Proterozoic rocks in the hanging wall and Cenozoic rocks in the footwall. The present study focuses on identifying the MBT contact across Gambar River section in Himachal Pradesh, India, using alternative methodologies, such as the meter‐scale litho‐structural mapping, followed by detrital zircon U–Pb geochronology to precisely identify the thrust contact and provide insights on the deformation history of the MBT zone. We have identified a sharp change in the age (from ~600 to ~61 Ma) of the sedimentary units along a narrow zone in the study area by detrital zircon U–Pb geochronology using LA‐ICP‐MS. The sharp change in the detrital zircon U–Pb age data thus delineate the MBT occurring in the area along a < ~1 m thickness. The lithological assemblage and the age data indicate the unified maximum depositional age from ~700 to ~600 Ma for the hanging wall rocks, which have been equated with the Krol Group of the Lesser Himalayan Sequence (LHS). In comparison, the footwall rocks exhibit the maximum depositional age of ~61 Ma and have been equated with the Cenozoic Subathu Formation of the Sub‐Himalayan Sequence (SHS).

A multi-disciplinary geophysical approach reveals a Paleoproterozoic cryptic suture in the Amazon Craton – Implications for Proterozoic Supercontinent reconstructions

The Amazon Craton evolved in the Paleo-Mesoproterozoic through successive continental magmatic arcs, which ages decrease westwards. However, when and if these magmatic arcs collided with another continent, closing an ocean basin is a subject of ongoing debate. It is speculated that the Baltica and Amazon cratons formed a purely long-lived accretionary margin for at least 900 Ma and only collided during the formation of the Rodinia Supercontinent at 1.2 – 1.0 Ga. Identifying relicts of suture zones is challenging since Proterozoic orogens tend to preserve the continental arc and the reworked components so that less than 10 % of the orogens are composed of oceanic evidence. Moreover, the Phanerozoic cover overlays more than 50 % of the proterozoic rocks in the Amazon Craton. Therefore, we propose a robust geophysical assessment to investigate the upper and lower crusts. We acquired new magnetotelluric data and integrated it to the available potential field (satellite, airborne, and ground), geochronological data (magmatic and metamorphic ages), crustal thickness and depth to Curie temperature estimates. A major crustal-scale structure (more than 1000 km in extension) separates two tectonic fragments with remarkable differences in the magnetic gradient and structure trend. We also find along this major boundary high and low Bouguer anomalies of long-wavelength paired, a step in the Mohorovic discontinuity, and a lower crust conductor. This evidence supports the hypothesis of a collision in Paleoproterozoic times. Our findings suggest that the orogen did not remain purely accretionary and evolved to a collisional type between 1.63 and 1.69 Ga. This contrasts with the purely accretionary orogens described in the Baltica Craton, thereby undermining the hypothesis that the Amazon and Baltica cratons shared the same peri-Nuna subduction system from 2.0 to 1.3 Ga.

Resource potential of the Proterozoic–Paleozoic Carrara depocentre, South Nicholson region, Australia: Insights from stratigraphic drilling

A comprehensive geochemical program was carried out on rock samples collected in the NDI Carrara 1 drill hole, the first stratigraphic test of the newly discovered Carrara Sub-basin located in the South Nicholson region of northern Australia. The drill hole recovered continuous core from 284 m to total depth at 1750 m and intersected approximately 1120 m of Proterozoic sedimentary rocks, unconformably overlain by 630 m of Cambrian Georgina Basin carbonate-rich rocks. Total organic carbon (TOC) contents from Rock-Eval pyrolysis highlight the potential for several thick black shales to be a source of petroleum for conventional and unconventional plays. Cambrian rocks contain an organic-rich section with TOC contents of up to 4.7 wt.% and excellent oil-generating potential. The Proterozoic section is overmature for oil generation but mature for gas generation, with potential for generating gas in carbonaceous mudstones showing TOC contents up to 5.5 wt.% between 680 and 725 m depth. A sustained release of methane (up to 2%) recorded during drilling from 1150 to 1500 m suggests potential for an unconventional gas system in the Proterozoic rocks from 950 to 1415 m depth, which exhibit favourable organic richness and thermal maturity. The Proterozoic rocks, which are comparable in age to the sediment-hosted deposits of the Century Mine, contain local occurrences of lead, zinc and copper sulfide minerals providing hints of mineralisation. The combined geochemical results offer the promise of a potential new resource province in northern Australia.

МИНЕРАЛОГИЧЕСКИЕ ОСОБЕННОСТИ РУДНЫХ ПРОЯВЛЕНИЙ ЗОЛОТА СОЛОЛИЙСКОГО ПОДНЯТИЯ ОЛЕНЕКСКОГО СВОДА

The paper deals with studied mineralogical and geochemical features of gold-sulphide manifestations of the Sololi uplift of the Olenek arch (northeast of the Siberian Platform) in order to identify their mineralogical features and prospects for their further research. The identical mineral composition of gold-sulphide formations developed by Early Proterozoic rocks of the Eekitian series and rhyolites of the same age gave reason to assume the simultaneity of their formation and to refer them to the general stage of ore formation. According to the established mineral paragenesis, described gold sulphide manifestations are attributed to the gold-quartzsulphide formation type. The discovery of gold sulphide mineralization in the zone of tectonic fracturing of Early-Middle Permian sandstones indicates the formation of ore occurrences in the area associated with the processes of tectonomagmatic activation of the Mesozoic. The identification of indicators of low-temperature hydrothermal processes as mineral paragenesis – mercury-containing gold, cinnabar and barite, testifies to shallow and low-temperature conditions for the formation of this ore manifestation. It was found that according to the Co/Ni ratio, 28% of pyrites (including gold-bearing ones) are associated with hydrothermal processes. In general, the detection of native gold in quartz veins in free form and as microinclusions and impurities in pyrite, as well as the discovery of relatively large poorly rounded attachments of gold to quartz in stream flows dewatering the studied ore formations, allows researchers to positively assess the prospects for further study of ore manifestations of the Sololi uplift of the Olenek arch.

Laramide bulldozing of lithosphere beneath the Arizona transition zone, southwestern United States

Abstract The northwest-trending transition zone (TZ) in Arizona (southwestern United States) is an ~100-km-wide physiographic province that separates the relatively undeformed southwestern margin of the Colorado Plateau from the hyperextended Basin and Range province to the southwest. The TZ is widely depicted to have been a Late Cretaceous–Paleogene northeast-dipping erosional slope along which Proterozoic rocks were denuded but not significantly deformed. Our multi-method thermochronological study (biotite 40Ar/39Ar, zircon and apatite [U-Th-Sm]/He, and apatite fission track) of Proterozoic rocks in the Bradshaw Mountains of the west-central Arizona TZ reveals relatively rapid cooling (~10 °C/m.y.) from temperatures of >180 °C to <60 °C between ca. 70 and ca. 50 Ma. Given minimal ca. 70–50 Ma upper-crustal shortening in the TZ, we attribute cooling to exhumation driven by northeastward bulldozing of continental lower crust and mantle lithosphere beneath it by the Farallon flat slab. Bulldozing is consistent with contemporaneous (ca. 70–50 Ma) underplating and initial exhumation of Orocopia Schist to the southwest in western Arizona and Mesozoic garnet-clinopyroxenite xenoliths of possible Mojave batholith keel affinity in ca. 25 Ma TZ volcanic rocks.

Sediment provenance and dispersal in the early Eocene Dongying Depression, Bohai Bay Basin, Eastern China: Evidence from detrital zircon geochronology, geochemistry and petrology

Comprehensive provenance studies of syn-rift basin fills are required to better understand possible sources of clastic detritus and sediment routing systems. The Eocene fill of the Bohai Bay Basin in eastern China represents a syn-rift succession, where subsurface datasets permit investigation of sediment sources and sinks. New detrital zircon U–Pb samples (441 detrital zircon grains) from six wells were combined with elemental geochemical analysis of siliciclastic sediment, sandstone petrography and palynology to investigate depositional ages and provenance. This study demonstrates the importance of integrating geochronometry, geochemistry, petrology and palynology datasets to fully unravel syn-rift sediment provenance and routing. Zircons of the Dongying Depression were derived principally from an active continental margin island-arc setting characterized by felsic acid magmas, and subordinately from a recycled orogenic belt. The Shicun and Binnan faults controlled the relationship between sediment routing systems and source areas, explaining spatial differences of provenance signals in the depression. The routing system around the Shicun fault is characterized by a dominance of late Paleozoic and Paleoproterozoic zircons and subordinate Mesozoic zircons. In contrast, increased Mesozoic zircons in samples from south of the Binnan fault provide evidence of Mesozoic magmatism in this area. The early Eocene sediments record the signal of early Cretaceous magmatism in the North China Craton, but lack a record of substantial syn-depositional magmatic activity since the Paleogene in the Dongying Depression. Zircons from early Cretaceous strata and pre-existing zircons from the Xing-Meng Orogenic Belt and Inner Mongolia paleo-uplift were transported to the Yanshan and Luxi areas together. These sediments entered the Dongying Depression in the early Eocene, following a period of recycling. In addition, a Precambrian basement signal indicates possible denudation of Neoarchean to Proterozoic rocks in the Luxi uplift and from the Binnan fault footwall. Overall, the provenance signals of the basin reflect zircon recycling from an Early Cretaceous succession, denudation of ancient magmatic rock masses and the absence of syn-sedimentary magmatism.

Sediment provenance signal of the discontinuous retroarc topography in the northern Andes during the Early Cretaceous

AbstractA large dataset of detrital zircon U–Pb ages (N = 5940) of Aptian‐Albian strata of the Colombian‐Ecuadorian retroarc region suggests that these rocks were sourced from Proterozoic cratonic and Permian–Triassic to Cretaceous rocks of the Andean proto‐Cordilleras. The nonconformity between Aptian‐Albian strata and Proterozoic rocks of the Garzón Massif indicates the existence of positive relief in this region. Topographic highs could have caused local basin compartmentalization and the prevalence of a localized provenance of coeval strata. Areas of positive relief were seemingly exposed to intense chemical weathering as suggested by the high Chemical Index of Alteration values, and low Rb/Sr and SiO2/Al2O3 ratios of analysed strata. Our results highlight the value of provenance analysis to study the ancient topography of retroarc systems and open the avenue for further research on the role of extensional tectonics in the topographic and basin evolution in the Andes.

Proterozoic evolution of the Alxa block in western China: A wandering terrane during supercontinent cycles

The tectonic affinity of the Alxa block in the western North China craton remains controversial because of the widespread Cenozoic sediments and limited Proterozoic geochronological data, especially in the western Alxa block. To establish the tectonic affinity of the Alxa block with neighbor units, we systematically investigated U-Pb age and Hf isotope composition of zircon and U-Th-Pb age of monazite from Proterozoic rocks in the southwestern Alxa block. Ages of zircon and monazite from granitic gneiss and mica schist indicate that the Paleoproterozoic Longshoushan Complex recorded a magmatic event at ∼2.0 Ga and two metamorphic events at ∼1.95–1.90 and ∼1.85–1.80 Ga. Detrital zircons from quartz schist of the Longshoushan Complex show an age peak at ∼2.0 Ga with εHf(t) values between −4.048 and 6.432. By contrast, detrital zircons from the Late Paleoproterozoic Dunzigou Group yield different Paleoproterozoic age peaks and εHf(t) values, suggesting a provenance change. Detrital zircons from the Neoproterozoic Hanmushan Group exhibit three age peaks at ∼2.5 Ga, ∼1.85 Ga and ∼0.8 Ga, and two types of εHf(t) values with wide variations. The depositional ages of the Dunzigou Group and Hanmushan Group are constrained as 1.8–1.7 Ga and 845–830 Ma, respectively. The detrital zircon age populations suggest that the Longshoushan area experienced a transition from a convergent setting (less polymodal age spectra) at ∼1.98–1.95 Ga to a divergent setting (polymodal age spectra) at 1.8–1.7 Ga and then intracontinental rifting at ∼830 Ma. Compared with previous zircon studies, we propose that the Alxa block may be connected with the Khondalite belt at ∼1.98–1.90 Ga during assemblage of the Columbia supercontinent, and located between the North China craton and the northern Siberian craton at 1.8–1.7 Ga in a divergent setting. The Alxa block probably separated from the North China craton in the Late Mesoproterozoic and moved next to the northwestern Yangtze craton in the Early Neoproterozoic during the assemblage and breakup of the Rodinia supercontinent.

Exotic mineral products from the Chupa gneisses of the Belomorides

Exotic mineral products (EМP) in the Chupa paragneisses of the Belomorian rock complex occur as an association of submicroscopic “spherules”, “pipes”, “chips” and their combined varieties. EМPs were derived by multi-stage allochemical stress-metamorphism of host gneisses at high pressure in temperature regimes typical of amphibolite (in the Late Archean) and epidote-amphibolite (in the Early Proterozoic) facies. One distinctive feature of EMPs is the presence of carbon in all of their mineral phases. The metal-like substance of “chips” and the cores of “spherules” and “pipes” is comparable in chemical composition and the Ме/С index (Ме = Fe + impurity elements) with known iron carbides, and is similar in composition to terrestrial minerals, such as chalypite, yarlongite, cohenite, and haxonite. The average bulk carbon content of core-free EMPs and core margin substance (at.%) is 7 for Archean and 7-11 for Proterozoic rocks. The average carbon content of iozite, a mineral phase prevalent in “spherules” and “pipes”, is 11 at.% for Archean and 14 at.% for Proterozoic units. The carbon content of the mineral fragments of host rocks (quartz, plagioclase, garnet, and kyanite), which form inclusions in EMPs, is 7-20 to 46 at. %. The distinctive characteristics of the EMPs, publications on the gas composition and soluble carbonaceous endogenic material of gas-liquid inclusions in the rock-forming minerals of Chupa gneisses, and the presence of graphite therein provide evidence for the significant, yet poorly understood contribution of carbon and its chemical compounds as fluid constituents to the highgrade metamorphism of the Belomorides.

Organic matter sulfurization and organic carbon burial in the Mesoproterozoic

Throughout the Proterozoic Era, sedimentary organic carbon burial helped set the pace of global oxygenation and acted as a major modulator of atmospheric CO2 and climate. Although Proterozoic rocks generally contain low concentrations of organic matter (OM), there are key exceptions to this rule, including the relatively OM-rich Arctic Bay shales from Baffin Island, Canada (Bylot Supergroup, Borden Basin, ∼1.05 Ga). The mechanisms driving elevated OM concentrations in these and other Proterozoic shales remain poorly understood. In the Mesozoic and Cenozoic, organic matter sulfurization can be a major driver of enhanced OM burial across a range of redox conditions comparable to those inferred for many Proterozoic environments. Therefore, in this study, we evaluate the role of sulfurization in driving OM preservation in the Mesoproterozoic Borden Basin and discuss its relevance to Proterozoic systems in general. We present the first evidence for syngenetic-to-early-diagenetic OM sulfurization in a Proterozoic basin, which begins to fill a several-billion-year gap in our record of organic S across Earth history. We find that OM sulfurization was particularly extensive in shales from a relatively shallow-water section (Alpha River) but less extensive in shales deposited in deeper water (Shale Valley), which is consistent with models that infer sulfidic ‘wedges’ or O2-minimum-zone-type structures on shelf margins at least intermittently at this time. At the shallower site, organic S and pyrite are similarly 34S-depleted and thus likely formed at roughly the same time near the sediment–water interface under conditions previously interpreted to have been ferruginous to intermittently sulfidic. In contrast, at the deeper-water site, large S-isotope differences between pyrite and organic S along with low apparent OM sulfurization intensities indicate that pyrite formation was favored over OM sulfurization during early sedimentation under variable but primarily ferruginous conditions. Although Mesoproterozoic biomass can be substantially sulfurized, indicators of sulfurization intensity are not correlated with OM concentrations, and therefore sulfurization does not appear to have been the primary driver of enhanced OM concentrations in Arctic Bay Formation shales. The link between sulfurization and total OM preservation may have been modulated during the deposition of Arctic Bay Formation shales by differences in iron availability, nutrient cycling, and particle dynamics in the Mesoproterozoic.

PROVENANCES AND SEDIMENTATION SETTINGS OF THE VENDIAN TERRIGENIC ROCKS OF THE SOUTHERN PART OF NEPA-BOTUOBA ANTECLISE (SIBERIAN PLATFORM)

This paper presents the results of petrographic, lithogeochemical, and U-Pb geochronological studies of detrital zircons in deep core sediments from the Vendian terrigenous sequences of the Nepa and Tira formations in the interior of the Siberian Platform. The section of the Nepa formation is characterized by a terrigenous sediment, the Tira Formation is composed mainly of carbonate rocks. The terrigenous rocks of the Nepa formation are composed of ill-sorted and poorly rounded clasts. Towards the higher part of the section of the Tira formation, there is observed an increase in the degree of sorting and roundness of detrital grains. It has been established that clastic rocks of the lower Nepа subformation are the products derived from rocks of mixed (acidic, basic) composition. The terrigenous rocks of the upper Nepa Subformation and the Tira formation accumulated mainly due to acid decomposition of rocks. The U-Pb (LA-ICP-MS) geochronological studies of detrital zircons from terrigenous rocks of the Nepa and Tira formations led to the conclusion that the sedimentary basin of these formations was contributred to by both the Archean-Early Proterozoic rocks in the basement of the Siberian Platform and the adjacent areas of the Central Asian foldbelt. It is assumed that the sediments of the lower Nepa subformation deposited in the residual basin, formed by joining of terrains and island arcs in the Paleoasian Ocean to the southern margin of the Siberian Platform during the Vendian accretionary-collisional events. This was followed by a transformation into a peripheral sedimentary basin and an occurrence of a transgression of the sea, which favored the accumulation of terrigenous-carbonate rocks of the Tira formation.

A comprehensive genetic model for the world’s largest Sb deposit (Xikuangshan, China)

AbstractThe Mesozoic (160–130 Ma), fault-controlled Xikuangshan Sb deposit within Devonian limestone strata of Hunan Province, Southern China is the world’s largest Sb deposit containing a proven reserve of ~2.5 m Sb. Although mined for over a century, its genesis remains poorly understood. Here we use new He-Ar isotope data of hydrothermal stibnite and both new and existing C-O-Sr-Nd isotopes of hydrothermal calcite with known stages to decipher its genesis and the major constraints on mineralization intensity. The 3He/4He and 40Ar/36Ar ratios of fluid inclusions trapped in stibnite are from 0.01 to 0.04 Ra (Ra: atmospheric 3He/4He ratio) and 304–1077, respectively, indicating the ore-forming fluids at Xikuangshan were dominated by air-saturated meteoric groundwater after interaction with crustal rocks. Ore-stage calcite C and O isotopes indicated that most CO2 in the fluids was acquired from marine carbonate rocks by dissolution; whereas Sr and Nd isotopes differed from deposited Devonian country rocks but were similar to the underlying regional Proterozoic clastic rocks in the region. Calcite from early and late stages showed a strong positive correlation between δ18O and 87Sr/86Sr, consistent with the mixing between the circulating groundwater and compounds released from the Proterozoic rocks due to extensive fluid-rock interaction. The 3He/Q ratios of the fluid inclusions are low, varying from 4.3 to 18.5 × 10−15 cm3 standard temperature and pressure (STP) J−1, indicating deep-seated magma could have provided heat by conduction but no volatiles into the ore-forming fluids. Based on these new results, we suggest that deep-seated granitic magma heated the down-going meteoric groundwater along fault zones, after which the groundwater extensively interacted with and extracted Sb from the Proterozoic Sb-rich rocks to form Sb-rich fluids. The Sb-rich fluids then ascended through regional faults and deposited Sb as stibnites at favorable structural traps to form the Xikuangshan Sb deposit. This study highlights that extensive water-rock interaction is essential to form the deposit, and more intensive water-rock interaction at an early stage allowed for early-stage mineralization yielding higher Sb reserves (>80%) at Xikuangshan.

Principles governing the selection of ore regions for gold deposit search

Background. In 2020, the Auditing Chamber of the Russian Federation, based on a representative report by its expert analysts M. Men’ and A. Kaulbars, proposed expanding the search for deposits of a number of metals, including gold. Since the fund of easily discovered gold deposits coming to the surface has been significantly reduced, prospecting of ore regions most promising in terms of gold deposits becomes highly relevant.Aim. In connection with the above task, it seems expedient to outline territories within the currently known gold provinces, where the prerequisites and signs of gold ores could be detected with minimal expenditures for their subsequent verification by drilling.Materials and methods. The study involved calculating the shares of the world’s gold reserves attributable to each type of distinguished deposits located in various rocks. The extensive experience of prospecting work carried out in recent decades and culminated in the discovery of the Muruntau deposit in Uzbekistan, a world leader in the development of gold deposits, and Russian largest and giant deposits – Natalka, Sukholozhskoe, Nezhdaninsky, Degdekansky, Maisky, Oktyabrsky, Pavlik, Kupol, as well as the Bakyrchik deposit in Kazakhstan and others, clearly shows that the most promising and largest deposits are selectively localized in favourable rocks and largescale disjunctive dislocations, or faults. To identify favourable rocks, the distribution of the world’s gold reserves in various rocks was analysed, as well as the distribution of reserves in the areas of the largest deposits in ore-bearing faults as the most favourable fault types containing the most powerful and extended gold ore bodies.Results. A comparison of the shares of reserves has shown that the most promising for prospecting are those rock complexes containing the highest proportions of the world’s gold reserves. The article presents the distribution of shares of the world’s gold reserves for near surface and deep-formed deposits of various types. The main share of the world’s gold reserves in deeply formed deposits is shown to be concentrated in the Phanerozoic strata of sandy-clayey composition and in the interlayers of easily replaceable carbonate and amphibolite shales developed in the Proterozoic quartzite-phyllite rocks. The reserves of near-surface gold-silver and gold-telluride ores are selectively located in the young Mesozoic-Cenozoic volcanic rocks of basalt-andesite composition, accompanied by dikes, subvolcanic stocks and the pipes of explosive breccias with large-scale gold ore bodies confined to them. Thus, the most promising areas for prospecting new deposits are those composed of the mentioned rock complexes and crossed by ore-bearing faults, in the zones of which the main largest ore bodies are concentrated. Conclusion. The conducted study into the distribution of the world’s gold reserves in the deposits of various types, as well as the selective localization of ores in deposits with the main and large scale ore bodies in the zones of ore-bearing faults (and, accordingly, containing the concentration of the main share of reserves in a particular deposit), provides a basis for discovering new deposits of the precious metal with minimal expenditures, thus contributing to increasing the resource base of the Russian Federation.

Methodological Development of a Combined Preparation for Micropaleontological and Sedimentological Studies of Samples From the Proterozoic Record

The recovery of microfossils from Proterozoic rocks is commonly challenging because of metamorphism. In this study, an application of different methods usually applied on Phanerozoic rocks to test efficiency on recovering microfossil from Proterozoic units is presented. Chemical, physical, and biological factors can influence the recovery of microfossils, thereby becoming a barrier for biostratigraphic and paleoecological studies. Furthermore, low-cost projects with a reduced amount of sample collected, such as drill core sampling, need to optimize the preparation time and sample needed for different analyses. To overcome this challenge, the classical procedure of mineralized microfossil preparation, the palynological technique, and the study of clay mineralogy with the analyses of diagenetic alteration and the search for possible microfossils in thin sections were combined. Three Proterozoic lithostratigraphic units were selected to develop an integrated procedure for preparing samples for micropaleontologic and sedimentologic studies: the Paranoá Group, Mesoproterozoic, and the Bambuí Group, Ediacarian-Cambrian, Brazil, and Nama Group, Ediacaran-Cambrian, Namibia. Recovering individual microfossils from the Paranoá and Bambuí groups has been a challenge for paleontologists. Therefore, most micropaleontological studies have been done as a part of microbiofacies analyses in thin sections. All sediment fractions were studied in trial for the examination (and picking) of mineralized microfossils, even the finest ones. The microfossil picking was conducted using a stereomicroscope. Three species were recovered following this procedure: Vetronostocale aff V. amoenumSchopf and Blacic, 1971, Myxococcoides sp., and Melanocyrillium sp. Analyses in whole rock samples of residues from water (H2O) and hydrogen peroxide 30% (H2O2) procedures showed similar results when the clay fraction studied was obtained as part of micropaleontological preparation compared with the results from the standard clay mineral preparation method. The clay fraction diffractograms showed that the micropaleontological preparation with H2O and H2O2 caused an increase in the intensity of the quartz reflections compared with untreated samples. Moreover, detailed protocols for organic-walled microfossil preparation and low concentrated acetic and formic acids attacks for mineralized microfossil extraction were presented.