Precambrian Research最新文献

The Haerdaban Pb–Zn deposit, located in the eastern West Tianshan Orogen, hosts stratiform and vein-type mineralization within Precambrian carbonate rocks. However, there has been limited research on the distinctions and relationships between these two mineralization styles. In this study, we compared trace element distributions, fluid conditions, material sources, and mineralization ages between stratiform and vein-type mineralization and reconstructed a detailed genetic model. Two episodes and four stages of mineralization were identified, including five generations of pyrite and two generations of sphalerite. The sedimentary exhalative episode represents stratiform mineralization including Stage I pyrite–pyrrhotite layers (Py-1, Py-2a, Py-2b, and Py-3) and Stage II sphalerite–galena layers (Sph-1). The magmatic–hydrothermal episode represents vein-type mineralization including Stage III pyrite–quartz–calcite veins (Py-4) and Stage IV sphalerite–galena–quartz–calcite veins (Sph-2). LA–ICP–MS analysis of Se–Co–Ni–As–Ag–Sb–Bi contents in pyrite suggests that Py-1 to Py-3 formed under relatively low temperatures (280 ± 8 °C) and high ƒO₂ sedimentary conditions. Py-4 formed under relatively high temperatures (339 ± 18 °C) and low ƒO₂ hydrothermal conditions. Variations in Fe–Mn–In–Ga–Ge–Cd–Cu contents in sphalerite indicate that Sph-1 formed under relatively low temperature (211 ± 7 °C), intermediate ƒS₂ (logƒS₂ = −11.1 ± 0.5), and moderate pH sedimentary exhalative conditions. Sph-2 formed under relatively high temperature (292 ± 5 °C), high ƒS₂ (logƒS₂ = −7.4 ± 0.2), and low pH hydrothermal conditions. In situ analysis of sulfide S–Pb suggests that ore-forming materials for stratiform mineralization were primarily derived from marine sediments, while those for vein-type mineralization primarily originated from magmatic sources. Sph-1 from stratiform mineralization yielded an Rb–Sr isochron age of 719 ± 16 Ma, while Sph-2 from vein-type mineralization exhibited an Rb–Sr isochron age of 380.3 ± 7.7 Ma. Random Forest classification of trace elements in pyrite and sphalerite predicted that stratiform mineralization is of sedimentary genesis, whereas vein-type mineralization is of magmatic–hydrothermal genesis. Based on our results, we identified a two-episode mineralization history for the Haerdaban Pb–Zn deposit: Neoproterozoic syngenetic sedimentary exhalative mineralization overprinted by Devonian epigenetic magmatic–hydrothermal mineralization. The results of this study highlight the importance of considering multiple geological events in ore-forming processes and will facilitate the exploration of similar deposits in the West Tianshan Orogen.

The contact between the Archean Bastar craton (BC) and Proterozoic Eastern Ghats Province (EGP), central India, is marked by a suture zone termed Terrane Boundary Shear Zone (TBSZ). BC in this area is largely composed of hornblende-biotite granite with some mafic dykes. Rocks in the TBSZ include quartzofeldspathic (leptynite) gneiss, garnet-orthopyroxene-bearing granitoid, mafic granulites (Group A of cratonic affinity, and Group B of EGP affinity), Mg-Al granulite and an isolated exposure of orthopyroxene-bearing gneiss. Detailed geochemical analysis shows remarkable similarity between Hbl-Bt granite and Grt-Opx-bearing granitoid, with A-type affinity, and between mafic dykes and Group A mafic granulites. However, the Opx-bearing gneiss is geochemically distinct having I-type affinity, similar to TTG gneisses described from BC. Metamorphic phase equilibria analysis and trace element modelling shows that (i) melting of Opx-bearing gneiss would produce a ferroan granitic melt resembling the Hbl-Bt granite, (ii) metamorphism at appropriate P-T conditions would convert the granite to Grt-Opx-bearing granitoid and the mafic dyke to Group A mafic granulite. U-Pb geochronology of zircon constrains emplacement ages of the magmatic precursors of Opx-bearing gneiss and Grt-Opx-bearing granitoid as ca. 2.73 and ca. 2.5 Ga, respectively. These rocks were subjected to an early granulite facies metamorphism, followed by an amphibolite facies metamorphism, shearing and hydrous fluid flux. Geochronological data shows that the latter event took place at ca. 0.52 Ga, while the earlier granulite facies event can only be tentatively suggested to be of late Stenian/Tonian age. Collating all the evidence (including published geophysical and geochronological data), we suggest that the initial collision between BC and EGP took place during late Stenian/Tonian time as a consequence of formation of the Greater Indian Landmass, a part of Rodinia supercontinent. The TBSZ, probably initiated due to the late Stenian/Tonian collision, was reactivated and reworked tectonothermally at ca. 0.52 Ga, caused by far field stress effect of the Kuunga orogeny, which was strong enough to obliterate most of the imprints of the late Stenian/Tonian orogeny.

The Vestfold Block, a typical polymetamorphic Archean terrane in East Antarctica, is a key area to understand amalgamations of Rodinia and East Gondwana continents. However, multiphase overprinting makes it difficult to determine the timing and nature of each tectonothermal event. In this study, we present P–T estimates, zircon, monazite U(–Th)–Pb and biotite/K–feldspar Rb–Sr isochron ages of paragneisses from the SE Vestfold Block. One paragneiss sample, which is assigned to the Chelnok Paragneiss, has experienced a protracted metamorphism from the Neoarchean to the early Paleoproterozoic. Phase equilibria modeling constrained the peak P–T conditions to 7.2–9.6 kbar and 850–880 ℃, and the post–peak metamorphism to 4.2–5.6 kbar and 720–790 ℃, respectively. On the other hand, a paragneiss sample close to the ice sheet documented a high–grade metamorphic event at 918 ± 23 Ma, with peak P–T conditions of 6.0–8.0 kbar and 860–880 ℃. Biotite/K–feldspar Rb–Sr dating for these two samples yields isochron ages of 474 ± 12 and 442 ± 7 Ma, respectively, representing the cooling ages of the Pan–African reworking. Collectively, an integrated application of diverse chronometers, combined with published data, indicates that the Vestfold Block may have experienced at least three major thermal events with variable intensities and extents. Initially, the supracrustal rocks in this region pervasively underwent a protracted high–grade thermal event from the Neoarchean to the early Paleoproterozoic, which formed the backbone of the block. Thereafter, the southern Vestfold Block experienced a Grenvillian granulite facies metamorphism, indicating that the Vestfold Block has been locally involved in the Rayner orogeny (i.e. the late Mesoproterozoic/early Neoproterozoic collision between the Indian craton and East Antarctica). Ultimately, the whole Vestfold Block may have been reworked under relatively low temperatures during the Pan–African Prydz tectonic event.

The Union Tin Member, comprising a succession of pyroclastic and shale-dominated sedimentary rocks associated with the 2.05 billion-year-old Bushveld Large Igneous Province (LIP), is well-exposed particularly in the Union Sn field located 150 km north of Pretoria, South Africa. The Union Tin Member is a laterally extensive (>200 km-wide) marker horizon situated between the rhyolitic Kwaggasnek and Schrikkloof formations of the Rooiberg Group. The shale unit exhibits low total organic carbon (TOC) contents (168 ± 70 ppm) along with a narrow range of δ¹³C values (−27.7 ± 1.7 ‰) and δ¹⁵N values mostly around 0 ‰, resembling characteristics typical of marine shales. These siliciclastic sedimentary rocks were deposited in a shallow-marine sedimentary environment subjected to sea-level fluctuations, suggesting widespread submergence of the Kaapvaal Craton despite ongoing LIP magmatism. The whole-rock compositions, particularly rare earth elements (REE), Th, Sc, Zr, Ni, V and TiO₂, indicate a provenance dominated by the underlying Rooiberg Group, which may also include the Paleoproterozoic Transvaal Supergroup, Archean tonalite-trondhjemite-granodiorite (TTG) suites and greenstones. Variable enrichments of the shale in Sn, Li, Cs, W and U are primarily associated with chlorite, sericite, illite and hematite. Notably, the rhyolites, immediately underlying and overlying the Union Tin Member, are similarly altered to a mineral assemblage dominated by the Fe-Al-Mg-bearing phases with relative enrichments especially in Sn. Alteration is linked to large-scale Sn-bearing hydrothermal fluids derived from the Lebowa Granite Suite. The periodic emplacement of extensive volcanic rocks of the Rooiberg Group is likely to have resulted in a downsagged >200 km-wide basin underlain by the Transvaal Supergroup. The deposition of the Union Tin Member within this basin could potentially represent the surface expression of a deep-seated, exceptionally large magma reservoir suggested to have contributed to the formation of the entire Bushveld LIP.

The genesis of many giant mercury (Hg) deposits/belts is widely debated largely because of a lack of reliable ore-forming age constraints. The LA-ICP-MS/MS dolomite U-Pb were employed to date the large Chatian Hg district in Xiangxi-Qiandong Hg belt of South China. The mineralization of this district can be divided into three stages: an initial Pyrite-Quartz stage before mercury deposition, a main Cinnabar-Sphalerite-Dolomite (Dol I) stage, and a subsequent Dolomite vein (Dol II) stage after ore formation. Three Dol I samples from the Chashula and Touponao Hg deposits were chosen for U-Pb dating, yielding ages of 509.8 ± 7.4 Ma, 509.4 ± 4.7 Ma and 507.4 ± 4.0 Ma respectively, indicating the mineralization occurred in the Cambrian Miaolingian, concurrent with or shortly after the deposition of the host-rock. These ages correspond with the emplacement periods of two Large Igneous Provinces (LIP) (the Pinghe silicic SLIP and the Kalkarindji LIP) in northeastern Gondwana, as well as the mantle-derived rocks in the study region. Combined with previous mercury isotopic data, our study proposes that the large-scale Hg mineralization in the area most likely resulted from the mantle-derived fluids, which would have directly contributed a significant amount of Hg for ore formation. Furthermore, the close timeframe between the Cambrian Series 2-Miaolingian trilobite extinction events and Hg mineralization in South China, may suggests a potential direct connection between the widespread mercury release and mineralization and the Cambrian Series 2-Miaolingian trilobite crisis in the region.

Although the early Paleoproterozoic global magmatism appears to be rare, the Taihua Complex within the Xiaoqinling terrane of the southern Trans-North China Orogen (TNCO) contains abundant evidence for magmatic and tectonic activities during this period. In this contribution, we present whole-rock geochemical and Sr-Nd isotopic data for the gneisses from the Taihua Complex within the Xiaoqinling terrane to investigate their petrogenetic processes, geodynamic setting, and provide a better understanding of the tectonic evolution in the southern margin of the TNCO. Based on their patterns of geochemical characteristics, the studied samples can be broadly subdivided into three groups. The Group-1 and −3 gneisses have relatively low Sr/Y ratios, and negative Eu anomalies, indicating plagioclase was a residual phase during partial melting, which requires relatively low pressure conditions. The majority of the samples in these groups have negative whole-rock εNd(t) values, indicating the magma source was mainly ancient crust. However, these Group-2 gneisses have moderate to high Sr/Y ratios, and high Mg#, which may related to mantle-melt interactions. This study shows that the protoliths of studied gneisses from the Taihua Complex within the Xiaoqinling terrane were probably formed by either partial melting of pre-existing arc lower crust or partial melting of the delaminated thickened lower crust and then interacted with the mantle components in a post-collisional extension tectonic regime, which is consistent with the global tectonic regime at this time.

The Archean domain of the Southern São Francisco Craton (SFC) acted as a foreland for the docking of the Mineiro Belt (2.47–2.05 Ga) through soft-collision processes during the Paleoproterozoic. Much later, during the Neoproterozoic, the newly formed Archean-Paleoproterozoic protocraton again served as a foreland for the development of thrust belts that contributed to the formation of the Gondwana supercontinent. From the Paleoproterozoic to the Neoproterozoic, pressure–temperature (P-T) conditions in the SFC crust varied from prograde amphibolite to retrograde amphibolite-greenschist metamorphic facies. In this paper, we reinterpret past ⁴⁰K/⁴⁰Ar data and present new ⁴⁰Ar/³⁹Ar dates in biotite and amphibole—evaluating the P-T conditions affecting metagranitoids, orthogneisses, and metamafic rocks of the southern SFC basement—to reconstruct its tectonothermal history during the assembly of the Gondwana supercontinent. Recalculated ⁴⁰K/⁴⁰Ar* and new ’total fusion’ ⁴⁰Ar/³⁹Ar dates facilitated the creation of a contour map with a 900 Ma isochron, roughly corresponding to the tectonic boundary between the Archean and Paleoproterozoic domains, with the Neoproterozoic tectonothermal imprint recorded south of the 900 Ma line. The younger ⁴⁰Ar/³⁹Ar plateau dates from 651 to 526 Ma obtained on biotite crystals delineate a 600 Ma isochron within the Mineiro Belt area, approximately coinciding with the Lenheiro Shear Zone (LSZ). This comprehensive dataset, encompassing dates from 900 to 526 Ma, underscores a polycyclic tectonothermal reactivation that either delimits the Mineiro Belt area or occurs along the LSZ. This prominent structure within the belt differentiates deeper plutonic rocks from shallower (sub)volcanic rocks to its north and south regions, respectively. These relatively deeper to shallower crustal positions suggest that Neoproterozoic collisional fronts developed progressively from northwest (earlier collisions) to southeast (later collisions), corresponding to older and younger dates, respectively. Additionally, Discordia lower-intercepts in Wetherill diagrams from 977 to 488 Ma, obtained in previous U-Pb investigations within the Mineiro Belt, align with our new ⁴⁰Ar/³⁹Ar plateau and ’total fusion’ dates, providing further evidence for reheating and partial to complete resetting of magmatic biotite-amphibole crystals and neoformed metamorphic biotite. Our results confirm that the Mineiro Belt did not behave as a stable continental landmass during the Neoproterozoic Brasiliano Orogeny, instead experiencing significant tectonothermal overprinting.

Some of the oldest, well-preserved exhalative sedimentary rocks (ferruginous cherts and jaspilites) occur in volcanic-dominated Paleoarchean sequences in the northern Pilbara Craton, Australia. Jaspilites contain fine-grained hematite particles that have been interpreted to have formed following seawater oxidation of vent-derived Fe²⁺(aq) by photoautotrophs. However, recent studies suggest that most of the iron in the jaspilites was originally deposited as Fe(II)-rich silicates such as greenalite, sparking renewed debate about how iron was precipitated in the early oceans. Here we show that rounded clasts of dusty ferruginous chert in basal conglomerates of the 3.43–3.35 Ga Strelley Pool Formation, North Pole Dome, Pilbara Craton, Australia, contain abundant nanoparticles of greenalite and apatite that are texturally, mineralogically and chemically near-identical to occurrences in Neoarchean banded iron formations in the Hamersley Group, Australia. Hematite where present occurs in trace amounts and its distribution along the edges of clasts implies that at least some of the hematite formed during weathering. The greenalite-rich clasts display a pronounced positive shale-normalized Eu anomaly, small positive Y anomaly and lack a positive La anomaly, features typical of Paleoarchean exhalites. The co-occurrence of greenalite- and apatite-rich chert-pebbles with clasts of hydrothermal black chert and silicified felsic volcaniclastic sedimentary rocks points to a volcanically active provenance with vigorous hydrothermal activity, consistent with derivation from the underlying 3.43 Ga Panorama Formation. We argue that the greenalite and apatite nanoparticles precipitated during mixing between hydrothermal vent fluids and anoxic seawater and were silicified on the seafloor. The round to oval shape of the chert clasts indicates that the silica cement had recrystallized prior to erosion, consistent with the presence of polygonal shrinkage structures in the dusty chert clasts. Our findings imply that greenalite, contrary to recent suggestions, not only formed in Paleoarchean seawater, but was also stable during deposition, diagenesis, and resedimentation. The precipitation and non-dissolution of apatite nanoparticles in Paleoarchean seawater points to elevated phosphate concentrations, whereas the absence of primary hematite is consistent with models advocating a role for abiotic iron deposition during the Archean.