Post-depositional Metamorphism Research Articles

New insights into the origin and depositional setting of the Haerdaban Pb[sbnd]Zn deposit, Chinese western Tianshan: Evidence from geology, chert geochemistry, and detrital zircon U[sbnd]Pb geochronology

The Haerdaban PbZn deposit (with an ore reserve of 10.93 Mt. at 1.0–25.65 % Zn and 0.7–12.29 % Pb) is hosted in weakly metamorphosed clastic‑carbonate rocks from the Proterozoic Haerdaban Group. It represents a significant addition of the sediment-hosted PbZn deposits in the Yili block, Chinese western Tianshan. Currently, there are ongoing debates regarding its genesis, with a particular focus on the crucial metallogenic mechanism (syngenetic sedimentary exhalation or epigenetic reworking) responsible for the primary sulfide mineralization. Mineralization at Haerdaban primarily occurs as banded to stratiform ore layers or lenses conformably sandwiched in their host rocks. Vein and stockwork ores occur locally below the stratiform ore layers. A syn-sedimentary fault trending SN was identified based on abrupt lateral changes in lithofacies and thickness of the stratigraphic units. The ore mineralogy is dominated by sphalerite, galena, quartz, and dolomite, with a small amount of pyrite, barite, and organic matter. Detrital zircon LA-ICP-MS UPb dating of the Haerdaban siltstones obtained a maximum depositional age of about 604 Ma. Their geochemical composition similar to the passive continental margin signatures, with rare earth element (REE) patterns enriched in LREE and negative Eu anomalies (Eu/Eu* = 0.50–1.14). Stratiform beds of chert that host disseminated ores have relatively high contents of hydrothermal components (e.g., Ba, Zn), with apparent positive Eu anomalies (Eu/Eu* = 7.38–49.34) and negligible negative Ce anomalies (Ce/Ce* = 0.85–0.98). They are thus interpreted to be hydrothermal sedimentary rocks (exhalites) deposited in a suboxic-anoxic environment proximal to the hydrothermal vents. Integrated geological and geochemical evidence indicates that the Haerdaban PbZn deposit is a typical vent-proximal sedimentary exhalative (SEDEX) deposit formed in a Neoproterozoic Sinian (Ediacaran) passive continental margin rift basin. Post-depositional metamorphism and deformation in the Paleozoic may have caused partial remobilization of primary ores but did not significantly alter the morphology of the orebodies. Furthermore, establishing a genetic model for the Haerdaban deposit has important implications for the exploration of similar deposits preserved in the equivalent stratigraphy within the Chinese western Tianshan region.

Geochemistry, U-Pb zircon geochronology, and Fe isotopes of the Shanzhuang banded iron formation in the North China Craton: Implications for genesis and depositional environment

The Shanzhuang banded iron formation (BIF) occurs in the Shancaoyu Formation, Taishan Group, in the western Shandong Province (WSP), eastern North China Craton (NCC). We constrained the Shanzhuang BIF depositional age of ∼2.50 Ga based on the zircon U-Pb dating of the leptynite interlayers and the cross-cutting metagranitoid. After deposition, the Shanzhuang BIF was subsequently metamorphosed into amphibolite facies at 2.50–2.45 Ga. According to different mineral assemblages, the Fe ores can be divided into three types: magnetite + quartz ± hornblende (type-1 ore), magnetite + quartz + hornblende + garnet ± hematite (type-2 ore), and quartz + hematite ± magnetite ± hornblende (type-3 ore). In the Post-Archean Australian Shale (PAAS)-normalized rare earth element and yttrium (REY) patterns, the three ore types show consistent flat REY curves. All ore types have moderate ƩREY concentrations (27.24–143 ppm), (La/Yb)N values of 0.29–2.31, significant positive Eu anomalies (1.05–3.47), slightly negative or positive Ce anomalies (0.87–1.26), slightly positive Y anomalies (1.16–1.48), and no apparent fractionation between light REEs (LREEs) and heavy REEs (HREEs), indicating mixing of hydrothermal fluid and seawater. The similar REY patterns and heavy Fe isotopes (0.47–1.02‰) of the three ore types suggest a certain amount of continental detritus was incorporated during the precipitation of the Fe phases in the Neoarchean-Paleoproterozoic oceans. The post-depositional metamorphism did not significantly affect the original REY geochemical signature of the BIF. The Shanzhuang BIF ores are not highly pure chemical sediments, but the type-1 ore is relatively pure and more representative of the original sedimentary composition. By contrast, the type-3 ore tends to have higher δ56Fe ratios (0.74–1.02‰) but lower δEu (1.27–1.97‰), implying substantial post-depositional hydrothermal alteration. The Shanzhuang Fe deposit is an Algoma-type BIF, and the surrounding rock consists of pyroclastic materials. Our model indicates that the Shanzhuang BIF was probably deposited in a redox stratified paleo-ocean environment.

The extensive hydrocarbon-mediated fixation of hydrothermal gold in the Witwatersrand Basin, South Africa

There is a close spatial relation between high-grade gold mineralization in the Witwatersrand basin and carbonaceous nodules, veins and seams. Hydrocarbons thus may well have been essential in ore genesis. We have sampled four major gold-, uranium- and hydrocarbon-bearing ore horizons, namely the Carbon Leader, Vaal, B and Black reefs, to determine the role of hydrocarbons in the accumulation and hydrothermal fixation of gold. Our multipronged approach included high-resolution scanning and transmission electron microscopy (SEM, TEM), nanotomography with video clips, and geochemical modeling. Post-depositional hydrothermal activity at the peak of regional metamorphism produced an assemblage of quartz, phyllosilicates, brannerite, crandallite, florencite, monazite and gold in all four reefs. The gold, hydrocarbons and associated mineral assemblages are closely related on the micro to nano scale. Gold deposition occurred in interstices, as fracture fillings in detrital minerals, and on the surface of migrated solid hydrocarbon residues. The spherical to elliptical inclusions in the gold consist of an outer pyrobitumen phase and a central void space, partially associated with nanometric gold, uraninite, coffinite and silica. The hydrocarbon-bearing inclusion likely formed by the entrapment of a fossil liquid oil precursor during gold precipitation. The oil was subsequently thermally altered and converted into the final pyrobitumen and gaseous residues. Geochemical calculations to simulate the interaction of an invading hot hydrothermal fluid with the hydrocarbons in the reefs reveal that a very small amount of hydrocarbons will drastically decrease the aqueous solubility of gold and hence cause its instant precipitation.We extend our genetic model for the epigenetic formation of gold in the Witwatersrand. Regional metamorphism promoted the extensive and likely basin-wide circulation of hydrothermal fluids; these were capable of mobilizing substantial amounts of gold. The liquid, gaseous and solid hydrocarbons in the reefs acted as efficient chemical traps for the concentration of gold. Being strong chemical reductants, they caused the rapid precipitation and accumulation of gold on the surface of the fossil oil droplets and already solidified hydrocarbons. The release of the gases from accessible hydrocarbons into the sediments away from their source buffered the redox state of the hydrothermal solutions even at a considerable distance from the pyrobitumen seams and veins, likely resulting in the deposition of gold in the absence of visible hydrocarbons. Although our findings do not explain the ultimate origin and exceptional endowment of gold in the Witwatersrand, we do provide intriguing evidence for the large-scale hydrothermal mobilization, accumulation and fixation of gold mediated by hydrocarbons during post-depositional metamorphism.

New zircon U–Pb and Hf–Nd isotopic constraints on the timing of magmatism, sedimentation and metamorphism in the northern Prince Charles Mountains, East Antarctica

The northern Prince Charles Mountains (PCM) in East Antarctica represent the largest continuously exposed section of the Rayner Complex and may provide important insights into the tectonic evolution of the Rayner orogen. We present new U–Pb and Hf isotopic data for zircons from felsic orthogneisses, mafic granulites, paragneisses and charnockites and additional Nd isotopic data for the former two rock types from the Beaver Lake area in the northern PCM. Zircons from the felsic orthogneisses document protolith ages of ca. 1170–1070Ma, with Hf and Nd model ages of 1.99–1.74Ga, suggesting the generation of the felsic magmas by partial melting of crustal rocks that were extracted from the mantle during the Paleoproterozoic. Detrital zircons from one paragneiss sample yield a major age population at ca. 1480–1140Ma and three subordinate populations at ca. 2130–1850, 1780–1620 and 1010–860Ma, whereas those from another paragneiss sample produce a major age population at ca. 1180–830Ma and a subordinate age population at ca. 1370–1230Ma. Discounting the effects of zircon recrystallization during post-depositional metamorphism, we infer that the sedimentary precursors to the aforementioned paragneiss samples were deposited after ca. 1200 and 1020Ma, respectively, in intra- or back-arc basins of the Rayner continental arc. Charnockites were either emplaced at ca. 980Ma or episodically at ca. 1050 and ca. 950Ma, with Hf model ages of 1.97–1.90Ga. They were derived from partial melting of a Paleoproterozoic source region similar to the surrounding felsic orthogneisses at deeper levels. Zircon overgrowth domains from all of the studied rock types indicate that high-grade metamorphism took place at ca. 945–915Ma. Only one paragneiss sample from the Else Platform preserves evidence of Cambrian metamorphic reworking. Based on published data from the Rayner Complex and the Eastern Ghats Belt of India, we speculate that long-lived convergent processes between the Indian craton and East Antarctica lasted from ca. 1500 to 900Ma. Therefore, the Rayner Complex may represent the exposed orogenic root of a large Meso-Neoproterozoic accretionary orogen.

Age, origin and significance of nodular sulfides in 2680 Ma carbonaceous black shale of the Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia

Sulfide nodules composed of pyrite occur within black shales in the Lucky Bay area of the Bulong Domain, Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia that correlate with the Neoarchean Black Flag Group. Detailed petrography reveals a variety of shapes and textures, from spherical to ovoid and zoned to uniform. Some nodules have recrystallized, resulting in a later generation of pyrite. All have been affected by post-depositional metamorphism, as evidenced by pressure shadows containing quartz±mica±carbonate assemblages. LA-ICP-MS analyses show that these nodules are enriched in a range of trace elements, including Co, Ni, Cu, Ag, Sb, Te, Au, Tl, Pb, and Bi. Gold and Te concentrations range from 0.3 to 1ppm and 10 to 50ppm, respectively. The Black Flag Group equivalents stratigraphically and structurally underlie a younger basin east of Lucky Bay that contains turbidites, sandy debris flows and BIF, the latter of which hosts several gold deposits at Randalls, 60km SE of Kalgoorlie. It is likely that the lateral equivalents of the nodular sulfide-bearing black shale originally occurred stratigraphically beneath what is now the ore horizon at Randalls. Given its anomalous Au and Te content, this lithology is a potential contributor of gold to the BIF-hosted gold deposits of the Randalls Goldfield, as well as other similar gold resources elsewhere in the Eastern Goldfields Superterrane.

The Tugela Terrane, Natal belt: Kibaran magmatism and tectonism along the southeast margin of the Kaapvaal Craton

This paper is a preliminary report on the geology of the Mandleni tectonostratigraphic package (MTP) part of the Tugela terrane, the northernmost element of the Natal belt in southeastern Africa. The Dondwana gneiss unit, a metamorphosed bimodal magmatic sequence comprising feldspathic gneiss, with lesser amounts of amphibolite, underlies much of the MTP. All primary structures in these rocks have been obliterated during post-depositional metamorphism and deformation. Fabrics attributable to the oldest recognised tectonic event (D1) have largely been overprinted during subsequent deformation. D2 was characterised by syn-kinematic high-pressure (P>9 kbar) granulite facies metamorphism. The dominant S2 planar fabric is a penetrative gneissic foliation that parallels lithological contacts. It is defined by colour and compositional banding, and by parallelism of coarsely crystalline mineral grains. D3 tectonism produced asymmetric north-verging folds, faults and shear zones, which accommodated overthrusting of the MTP onto the Kaapvaal craton. Plagioclase ± biotite mantles on garnet developed at this time and are attributable to uplift of the MTP from the mid- to deep-crustal levels established during D2. Ultramafic rocks (the Evuleka ultramafics) tectonically interleaved with the Dondwana gneiss unit during D3, consist of talc schist, and massive serpentinite with disseminated chromite and magnetite. These ultramafic rocks are interpreted as altered and sheared mantle or cumulate from the lower portion of an ophiolite sequence. Granitoid magmatism associated with D3 is related to subduction beneath the MTP. This subduction led to basin-closure and emplacement of the MTP onto the Kaapvaal craton. The final tectonic event (D4) produced broad open south-plunging folds and sinistral shear zones.

Alteration Rinds on Glacial Diamictite Clasts in the Gowganda Formation: Possible Indicators of Low Atmospheric Oxygen ca. 2.3 Ga

The Coleman Member of the ea. 2.3 Ga Gowganda Formation is beautifully exposed in a glacially polished outcrop near Cobalt, Ontario. This lithologic unit is a glacial diamictite, or tillite, consisting of clasts up to 1.5 m in diameter embedded in a silty to sandy matrix. More than half of the clasts display alteration rinds. On granite clasts these consist of a pale pink outer rim and a dark inner rim, in which orthoclase has been replaced by what is now chlorite. The rinds are inferred to have initially developed as a result of chemical weathering before the clasts were deposited as part of the tillite, and to have been modified by post-depositional metamorphism. The chlorite in the rinds contains Mg and Fe in approximately the same proportion as in the unweathered clasts. The chlorite was probably generated by the low-grade metamorphism of smectites produced by the reaction of feldspars with O2-free groundwater carrying Fe+2 and Mg+2. The absence of O2 in these ground waters is consistent with the deposition of the Gowganda Formation before the rise of O2 in the atmosphere ca. 2.25 Ga.

Boron isotope systematics of tourmaline formation in the Sullivan Pb–Zn–Ag deposit, British Columbia, Canada

We report here the results of 54 boron isotope analyses of tourmaline associated with the giant Sullivan Pb–Zn–Ag deposit in southeastern British Columbia, Canada. The δ11B values range from −11.1 to −2.9‰, which is almost as great as the range found worldwide in tourmalines from 33 massive sulfide deposits and tourmalinites in dominantly clastic metasedimentary terranes. The major control on the overall δ11B values of the Sullivan tourmalinites is the boron source. Potential controls over the large range of the data also include: (1) differences in formation temperatures of the tourmalinites, (2) different stages of tourmaline formation, (3) variations in the proportions of dissolved boron incorporated into the tourmaline (Rayleigh fractionation), (4) seawater entrainment, and (5) post-depositional metamorphism. The boron isotope data at Sullivan are consistent with boron derivation from leaching of footwall clastic sediments. However, the great abundance of tourmaline in the Sullivan deposit suggests that the local clastic sediments were not the sole source of boron, and we argue that non-marine evaporites, buried deep below the orebody, are the most viable source of this additional boron. It is likely that some of the variation in tourmaline δ11B values reflect mixing of boron from these two sources. Comparison of the potential effects of these controls with geologic and other geochemical evidence suggests that major causes for the wide range of δ11B values measured at Sullivan are seawater entrainment and Rayleigh fractionation, although in places, post-depositional alteration and thermal metamorphism were important in determining δ11B values of some of the recrystallized tourmalinites.

Scanning Electron Microscopy of Quartz in Precambrian Cherts and Dolomites from Southern Africa

AbstractScanning electron microscopic (SEM) examination of the quartz isolated from chert of Transvaal and Swaziland, of 2000 and 3400 million year (MY) ages, respectively, in southern Africa revealed marked differences in quartz morphology. Well-defined individual euhedral quartz crystals, with polyhedral triple-point faces, were clearly evident on freshly fractured Transvaal chert surfaces as well as with the quartz isolates from the chert. The morphology and coarseness suggest crystal growth with little, if any, metamorphism; however, the δ18O values of 23.8–24.1‰ suggest crystallization temperatures of perhaps 40–45°C. In contrast, fracture surfaces of the older, strongly metamorphosed Swaziland cherts revealed a high degree of grain intergrowth which inhibited fracture between quartz grains. Quartz isolates from these showed strongly interlocked quartz crystal clusters and elongated chips of quartz with poorly defined irregular faces. Intercalation of mineral veins in the cherts on a mm scale and the intergrown character of the quartz grain boundaries provide evidence that the latter cherts have been strongly metamorphosed and recrystallized, in keeping with 14.6–15.1‰ δ18O values, corresponding to 80°C fractionation with water. The SEM micrographs of the fine quartz (1–10 µm) isolated from the Otavi dolomite formation of the 700-MY Damara System and from the 2000-MY Transvaal Dolomite Series revealed well-defined subhedral and euhedral quartz crystals of small size which, together with the 26.9–27.8‰ and 23.8‰ δ18O values, respectively, indicate that these dolomites have been affected little, if any, by post-depositional metamorphism; their crystallization temperatures fall in the range of 25–30 and 40–45°C, respectively.