Authigenic Monazite Research Articles

Hydrothermal mineral replacement in the apatite-rhabdophane-monazite system: Experiments, reaction mechanisms and geological implications

Apatite, rhabdophane and monazite are actinide- and rare-earth element (REE)-bearing phosphate minerals with diverse geoscientific applications. However, their mineral-fluid reaction mechanisms below 200 °C are understudied. Insufficient understanding as to how these minerals behave during hydrothermal alteration impedes useful interpretations of REE-U-Th mobilisation in regolith profiles, diagenetic environments and low-temperature hydrothermal systems. This work experimentally investigates the replacement of apatite by rhabdophane in a Ce-doped acidic fluid at 30 °C, and of rhabdophane by monazite in REE-barren acidic fluids at 180 °C. Rhabdophane replaced apatite via the coupled dissolution-precipitation mechanism, forming nanoscale prisms clustered into spherulitic aggregates. Interstitial voids that formed between rhabdophane and apatite indicate how rhabdophane precipitation was the rate-limiting step. Void widths varied in response to apatite's anisotropic dissolution; rapid dissolution parallel to the c-axis formed the widest voids (50–125 μm) and thickest rhabdophane layers (20–50 μm), while slow dissolution perpendicular to the c-axis formed narrower voids (< 1 μm) and thinner layers (< 5 μm). At 180 °C, monazite also replaced rhabdophane via the dissolution-precipitation mechanism. Replacement rates increased with increasing phosphate concentrations, implying that monazite precipitation was the rate-limiting step. However, microscale rhabdophane aggregates were pseudomorphically preserved. This is attributed to the shared orientation of rhabdophane prisms (not pseudomorphically preserved) and their intergranular boundaries which, as microreactors, guided monazite precipitation. Both replacement reactions were accompanied by a loss of U to solution, which is attributed to the stabilisation of uranyl-phosphate complexes under acidic conditions. Results indicate that: 1) anisotropic dissolution can govern the extent of rate coupling between dissolution and precipitation during mineral replacement; 2) the pseudomorphic replacement of mineral aggregates can occur when precipitation is the rate-limiting step; 3) rhabdophane is a feasible (and perhaps necessary) metastable precursor to authigenic monazite precipitation in low temperature hydrothermal systems; 4) highly porous aggregates of both rhabdophane and monazite generate insufficient EPMA totals, such that they cannot be differentiated by EPMA alone, and; 5) phosphate complexes can mobilise U under certain hydrothermal conditions, with implications for geochronology, nuclear waste management and the formation of REE deposits.

Mineralogical and Geochemical Constraints on the Occurrence Forms of REEs in Carboniferous Karst Bauxite, Central Guizhou Province, Southwest China: A Case Study of Lindai Bauxite

Karst bauxite is a major source of aluminum and contains high concentrations of trace elements such as Li, Ga, Sc, and rare earth elements (REEs). It is regarded as a potential non-conventional REE source due to the increasing demand for REEs. This study provides new mineralogical and geochemical evidence of a Lindai bauxite deposit in central Guizhou Province, aiming to analyze the genesis of REE minerals and reveal the forms of REEs occurring in these deposits. The results indicate that a large number of detrital and authigenic rare-earth phosphate minerals, including monazite and xenotime, were identified. In terms of their genesis, the authigenic monazite was mainly precipitated under alkaline and reducing conditions, whereas the authigenic xenotime was formed in acidic and reducing conditions. The mineralogical evidence detected by scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) and a TESCAN Integrated Mineral Analyzer (TIMA) suggests that the REEs in the Lindai bauxite exhibit multiple forms, including rare-earth phosphate minerals (monazite and xenotime) and REE scavenging by hematite and anatase phases. Among them, the anatase controls some LREE (such as La, Ce, and Pr) distributions in addition to monazite, whereas the hematite also controls relatively large amounts of Sm, Eu, and HREE (such as Gd, Tb, Dy, Ho, Er, Tm, and Lu) distributions. According to the calculation of the percentage of REE-bearing mineral phases obtained by a TIMA analysis, REE scavenging by hematite accounted for 93.28% of all forms of REEs, REE scavenging by anatase accounted for 5.88%, monazite accounted for 0.67%, and xenotime accounted for 0.17%. This study provides new evidence of the forms of REEs occurring in Carboniferous karst bauxite in central Guizhou Province.

U-Pb dating reveals multiple Paleoproterozoic orogenic events (Hamersley orogenic cycle) along the southern Pilbara margin (Australia) spanning the onset of atmospheric oxygenation

Abstract The early Paleoproterozoic was a time of unprecedented change in Earth's climate and surface environment. The key to resolving some of the controversies surrounding the timing and causes of these changes lies with supracrustal sequences, such as the 2.45–2.22 Ga Turee Creek Group in the southern Pilbara craton, northwestern Australia. The group preserves a predominantly siliciclastic sequence; however, its precise age, tectonic setting, and postdepositional history are disputed. Although it is interpreted to have been deposited in a foreland basin setting shortly after 2.45 Ga, the oldest well-recognized deformational event, marked by northward folding and thrusting, is the 2.20–2.15 Ga Ophthalmia orogeny. Evidence for a pre-Ophthalmia fabric-forming tectonic event north of the Archean Sylvania Inlier, southeast Pilbara craton, which is marked by tight to isoclinal folding, has been largely overlooked. In this area, we report in situ U-Pb geochronology of authigenic monazite and xenotime in shale with a well-developed tectonic cleavage from the ca. 2.63 Ga Jeerinah Formation. Monazite porphyroblasts, which are locally wrapped by strain fringes aligned in a tectonic cleavage, yielded weighted mean 207Pb/206Pb ages at 2370 ± 11 Ma and 2312 ± 8 Ma, whereas xenotime, which overprints a crenulation cleavage, gave a weighted mean 207Pb/206Pb age of 2291 ± 11 Ma, constraining fabric development to between 2.31 Ga and 2.29 Ga. Our results confirm the existence of a pre-Ophthalmia deformational event in the southeastern Pilbara craton, herein referred to as the Sylvania orogeny, which is part of an ~300 m.y. interval (2.45–2.15 Ga) of northward-directed compression (“Hamersley orogenic cycle”). This orogenic cycle is marked by east-west and northwest-southeast folding, cleavage development, veining, hydrothermal gold mineralization, and isotopic resetting across the southern Pilbara craton. Our results indicate that the syn–Great Oxidation Event Turee Creek Group was deposited in one or more foreland basins after 2.45 Ga. Our results provide a new tectonostratigraphic and geodynamic framework for understanding the timing and origin of geochemical records in a key succession deposited during an interval of global environmental change.

SIMS Pb-Pb dating of phosphates in the Proterozoic strata of SE North China Craton: Constraints on eukaryote evolution

Meso-Neoproterozoic sedimentary successions in the southeastern margin of North China Craton contain a wealth of fossil and chemostratigraphic information, which aids in our understanding of early multicellular life and environment coevolution. However, a lack of reliable age constraints means the timing of these fossil occurrences and the corresponding evolutionary tempo of multicellular life remain disputed. This study aims to provide new age constraints by means of authigenic xenotime and monazite geochronology, coupled with morphological observation and mineral chemical analyses. Morphological observation and mineral chemistry allows differentiation of authigenic xenotime and monazite from detrital origins. Authigenic xenotime from the Shijia Formation, upper part of Meso-Neoproterozoic sedimentary successions, gave a SIMS Pb-Pb age of 919 ± 23 Ma, whereas authigenic monazite from the Xinxing and Liulaobei formations, lower part of Meso-Neoproterozoic sedimentary successions, produced SIMS Pb-Pb ages of 1086 ± 17 Ma and 1086 ± 120 Ma, respectively, all of which are interpreted as minimum depositional ages. The new ages suggest the crown group multicellular eukaryotes have highly evolved and diversified prior to 1.0 Ga, and confirm the use of Trachyhystrichosphaera aimika as an index fossil for the Mesoproterozoic-Neoproterozoic transitional period.

Minerals of Rare Earth Elements in High-Phosphorus Ooidal Ironstones of the Western Siberia and Turgai Depression

The aim of this research was to study the rare earth (REE) minerals in ooidal ironstone deposits of the West Siberian basin and the Turgai depression. Authigenic minerals (monazite and cerite) were described, and their main mineral form was identified as light rare earth element phosphate (LREE-phosphate) in this study. LREE-phosphate is included in ferruginous ooids, peloids, and oncoids and forms a consistent mineral association with Fe-hydroxides (goethite and its hydrated amorphous derivatives) and Fe-rich layered silicates (Fe-illite-smectite, chamosite, berthierine). The constancy of the mineral association in two deposits of different ages indicates a general mechanism behind the formation of these minerals. LREE-phosphates (authigenic monazite) are characterized by microscopic sizes (up to 24 μm), diverse morphology (mainly spherical or xenomorphic), and occupy spaces between the micro-cortex in ferruginous spheroids. This mineral can be found in other deposits of ooidal ironstone. According to its mineralogical and chemical characteristics, LREE-phosphate mainly belongs to the authigenic (nodular or “gray”) monazite. However, the incomplete (not 100%) correspondence of Kikuchi bands with the reference monazite does not allow its reliable identification. Based on its small size, chemical leaching or bacterial interaction is recommended to extract REE from ooidal ironstone while predicting the associated removal of phosphorus from iron ore due to its dominant phosphate mineral form. Ooidal ironstone should be considered a complex deposit and an unconventional natural type of REE ores as an example of the largest Bakchar and Lisakovsk deposits.

The Mesoproterozoic Baoban Complex, South China: A missing fragment of western Laurentian lithosphere

Abstract New age data for the Baoban Complex, South China establishes that it lay outboard of western Laurentia in the early Mesoproterozoic but was not part of the Cathaysia Block, with which it is traditional linked, until the mid-Paleozoic. Our geochronology data for detrital zircon and authigenic monazite grains from metasedimentary rocks indicate accumulation between ca. 1.55 Ga and 1.45 Ga for the Gezhencun succession of the Baoban Complex and ca. 1.45 Ga and 1.30 Ga for the Ewenling succession. The former unit is dominated by detrital zircon populations between 1900 Ma and 1500 Ma with two peaks at 1780 Ma and 1580 Ma. The Ewenling succession has detrital zircon peaks at 1720 Ma and 1450 Ma. Newly discovered gneissic granites were emplaced at 1550 Ma and intruded by 1450 Ma leucogranite dykes that are coeval with 1460–1430 Ma bimodal magmatism. The whole Baoban Complex was metamorphosed over the range of 1.3–0.9 Ga based on ages of authigenic zircon and monazite. Depositional ages of metasedimentary rocks are coeval with successions of the Belt-Purcell Basin, western Laurentia. Detrital zircon from the two regions have similar age populations and Lu-Hf compositions, and display a synchronous provenance shift at ca. 1.45 Ga. Basement lithologies on Hainan Island range in age from ca. 1.55–1.43 Ga and underwent metamorphism during 1.3–0.9 Ga. This is younger than basement rocks on the mainland of the Cathaysia Block in South China, suggesting the two regions are spatially unrelated at this time and hence the Mesoproterozoic record of the island cannot constrain the location of the Cathaysia Block in the Nuna and Rodinia supercontinents.

High-Grade Magnetite Mineralization at 1.86 Ga in Neoarchean Banded Iron Formations, Gongchangling, China: In Situ U-Pb Geochronology of Metamorphic-Hydrothermal Zircon and Monazite

Abstract Although less common than hematite ores, high-grade magnetite ores represent a distinct type of iron mineralization hosted by banded iron formations (BIFs). The Gongchangling iron deposit hosted in ~2.55 Ga BIFs in the North China craton represents one of the most economically important iron deposits in China. Located in mining area II, it is a high-grade (&gt;50 wt % Fe) magnetite deposit and one of the largest of its type in the world. However, the lack of reliable age constraints on iron mineralization has hindered the testing of competing genetic models for the formation of the Gongchangling deposit. In situ U-Pb geochronology of monazite and zircon intergrown with garnet from the proximal alteration zone of a high-grade iron orebody yielded an age of 1.86 Ga, which represents the timing of formation of high-grade magnetite mineralization. This age is coeval with a tectonic extension event recorded in the northeastern North China craton. Our results preclude the previously suggested genetic link between high-grade magnetite mineralization and ~2.50 Ga regional metamorphism. Growth of authigenic monazite and zircon is likely related to the breakdown of detrital zircon, which has undergone metamictization. In combination with previously published data, we propose that the development of zoned alteration associated with the deposit, which is characterized by the garnet-amphibole-magnetite assemblage in the proximal zone changing to a chlorite-quartz–dominated assemblage in the distal zone, can be attributed to a gradual decrease in temperature from &gt;550° to ~250°C and to alteration minerals forming from leaching of the BIFs instead of by replacement of the wall rocks. Magnetite mineralization was controlled by the well-developed faults that cut the BIFs and provided conduits for silica-undersaturated alkaline meteoric fluids. Fluid flow likely took place in an extensional tectonic regime, similar to that invoked elsewhere for hematite mineralization but at greater depths.

Fingerprinting Proterozoic Bedrock in Interior Wilkes Land, East Antarctica

Wilkes Land in East Antarctica remains one of the last geological exploration frontiers on Earth. Hidden beneath kilometres of ice, its bedrock preserves a poorly-understood tectonic history that mirrors that of southern Australia and holds critical insights into past supercontinent cycles. Here, we use new and recently published Australian and Antarctic geological and geophysical data to present a novel interpretation of the age and character of crystalline basement and sedimentary cover of interior Wilkes Land. We combine new zircon U–Pb and Hf isotopic data from remote Antarctic outcrops with aeromagnetic data observations from the conjugate Australian-Antarctic margins to identify two new Antarctic Mesoproterozoic basement provinces corresponding to the continuation of the Coompana and Madura provinces of southern Australia into Wilkes Land. Using both detrital zircon U–Pb–Hf and authigenic monazite U–Th–Pb isotopic data from glacial erratic sandstone samples, we identify the presence of Neoproterozoic sedimentary rocks covering Mesoproterozoic basement. Together, these new geological insights into the ice-covered bedrock of Wilkes Land substantially improve correlations of Antarctic and Australian geological elements and provide key constraints on the tectonic architecture of this sector of the East Antarctic Shield and its role in supercontinent reconstructions.

REEs and U distribution in P-rich nodules from Gelasian Apulian Tethyan carbonate: A genetic record

In the Tethyan realm the carbonate-dominated Meso-Cenozoic South Tethyan Phosphogenic Province is of considerable economic importance since it represents the greatest accumulation of sedimentary phosphorites. In southern Italy, in the Salentine peninsula (the southern part of the Apulian Carbonate Platform, ACP), is well documented the occurrence of Cenozoic P-rich levels consisting of nodules and pebbles and showing a large P2O5 fluctuation (4.07–22.07 wt%), due to variable calcite abundance. The mainly observed P-bearing minerals are hydroxyapatite and carbonate fluorapatite and U, together with Sr and Pb, preferentially acts as Ca substitutes in both lattices. Minor authigenic monazite (LREE-bearing phosphate) and xenotime (HREE+Y-bearing phosphate), likely formed during sediment burial compaction and diagenesis. The total REEs' abundances and the shape of the shale-normalized REE-patterns in the P-rich nodules are in the range of those typically observed in Paleocene-Eocene through Pleistocene-Recent P-rich sediments, supporting the idea of a broadly consistent ocean chemistry in this span of geological time. The (La/Yb)N proxy is within the modern seawater range, signifying early diagenetic adsorption played only a minor role in affecting the REEs' distribution. The Ce and Pr anomalies suggest some P-rich nodules experienced very localized hypoxic to anoxic conditions promoting Ce/Ce* spurious results in a generally oxic to hypoxic environment causing real negative Ce anomaly. This scenario is reinforced by the lack of the coupled uranium-vanadium enrichment typically observed in an anoxic environment. Since the Pliocene the eastern Mediterranean was variously affected by oxygenation and productivity and it is likely the ACP phosphate-rich sediments formed under low sedimentation rates and authigenesis in a bottom current-dominated regime, as also indicated by the glauconite occurrence. Transgressions and sea levels rising following major glaciations may have favored the deposition of phosphate sediments by creating new restricted basin configurations and increased nutrient input promoted by upwelling processes.

In situ U–Pb geochronology of xenotime and monazite from the Abra polymetallic deposit in the Capricorn Orogen, Australia: Dating hydrothermal mineralization and fluid flow in a long-lived crustal structure

The Proterozoic Capricorn Orogen is a major tectonic zone that records the assembly and subsequent reworking of the West Australian Craton. Recent seismic transects across the orogen have identified major crustal structures, some of which are spatially associated with hydrothermal mineral deposits. The sedimentary-rock-hosted Abra deposit, which is the largest base-metal accumulation in the Capricorn Orogen, is localized within the crust-cutting Lyons River–Quartzite Well fault zone. Robust radiometric dates for the timing of sediment deposition and hydrothermal mineralization are essential for understanding the geological history of this long-lived orogen and the processes that formed the ore deposits. In situ U–Pb SHRIMP geochronology of xenotime intergrown with magnetite–hematite–galena from the Abra ore zone yields a weighted mean 207Pb/206Pb age of 1594±10Ma (n=14, MSWD=2.6) which is interpreted to represent a period of xenotime growth during the hydrothermal activity responsible for the mineralization. An older coherent cluster within this group gives a weighted mean age of 1610±16Ma (n=5, MSWD=1.5), which constrains the depositional age of the lower Edmund Group sediments to between c. 1680Ma (maximum age of the basal Mt Augustus Sandstone) and c. 1610Ma. Authigenic monazite from the ore zone gives 207Pb/206Pb ages of 1375±14 (n=16, MSWD=0.99)Ma, interpreted to represent a hydrothermal event postdating the main phase of mineralization. Monazites in samples distal to mineralization yield weighted mean 207Pb/206Pb ages of 1221±14Ma (n=5, MSWD=1.04) and 995±18Ma (n=6, MSWD=1.3), interpreted as records of discrete episodes of hydrothermal fluid flow. Our results suggest that the Lyons River–Quartzite Well Fault, which is one of the principal structures in the Capricorn Orogen, has a long history of tectonic reactivation, spanning more than 600 million years and involving crustal extension and sediment deposition, hydrothermal mineralization and multiple episodes of fluid flow. Xenotime and monazite represent ideal chronometers for investigating the complex histories of hydrothermal mineralization and fluid flow in major crustal structures, and helping to unravel the geological evolution of intracratonic orogens.

Authigenic monazite and detrital zircon dating from the Proterozoic Rocky Cape Group, Tasmania: Links to the Belt-Purcell Supergroup, North America

The oldest known rocks in Tasmania occur in the Proterozoic Rocky Cape Group, a ∼10km thick quartzarenite-siltstone-pelite-dominated succession, previously constrained to have been deposited between 1450Ma and 750Ma. The Rocky Cape Group contains the enigmatic fossil Horodyskia (‘string of beads’) and has the potential to place Tasmania within supercontinent reconstructions. Detrital zircon and authigenic monazite grains dated via U–Pb laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analysis yield a deposition window between c. 1450Ma (youngest zircon populations) and c. 1330Ma (oldest authigenic monazite population) for the ∼9km thick lower-middle units (Pedder River Siltstone, Lagoon River Quartzite, Balfour Subgroup which hosts Horodyskia, Detention Subgroup). The upper units (∼1km) include the Irby Siltstone, which is younger than c. 1310Ma; this unit is likely separated from both the lower-middle units and the overlying c. <1010Ma Jacob Quartzite by disconformities. The lower-middle Rocky Cape Group is dominated by detrital zircon populations between 1600 and 1900Ma, with subordinate populations at c. 1450Ma and a spread of older grains mostly between 2300 and 2900Ma. The overlying Irby Siltstone has a bimodal detrital zircon distribution with a dominant peak at c. 1340Ma and a secondary peak at c. 1720Ma; no >1900Ma grains were identified.Authigenic monazite age distributions are complex, with multiple age domains within most samples. The common Pb corrected 206U/238Pb ages, defined by oldest grains in each sample, identify three statistically significant groups: (1) 1358–1292Ma (inclusive of two sigma errors) (Lagoon River Quartzite and Pedder River Siltstone), (2) 1283–1239Ma (Cowrie Siltstone and Balfour Subgroup), and (3) 1085±9Ma (Detention Subgroup). We suggest monazite was precipitated during episodic fluid flow events at these three stages in various parts of the basin. The original source for REE-bearing fluids could be detrital monazite, which is rarely preserved, and/or organic matter from the interbedded carbonaceous shales.The Rocky Cape Group has a shared provenance with the higher-grade metasediments (Surprise Bay and Fraser formations) of nearby King Island; the newly derived depositional ages also overlap and support the correlation of these rock associations. On the basis of current datasets, there are no obvious correlations that can be made with Mesoproterozoic basins preserved in mainland Australia. Instead, an overlap in the timing of deposition, similarities in detrital zircon signatures and analogous depositional environment suggests the c. 1.45–1.37Ga upper Belt-Purcell Supergroup (Missoula and Lemhi groups) of western North America constitutes a plausible correlation with the Tasmanian Mesoproterozoic succession. If the (unexposed) Palaeoproterozoic basement of Tasmania correlates with the Transantarctic Mountains region of East Antarctica as previously proposed, we suggest that the overlying Mesoproterozoic sequences were deposited during rifting of the supercontinent Nuna, between proto-Australia (including the Mawson craton of Antarctica) and Laurentia as predicted by the most recent palaeogeographic reconstructions. Both the Tasmanian and western Laurentian packages were affected by episodic post-depositional fluid flow events between c. 1.35 and 1.05Ga, possible thermotectonic imprints of the subsequent assembly of Rodinia.

Dating fluid flow and Mississippi Valley type base-metal mineralization in the Paleoproterozoic Earaheedy Basin, Western Australia

The ages of deposition and metamorphism of low-grade Precambrian metasedimentary sequences can be difficult to define in the absence of interlayered volcanogenic rocks. Monazite and xenotime can grow at temperatures below 400°C and can give vital evidence for the timing of diagenesis, hydrothermal fluid flow, and low-grade deformation and metamorphism in Precambrian basins. The histories of sedimentary basins in the eastern Capricorn Orogen of the West Australian Craton are generally poorly constrained. However, sandstones and siltstones of the basal Yelma Formation in the Earaheedy Basin contain authigenic monazite and xenotime associated with sulphide minerals related to Mississippi Valley Type base-metal mineralization hosted within carbonate of the overlying Sweetwaters Well Member. In the Teague area, within the Earaheedy Basin, siliciclastic rocks overlying granites of the Yilgarn Craton contain authigenic monazite that gives an in situ SHRIMP 207Pb/206Pb age of 1811±13Ma. Monazite in weathered sandstones and siltstones of the Cano secondary Pb deposit on Magellan Hill, overlying the Yerrida Basin, gives an indistinguishable age of 1815±13Ma. The Cano monazite is interpreted to have been intergrown with primary sulphide minerals and therefore to record the age of mineralization. Xenotime outgrowths on detrital zircons from Cano have high common Pb but define an isochron with an age of 1832±36Ma. The isochron is co-linear with Pb isotope ratios from Pb ore at Magellan, suggesting that xenotime growth is also related to mineralization. The timing of fluid flow is synchronous with the 1820–1770Ma Capricorn Orogeny, and is the first evidence for activity of this age in the eastern part of the Capricorn Orogen. The age of mineralization provides a firm minimum age for the Yelma Formation, and constrains its deposition to between ∼2.0Ga, the youngest age of detrital zircons, and the mineralization age.

Occurrence and paragenesis of diagenetic monazite in the upper Triassic black shales of the Marvast region, South Yazd, Iran

Upper Triassic-Jurassic black shale at Marvast, Iran, contains grey to green-grey ellipsoidal nodules of monazite ranging from 0.1 to 2 mm across. The presence of host-rock mineral inclusions within the monazite grains, low Th content in the monazite, lack of relict yellow cores (characteristic of igneous monazite), and the absence of monazite in the other sedimentary sequences in the Marvast area rule out a detrital origin for the monazite nodules and suggest authigenic crystallization during sediment compaction. Enrichment of the cores of the monazite grains in mid-range to heavy rare-earth elements (REE) and their rims in La + Ce point to variations in the degree of REE mobility and/or evolving composition of the diagenetic mineralizing fluid during nodule growth. The phosphorus and REE required for monazite crystallization were probably derived from seawater and adsorbed on clays and Fe-Mn hydroxides. The interstitial fluids expelled from the sediments during burial compaction and diagenesis became enriched in P and REE through complexing. The association of the Marvast monazite nodules with the black shale may indicate that organic complexes aided in the mobilization and transport of the REE into the pore fluids. Detailed field investigations in the study area and vicinity show that authigenic monazite in the upper Triassic-Jurassic shale sections is spatially associated with quartz lenses. It is likely that these lenses are surface expressions of shallow intrusive magmas, which provided the heat that promoted the mobilization and redistribution of the REE and P, and initiated precipitation of monazite in the overlying sediments.

Formation of authigenic monazite-(Ce) to monazite-(Nd) from Upper Carboniferous graywackes of the Drahany Upland: Roles of the chemical composition of host rock and burial temperature

Irregular aggregates of monazite-(Ce) to monazite-(Nd), up to 150μm in diameter, have been found in graywackes from the Protivanov Formation, Drahany Upland, Czech Republic. Monazite aggregates enclose a variety of detrital and authigenic minerals. Textural evidences suggest that they are of authigenic origin and grew during very low-grade metamorphism at a maximum temperature ~200–300°C in relation to a deep burial. Rarely, they enclose relics of detrital monazite with a chemical composition typical for high-grade monazite. Intensive fluid-related REE-remobilization occurred in relation to this very low-grade metamorphism. Authigenic monazite appears to form only in Ca-poor rocks. In Ca-rich graywackes, which underwent the same low-temperature metamorphic overprint, authigenic synchysite-(Ce) or parisite-(Ce) are common instead. In the overlaying Rozstání and Myslejovice Formations (T~150–200°C), remobilization of REEs was low and authigenic monazite did not form even in the Ca-poor graywackes. Authigenic monazite shows low concentrations of Th, U, Ca, Y+HREE, and a low Eu anomaly. The central parts of the aggregates are enriched in Nd+MREE, whereas the outer parts are enriched in La and Ce. Zoning pattern suggests an evolution in fluid chemistry during diagenesis/very low-grade metamorphism or it relates to different solubility of the individual REE-phosphates. The major source of REEs in the pore fluids are probably detrital minerals mainly monazite. Other minor sources of REEs probably include Fe-oxide/hydroxide and/or clay minerals, which could have contributed to the Nd+MREE-rich cores in the authigenic monazite.

Th–U–total Pb geochronology of authigenic monazite in the Adelaide rift complex, South Australia, and implications for the age of the type Sturtian and Marinoan glacial deposits

The Adelaide rift complex in South Australia contains the type sections for Sturtian and Marinoan glacial deposits. The litho- and chemo-stratigraphy of these deposits play a central role in evaluating global Neoproterozoic ice age hypotheses and Rodinia supercontinent reconstructions, but reliable depositional age constraints have been extremely limited. We report results of in situ Th–U–total Pb (electron microprobe) dating of detrital and authigenic monazite in two samples from the Umberatana Group (Sturtian Holowilena Ironstone and pre-Marinoan Enorama Shale) in the Central Flinders Ranges. Several texturally and chemically distinct detrital and authigenic populations are recognized. Detrital dates range from 1600 Ma to 760 Ma and most relate to well-known orogenic or igneous events in surrounding cratonic regions. Authigenic monazite grew in three or more pulses ranging from 680 Ma to 500 Ma. The date of 680 ± 23 Ma (2 σ) for the earliest generation of authigenic monazite in sandstone from the Enorama Shale (1) provides an estimate for the age of the base of the Trezona carbon isotopic anomaly just beneath the Marinoan glacial deposits, (2) provides an absolute minimum age constraint on the underlying Sturtian glacial deposits, and (3) supports proposed correlations between type Marinoan deposits and precisely dated glacial deposits in Namibia and China, which bracket the presumed Marinoan equivalents between 655 and 635 Ma. This age is inconsistent with a Re–Os isochron age of 643 ± 2.4 Ma (2 σ) on shales near the bottom of the Sturtian–Marinoan interglacial succession, stratigraphically > 3000 m below the Enorama Shale sample, and militate against the hypothesis that the type Marinoan is correlative with the 580 Ma Gaskiers glaciation. Monazite growth near 600 Ma and again at about 500 Ma probably represent hydrothermal fluid-flow events, the latter of which also corresponds to the well-known Delamerian Orogeny during which the Adelaide sediments were folded into their present structural pattern.

Geochemistry of Late Triassic coals in the Changhe Mine, Sichuan Basin, southwestern China: Evidence for authigenic lanthanide enrichment

Abstract Five thin coals of Late Triassic age occur in the Changhe Mine, Sichuan Basin, southwestern China. Petrology and geochemistry of these coals and their roof, parting, and floor samples were studied. The results show that the Changhe coals are enriched in As (8.9–11.8 μg/g), W (2.2–11.2 μg/g), Pb (20.8–48.2 μg/g), and Th (7.5–18.8 μg/g). The source of these elements is possibly the Xuefeng old land which is an early Paleozoic source area southeast of the Sichuan Basin. Two tonsteins were found in the Changhe Mine. One is the roof (K8R) of the K8 coal which is a pure kaolinitic claystone. The other is the claystone (K9C) above the roof of the K9 coal which is mainly made up of mixed illite–smectite minerals. The REE content (287.9 μg/g) of the K8 coal is much higher than that of its roof (153.6 μg/g). SEM-EDX results show that authigenic monazite and kaolinite are the main carriers of REE in the K8 coal. On the basis of coal chemistry and occurrence of the precipitated minerals, it is likely that leaching of the K8R tonstein occurred. It is possible that trace elements including Be (8.4 μg/g), Zr (302 μg/g), Nb (26 μg/g), Hf (9.7 μg/g), Th (18.8 μg/g), U (5.8 μg/g), Ga (16.1 μg/g), and Pb (48.2 μg/g) in the K8 coal were also elevated by the leachate of its overlying tonstein (K8R). The K9C tonstein with a total thickness of 16 cm is widely distributed in the Changhe Mine and serves as a distinct marker bed for correlation of coal seams. Besides mixed illite–smectite minerals, trace amounts of euhedral zircon, anatase, apatite, and massive silica with vesicular structure were also examined, these constitute strong evidence for its felsic volcanic ash origin.

The Rožná uranium deposit (Bohemian Massif, Czech Republic): shear zone-hosted, late Variscan and post-Variscan hydrothermal mineralization

Three major mineralization events are recorded at the Rožna uranium deposit (total mine production of 23,000 t U, average grade of 0.24% U): (1) pre-uranium quartz-sulfide and carbonate-sulfide mineralization, (2) uranium, and (3) post-uranium quartz-carbonate-sulfide mineralization. (1) K–Ar ages for white mica from wall rock alteration of the pre-uranium mineralization style range from 304.5 ± 5.8 to 307.6 ± 6.0 Ma coinciding with the post-orogenic exhumation of the Moldanubian orogenic root and retrograde-metamorphic equilibration of the high-grade metamorphic host rocks. The fluid inclusion record consists of low-salinity aqueous inclusions, together with H2O-CO2-CH4, CO2-CH4, and pure CH4 inclusions. The fluid inclusion, paragenetic, and isotope data suggest that the pre-uranium mineralization formed from a reduced low-salinity aqueous fluid at temperatures close to 300°C. (2) The uraniferous hydrothermal event is subdivided into the pre-ore, ore, and post-ore substages. K–Ar ages of pre-ore authigenic K-feldspar range from 296.3 ± 7.5 to 281.0 ± 5.4 Ma and coincide with the transcurrent reorganization of crustal blocks of the Bohemian Massif and with Late Stephanian to Early Permian rifting. Massive hematitization, albitization, and desilicification of the pre-ore altered rocks indicate an influx of oxidized basinal fluids to the crystalline rocks of the Moldanubian domain. The wide range of salinities of fluid inclusions is interpreted as a result of the large-scale mixing of basinal brines with meteoric water. The cationic composition of these fluids indicates extensive interaction with crystalline rocks. Chlorite thermometry yielded temperatures of 260°C to 310°C. During this substage, uranium was probably leached from the Moldanubian crystalline rocks. The hydrothermal alteration of the ore substage followed, or partly overlapped in time, the pre-ore substage alteration. K–Ar ages of illite from ore substage alteration range from 277.2 ± 5.5 to 264.0 ± 4.3 Ma and roughly correspond with the results of chemical U–Pb dating of authigenic monazite (268 ± 50 Ma). The uranium ore deposition was accompanied by large-scale decomposition of biotite and pre-ore chlorite to Fe-rich illite and iron hydrooxides. Therefore, it is proposed that the deposition of uranium ore was mostly in response to the reduction of the ore-bearing fluid by interaction with ferrous iron-bearing silicates (biotite and pre-ore chlorite). The Th data on primary, mostly aqueous, inclusions trapped in carbonates of the ore substage range between 152°C and 174°C and total salinity ranges over a relatively wide interval of 3.1 to 23.1 wt% NaCl eq. Gradual reduction of the fluid system during the post-ore substage is manifested by the appearance of a new generation of authigenic chlorite and pyrite. Chlorite thermometry yielded temperatures of 150°C to 170°C. Solid bitumens that post-date uranium mineralization indicate radiolytic polymerization of gaseous and liquid hydrocarbons and their derivatives. The origin of the organic compounds can be related to the diagenetic and catagenetic transformation of organic matter in Upper Stephanian and Permian sediments. (3) K–Ar ages on illite from post-uranium quartz-carbonate-sulfide mineralization range from 233.7 ± 4.7 to 227.5 ± 4.6 Ma and are consistent with the early Tethys-Central Atlantic rifting and tectonic reactivation of the Variscan structures of the Bohemian Massif. A minor part of the late Variscan uranium mineralization was remobilized during this hydrothermal event.

Isotopic dating of the migration of a low-grade metamorphic front during orogenesis

The southern margin of the Pilbara Craton in northwestern Australia underwent regional heating, folding, and thrusting as well as extensive fluid flow during collision in the Paleoproterozoic. However, the precise timing of this event is uncertain. Previous geochronologic studies of Archean basement rocks have yielded a wide range of younger radiometric dates (from ca. 2.4 Ga to ca. 2.0 Ga), interpreted to record multiple hydrothermal alteration events. In situ U-Pb geochronology of authigenic monazite and xenotime in very low grade Archean metasedimentary rocks from across the craton suggests that the Pilbara was affected by at least two cryptic thermotectonic events, the first between ca. 2430 Ma and ca. 2400 Ma, and the second between ca. 2215 Ma and ca. 2145 Ma. The older event is restricted to three localities in the west, and its cause is unknown. The younger event affected most of the craton (>100,000 km 2 ), spanning a 70 m.y. period from ca. 2215 Ma nearest the collisional margin in the south, to ca. 2145 Ma toward the craton interior in the north. The widespread geographic and stratigraphic distribution of ca. 2.2 Ga phosphates suggests that fluid flow was pervasive. This event was probably driven by the northward-advancing Ophthalmian fold-and-thrust belt that developed during protracted collision. The associated low-grade metamorphic front migrated over ∼350 km at an average rate of ∼5.0 mm/yr, placing important constraints on the rate and duration of deformation and metamorphism in orogenic settings.

Dating diagenetic monazite in mudrocks: constraining the oil window?

Authigenic monazite nodules, increasingly recognized in Pre-Mesozoic mudrock successions, provide a means for dating diagenesis by exploiting trace uranium within the monazite structure. But, dating is difficult in practice because of zonation and abundant host rock inclusions. We report here a refined technique that has yielded a date of 419 ± 14 Ma 2 σ MSWD = 1.2 for a 17-point Pb–Pb regression age from a Telychian (Silurian: late Llandovery) mudrock from central Wales. SHRIMP analyses of monazite nodules from the same horizon are consistent with this, and additionally demonstrate concordance of the U–Pb and Th–Pb systems. These results open the door to providing robust dates for a phase of burial-related diagenesis which, in Wales, seems approximately coincident with, and perhaps was related to, hydrocarbon expulsion.

U[sbnd]Pb, Pb[sbnd]Pb and Sm[sbnd]Nd dating of authigenic monazite: implications for the diagenetic evolution of the Welsh Basin

Authigenic monazite nodules from Silurian turbiditic rocks in the central Welsh Basin are zoned with respect to Nd isotope composition. The cores are relatively radiogenic, and the rim-to-core trend projects toward the composition of associated, very early authigenic, apatite concretions. These authigenic phases therefore equilibrated with changing fluid composition as they formed. The apatites and the monazite nodule cores may have included significant Nd from organic matter or seawater, whereas the REEs in the outer parts of the monazite nodules were derived from clastic detritus. The apparent SmNd ‘age’ obtained from these phosphates is a mixing line, which gives an Ordovician age for these Silurian rocks. The UPb isotope system does not show similar trends, and shows promise as an alternative method for dating such phosphates: a date of 417 ± 11 Ma 2σ (MSWD = 107) agrees well with recent estimates of the time of the diagenetic, smectite-illite transition in the rocks of this study. A PbPb isochron gave an age of 422 ± 24 Ma 2σ (MSWD = 0.9) this shows that the Pb is in isotopic equilibrium and is undisturbed, and that the elevated MSWD of the UPb regression is caused by slight subsequent mobility in the UPb system. The event dated by the monazite is probably compactional dewatering during accelerated mid-Silurian sediment loading in the central Welsh Basin.