Alkaline Complex Research Articles

Petrogenesis and rare-metal mineralization of the Ririwai alkaline granitoids, north-central Nigeria: mineralogical and geochemical constraints

ABSTRACT The Ririwai anorogenic alkaline complex harbours extensive rare metal (Nb-Zr-Sn-Zn) mineralization. However, uncertainty still surrounds their origin and magmatic-hydrothermal processes responsible for their rare metal mineralization. The complex is composed of peralkaline arfvedsonite ± albite-bearing granites, mildly peralkaline fayalite-bearing granite porphyries, and weakly peraluminous biotite-bearing granites, all of which show diagnostic A1-type characteristics. Zircon U-Pb dating reveals emplacement ages between 181.4 ± 0.81 and 188.6 ± 7 Ma for the three rock units. Sm-Nd and Pb isotopes (εNd (t) = –0.6 to −5.4, 206Pb/204Pb = 17.68–17.93) constrained the Ririwai A-type granites to represent highly fractionated products of enriched mantle-derived magmas. Mass-balance modelling suggests that magma evolution was fuelled by extensive fractional crystallization of 50 ~ 60 vol.% K-feldspar, 30 ~ 35 vol% fayalite, 5 ~ 10 vol.% amphibole, and ~ 5 vol.% apatite under reduced and water-undersaturated (ΔFMQ = –1.55; logfo2 = –18.63; H2O = <2.0 wt.%) conditions and assimilation of ~ 50 vol.% crust. Whole rock and amphibole compositions revealed an increasing degree of fractionation from the fayalite-bearing granite porphyries through biotite-bearing granites to the arfvedsonite ± albite-bearing granites. This extended fractionation at the late stage of magma differentiation, facilitated by fluorine dissolution and concentration of excess HFSE and REE as alkali-fluorocomplexes in the fluorine-rich silicate melt, allowed the accumulation of economic rare metal pyrochlore and columbite mineralizations in the highly fractionated arfvedsonite± albite- and biotite-bearing granites, respectively. Nonetheless, the metasomatic reaction rim textures around the magmatic pyrochlore, coupled with the M-type lanthanide tetrad effect (TE1,3 >1.10) indicate the subsequent effect of hydrothermal processes. Tectonically, reactivated shear fractures and transcurrent faults following lithospheric stresses in a transtensional regime allowed magma uprising and emplacement of A-type granites in the Ririwai complex.

Petrology, U-Pb titanite dating and Sr-Nd isotope geochemistry of a shoshonitic lamprophyre dyke near the Western Dharwar Craton-Southern Granulite Terrane contact, southern India: Implications for long-lived enriched mantle, widespread Tonian-Cryogenian rifting, and Rodinia configuration

We present petrology, U-Pb titanite geochronology, bulk-rock and Sr-Nd geochemistry of an undeformed and unmetamorphosed shoshonitic lamprophyre (spessartite) from the Mysuru area (Halaguru- Harohalli alkaline province) located approximately 80 km north of the tectonic contact between the Western Dharwar Craton and the Southern Granulite Terrane, southern India, to understand its origin and to evaluate its geodynamic significance. The lamprophyre shows a typical porphyritic-panidiomorphic texture with amphibole phenocrysts. Clinopyroxene occurs only as xenocrysts with feldspar, apatite, titanite and titano-magnetite confined to the groundmass. Amphibole mineral composition classifies them as pargasite and tremolite, and the groundmass is essentially constituted of alkali feldspar. High whole-rock Mg# (56–65), Ni + Cr (245 to 730 ppm), a lack of correlation between SiO2 and Rb/Sr as well as ɛNd(i) indicates that crustal contamination experienced by the lamprophyre magma is limited. U-Pb titanite (n = 18) SHRIMP-II dating yielded a Neoproterozoic Tera-Wasserburg concordia age of 820 ± 15 Ma (MSWD = 5.9). Whole-rock derived 87Sr/86Sr(i) (0.70425–0.70530) and ɛNd(i) (-6.3 to −4.3) suggest origin of the lamprophyre from an enriched lithospheric mantle source. Paleoproterozoic depleted-mantle model ages of ca. 1.8 Ga suggest the existence of a long-lived enriched mantle which correspond to the timing (ca.1.9–1.7 Ga) of widespread mafic dyke swarms’ emplacement in the Dharmapuri-Agali-Tiruvannamalai domain in the Salem block at the northernmost portion of the Southern Granulite Terrane. The Tonian emplacement age of the studied lamprophyre is synchronous with several co-spatial Neoproterozoic (i) alkaline complexes ± carbonatite ± lamprophyre from the northern part of the Southern Granulite Terrane, and (ii) other mafic alkaline dykes of the Halaguru-Harohalli alkaline province from the southern part of the Eastern Dharwar Craton. Our results highlight the importance of this lamprophyre dyke, present close to the’Fermor Line’ as well as near the boundary between Eastern and Western Dharwar Cratons, which provides a rare opportunity to understand its spatio-temporal link with alkaline dykes of the Southern Granulite Terrain. Our findings also help understand the role of repeated large-scale rifting, associated with the Rodinia break-up, leading to the generation of a wide spectrum of Tonian-Cryogenian alkaline magmas in southern Indian Shield. The new age data also have implications for the configuration of India’s paleoposition during the Neoproterozoic and support the idea that South China Block-India were located on the periphery of Rodinia or were already detached from the supercontinent.

Genesis of the Yangyugou syenite-carbonatite complex-related REE deposit in the Qinling Orogenic Belt, Central China: Constraints from zircon U-Pb geochronology, Hf isotopes, and mineral geochemistry

The Qinling Orogenic Belt (Central China) is a world-class rare earth element (REE) metallogenic belt in which REE deposits are mainly hosted by alkaline rocks and/or carbonatites. The newly discovered Yangyugou REE deposit, hosted by alkaline complexes in this belt, is unusual because the REE mineralization mainly takes the form of Ba- and REE-bearing minerals, such as barytocalcite. In this study, we present whole-rock major and trace elements, zircon geochronology, Hf isotopes, and mineral geochemistry to investigate the enrichment of REE and Ba and origin of this deposit. The REE-bearing alkaline complex at the Yangyugou deposit consists of syenite and carbonatite. Carbonatite exhibits higher concentrations of REE and Ba compared to syenite. The concentrations of REE and BaO in the carbonatite can be up to 6214 ppm and 22.4 wt%, respectively, and exhibit a positive correlation. The carbonatite contains numerous Ba-rich minerals, primarily comprising barytocalcite and a smaller amount of bazirite. The REE concentrations of barytocalcite range from 3059 to 10032 ppm, and the Ba concentration can be as high as 37.4 wt%. The barytocalcite is the primary carrier of REE and Ba in the Yangyugou carbonatite. Zircons from the syenite have a 206Pb/238U weighted mean age of 438.9 ± 3.3 Ma (MSWD = 1.08). The εHf(t) values of these zircons range from −8.28 to −3.17, with two-stage Hf model ages of 1.61–1.93 Ga. Apatite from the syenite has initial 87Sr/86Sr ratios of 0.70906 to 0.70984. Our study indicates that syenite-carbonatite complex was probably derived from the enriched mantle that has been metasomatized by a Ba-rich fluid, and that the early Silurian alkaline magmatism may be related to the opening of the Erlangping back-arc basin.

The origin of the Daly gap by fractional crystallization in a transcrustal plumbing system in the Passa Quatro alkaline complex (SE Brazil)

ABSTRACT The surface gap of tephriphonolite compositions in the Passa Quatro alkaline complex, in SE Brazil, is attributed to non-buoyancy of these high viscosity intermediate compositions during magmatic fractional crystallization at different depths. Thermodynamic and Rayleigh models for three alkaline suites of a strongly silica-undersaturated series were integrated to demonstrate that parental (basanite, nephelinite and alkaline basalt) and residual (phonolites) magmas are cogenetic by fractional crystallization at pressures of 10 kbar and 5 kbar, i.e. at magma chambers located above and below the Conrad discontinuity within the Upper Cretaceous-Palaeogene 40 km-thick continental crust in SE Brazil. The fractional crystallization of the parental magmas produced an ultramafic residual solid (wehrlite, olivine-bearing clinopyroxenite and clinopyroxenite) and residual liquids (evolved basanite and alkaline basalt, and phonotephrite). The latter moved up and built magma chambers at lower depths in the continental crust where those magmas initially crystallized clinopyroxenite extracts and high viscosity residual liquid that was entrapped and prevented to ascent towards the surface. Then, an abrupt change in the extract mineralogy produced a feldspathic-rich extract (leucocratic leucite monzosyenite, mesocratic syenite or leucocratic monzodiorite) and a residual phonolitic liquid with low viscosity. Following the feldspar crystallization, the magma viscosity decreased, acquiring adequate buoyancy that resulted in shallow-crust magma chambers in which phonolite diversification took place. These processes occurred due to the existence of a transcrustal, plumbing architecture where the high viscosity intermediate tephriphonolites were prevented to reach shallower depths, resulting in the Daly gap observed in the Passa Quatro Alkaline Complex.

Упорядочение катионов в минералах сложного состава: P3-сергеванит — второй представитель группы эвдиалита с примитивной ячейкой

Sergevanite with idealized formula (Na12KH3O)[Ca3(Mn2Ca)](Na2Fe)Zr3Ti0.5Si25.5 O72OH2.(H2O)2 (Z = 3) — a low calcium eudialyte group mineral from Lovozero alkaline complex was investigated repeatedly using obtained earlier the X-ray diffraction data. Its crystal structure with 174 sites was refined in the frame of trigonal primitive cell to R = 4.7 % on the base of 5613 averaged independent reflections with F &gt; 3(F). The unit-cell parameters were: a = 14. 2179(1), c = 30.3492(3) Å, V = 5313.1(1) Å3, space group P3. More detailed, close to real, cations distribution on sites was established. Studied sergevanite was the second mineral of eudialyte group with lowering symmetry from rhombohedral (R3) to trigonal (P3).

Major and Trace-Element Composition of Minerals in the Paleoproterozoic Tiksheozero Ultramafic–Alkaline–Carbonatite Complex, Russia: Insight into Magma Evolution

The Middle Paleoproterozoic (1.99 Ga) Tiksheozero ultramafic‒alkaline‒carbonatite complex in Northern Karelia is one of the Earth’s oldest alkaline complexes. The major and trace-element compositions of minerals were used to decipher the genetic relations between ultramafic cumulates, alkaline rocks, and carbonatites. Based on detailed analysis of clinopyroxenes from ultramafic cumulates, it was assumed that they were derived from an alkaline melt. It was estimated that ultramafic cumulates and alkaline rocks were formed at close moderate pressure, which in combination with the above facts, is consistent with their cogenetic origin. The REE patterns of clinopyroxenes are characterized by the high LREE/HREE fractionation, with slightly convex-upward LREE patterns (La/Nd &lt; 1), which are typical of deep-seated cumulates formed in an equilibrium with an alkaline basaltic melt. Two types of REE zoning were distinguished in apatite using cathodoluminescence imaging. The first type with an outward LREE decrease was found in apatite from silicate rocks of the complex and was likely produced by the closed-system overgrowth of apatite from a residual melt at the late magmatic stage. In contrast, apatite from carbonatite is characterized by a slight outward LREE increase, which is likely related to the re-equilibration of apatite with fresh batches of REE-enriched carbonatite magma. Precipitation of monazite along fractures and margins of apatite in complex with essential HREE and Y enrichment observed in syenite is indicative of the metasomatic interaction of this rock with fluid. Apatites from alkaline rocks and carbonatites define a common trend in the Y–Ho diagram, with a decrease in the Y/Ho ratio from foidolites to carbonatites. This fact together with the absence of signs of liquid immiscibility, and compositional variations in apatite in silicate rocks and carbonatites, are consistent with their origin through fractional crystallization rather than liquid immiscibility.

Magnesium isotopic constraints on the lithospheric mantle metasomatism beneath the craton margin

The magnesium (Mg) isotopic distinction between surficial carbonate and mantle rock makes the Mg isotopes a potential tracer for recycled crustal materials (especially the carbonate-bearing materials) within the mantle. However, whether or to what extent recycled carbonate-bearing crustal materials could influence the Mg isotopes of the lithospheric mantle is still in debate. Here, we report the Mg isotope data for three well-characterized alkaline complexes from the northern margin of the North China Craton, i.e., the Hekanzi, Yaojiazhuang and Saima alkaline complexes. These alkaline rocks were all derived from enriched lithospheric mantle sources beneath the craton margin that were variably metasomatized by carbonate-bearing recycled crustal materials. However, they have relatively homogeneous and normal mantle-like δ26Mg values, i.e., −0.23 to −0.25‰ for the Hekanzi, −0.20 to −0.30‰ for the Yaojiazhuang and − 0.25 to −0.30‰ for the Saima alkaline rocks, respectively. These Mg isotopic characteristics were unrelated to shallow crustal processes (i.e., surface alteration, fractional crystallization and crustal assimilation) but inherited from the mantle sources. The mantle-like Mg isotopes suggest that the recycled carbonate-bearing crustal materials did not change the Mg isotopic compositions of the lithospheric mantle overlying the subduction zone, which mainly ascribes to the large contrast of MgO contents between the recycled crustal materials and the lithospheric mantle peridotite. The MgO contents of calcite-rich carbonates in the metasomatic agents released from the subducted crustal materials are too low to induce Mg isotopic anomaly in the MgO-rich peridotite. Our work reveals that Mg isotopes might not be a sensitive tracer of carbonate melt/fluid metasomatism in the lithospheric mantle beneath the craton margins.

Petrogenesis of the Zijinshan alkaline complex in Shanxi Province, western North China Craton: Mixing of melts from multiple sources

The destruction of the North China Craton (NCC) is a well‐known dynamic event in the Mesozoic, but the western NCC, with only few magmatic activities, has lacked studies so far. The Zijinshan alkaline complex, tectonically located at the western part of the NCC and the eastern edge of Ordos Block/Basin, is one of the most typical alkaline intrusions during the NCC destruction, and consists of monzonite, aegirine–augite syenite, nepheline syenite, pseudoleucite phonolite and trachyte porphyry. Geochronological, geochemical and in situ Sr–Nd–Hf isotopic analyses were performed on the various lithologies, in order to reveal the magma sources, petrogenesis and possible geodynamic processes. Zircon and titanite U–Pb dating shows that the Zijinshan complex was emplaced at ~130 Ma. The main rocks are enriched in light rare earth elements (LREEs) and large‐ion lithophile elements (LILEs; such as Rb, Ba, K, Sr), and depleted in heavy rare earth elements (HREEs) and high‐field‐strength elements (HFSEs; such as Th, U, Nb, Ta, Ti), without negative Eu anomaly. In addition, the obviously enriched and inhomogeneous Sr–Nd isotopic components occurred among the various lithologies, implying the primary magma of Zijinshan complex may originate from a deep mixed source. Specifically, the monzonites of Zijinshan complex come from the partial melting of the enriched lithospheric mantle with participation of crustal materials. Meanwhile, the nepheline syenites and alkaline volcanics originated from the enriched lithospheric mantle mixing with the depleted asthenosphere‐derived compositions. During the NCC destruction, continuous asthenospheric upwelling resulted in several times of mixing of melts from asthenospheric mantle, lithospheric mantle and lower crust in different proportions, and then the mixed magma of each episode migrated upward to the shallow part to form the Zijinshan pluton.

Concealed architecture, volcanism and tectonics of Deccan LIP related alkaline complexes of Rajasthan, NW India: Constrained from high resolution aero-geophysical data

In the north-western Indian shield two important alkaline complexes (Sarnu-Dandali and Mer-Mundwara) are linked to end-Cretaceous Deccan large igneous province (LIP) bear significance for REE mineralization, CO2 outgassing and mass extinctions near the K-Pg boundary. Although both of them bear numerous similarities, but, the geomorphology and magmatic emplacement are quite conflicting. Here, the subsurface crustal architecture of these two alkaline complexes is reported for the first time on the basis of high resolution airborne geophysical anomaly data (magnetic and spectrometric). Aeromagnetic anomalies distinguish prominent NW-SE and E-W trending linear deep seated crustal structures in the Sarnu-Dandali alkaline complex. The E-W trending structure mimics the older tectonic tendencies of the regional crustal-scale Narmada-Tapti tectonic zone. The NW-SE trending linear are linked to subsurface rift-oblique networks of fault systems of the petroliferous Barmer basin. The ensemble of numerous fracture patterns at various depths may attest to the rejuvenation of fracture systems and recurrent magmatism, leading to the polychronous nature of Sarnu-Dandali alkaline complex. On the contrary, the Mer-Mundwara alkaline complex exhibits broad magnetic circular features representing the deeper sections of a magmatic plumbing system. The geometry also resembles the subsurface magma chambers for Deccan basalt volcanism, occurring as rootless cones. Paleo stress pattern is derived from the fractures and when compared with the results obtained from geophysical data, these analyses demonstrate a high degree of compliance, reinforcing the validity and accuracy of both methods. Further, the inversion modeling, profile depth analysis and computed Euler depth solutions led us to infer that the subsurface magma sources of the Sarnu-Dandali are deeper than those of the Mer-Mundwara alkaline complex.

Eocene (∼ 53 Ma) alkaline complex in the southern Lhasa subterrane, Tibet: Implications for Neo-Tethyan slab breakoff

The Eocene alkaline magmatism in the southern Lhasa subterrane provides insights into the timing and dynamic mechanism of the Neo-Tethyan oceanic slab breakoff. Here, we present a combined study of whole-rock geochemistry, zircon U–Pb ages and Sr–Nd–Hf isotopes for the Mayum alkaline complex in the southern Lhasa subterrane. The complex is composed of amphibole syenites, quartz syenite, alkali feldspar granites and mafic microgranular enclaves (MMEs) in the host quartz syenites. Their crystallization ages are 52.2 ± 0.8 Ma, 52.2 ± 1.0 Ma, 49.6 ± 0.5 Ma and 53.1 ± 1.6 Ma, respectively, indicating that these rocks were simultaneously generated in the early Eocene. The hosted alkaline rocks are shoshonic with high SiO2 (64.32–70.49 wt.%), Na2O+K2O (6.63–9.43 wt.%) and low MgO (0.57–2.47 wt.%) contents. They are enriched in large-ion lithophile elements (LILEs: e.g., Rb, K), light rare earth elements (LREEs), Th and U, and are depleted in high field-strength elements (HFSEs) (such as Nb, Ta, and Ti) with strongly negative Eu anomalies (Eu/Eu* = 0.33–0.75). They have homogeneous whole-rock (87Sr/86Sr)i ratios (0.7051–0.7056), low εNd(t) values (–1.7 to –0.9), and positive zircon εHf(t) values (+1.69 to +7.72). Thus, the host alkaline rocks were generated from partial melting of the juvenile lower crust under the southern Lhasa subterrane, and went through a fractional crystallization process during magma ascending emplacement. Compared with the host alkaline complex, the MMEs have lower SiO2 contents (55.16–63.67 wt.%), very higher Fe2O3T (4.08–6.58 wt.%) contents with variable Cr (3.45–226.00 ppm), V (46–107 ppm) and Ni (2.66–6.16 ppm) contents. Meanwhile, the MMEs have similar patterns in trace and rare earth elements (such as obviously Eu anomalies, Eu/Eu* = 0.24–0.38) and similar whole-rock Sr-Nd isotopes ((87Sr/86Sr)i = 0.7051–0.7056, εNd(t) = –2.1 to –1.4) with the host alkaline rocks. Thus, the MMEs might be derived from a mantle source, and magma mixing occurred between the MMEs and the host alkaline rocks. The Mayum alkaline rocks and associated MMEs were generated under an extensional tectonic setting related to the Neo-Tethyan oceanic slab breakoff at ca. 53 Ma.

Olivine composition of calcite-carbonatite from Sevvattur carbonatite alkaline complex, Dharmapuri Rift Zone, Southern Granulite Terrain, India

Olivine in carbonatites worldwide is mainly of Mg-rich composition (forsterite: Fo85 and Fo99). However, the occurrence of fayalite (Fo<50) is extremely rare except few localities. The Sevvattur carbonatite alkaline complex (SCAC) is part of large Tirrupattur carbonatite-alkaline complex (TCAC), Southern India, which consists of early to late differentiated/fractionated products from dunite-wehrlite-shonkinite-carbonatite to ferrosyenite. In TCAC olivine composition shows decreasing Fo93 to Fo06 contents from early dunite to more evolved ferrosyenite. We report here an occurrence of low-Ni, olivine with intermediate Fo60-65 content and low 100*Mn/Fe, Ni/Co, V/Sc ratios from the SCAC. Textural and compositional evidence reveals the xenocrystic (exotic) nature of olivine and possibly linked with multiple stages of crystal melt interaction in the protracted magmatic evolution and associated metasomatic processes during the interaction of peridotite lithosphere with carbonatite magma in the TCAC at low temperature and shallow depth. It is concluded that the olivine in the Sevvattur Carbonatite Olivine provide the missing link or compositional gap during the evolution of the TCAC of early to late differentiated products from dunite (Fo90-91)-wehrlite (Fo70-72)-shonkinite (Fo72-77)-carbonatite (Fo∼90and 63–65 present study) to ferrosyenite (Fo03-06).

Origin of heavy rare earth element enrichment in carbonatites

Heavy rare earth elements (HREE) are currently in high demand for use in high technology, renewable energy and low-carbon transport, but they are the least abundant in nature. Carbonatites are the primary source of REE; however, they are dominated by light REE (LREE). It remains unknown whether carbonatites have the potential to form economic HREE mineralization. Here we report a xenotime-bearing carbonatite in the Bachu REE deposit, northwestern Tarim Large Igneous Province (TLIP), China, and infer the origin of HREE in carbonatites. The rocks evolved from dolomite to calcite carbonatites, and their HREE content correspondingly increased. Both types of rocks have similar monazite U-Pb ages (ca. 300 Ma), and are older than the major eruption of flood basalt of the TLIP, and associated alkaline complexes. They contain higher εNd(t) (2.4–4.1) and lower initial Sr isotopic ratios [(87Sr/86Sr)i = 0.7036–0.7041] than the basalts but similar values to those of younger alkaline rocks. The carbonatites are inferred to be directly derived from low-degree melting of lithospheric mantle sources induced by a deep-seated mantle plume. Calcite carbonatites contain characteristic xenotime, which is associated with burbankite, sulfates, and minor quartz. The rock-forming calcite shows high HREE abundance and flat REE patterns (La/Ybcn = 0.3–2.1). Apatite and LREE minerals in calcite carbonatites also have a higher HREE content (e.g., Y2O3 up to 2 wt%) than those in dolomite rocks. This finding indicates that the early dolomite carbonatite underwent strong fractionation of dolomite and LREE minerals, resulting in HREE and alkali enrichment in the evolved calcite rocks. High amounts of alkalis further enhance the solubility of REE, particularly HREE, in the residual melts. Silica assimilation from the country rocks facilitates the HREE mineralization by sequestering alkalis. Therefore, HREE enrichment in carbonatites may require substantial fractional crystallization of initial melts as well as alkali conservation during ascent.

The Beacon Heights “Tinguaite” (Phonolite): A recrystallised agpaitic ignimbrite in the Mesoproterozoic Pilanesberg alkaline complex, South Africa

AbstractAn extrusive rock unit in the Mesoproterozoic Pilanesberg alkaline complex that has been referred to as the “Beacon Heights Tinguaite” since the work of Shand (1928) has been reexamined in the field and studied by petrographic microscopy, scanning electron microscopy, and whole-rock major and trace element analysis. The new observations indicate that the unit (here informally renamed the “Beacon Heights Phonolite” to comply with current petrographical nomenclature) is a laminated, volcanic rock in which mafic layers rich in sodic pyroxene alternate on any scale with discontinous, felsic layers consisting of alkali feldspar, nepheline, sodalite and their alteration products natrolite and analcime. Highly strontian apatite, fluorite, a manganiferous pectolite-group mineral, sphalerite and pyrrhotite are minor to accessory minerals. In addition, the rock carries indicator minerals typical of agpaitic rocks (including lamprophyllite and eudialyte-group minerals and their replacement products). The rock contains abundant crystal fragments of felsic minerals (alkali feldspar, nepheline and sodalite), fragments and elongated lenses of nepheline syenite, some of which contain primary magmatic lamprophyllite, and rare armoured lapilli. The overall structure of the rock is that of a welded ashflow tuff (igmimbrite) affected by post-magmatic recrystallisation processes. Whole-rock major and trace element compositions suggest a compositional affinity with members of the agpaitic, intrusive Green Foyaite Suite of the Pilanesberg Complex, and probably also a genetic relationship. The Beacon Heights Phonolite is a rare, possibly unique, example of an ashflow deposit of agpaitic, highly silica-undersaturated composition.

Middle Devonian gold mineralization at the northern margin of the North China craton: Constraints from hydrothermal titanite, garnet, and zircon U-Pb geochronology of the Jinjiazhuang gold deposit

The northern margin of the North China craton (NCC) is adjacent to the Central Asian orogenic belt. The region underwent arc-continental collision during the Middle Paleozoic, oceanic plate subduction and closure during the Late Paleozoic to Early Mesozoic, and tectonic reactivation during the Late Mesozoic. The Late Paleozoic–Early Mesozoic and Late Mesozoic tectonothermal events formed numerous gold deposits along the northern margin of the NCC. However, until now, it was unclear whether any gold mineralization formed during the Middle Paleozoic arc-continental collision. The Jinjiazhuang gold deposit is located in the Zhang–Xuan district at the center of the northern margin of the NCC. Gold mineralization is structurally controlled by NWW-SEE-striking faults cutting the Xiaozhangjiakou ultramafic intrusion and Da’nanshan alkaline complex, which exhibit zircon U-Pb ages of 400 ± 5 Ma (2σ, MSWD = 0.3) and 389 ± 3 Ma (2σ, MSWD = 0.8), respectively. Petrographic and geochemical data for titanite, garnet, and zircon grains from Jinjiazhuang gold ores indicate that these minerals have a hydrothermal origin and coprecipitated with the gold. Laser ablation inductively coupled plasma mass spectrometry analyses of the hydrothermal titanite, garnet, and zircon grains yielded reproducible U-Pb dates of 383 ± 12 Ma (2σ, MSWD = 1.4), 392 ± 2 Ma (2σ, MSWD = 0.79), and 391 ± 3 Ma (2σ, MSWD = 1.4), respectively. These new data suggest that the Jinjiazhuang gold deposit formed in the Middle Devonian and was coeval with the emplacement of the Da’nanshan alkaline complex, the genesis of which is related to post-collisional extension across the Bainaimiao island arc and NCC. The recognition of Middle Devonian gold mineralization related to alkaline complexes is of great significance for targeting similar gold deposits along the Fengning–Longhua fault, along which numerous Middle Devonian alkaline intrusions are located.

The genesis of calcite and dolomite carbonatite-forming magma by liquid immiscibility: a critical appraisal

AbstractLiquid immiscibility has become the preferred mode of genesis for the carbonatite rocks, which commonly, but not exclusively, accompany silicate rocks in alkaline-rock complexes. This concept has been universally based on the presumption that nephelinitic and phonolitic magmas can evolve to a stage where two conjugate immiscible liquids separate. It is assumed that these two liquids separate quickly, or even instantaneously, into discrete bodies of magma capable of being intruded or extruded with subsequent independent crystallization. Supporting evidence generally given is: alleged consanguinity as discrete occurrence of the two rock types; similarity of radiogenic isotope ratios; trace element contents similar to those predicted from experimentally derived partition coefficients. We do not accept that a general case for liquid immiscibility has been demonstrated; although we do accept that silicate and carbonate liquids are inherently immiscible, we maintain that they are not conjugate in a petrogenetic context. We have reviewed and critically examined the experimental data purporting to establish liquid immiscibility and find that when applied to natural rocks, they are based on inappropriate experimental designs, which are not relevant to the genesis of calcite or dolomite carbonatites, although they might have some relevance to Oldoinyo Lengai nyerereite–gregoryite lavas. The design of these experiments guarantees immiscibility and ensures that the carbonate liquids formed will be calcitic or sodium-rich. We dispute the validity of comparing the trace element contents of natural rocks, which in many instances do not represent liquid compositions, to experimentally determine partition coefficients. We consider that experimental design inadequacies, principally assuming but not proving, that the liquids involved are conjugate, indicate that these coefficients are merely an expression of the preference of certain elements for particular liquids, regardless of how the liquids formed. Proof of consanguinity in alkaline complexes requires more accurate age determinations on the relevant rock types than has generally been the case, and in most complexes, consanguinity can be discounted. We dispute the contention that melt inclusions represent parental melts, although they might elucidate the character of magmas undergoing fractional crystallization from magmatic to carbothermal stages. Radiogenic isotope data are shown to be too widely variable to support a case for liquid immiscibility. We address the contention that calcite cannot crystallize from a dolomitic liquid formed by direct mantle melting, and must therefore have crystallized from a calcite carbonate liquid generated by liquid immiscibility, and demonstrate that it is an unsupported hypothesis as calcite can readily crystallize from dolomitic liquids. We observe that, because immiscible dolomite liquids have never been produced experimentally, the liquid immiscibility proposition could at best be applied only to calcite carbonatites, thus leaving unexplained the large number of dolomite carbonatites and those of either type, which are not accompanied by alkaline silicate rocks. The assumed bimodality of alkaline-rock carbonatite complexes is considered to be fallacious and no actual geological or petrographic evidence for immiscibility processes is evident in these complexes. Several examples of alkaline rock carbonatite complexes for which immiscibility has been proposed are evaluated critically and shown to fail in attempts to establish them as exemplifying immiscibility. We conclude that no actual geological or experimental data exist to establish liquid immiscibility being involved in the genesis of calcite or dolomite carbonatite-forming magmas.

Unsupervised machine learning-based prospectivity analysis of NW and NE India for carbonatite-alkaline complex-related REE deposits

Rare Earth Elements (REE) form critical raw materials in environment-friendly, high-technology devices and components, and therefore have been classified as “critical minerals and metals” by most countries. About 62% of the global resources of REE occur associated with carbonatite-alkaline complexes; however, the entire production of REE in India currently comes from secondary deposits, even though India contains a variety of REE-enriched primary source rocks, particularly carbonatites and alkaline complexes. There is, therefore, a significant potential in the county for new REE deposit discoveries associated with carbonatite-alkaline complexes. This research attempts to identify exploration targets for REE associated with carbonatite-alkaline complexes in northern India utilising a Self-Organising Maps (SOM)-driven workflow. This unsupervised machine-learning-based workflow eliminates the hand-crafting of input predictor features. The algorithm creates clusters of features directly from primary gridded geophysical and topographical datasets. The obtained clusters are then analysed based on available geological knowledge and empirical spatial associations with known occurrences in the study areas to identify prospective clusters and generate prospectivity maps. Nine new targets are identified across the Shillong plateau in northeastern and Western Rajasthan in northwestern India. These new targets, in addition to the known carbonatite-alkaline complexes, are recommended for further data collection and follow-up exploration. It is noteworthy that these targets conform to the targets identified by Aranha et al. (2022a, 2022b) using mineral systems-guided fuzzy inference systems.

Magmatic rare earth element mineralization and secondary fluid-assisted redistribution in the Kamthai carbonatite complex, NW India

Calcite carbonatites from the Kamthai alkaline complex in western India are highly enriched in Sr and the rare earth elements (REE) owing to their formation from highly differentiated carbonatitic magma. Pegmatitic calcite carbonatites preserve rare magmatic intergrowth of rod/pod-like carbocernaite [(Ca, Na)(Sr, REE, Ba)(CO3)2] and REE + Sr-rich calcite. This intergrowth is interpreted to be the result of rapid simultaneous crystallization of the phases from a brine-rich carbonatitic magma. Primary carbocernaite, calcite, and britholite [(LREE, Ca)5(SiO4, PO4)3(OH, F)] sequestered Sr and REE from the brine-rich melt and constitute the major sources of these elements during hydrothermal redistribution. Replacement textures suggest fluid-assisted, post-magmatic hydrothermal REE redistribution through dissolution-reprecipitation process that concentrated the REE into secondary REE-(fluor/hydroxyl)carbonates, cerianite (ideally Ce4+O2), ancylite [(Sr, Ca)LREE(CO3)2(OH)∙(H2O)], and monazite. Hydrothermal alteration of magmatic calcite leached out the LREE, whereas in case of carbocernaite and britholite, an increase in their REE contents is noticed, followed by pseudomorphic replacement by REE-(fluor/hydroxyl)carbonate minerals.The carbonatite-derived hydrothermal fluids initially transported the REE as sulfate and later as hydroxyl‑carbonate complexes. The ligand transition was a consequence of the precipitation of baryte, celestine, and pyrite, which led to a reduction in fluid sulfur content and increase in its pH due to interaction with carbonates. Precipitation of secondary REE minerals was triggered by decrease in ore fluid temperature due to interaction with relatively cooler, oxidized fluid and interaction with carbonates. Carbocernaite, ancylite, cerianite, and Ce-bearing REE-(fluor)carbonates constitute early precipitated secondary REE minerals along with baryte, celestine, and pyrite as by-product phases, whereas both Ce- and La-rich, hydroxyl-dominated members of bastnäsite, parisite, and synchysite, along with monazite were the dominant REE minerals which precipitated during the advanced hydrothermal stage, with siderite as a by-product.

Soluble Biobased Substances in soil or Salicylic Acid on leaves affect the foliar pH and soil biovariability of Grapes-as explained by the NIR Spectroscopy of Litterbags and Teabags

The purpose of the trial was to check the effects of two grapevine treatments on the plant activity and on the bioactivity and biovariability of the soil. An alkaline complex of Soluble Biobased Substances (SBS) was used in soil at 30 g per plant in a single solution. Salicylic Acid (SA) was used on leaves at 150 mg l-1 every two weeks at 50 ml plant. The plants were examined for their foliar pH and NIR spectra. The soil bioactivity was monitored by means of hay-Litterbag-NIRS (LBN) in combination with the Teabag Index (TBI), using rooibos and green tea that had been buried for 60 days. The evolution of the TBI presented here concerns the TBI-NIRS spectroscopic method used for discriminant analysis. A new algorithm was used to estimate the soil microbiome from the green Teabag spectra. The obtained results showed that the plants and the soil responded to the treatments. In fact, SBS, but not SA, lowered the leaf pH by 5%, an unexpected and original result. Both treatments increased the variability of the leaf composition, with a lower discrimination, based on the NIR spectra, from the Control (75%) to 44% (SA) and 38% (SBS). The TBI method, which is based on weights, was less efficient (67%) than the TBI-NIRS of rooibos (96%) or the LBN of hay (80%), but it was like the TBI-NIRS of green tea (74%). The LBN analyses indicated that the mycorrhizal index had increased by 8% in SA but had reduced by 7% in SBS, while both treatments reduced the activity of the microbes, which did not affect the soil respiration rate. The mineral N in the soil was substantially raised by about 11÷69%. The Taxa profiles showed marked deviations from the Control. Moreover, the SBS treatment reduced the Glomeromycota by 35%, which matched to the reduction in the mycorrhizal index. The most favored Bacteria from the treatments were Proteobateriaand Actinobacteria, with Mortierellomycota being the most penalized. Neither treatment affected the production, but both delayed the technological maturity by 9-11%, while the SBS retarded the phenolic maturity by about 18%. It has been concluded that a simple treatment of vines can affect the bioactivity in the leaves and berries as well as the biovariability of the soil.

The Crystal Structure of Mg–Al–CO3 Layered Double Hydroxide

The crystal structure of quintinite, Mg4Al2(OH)12(CO3)·3H2O, from the Jacupiranga alkaline complex (Cajati, São Paulo, Brazil), was refined for two samples (91002 and C7029) using single-crystal X-ray diffraction data. The mineral crystallizes in the P-3c1 space group, a = 5.246/5.298, c = 15.110/15.199 Å for samples 91002/C7029. The crystal structure consists of octahedral sheets with Mg and Al ordering according to a 3 × 3 superstructure. The Mg and Al atoms are coordinated by six hydroxylated oxygen atoms; the average &lt;Mg–O&gt; and &lt;Al–O&gt; bond distances are in the ranges 2.022–2.053 Å and 1.974–1.978 Å, respectively. The interlayer structures are identical (in contradiction to the previous assumptions), and consist of disordered (CO3)2− groups and (H2O)0 molecules. The samples from Jacupiranga can be identified as quintinite-2T, which is the second finding of this polytype after the Kovdor alkaline complex (Kola peninsula, Russia). The powder X-ray diffraction pattern of quintinite-2T contains weak superstructure reflection at 4.57 Å (010), indicative of Mg and Al ordering. An important crystal-chemical criterion of quintinite is the interlayer distance (d00n-value) of ~7.56 Å, which is steady among natural specimens from various findings worldwide.

The Origin of Ultramafic Complexes with Melilitolites and Carbonatites: A Petrological Comparison of the Gardiner (E Greenland) and Kovdor (Russia) Intrusions

Abstract In many alkaline complexes, large amounts of ultramafic rocks occur together with carbonatites, melilitolites and other alkaline silicate rocks. There is an ongoing debate if and how these contrasting lithologies were formed by differentiation of a common, mantle-derived silicate magma or rather by metasomatic processes between carbonatite and country rocks. In order to find petrological evidence for one or the other, two key examples, the Gardiner (E Greenland) and Kovdor (Russia) complexes are compared in this study. Despite their similar tectonic setting and succession of rock types, they show significant differences in the texture and mineral composition of ultramafic rocks. Ultramafic rocks from Kovdor include calcite- and biotite-rich dunites and pyroxenites without typical cumulate textures. They consist of Ni-poor forsterite, Cr-poor diopside and Ni-Cr-poor spinel and are possibly metasomatic reaction products between mantle-derived carbonatite melts and silicic host rocks. Similar ultramafic rocks are associated with carbonatites e. g. at Palabora (South Africa), Afrikanda (Russia), and Salitre (Brazil). In contrast, the ultramafic rocks from Gardiner show well-preserved cumulate textures and consist of Ni-rich forsterite, Cr-rich diopside as well as Cr-Ni-Ti-rich spinel and also contain F-Cl-rich apatite. They record an increase in aSiO2 from dunite to pyroxenite at similar fO2 (ΔFMQ ~ +1.2, with FMQ = fayalite-magnetite-quartz buffer), indicating that these rocks represent cumulates of an evolving, moderately oxidized mafic melt derived from a Ti-rich mantle source, similar to other rocks of the North Atlantic igneous province. In contrast to systems like Kovdor where carbonatite metasomatism is likely dominant, Ti-rich parental silicate magmas can abundantly crystallize Ti phases, as recorded by massive perovskite cumulates in Gardiner melilitolites. This can effectively scavenge HFSE from the magmatic system early in its evolution and likely explains HFSE-barren carbonatites at Gardiner, while those from Kovdor are highly HFSE-enriched. In summary, the results of our study provide strong textural and mineral chemical evidence that ultramafic rocks in alkaline complexes can be of both cumulate and metasomatic origin; the specific type has an important bearing on their HFSE enrichment and on the types of ores present in such complexes.