Mineralogy and Petrology最新文献

Mineral assemblages formed at low oxygen fugacity are commonly confined to the lithologies of extraterrestrial or deep Earth origin. The occurrences of reduced mineral phases in upper crustal rocks [formed under oxygen fugacity conditions below the iron-wüstite (IW) buffer] are rare. However, they are important for understanding the chemical drivers of natural redox processes. Here, we present detailed studies of reduced mineral assemblages, which were found in situ in superficial combustion metamorphic (CM) rocks of west-central Jordan and compare them to reduced mineral assemblages found in situ in the CM rocks of south-central Israel. The studied assemblages contain a suite of exotic phases more typical of meteorites: native iron, phosphides (schreibersite, Fe₃P; allabogdanite, Fe₂P; transjordanite, Ni₂P; murashkoite, FeP; halamishite, Ni₅P₄; zuktamrurite, FeP₂; polekhovskyite, MoNiP₂), and sulphides (daubréelite, FeCr₂S₄; oldhamite, CaS; troilite, FeS), part of which (native iron, allabogdanite, halamishite, polekhovskyite, daubréelite) have not previously been discovered in the CM rocks of west-central Jordan. The mineralogical diversity of terrestrial phosphides and the occurrence of Ni- / Mo-rich phases can be explained by (1) high P, Ni, and Mo content in the sedimentary protolith, (2) transformations of primary Fe₃P / Fe₂P, (3) extreme disequilibrium of the processes, and (4) crystal-chemical control of Ni- / Mo- speciation.

Tourmaline is a valuable forensic mineral that contains a wide range of elemental components capable of reconstructing its geologic evolution. In this study, we developed detailed petrographic and geochemical research on tourmaline-bearing rocks and tourmalinites found surrounding the ca. 3.30 Ga rhyolite of the Gavião Block, northern São Francisco Craton. Two distinct types of mineral assemblages in the tourmaline-bearing rocks are recognized based on the presence of sulfides, oxide minerals, and carbonaceous matter. Along with these tourmaline-bearing rocks, tourmalinite samples are found. The mineral chemistry of tourmalines from the two types of tourmaline-bearing rocks reveals mainly schorl-dravite as tourmaline from a mineral assemblage containing quartz, tourmaline, pyrite, and hematite, and a diverse composition ranging from schorl-dravite, feruvite-uvite, and schorl-feruvite solid solutions for tourmalines present in the mineral assemblage that contains quartz, pyrite, and carbonaceous matter. The tourmaline composition is essentially foitite-schorl in tourmalinite samples. Most of the metasedimentary rocks studied have felsic and mafic source compositions. The interaction of hydrothermal fluid was crucial for the formation of sulfide and tourmaline minerals. However, distinct tourmaline origins are recognized in tourmaline-rich rocks, pointing out the relevance of sedimentary and metamorphic processes.

Juína is the second-largest diamond-producing municipality in Brazil and is globally known for its outstanding sublithospheric diamond occurrences in both placer and kimberlite-hosted deposits. However, the scarcity of petrological data for Juína kimberlite pipes hampers understanding the nature and mantle source of these primary diamond sources in this region. Here, we present a textural and mineralogical study of ten kimberlite pipes from the Juína area. Based on petrographic features and mineral compositions, we interpret the studied Juína pipes as archetypal kimberlites with pyroclastic emplacement styles filled with resedimented volcaniclastic kimberlite and Kimberley-type pyroclastic kimberlite variants. The composition and texture of the magmatic phases, particularly spinel and phlogopite, suggest crystallisation from kimberlite sensu stricto magmas. The presence of high-Na eclogitic garnets and the absence of high-Cr low-Ca G10 garnets within the mantle xenocryst suite suggest the likelihood of eclogitic diamonds among Juína's lithospheric diamond populations. The Zr and Y contents, Ti/Eu and Zr/Hf ratios in the peridotite garnets, and Zr contents, Ca/Al, La_N/Yb_N (primitive-mantle normalised), Ti/Eu, and Zr/Hf ratios in the clinopyroxenes suggest a solid connection to kimberlite melt-related mantle metasomatism. Thermobarometry calculations indicate a relatively narrow stability window (825–936 ºC and 32–36 kbar) for lithospheric diamonds in the Juína region. Our findings have important implications for regional diamond exploration programs, shedding light on the primary sources of Juína's diamonds and contributing to understanding the deep geological processes in the underlying lithospheric mantle beneath the Amazonian craton.

Veloso is one of numerous and poorly documented auriferous deposits of Ouro Preto, the town that symbolises the gold rush in Brazil at the turn of the seventeenth century. We present the results of underground geological mapping, combined with a boron (B) isotopic study of tourmaline, an elusive mineral in the auriferous quartz lode of the historical Veloso deposit. Its lode is characteristically brecciated in a host rock that shows no cataclastic fabric. The host rock is itabirite, a metamorphosed banded iron formation. Tourmaline is essentially dravite and locally occurs as abundant crystals in breccia-cementing pockets of goethite, formed from the oxidation of sulfide minerals. Gold is spatially associated with tourmaline in the goethite-rich pockets. In situ measurements for B isotopes yielded δ¹¹B values in the range of −21 to −9‰. This range is similar to that reported for tourmaline of the Passagem de Mariana deposit, the best documented auriferous lode deposit at the south-eastern edge of the Quadrilátero Ferrífero. The tourmaline B isotopic data reflect auriferous fluids of crustal origin that sourced B from metasedimentary rocks, which may include a non-marine evaporitic component.

A new occurrence of epidote-(Sr) CaSr(Al₂Fe³⁺)[Si₂O₇][SiO₄]O(OH), Sr-REE-rich epidote and Sr-rich allanite-(Ce) is located in Lower Cretaceous, Sr-rich hydrothermally altered leucocratic dykes penetrating alkaline igneous rocks (teschenites) near the Nový Jičín town (the Silesian Unit, Outer Western Carpathians). The dykes contain phenocrysts of pyroxene, amphibole, biotite, fluorapatite and dominant felsic groundmass consisting mostly of alkali feldspars and zeolites (analcime, natrolite and thomsonite-Ca). Accessory minerals include Ti-rich magnetite, prehnite, chamosite, pyrite, calcite, (OH, F)-rich grossular, epidote-group minerals, HFSE-, REE-rich minerals, Sr-rich baryte and slawsonite. The Sr-rich epidote forms columnar crystals or irregular aggregates, which are mostly spatially related to chamosite-titanite pseudomorphs; it contains 0.15–0.81 apfu Sr, ≤ 0.55 apfu REE; Fe³⁺/(Fe³⁺ + Al) = 0.16–0.48. The Sr²⁺ substitutes Ca²⁺ in the A2 site by a coupled substitution involving other A2 (REE³⁺, Th⁴⁺) or M (Al³⁺, Fe³⁺, Fe²⁺) cations. The Sr-rich epidote crystallized from hydrothermal solutions, probably at temperatures between ~ 250–430 °C, during cooling of the host rock. The dykes show higher Sr contents (5680–7830 ppm) and ⁸⁷Sr/⁸⁶Sr_i(120 Ma) ratios (~ 0.7046–0.7047) compared to host mesocratic teschenites (1310–1470 ppm Sr and ~ 0.7038–0.7045, respectively). The Sr isotopes indicate origin of most Sr from primary magmatic plagioclase in parent teschenite. Nevertheless, there also participated external fluids, derived from the Lower Cretaceous seawater or diagenetic waters related to associated siliciclastic sediments with ⁸⁷Sr/⁸⁶Sr_i(120 Ma) = ~ 0.7073–0.7083. These more radiogenic sources contributed at least 6–21% of the bulk Sr budget of the studied Sr-rich epidote-bearing leucocratic dykes.

A report of the supergene mineral mallestigite, Pb₃Sb⁵⁺(SO₄)(AsO₄)(OH)₆·3H₂O, from the type locality Neufinkenstein-Grabanz district, Mallestiger Mittagskogel, Carinthia, Austria, is given in order to close gaps that have been left open since its first description. At the type locality, the dump of an abandoned small copper-lead-zinc mine, the mineral occurs frequently in up to ⁓2 mm long clear colourless idiomorphic hexagonal prisms {100} terminated by the bipyramid {101}. Details of the occurrence, paragenesis, physical properties, a FTIR spectrum and X-ray powder diffraction data are reported. Chemical analysis in a scanning electron microscope in conjunction with a crystal structure analysis gave the empirical formula Pb_3.02Sb_1.06(SO₄)_0.96(AsO₄)_0.97(OH)₆·3H₂O. An ab-initio crystal structure determination with CCD diffractometer data, MoKα radiation, and a nearly untwinned crystal proved that mallestigite is hexagonal, space group P6₃, a = 8.9326(4), c = 11.1044(5) Å, V = 767.33(8) ų, Z = 2, and gave a final R1 = 0.0184 for 1632 independent reflections with I > 2σ(I). Mallestigite belongs to the fleischerite group. The structure is built up from [Sb⁵⁺(OH)₆]^1– octahedra which are linked by triplets of edge-sharing Pb²⁺ into chains [Pb₃Sb(OH)₆]⁵⁺ extending along a 6₃ axis. Each three of such chains are bridged by triplets of H₂O and by rows of SO₄^2– and AsO₄^3– tetrahedra alternating along a 3-fold axis and showing very different links to the Pb atom. Pb²⁺ is in a strongly distorted one-sided [9]-coordination by O atoms that makes the structure clearly hemimorphic. Structural relationships to the fleischerite group minerals schaurteite, genplesite, despujolsite and fleischerite as well as to synthetic Ba₃Sn⁴⁺(SeO₄)₂(OH)₆·3H₂O are outlined and changes in the space group symmetry of despujolsite (P(overline6)2c → P6₃/mmc) and fleischerite (P(overline6)2c → P6₃) are proposed.

The post-tectonic and post-orogenic mafic rocks from the Spanish Central System (SCS) (Iberian Massif) include dyke swarms of shoshonitic (microgabbros) and alkaline (lamprophyres and diabases) geochemical affinity, which register the nature of the metasomatic lithospheric mantle under central Spain. Such magmas sometimes show a direct (or indirect) relationship with the formation of orogenic and intrusion-related gold deposits, which are relatively abundant in the Iberian Massif. The noble and base metal composition of these intrusions shows Primitive Mantle-normalized patterns characterized by positive Au and Co anomalies and fractionated platinum group elements (PGE): from lower Ir-group PGE (IPGE; Ir–Ru) to higher Pd-group PGE (PPGE; Rh–Pt–Pd). The low contents of PGE, together with the base metal contents of pyrite (which is the dominant sulphide phase in the alkaline dykes), is in accordance with low degrees of mantle partial melting and the early segregation of sulphides during magma differentiation. The scarcity of PGE mineral deposits in the Iberian Massif could be explained in part by the apparent lack of PGE enrichment in the Iberian lithospheric mantle. On the contrary, the positive Au anomaly of the SCS mafic dykes represents relatively high Au contents, similar to (and higher than) those of mafic rocks derived from metasomatized subcontinental lithospheric mantle underlying Au-endowed cratons. Several geochemical features point to subduction-related metasomatism of either oceanic or continental nature as the main source of Au enrichment. The Au re-fertilization of the lithospheric mantle under central Spain makes it a potential source in the formation of gold mineralizations.

Geochemical and mineralogical investigations of the Lower Permian Kemmlitz rhyolite within the NW-Saxonian Basin (Germany) and associated lithophysae (high-temperature crystallization domains) as well as agates were carried out to constrain the genesis and characteristics of these volcanic rocks and the origin of the agate-bearing lithophysae. The volcanic rocks of rhyolitic composition are dominated by quartz, sanidine, and orthoclase and most likely derive from lava flows. Agate-bearing lithophysae were exclusively formed in a glassy facies (pitchstone) of the rhyolites, which was afterwards altered to illite-smectite mixed-layer clays. The results of this study show that agate formation can be related to the alteration of the volcanic rocks accompanied by the infill of mobilized silica into cavities of lithophysae. Fluid inclusion studies point to temperatures of agate formation above 150 °C, indicating that the mobilization and accumulation of silica started already during a late phase of or soon after the volcanic activities. Remarkable high concentrations of B (29 ppm), Ge (> 18 ppm), and U (> 19 ppm) as well as chondrite-normalized rare earth element (REE) distribution patterns of the agates with pronounced negative Eu-anomalies, slightly positive Ce-anomalies and enriched heavy rare earth elements (HREE) indicate interactions of the host rocks and transport of SiO₂ with magmatic volatiles (F/Cl, CO₂) and heated meteoric water. Characteristic yellow cathodoluminescence (CL), heterogeneous internal textures as well as high defect density of micro- and macrocrystalline quartz detected by electron paramagnetic resonance (EPR) spectroscopy point to crystallization processes via an amorphous silica precursor under non-equilibrium conditions.