Mineralogy and Petrology最新文献

The 3.30 Ga high-silica volcanic system of the Gavião Block, São Francisco Craton, represents the remnants of within-plate magmatism related to an intracontinental rift. However, the petrogenetic processes that may have taken place in the relatively shallow primitive continental crust has not been fully constrained due to a scarce record. Petrographic and chemical analyses in biotite, as well as in-situ Sr isotope ratios in plagioclase, were used to trace petrogenetic processes and physicochemical conditions of the magmatic system. The subvolcanic rock has a well-preserved primary volcanic feature represented by magma flow textures, euhedral to subhedral plagioclases, rapakivi microstructures, and glomerocrysts. Plagioclase populations formed at two distinct stages recorded by trace elements and Sr isotope. Plagioclase phenocrysts and rapakivi phenocrysts have a slight enrichment of light rare earth elements (LREE), Sr/Ba ratio, and slight variation of Sr isotopes composition. Meanwhile, other phenocrysts and rapakivi crystals have low LREE, Sr/Ba, and a limited variation of Sr isotope ratio. Mineral chemistry evidence points to country rock assimilation during plagioclase formation and a crustal source for primary biotites under oxidized conditions.

The Yidun Terrane hosts huge amounts of Late Triassic granitic plutons. While the southern Yidun Terrane, also referred to as the Zhongdian arc, mainly includes Late Triassic porphyry and related porphyry Cu-Au deposits, rare mineralization was found related to the contemporary granitic batholiths in the northern Yidun terrane (NYT). This paper has explored the chemical compositions of the apatite from these intrusions, which provide insights into the petrogenesis and the mineralization potential. The higher Mg and V concentrations and La/Sm ratios of the Zhongdian apatite than the Cuojiaoma and Daocheng apatite in the NYT suggestes that the Zhongdian porphyries were derived from a mixed source with both mantle and crustal contributions, while the Cuojiaoma and Daocheng granites are dominantly crustal derived. Compared with the Cuojiaoma and Daocheng apatite, the Zhongdian apatite are characterized by higher Sr, Eu/Eu* [Eu_N/(Sm_N×Gd_N)^0.5], S and Cl, and low Mn and Y concentrations with adakitic affinity, indicating an environment with high oxygen fugacity and S and Cl contents for their parental magmas, which is favorable for the formation of porphyry Cu-Au deposits. By comparing the chemical characteristics of apatite from the ore-bearing porphyries and ore-barren granites in the Yidun Terrane, this study proposes that the apatite from the ore-bearing rocks commonly has relatively high Sr, S and Fe concentrations and low Mn, Y and Si/S ratio. These geochemical characteristics can be good indicators for predicting the metallogenic potential of the rocks.

The Hemrin Basalt (HB) is an abnormal occurrence because of its genesis, mineralogy, textures, rare native metals content, and odd geographic-geologic-tectonic setting. It occurs as a caprock for few NW/SE trending hills in a nearly flat-lying area. The HB is a high-K calc-alkaline, collision-related, continental-arc basalt and consists of diopside, labradorite and volcanic glass as the predominant constituents displaying rare firework and feathery textures. The accessory minerals in HB comprise small, discrete grains of native Ni, Fe, Cu, and Sn metals, and scattered grains of sulfides (pyrrhotite, pentlandite, molybdenite), oxides (magnetite, ilmenite, Cr-spinel), and apatite. These minerals occur as disseminated individual or composite grains of variable shapes within the groundmass of HB. Vesicle-filling secondary minerals are zeolite, calcite, anhydrite, and gypsum. The native metals were formed under reducing conditions. The reducing conditions were created as a result of assimilation of C- and S-rich sedimentary rocks by the ascending magma from the continental lithospheric mantle through the thick pile of sedimentary formations. The possible C- and S-rich rocks include the carbonates and evaporites of Fatha Formation and the Hemrin Coal Seam within the upper part of Injana Formation hosting the HB and overlying the Fatha Formation, and possibly the petroleum source, reservoir and caprocks forming the sedimentary column underlying the area. Assimilation of S from these rocks created sulfides, while assimilated C resulted in the formation of native metals.

The Kenticha pegmatite field comprises suites of barren to fertile pegmatite swarms. It shows textural, mineralogical, and geochemical variations. The pegmatites are structurally controlled and emplaced in mafic–ultramafic belts during the Neoproterozoic. This study aims to constrain the genetic and tectonic setting of the Kenticha rare-metal (RM) pegmatite and the associated granite. The presence of minerals such as biotite, muscovite, spodumene, spessartine-almandine garnet, gahnite, beryl, tourmaline, cordierite, and topaz indicate the peraluminous nature of the granite and associated pegmatites. The Kenticha rare-metal granite-pegmatite shows ASI > 1.1, low V, Y, very low Sr, Ba, Th, Zr and REE, very high Be, Li, Rb, Cs, Ga, Nb, and Ta than the upper continental crust. The parental two-mica granite has higher Co, Rb, Ba, and Ce and lower V, Zr, Y, Nb, Ni, La, Pb, Sr, and Th than the upper continental crust. The Ta/Nb value in the two-mica granite is nearly equal to the average upper crustal value. In both parental granite and pegmatites, the normative corundum (C) is greater than 1%. The P₂O₅ content is low in the assumed parental granite. However, in the main ore body, the P₂O₅ content increases towards the more fractionated pegmatite. Due to its peraluminous character and high content of Be, Cs, Li, Ta, and Rb, we can categorise the granite-pegmatite under the LCT subclass of the rare-element pegmatite. It's associated with S-type granite. HFSE and LIL elements show characteristics of bulk continental crust composition and syn-collisional felsic magmatism. Biotite composition suggests partial melting of the metasedimentary protolith as the source of the two-mica granite. The plots of Y vs. SO₂, P₂O₅ vs. SiO₂, % normative C vs. Rb, Y vs. Rb, and Th vs. Rb support the S-type magma source. Geochemical and mineralogical data show a nearly vertical evolutional trend of the granite-pegmatite. It varies from the basal granitic unit to the pegmatite core unit. The mineralization of rare metals in the granite-pegmatite was mainly controlled by genetic (processes).

Prachařite, ideally CaSb⁵⁺₂(As³⁺₂O₅)₂O₂·10H₂O, is a new mineral found in underground workings of the Plaka Mine No. 80, Plaka, Lavrion Mining District, Attica, Greece. It occurs as colourless to white, thin tabular hexagonal, in general sharp crystals up to 2.5 mm in diameter, and is associated with pharmacolite, sulphur and very rare smamite {Ca₂Sb(OH)₄[H(AsO₄)₂]·6H₂O} on a matrix composed of sphalerite, galena and carbonate gangue. Prachařite is translucent to transparent, with a glassy lustre, white streak, a good cleavage parallel to {0001} and a distinct cleavage parallel to {10(overline{1 })0}. It is non-luminescent, brittle, and has an uneven fracture, a Mohs hardness of 2–2.5 and X-ray density D_x = 2.848 g/cm³, D_calc. = 2.836–2.853 g/cm³ (for two measured compositions). Optically, it is uniaxial negative, with ω = 1.619(1) and ε = 1.553(1). Prachařite is trigonal, space group P(overline{3 })c1 (no. 165), with a = 13.951(2), c = 19.899(2) Å, V = 3354.1(10) ų and Z = 6. Strongest lines in the X-ray powder diffraction pattern are [d in Å (I) hkl]: 9.894 (100) 002; 6.045 (8) 200; 5.156 (10) 202; 4.946 (11) 004; 3.297 (19) 311, 006, 222; 2.988 (22) 400, 313, 116. Two sets of independent electron probe micro-analyses yielded (wt%): CaO 6.28/7.12, MgO 0.09/-, Zn -/0.01, Sb₂O₅ 39.22/40.19, As₂O₃ 47.59/47.39, SO₃ -/0.02, H₂O 21.65/22.04 (calculated on the basis of ideal composition derived from crystal-structure determination), total 114.83/116.77; the total is reproducibly high due to a loss of a third of all water molecules under the electron beam. The empirical formulae, based on O = 22 atoms per formula unit, for the two datasets are very similar, (Ca_0.93Mg_0.02)_Σ0.95Sb_2.02(AsO₃)_4.00·10H₂O and Ca_1.04Sb_2.03(AsO₃)_3.92·10H₂O. The ideal formula is CaSb⁵⁺₂(As³⁺₂O₅)₂O₂·10H₂O, determined with the help of a crystal-structure determination based on single-crystal X-ray diffraction datasets collected at room temperature (R1 = 2.3%). The atomic arrangement of prachařite is unusual; it is based on two different layers containing a six-membered ring of corner-sharing SbO₆ octahedra, an eight-coordinated Ca1 atom in the centre of the ring, two non-equivalent AsO₃ groups corner-linked to form a (As₂O₅)⁴⁻ diarsenite group, and, on interlayer sites, a seven-coordination Ca2 atom and three water molecules (all only w