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Item type:Publication, Magma mingling in kimberlites: Evidence from the groundmass cocrystallization of two spinel-group minerals(MDPI, 2020-09-01)We present the results of a detailed petrographic study of fresh coherent samples of the Menominee kimberlite sampled at site 73, located in Menominee County, MI, USA. Our objective is to account for its unusual and complex paragenetic sequence. Several generations of olivine, ilmenite, and spinel-group minerals are described. Early olivine and ilmenite are xenocrystic and were replaced or overgrown by primary minerals. Zoned microcrysts of olivine have a xenocrystic core mantled by a first rim in which rutile, geikielite, and spinel s.s. (spinel sensu stricto) cocrystallized. The in situ U–Pb dating of a microcryst of primary rutile yielded 168.9 ± 4.4 Ma, which was interpreted as the age of emplacement. The groundmass consists of olivine, spinel s.s., a magnesian ulvöspinel–ulvöspinel–magnetite (MUM) spinel, calcite, and dolomite. An extremely low activity of Si is suggested by the crystallization of spinel s.s. instead of phlogopite in the groundmass. The presence of djerfisherite microcrysts indicates high activities of Cl and S during the late stages of melt crystallization. The occurrence of two distinct spinel-group minerals (spinel s.s. and qandilite-rich MUM) in the groundmass is interpreted as clear evidence of the mingling of a magnesiocarbonatitic melt with a dominant kimberlitic melt. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Trace element composition and U-Pb ages of cassiterite from the Bolivian tin belt(Springer Science and Business Media Deutschland GmbH, 2021-11-01)The Bolivian tin belt is a metallogenic province in the Eastern Cordillera of the Andes known for its Sn, W, Ag, and base metal deposits. Cassiterite, which is a major constituent in many magmatic-hydrothermal ore deposits from the Bolivian tin belt, can incorporate dozens of elements within its crystal lattice, making it a useful geological tracer mineral and also a potential host of critical elements. New U-Pb dating of cassiterite yields Late Triassic (Kellhuani deposit) and Late Oligocene to earliest Miocene (Viloco, Huanuni, and Llallagua deposits) ages. These ages confirm that Sn mineralization in the Bolivian tin belt occurred at least in two separate events during two major magmatic episodes apparently triggered by mantle upwelling, decompression melting, and basalt production promoting high heat flow into the overlying crust. The composition of studied hydrothermal cassiterite yields some geochemical trends that are attributed to its distance to the causative intrusion and/or level of emplacement. For example, cassiterite is generally enriched in Nb and Ta and yields higher Ti/Zr and Ti/Sc ratios in samples from xenothermal ore deposits located adjacent to intrusive complexes relative to shallow xenothermal and epithermal ore deposits. Therefore, these geochemical trends in cassiterite are useful tracers pointing to magmatic-hydrothermal centers. REE distribution in cassiterite was likely influenced by boiling processes, which resulted in tetrad-type irregularities. Cassiterite from the Bolivian tin belt is unattractive as a source for Nb (interquartile range [IQR] 4.84–0.037 ppm), Ta (IQR 0.0924–0.0126 ppm), and Ge (IQR 3.92–0.776 ppm). Some deposits, however, contain cassiterite relatively enriched in In (IQR 96.9–9.78 ppm, up to 1414 ppm) and Ga (IQR 92.1–3.03, up to 7437 ppm), that could constitute an attractive supplementary source for these elements in addition to sulfide minerals in the same deposits. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Petrology, metallogeny and U-Pb geochronology of the Paleoproterozoic mafic-ultramafic Hamutenha intrusion, Angolan Shield(Elsevier BV, 2022-10-12)The Hamutenha intrusion (Huíla province, SW Angola) is a 3-km-long elongated magmatic body defined by a set of two intrusive units. The northern unit is formed by alternating bands of dunites and olivine gabbros, while the southern unit is composed of amphibole diorites. The Hamutenha body is hosted by the Paleoproterozoic Epupa Complex in the Angolan Shield, SW margin of the Congo Craton. A mineralogical, petrological and geochemical study of the Hamutenha intrusion has been performed. The constrained compositional features of the parental melt suggest interaction of tholeiitic magmas with a significant enrichment in Fe and Ti. Although disseminated Fe–Ni–Cu-(Co) secondary sulphides are observed, both the crustal contamination parameters and generally low bulk-rock metals contents indicate that these elements were most likely previously extracted from the parental melt. U–Pb zircon dating of amphibole diorite samples from the southern unit yields a date of 1844 ± 14 Ma. These data suggest that the emplacement of these rocks was previous to the KC emplacement and it was probably related to the Paleoproterozoic Epupa Complex magmatism. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Tin Mineralization in the Triassic Chacaltaya District (Cordillera Real, Bolivia) Traced by In Situ Chemical and δ18O-δ11B Compositions of Tourmaline(Society of Economic Geologists, Inc, 2023-12-19)We present a petrographic and geochemical study of tourmaline from the Triassic Chacaltaya Sn-polymetallic district in the Cordillera Real of Bolivia. Tourmaline is associated with greisens, breccias, and veins, which occur around the Triassic Chacaltaya peraluminous granitic stock hosted by Silurian metasedimentary rocks. Three main petrographic types of hydrothermal tourmaline have been identified: pre-ore greisen-related (Tur-1), syn-ore breccia-related (Tur-2), and syn-ore vein-related (Tur-3). The three types of tourmaline belong to the alkali group and have Fe-rich compositions mostly close to the schorl end member. Overlapping Fe/(Fe + Mg) ratios suggest broadly similar compositions of the hydrothermal fluids during the deposition of tourmaline. The most notable differences in minor and trace element contents include relative enrichment in Zn and Li in Tur-1 and relative enrichment in Ca, Sc, V, Cr, Sr, Sn, Y, Cs, Be, and Zr in Tur-3, with Tur-2 showing intermediate compositions between those of Tur-1 and Tur-3. The progressive enrichment in Sn from Tur-1 (avg = 14 ppm) through Tur-2 (avg = 311 ppm) and Tur-3 (avg = 476 ppm) indicates an increase of Sn concentrations in the hydrothermal system coinciding with cassiterite deposition in breccias and veins. The transition from high Li and Zn contents in Tur-1 to elevated Ca, Sr, V, and Cr contents in Tur-3 is interpreted as reflecting interaction between a hydrothermal fluid of magmatic origin and the metasedimentary country rocks. Strong and relatively steady positive Eu anomalies in all tourmaline types suggest dominantly reduced hydrothermal conditions. In situ δ18O and δ11B analyses of greisen-related Tur-1 reveal crystallization in isotopic equilibrium with magmatic water derived from a peraluminous S-type granite. In contrast, higher δ18O values of breccia-related Tur-2 and vein-related Tur-3 indicate crystallization in isotopic equilibrium with a fluid of metamorphic origin or a magmatic fluid that variably interacted with the metasedimentary host rocks. Geochemical modeling reproduces interactions between a fluid of magmatic origin and the host metasedimentary rocks at moderate water/rock ratios between 0.1 and 0.5. We conclude that cassiterite mineralization in the Chacaltaya district was formed primarily through interaction between B-Sn–rich magmatic fluids and the metasedimentary country rocks. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Germanium- and Gallium-Rich Sphalerite in Mississippi Valley–Type Deposits: The San Vicente District and the Shalipayco Deposit, Peru(Springer Science and Business Media Deutschland GmbH, 2023-06-01)Sphalerite in Mississippi Valley–type (MVT) deposits hosts significant resources of both germanium and gallium. Here, we provide a survey on the distribution of Ge, Ga, and other minor and trace elements in sphalerite from MVT deposits in the Eastern Cordillera and sub-Andean regions of Peru, including the San Vicente deposit and the nearby Chilpes and Huacrash prospects, and the Shalipayco deposit. We present also a micro- and nano-scale textural characterization of Ge-rich sphalerite. In situ laser ablation-inductively coupled plasma-mass spectrometry analyses yielded Ge contents (inter-quartile range [IQR] = 164–36 ppm for the San Vicente district and IQR = 425–101 ppm for the Shalipayco deposit) that overlap with the range reported for sphalerite from other MVT deposits elsewhere. The highest Ge contents (IQR = 1207–375 ppm, up to 1861 ppm) were found in Fe-poor orange sphalerite deposited during a volumetrically minor second mineralization step in the San Vicente deposit located mainly in steep veins that crosscut the main first-step mineralization dominated by darker sphalerite. Reddish-brown sphalerite from Chilpes (IQR = 445–22 ppm, up to 1745 ppm) and brownish orange sphalerite from Huacrash (IQR = 650–34 ppm, up to 855 ppm) also yielded remarkably high Ge values. In Shalipayco, the highest Ge contents were analyzed in late Fe-poor yellow sphalerite (IQR = 375–267 ppm, up to 1119 ppm). The highest Ga contents were determined in reddish-brown sphalerite from the Chilpes prospect (IQR = 1156–0.26 ppm, up to 3943 ppm), although Ga contents are, in general, much lower than those of Ge in most analyzed sphalerite (IQR = 27–0.22 ppm in the San Vicente district and IQR = 2.8–0.081 ppm in the Shalipayco deposit). These figures place some of the analyzed sphalerite types among the Ge- and Ga-richest samples ever reported. Linkage of textural and compositional data points to light-colored, chiefly orange and yellow sphalerite generations crystallizing at lower temperatures and relatively late in the paragenetic sequences as those with the highest Ge contents. In contrast, the paragenetic control on Ga enrichment is unclear. Focused ion beam and high-resolution transmission electron microscopy (FIB-HRTEM) investigation combined with trace element content correlations reveal selective partitioning of Ge and Ga into sphalerite as structurally bound elements and their incorporation via substitution mostly coupled to monovalent cations triggering polytypism.