Application of Various Geochemical Proximity Indicators to the Tin Favorability of South-Sardinian Granites (original) (raw)
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This contribution examines two types of Mn deposits, which were exploited in the past in Sardinia. Both types are Tertiary in age, but not strictly coeval and they are both linked to a continental arc setting characterized by the presence of calcalkaline volcanics and shallow marine sediments. Mn-deposits formed in terrestrial or shallow marine environments are unusual compared to the most common deep oceanic ones, and therefore poorly studied. TheMn-ores of Sardinia occur either as stratabound deposits with stratiformgeometry associated to ruditeswith littoral facies or as vein-type deposits hosted within volcanic rocks. In the stratiformdeposits the Mn ores form the matrix or cement of a transgressive conglomeratic bed overlain by shallow water limestones and underlain by calc-alkaline volcanics. The vein-type deposits consist of Mnoxides hosted by latite lava domes, comenditic lava flows and rhyolitic ignimbrites. Although these geological settings are markedly different, the main Mn-bearing phases are the same, namely pyrolusite and hollandite. The deposits differ in the presence of accessory minerals, such as todorokite, manganite, coronadite, cryptomelane and gangue minerals. Rare earth element geochemistry is themost effective tool in discriminating the two types ofMn deposits. These analyses indicate that the vein-type deposits were formed from geofluid mixtures linked to the Oligocene– Miocene calcalkaline volcanic cycle. Mineralogical and geochemical data indicate that the crystallization of the Mn oxides in the stratabound deposits was diagenetic, related to sudden changes in Eh and pH in coastal, meteoric waters that came in touch with the marine coastal water after having flushed the calcalkaline volcanics.
Geochemical characteristics of potassic volcanics from Mts. Ernici (Southern Latium, Italy
Contributions To Mineralogy and Petrology, 1981
Major elements, trace elements and 87Sr/86Sr data are reported for the Quaternary potassic alkaline rocks from the Mts. Ernici volcanic area (Southern Latium — Italy). These rocks are represented by primitive types which display high Mgv, low D.I., variable degrees of silica undersaturation and different K2O contents which allowed the distinction of a potassium series (KS) and a high potassium series (HKS). All the analyzed samples have high LIL element contents and high 87Sr/86Sr which ranges between 0.707–0.711. They also have fractionated REE patterns. The KS rocks have lower LIL element concentrations and 87Sr/86Sr ratios than the HKS rocks with a large compositional gap between the two series. Minor but still significant isotopic and trace element variations are also observed within both KS and HKS. The genesis cannot be completly explained either by crystal liquid fractionation, mixing or assimilation processes or by different degrees of equilibrium partial melting from a homogeneous source, thus indicating that both the KS and HKS consist of several geochemically and isotopically distinct magma types. The data suggest that the KS and HKS magmas originated by low degrees of melting of a garnet peridotite mantle heterogeneously enriched in LIL elements and radiogenic strontium, possibly accompanied by disquilibrium melting of some accessory phases. The occurrence of a geochemical anomaly within the mantle is believed to be due to fluid metasomatism probably generated by dehydration of a lithospheric slab subducted during the Late Tertiary development of the Apennine Chain.
Geologica Carpathica, 2018
The S-type accessory mineral assemblage of zircon, monazite-(Ce), fluorapatite and tourmaline in the cupolas of Permian granites of the Gemeric Unit underwent compositional changes and increased variability and volume due to intensive volatile flux. The extended S-type accessory mineral assemblage in the apical parts of the granite resulted in the formation of rare-metal granites from in-situ differentiation and includes abundant tourmaline, zircon, fluorapatite, monazite-(Ce), Nb-Ta-W minerals (Nb-Ta rutile, ferrocolumbite, manganocolumbite, ixiolite, Nb-Ta ferberite, hübnerite), cassiterite, topaz, molybdenite, arsenopyrite and aluminophosphates. The rare-metal granites from cupolas in the western segment of the Gemeric Unit represent the topaz-zinnwaldite granites, albitites and greisens. Zircon in these evolved rare-metal Li-F granite cupolas shows a larger xenotime-(Y) component and heterogeneous morphology compared to zircons from deeper porphyritic biotite granites. The zircon Zr/Hf wt ratio in deeper rooted porphyritic granite varies from 29 to 45, where in the differentiated upper granites an increase in Hf content results in a Zr/Hf wt ratio of 5. The cheralite component in monazite from porphyritic granites usually does not exceed 12 mol. %, however, highly evolved upper rare-metal granites have monazites with 14 to 20 mol. % and sometimes > 40 mol. % of cheralite. In granite cupolas , pure secondary fluorapatite is generated by exsolution of P from P-rich alkali feldspar and high P and F contents may stabilize aluminophosphates. The biotite granites contain scattered schorlitic tourmaline, while textural late-magmatic tourmaline is more alkali deficient with lower Ca content. The differentiated granites contain also nodular and dendritic tourmaline aggregations. The product of crystallization of volatile-enriched granite cupolas are not only variable in their accessory mineral assemblage that captures high field strength elements, but also in numerous veins in country rocks that often contain cassiterite and tourmaline. Volatile flux is documented by the tetrad effect via patterns of chondrite normalized REEs (T1,3 value 1.46). In situ differentiation and tectonic activity caused multiple intrusive events of fluid-rich magmas rich in incompatible elements, resulting in the formation of rare-metal phases in granite roofs. The emplacement of volatile-enriched magmas into upper crustal conditions was followed by deeper rooted porphyritic magma portion undergoing second boiling and re-melting to form porphyritic granite or granite-porphyry during its ascent.