Kaolinization — a tool to unravel the formation and unroofing of the Pleystein pegmatite–aplite system (SE Germany)

The Penouta Sn-Ta deposit, in the northwest of Spain, is a greisenized granitic cupola where Ta minerals occur mainly as disseminations in a leucogranite body intruded in Precambrian–Lower Cambrian gneisses and mica-schists. This leucogranite is a medium- to fine-grained inequigranular rock consisting mainly of quartz, albite, K-feldspar and muscovite. Accessory minerals are mainly of spessartine, zircon, cassiterite, Nb-Ta oxides, monazite, xenotime, native bismuth and pyrite. The alteration processes were mainly albitization, muscovitization and kaolinitization.This leucogranite is peraluminous and P-poor, with 0.03–0.07 wt.% P2O5, 900–1500 ppm Rb, 30–65 ppm Cs, 120–533 ppm Li, 80–140 ppm Ta, 51–81 ppm Nb and up to 569 ppm of Sn.Mineralogical characterization of Nb-Ta oxide minerals was determined by X-ray diffraction, scanning electron microscopy, electron microprobe analysis and mineral liberation analysis. Mn-rich members of the columbite-group minerals (CGM) are the most common Ta-bearing phases, but microlite, wodginite, tapiolite and Ta-rich cassiterite occur also. CGM crystals are commonly zoned concentrically, with a Nb-rich core surrounded by a Ta-rich rim, with a sharp boundary between them. Convoluted zoning occurs also. Dissolution textures resulting from the corrosion of columbite and tantalite rims, in particular, are common. The Mn/(Mn + Fe) ratio varies between 0.33 and 0.97 and the Ta/(Ta + Nb) ratio between 0.07 and 0.93. Wodginite and microlite formed as late replacements of CGM and occur associated with tantalite and cassiterite. Subhedral to anhedral cassiterite crystals, usually up to 200 μm across, occur in two generations: the earlier one is Nb,Ta-poor whereas in the later generation, the Ta content can reach >9 wt.% of Ta2O5 and 1.7 wt.% of Nb. The presence of a fluid phase in the apical zone of the granite, probably related to the separation of a fluid/vapour of the melt, could explain the sponge-like textures, the Ta enrichment associated with these textures, the occurrence of Ta-enriched mineral phases (microlite and wodginite) and their common interstitial character.

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“…Similar dissolution textures are evident in other Nb-Ta minerals from pegmatites (e.g. Wise and Brown, 2010; Dill et al ., 2015).…”

Section: Resultsmentioning

confidence: 99%

Textural and mineral-chemistry constraints on columbite-group minerals in the Penouta deposit: evidence from magmatic and fluid-related processes

et al. 2018

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“…Local hydrothermal activity and widespread Mesozoic and Cenozoic subtropical weathering led to intense alteration in central Europe (Dill et al 2015). In places, the NSVC volcanics were kaolinized up to 40 m depth (e.g., Kemmlitz Porphyry, Fig.…”

Section: Geological Settingmentioning

confidence: 99%

Evolution of the Lower Permian Rochlitz volcanic system, Eastern Germany: reconstruction of an intra-continental supereruption

Hübner

Breitkreuz

Repstock³

et al. 2021

Int J Earth Sci (Geol Rundsch)

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Extensional tectonics in the Late Paleozoic Central Europe was accompanied by rift magmatism that triggered voluminous intracontinental caldera-forming eruptions. Among these, the Lower Permian Rochlitz Volcanic System (RVS) in the North Saxon Volcanic Complex (Eastern Germany, Saxony) represents a supereruption (VEI 8, estimated volume of 1056 km3) of monotonous rhyolites followed by monotonous intermediates. Mapping, petrography, whole-rock geochemistry along with mineral chemistry and oxygen isotopes in zircon display its complex eruption history and magma evolution. Crystal-rich (> 35 vol%), rhyolitic Rochlitz-α Ignimbrite with strong to moderate welding compaction erupted in the climactic stage after reheating of the magma by basaltic injections. Due to magma mixing, low-volume trachydacitic-to-rhyolitic Rochlitz-β Ignimbrite succeeded, characterized by high Ti and Zr-values and zircon with mantle δ18O. Randomly oriented, sub-horizontally bedded fiamme, and NW–SE striking subvolcanic bodies and faults suggest pyroclastic fountaining along NW–SE-oriented fissures as the dominant eruption style. Intrusion of the Leisnig and the Grimma Laccoliths caused resurgence of the Rochlitz caldera forming several peripheral subbasins. In the post-climactic stage, these were filled with lava complexes, ignimbrites and alluvial to lacustrine sediments. Significant Nb and Ta anomalies and high Nb/Ta ratios (11.8–17.9) display a high degree of crustal contamination for the melts of the RVS. Based on homogenous petrographic and geochemical composition along with a narrow range of δ18O in zircon Rochlitz-α Ignimbrite were classified as monotonous rhyolites. For the Rochlitz-β Ignimbrites, underplating and mixing with basic melts are indicated by Mg-rich annite–siderophyllite and δ18O < 6.0 in zircon. The wide spectrum of δ18O on zircon suggests an incomplete mixing process during the formation of monotonous intermediates in the RVS.

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“…Geochemistry studies of Feldspar and Muscovite from pegmatite of the Gatumba area show a link to the Nb-Ta-Sn mineralisation. Kaolinisation was found as a tool to alter the primary emplaced pegmatite which released the Nb-Ta, Sn rare metals precipitated into columbite-tantalite and cassiterite [23]. Cassiterite overprinted the columbite-tantalite in granitic pegmatites by precipitation reactions.…”

Section: Implications For Nb-ta-sn Mineralisationmentioning

confidence: 99%

Geochemistry of feldspar and muscovite from pegmatite of the Gatumba area, Karagwe Ankole Belt: implications for Nb–Ta–Sn mineralisation and associated alterations

et al. 2020

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The studied pegmatites suggest a sodium-dominant pegmatitic melt where micas and albites are dominant over potassium minerals. The mapped pegmatites from the Gatumba area are of interest in Karagwe Ankole Belt for the study of rare-element pegmatites. Previous works on the Gatumba pegmatites included geochemistry and geological mapping; and further studies are required for focused exploration and mine planning. For this reason, field investigation, petrographic and mineral chemistry studies on pegmatites were carried out. Thin sections of the rock samples collected were studied under a transmitted light microscope. Electron Microprobe Analysis and Energy Dispersive X-Ray Spectroscopy were used for mineral chemistry and identification; Back Scattered Electron analysis was used for mineral texture. Major minerals identified in the pegmatite bodies are quartz, plagioclase and micas. Albites (An 0.1-0.8 ; Ab 98.7-99.6 and Or 0.3-0.5) are found as the dominant-plagioclase minerals in the pegmatites from the Gatumba area. Albites are precursors to kaolinites, and the albitisation and kaolinisation processes altered the primary emplaced pegmatites during the mineralisation of the Nb-Ta, Sn rare metals, which precipitated into columbite-tantalite and cassiterite. Muscovites are the most common mineralogical indicators of strongly peraluminous composition in plutonic rocks. Crystallisation, muscovitisation and kaolinisation accompanied the precipitation of prompted cassiterite and columbite-tantalite in the pegmatites of the study area.

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Kaolinization — a tool to unravel the formation and unroofing of the Pleystein pegmatite–aplite system (SE Germany)

Cited by 19 publications

References 38 publications

Textural and mineral-chemistry constraints on columbite-group minerals in the Penouta deposit: evidence from magmatic and fluid-related processes

Textural and mineral-chemistry constraints on columbite-group minerals in the Penouta deposit: evidence from magmatic and fluid-related processes

Evolution of the Lower Permian Rochlitz volcanic system, Eastern Germany: reconstruction of an intra-continental supereruption

Geochemistry of feldspar and muscovite from pegmatite of the Gatumba area, Karagwe Ankole Belt: implications for Nb–Ta–Sn mineralisation and associated alterations

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