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Mineralogía y geocronología de rutilo-(Nb,Ta) relacionado a casiterita y columbita-tantalita provenientes de rocas Mesoproterozoicas del Cratón Amazónico cerca de Cachicamo, Colombia

Abstract : In order to clarify if rutile-(Nb,Ta) mega-crystals in the Indigenous Cachicamo community (NE-Vichada-department, Colombia) are derived from rocks of the Parguaza Rapakivi Granite suite, samples were extracted from a ferruginous duricrust layer (composed of hematite, goethite, limonite, ± magnetite, ilmenite, zircon, and rock fragments) of a lateritic profile over granitoid basement rocks and analyzed together with soils and concentrates. Density and pXRF measurements of >250 rutiles revealed a strong variation from low-density (~3.4 g/cm3), nearly-pure TiO2-rutiles, to rutile-(Nb,Ta) crystals (up-to 5.9 g/cm3) enriched in Nb, Ta, Fe, Mn, Sn, V and W. Metallography, SEM-EDX, XRF and XRD exhibited up-to 0.2 mm- inclusions of ~17% columbite-(Fe) and 4% cassiterite in a ~78% rutile-(Nb,Ta) matrix, where EPMA values of ~72% TiO2, 11% Nb2O5, 9% Ta2O5, 6% Fe2O3 and 1.5% SnO2 confirm, together with low W and Mn, pegmatitic-pneumatolytic conditions. ICP-MS measurements yielded enough U for encouraging LA-ICP-MS dating in the University of Rennes 1, where, using rutile standards R10 and R19, in ten rutile-(Nb,Ta) crystals, concordant U/Pb-ages of 1512±12 My were determined. These ages confirm the former assumption based on field observation that the Cachicamo rutiles-(Nb,Ta) originally crystalized within the 1500-1550 My-old Parguaza Granite in Venezuela, where secondary deposits contain similar (sub-?) economic cassiterite, rutile-(Nb,Ta), columbite-tantalite and monazite. Additionally, the analysis of different horizons containing zircon, ilmenite and magnetite, showed that rutiles-(Nb,Ta) are restricted to the duricrust horizon. Detrital zircons from the A-horizon (duricrust) revealed a similar age span of 1525 My, which is coeval with the Parguaza Batholith reported in Venezuela, whereas ages of 1415 My of zircons from the C-horizon (regolith) coincide with the local Parguaza Rapakivi Granite ages (1340-1402 My). Hence, this study supports a nonlocal source for the Cachicamo rutile-(Nb,Ta) crystals, which probably had experienced a post-exhumation transport westward over several tens of kilometres through a zone which nowadays is deepened by the Orinoco-river valley.
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Submitted on : Friday, January 8, 2021 - 9:32:50 AM
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José Alejandro Franco, Thomas Cramer, Amed Bonilla, Antonio Jose Castañeda, Marc Poujol, et al.. Mineralogía y geocronología de rutilo-(Nb,Ta) relacionado a casiterita y columbita-tantalita provenientes de rocas Mesoproterozoicas del Cratón Amazónico cerca de Cachicamo, Colombia. Boletin de geologia, 2021, 43 (1), pp.100-126. ⟨10.18273/revbol.v43n1-2021005⟩. ⟨insu-03103339⟩

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