Geochemistry and U-Pb geochronology of columbite of the Cachoeirinha Deposit, Rondônia Tin Province, Brazil

The Cachoeirinha Sn ± (Nb-Ta) deposit is an important producer of the Rondônia Tin Province. It is inserted in the regional unit named Rondônia Intrusive Suite (995–956 Ma) and hosts granular accumulations of columbite group minerals (CGM) embedded in thick Cenozoic colluvial-alluvial covers. Cachoeirinha CGM have subrounded to angular grains, with irregular to regular internal zoning, micro-inclusion, and ex solution features. They have columbite-(Fe) as the dominant mineral phase, with variation in the Ta/(Ta+Nb) ratio and with Sn, Ti, W, U, Ba, Cr, REE, and V as chemical impurities. It has high total REE content (LREEN depleted, HREE_N enriched, Ce and Eu anomalies), δ¹⁸O = 2.78‰, and with two U-Pb age groups: 1052 ± 6 and 909 ± 7 Ma. This information indicates that physicochemical oscillations favored the ionic substitutions and resorption mechanisms between Nb ↔ Ta, as well as coupled substitutions with the entry of geochemical impurities, according to the equation: 3(Sn, Ti, U, Si)⁴⁺ + (W, Cr)⁶⁺ ↔ 2(V, Nb, Ta)⁵⁺ + 2(Al, REE)³⁺ + (Fe, Mn, Ba)²⁺, at the temperature range between 400 deg and 500 deg C. Finally, the Cachoeirinha Sn ± (Nb-Ta) alluvial deposit host ore is related to at least two Neo-Mesoproterozoic magmatic pulses. (author)