Occurrence and Composition of Columbite-(Fe) In the Reduced A-Type Desemborque Pluton, Graciosa Province (S-SE Brazil)

Astrid Siachoque,Rodolfo Garcia,Silvio R.F Vlach

doi:10.3390/min10050411

Abstract

Columbite-(Fe) is a post-magmatic accessory mineral occurring within syenogranites and greisens from the Desemborque Pluton. The petrographic (SEM) and geochemical (EPMA and LA-ICPMS) examination of this mineral shows two distinct textural types within both the rocks, named columbite-1 and columbite-2. The columbite-1 type is characterized by zoned crystals with two stages of crystallization: i) An early Nb-rich cores with low Ta/(Ta + Nb) and Mn/(Mn + Fe) ratios (0.02–0.08 and 0.17 to 0.21 apfu, respectively), and ii) a later Ta-rich rims with higher Ta/(Ta + Nb) ratios (0.11–0.26) and similar Mn/(Mn + Fe) ratios (from 0.14 to 0.22) relative to the former cores. On the other hand, the columbite-2 type is defined by irregular crystals with patchy textures and very low Ta/(Ta + Nb) ratios (0.008–0.038) and moderate Mn/(Mn + Fe) ratios between 0.20 and 0.38. Trace element compositions of all columbite-(Fe) crystals are relatively enriched in HREEs and HFSEs; however, the columbite-2 presents higher abundances of REEs, Y, Th, U, Pb, Sc, and Sn relative to the columbite-1. This study highlights a unique hydrothermal origin for both the columbite types, but the textural relations of the columbite-2 crystals indicated that its formation is related to fluid-induced alterations of post-magmatic fluorite and/or cassiterite crystals at the final stage of the post-magmatic evolution.

Highlights

Columbite is the main Nb-oxide in the orthorhombic columbite-tantalite group minerals (CGM)with the general formula AB2 O6, where Fe2+ or Mn2+ occupies the A-site, and Nb5+ or Ta5+ theB-site [1,2]
CGM are usually found in rare-metal granites and pegmatites [9,10,11,12,13,14,15] but these minerals are common in F- and Li-rich peraluminous granitic systems and associated greisens [16,17,18,19,20]
In the case of columbite-1 group, zoning patterns within crystals indicate two hydrothermal substages of crystallization during the post-magmatic evolution: the first stage is represented by the formation of Nb-rich cores crystals, while the second stage is characterized by the subsequent crystallization of the Ta-rich rims around the former cores

Summary

Introduction

Columbite is the main Nb-oxide in the orthorhombic columbite-tantalite group minerals (CGM)with the general formula AB2 O6 , where Fe2+ or Mn2+ occupies the A-site, and Nb5+ or Ta5+ theB-site [1,2]. End-members of this group are designated as columbite-(Fe), columbite-(Mn), tantalite-(Fe), and tantalite-(Mn) [3]. These minerals usually present a wide range of composition and structural variations including minor substitutions of Ti, Sn, W, Zr, Hf, Sc, and REEs (among others), which are related to cationic order-disorder phenomena between the A- and B-sites [4,5,6,7,8]. CGM are usually found in rare-metal granites and pegmatites [9,10,11,12,13,14,15] but these minerals are common in F- and Li-rich peraluminous granitic systems and associated greisens [16,17,18,19,20]. In Brazil, CGM are reported in pegmatites from the Borborema Province [21,22,23], the Lourenço-Amapá Province, and the São Joao do

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