Formation Conditions and 40Ar/39Ar Age of the Gem-Bearing Boqueirão Granitic Pegmatite, Parelhas, Rio Grande do Norte, Brazil

Strmić Palinkaš Strmić Palinkaš,Šoštarić Šoštarić,Bermanec Bermanec,Scholz Scholz,Palinkaš Palinkaš,Neubauer Neubauer

doi:10.3390/min9040233

Abstract

The Boqueirão granitic pegmatite, alias Alto da Cabeça pegmatite, is situated in Borborema Pegmatitic Province (BPP) in Northeast Brazil. This pegmatitic province hosts globally important reserves of tantalum and beryllium, as well as significant quantities of gemstones, including aquamarine, morganite, and the high-quality turquoise-blue “Paraíba Elbaite”. The studied lithium-cesium-tantalum Boqueirão granitic pegmatite intruded meta-conglomerates of the Equador Formation during the late Cambrian (502.1 ± 5.8 Ma; 40Ar/39Ar plateau age of muscovite). The pegmatite exhibits a typical zonal mineral pattern with four defined zones (Zone I: muscovite, tourmaline, albite, and quartz; Zone II: K-feldspar (microcline), quartz, and albite; Zone III: perthite crystals (blocky feldspar zone); Zone IV: massive quartz). Huge individual beryl, spodumene, tantalite, and cassiterite crystals are common as well. Microscopic examinations revealed that melt inclusions were entrapped simultaneously with fluid inclusions, suggesting the magmatic–hydrothermal transition. The magmatic–hydrothermal transition affected the evolution of the pegmatite, segregating volatile compounds (H2O, CO2, N2) and elements that preferentially partition into a fluid phase from the viscous silicate melt. Fluid inclusion studies on microcline and associated quartz combined with microthermometry and Raman spectroscopy gave an insight into the P-T-X characteristics of entrapped fluids. The presence of spodumene without other LiAl(SiO3)2 polymorphs and constructed fluid inclusion isochores limited the magmatic–hydrothermal transition at the gem-bearing Boqueirão granitic pegmatite to the temperature range between 300 and 415 °C at a pressure from 1.8 to 3 kbar.

Highlights

Pegmatites, plutonic igneous rocks characterized by extremely coarse crystals with a systematically variable size, represent an important source of industrial minerals, hi-tech mineral commodities (e.g., Li, Cs, Be, Nb, Ta, Sn), and gemstones
According to its mineralogical and geochemical characteristics, the Boqueirão granitic pegmatite has been classified as a member of the LCT pegmatite family, broadly widespread over the Borborema Pegmatitic Province (BPP)
The pegmatite shows a zonal structure with significant enrichment on incompatible elements from its outer rim toward the massive quartz core

Summary

Introduction

Pegmatites, plutonic igneous rocks characterized by extremely coarse crystals with a systematically variable size, represent an important source of industrial minerals (feldspars, quartz, spodumene, petalite), hi-tech mineral commodities (e.g., Li, Cs, Be, Nb, Ta, Sn), and gemstones. The pegmatite is hosted by Borborema Pegmatitic Province (BPP), which represents one of the world’s most important sources of tantalum and beryllium, as well as of gemstones, including aquamarine, morganite, and the high-quality turquoise-blue “Paraíba Elbaite” [2,3,4,5]. Mineralogical and geochemical features, characterized by a strong enrichment in numerous incompatible elements, including Li, Rb, Cs, Be, Sn, Ta, Nb (with Ta > Nb), B, P, and F, classify the Boqueirão granitic pegmatite into the lithium-cesium-tantalum (LCT) family [5,6,7,8,9]. According to previously published data, the BPP pegmatites were formed in a relatively narrow range of pressure (2.1 to 4 kbar) but within a wide range of temperatures (390–900 ◦ C) [10,11,12]

Methods

Results

Discussion

Conclusion