Phosphate sequence study of Boqueirâo pegmatite (Rio Grande Do Norte State, Brazil)

Abstract

The Boqueirão pegmatite is located in the Borborema Pegmatite Province in the state of Rio Grande do Norte (NE Brazil) hosted by Neoproterozoic taconglomerates of Equador Formation. The pegma tite occurs as a concentric body displaying characteristic units according their mineralogy. The border and wall zones are composed by quartz, microcline and muscovite often displaying skeletal and graphic textures with accessory tourmaline and garnet. The intermediate zone is divided in two distinct subzones and shows a complex mineralogy exhibiting a varied phosphate association, beryl, columbite -tantalite and zircon. The innermost zone is mainly constituted by a quartz core. In addition, typical replacement bodies of albite are recognized at the eastern part of the body. Lithiophilite triphylite represents the most abundant phosphate phase exhibiting exsolutions of sarcopside and replaced by sicklerite due to Lileaching alteration, best known as Quensel Mason sequence .The scarcity of heterosite -purpurite series is remarkable. Montebrasite and triplite are also primary phosphates observed. Subsequently ,the presence of alluaudite and varulite occurrences suggests that Na-metasomatism affected the pegmatite. The whole ensemble is crosscut by an extensive alteration that resulted in secondary phosphate minerals . Electron microprobe data of triphylite-lithiophilite shows Mn/(Mn+Fe)ratios between 0.21-0.24 but also up to 0.72 which seems to indicates that the earlier pegmatite magma could be slightly fractionated; consequently,ferrisicklerite shows Mn/(Mn+Fe) values between 0.69 -0.77 in agreement with high evolution degree and suggesting that pseudomorphism may be produced by high rock/fluid ratios. Alluaudite and varulite also record Mn and Fe contents up to 0.71 similar to precursor phosphates. Furthermor eight different secondary phosphates have been identified as replacement of primary phases either as pseudomorphs or crosscutting them. They form euhedral crystals of fine grain size, spherulites or extensive masses displaying vivid colours which are ea sily identifiable. Manrich secondary phases are hureaulite, eosphorite, whiteite, jahnsite and serrabrancaite while lipscombite, phosphosiderite and mitridatite correspond to Ferich secondary association. Supergenic late origin and entrance of Ca-K,Mgrich fluids from host rock may be invoked in order to explain their formation. They do not show any relationship with processes of pegmatite crystallization. Hf content of zircon from the intermediate zone has been analysed by electron microprobe as a pow erful indicator of pegmatite evolution degree together with Mn content of phosphates. Hf content obtained is low up to 5% wt. of HfO which which points out to low fractionated pegmatite magma. Besides, the presence of ferromagnesian silicates forming the pegmat ite units such as Ferich tourmaline, garnet and green beryl is in agreement of that hypothesis. Therefore, Boqueirão corresponds to rare element pegmatite of beryl columbite phosphate subtype.