Metasomatic Formation and Replacement of Apatite (Bamble Sector, South Norway)

Abstract

The Odegarden verk apatite deposit is hosted in scapolitised metagabbro. Chlorapatite (Cl < 6.8 wt%) occurs dispersed in the metagabbro and as coarse crystals in apatite–phlogopite veins, crystallised and deposited during scapolitisation. Secondary pseudomorphic replacement related to albitisation transformed chlorapatite to porous hydroxy-fluor-apatite. Mass balance calculations show that scapolitisation of the Odegarden gabbro caused extensive depletion in Fe2O3, progressive depletion in CaO and Al2O3 and increase in MgO and Na2O. The apatite–phlogopite veins are markedly enriched in P2O5, K2O and MgO. Albitisation shows extensive depletion in Fe2O3, depletion in TiO2, MgO and CaO, while an increase in Na2O, Al2O3 and SiO2. Bamble sector is characteristically affected by metasomatism along a 100-km long and 25-km wide area, resulting in chemical and mineralogical transformation of continental crust. Metasomatic formation and replacement of apatite at Odegarden verk is in accordance with the regional distribution of Ap-deposits occurring related to scapolitised metagabbros in the area.