Complex metamorphic and metasomatic history recorded by REE-phosphates in apatite-iron oxide ores from Svalbard

Abstract

A set of small iron oxide-apatite (IOA) ore bodies have been discovered within polydeformed and polymetamorphosed metasedimentary rocks on Prins Karls Forland, Svalbard. A complex tectonothermal history resulted in the development of various ore structure, varying from brecciated to mylonitised. Generally, the IOA ores are divided into two major geochemical subtypes: (1) fluorapatite-bearing with predominant low-Th monazite, and (2) F-Cl apatite-bearing with predominant high-Th monazite. Initial alteration of the ores resulted in liberation of REE and P from the apatite and redeposition as small (<30 mm), rounded monazite and xenotime crystals. REE and P in solution were likely transported during deformation that probably enhanced the transportation process. In fact, both the iron oxides and phosphates in the intensively deformed ores show features characteristic of fluid-assisted dissolution-reprecipitation creep. Subsequent stages of alteration caused either Th-enrichment of the monazite or even full replacement of low-Th monazite by a high-Th variety. In some of the ore bodies the original fluorapatite incorporated Cl, Mn, and Sr likely due to interaction with a Cl-rich fluid from the surrounding gabbroic and metasedimentary host rocks. The huge variability in the textures and the mineral assemblages from the ore bodies most likely reflect interaction with compositionally variable fluids that were liberated during the protracted tectonothermal evolution of the entire metamorphic complex. Hence, it is concluded that the IOA ores formed as a product of Fe, P, Ca, and REE fractionation from hypersaline fluids associated with the surrounding gabbros and metasedimentary rocks indicating that the ores were subjected to fluid activity during at least one metamorphic event.