Kiruna-Type Iron Oxide-Apatite Ores and "Apatitites" of the Bafq District, Iran, with an Emphasis on the REE Geochemistry of their Apatites

The Bafq metallogenic province in Iran contains world class Kiruna-type apatite-iron oxide ores, apatite-rich magmatic rocks called "apatitites", REE, Th-U, Pb-Zn and recently reported Cu(-Au?) mineralisation. The metallogenesis is related to magmatic events that accompanied a major late Precambrian rifting event within Gondwanaland. The magmatic activity is subvolcanic to volcanic, is characterised by thebimodal association of rhyolites and spilitic basalts and is accompanied by a regional alkali metasomatism.

The iron deposits are commonly hosted by hydrothermally altered and metasomatised rhyolitic rocks, which are either interstratified with volcano-sedimentary sequences, or form large subvolcanic and volcanic masses. The iron ore is dominantly a Ti-V-poor massive magnetite with subordinate hematite, and is commonly accompanied by apatite. Apatite also occurs within the magmatic "apatitites', which are spatially and temporally closely associated with the iron ores.

The behaviour of trace elements in the Bafq apatites from both associations is identical. Both apatites are enriched in REE up to 2 weight% REE oxides, and have a REE distribution pattern marked by a strong LREE/HREE fractionation, pronounced negative Eu and slightly positive Ce anomalies. They are also characterised by depletion in Sr (to 300-400 pptn) and Th (to 100 ppm) and are practically devoid of U and Pb.

The REE geochemistry of the apatites from the Bafq region is similar to other global apatite-magnetite ores, including those of Kiruna in Sweden, but also in part to both nelsonites and to the carbonatite association. However, they differ fundamentally from the sedimentary phosphorites, thus excluding a marine source.

The identical chemistry of apatites from both the iron ores and "apatitites" in the Bafq district reflects the composition of a homogeneous source reservoir and confirms the geological, mineralogical, and petrographic evidence that apatites of both associations are cogenetic. These observations strongly imply that fluids have played a crucial role in the formation of the iron ores and the "apatitites", putting the previously proposed iron-phosphorous liquid magma source into question. The fluid model also explains the large spectrum of modes of emplacement, from submarine hydrothermal to hydrothermal and metasomatic. The source of the mineralising fluids is assumed to be an evolved magmatic fluid rich in volatiles and incompatible elements, that was generated during late Precambrian intra-continental rifting within the Bafq district.