Magnetite-apatite deposits of the Bafq district, Central Iran: apatite geochemistry and monazite geochronology

The Bafq mining district is in the Early Cambrian Kashmar-Kerman volcano-plutonic arc in Central Iran and hosts important 'Kiruna-type' magnetite-apatite deposits. The hydrothermal magnetite-apatite mineralization occurs mostly as massive orebodies, metasomatic replacements, veins and stockworks. Apatite (low-Sr fluorapatite containing small amounts of hydroxyl) has undergone a partial hydrothermal overprint which involved leaching of Na, Cl and REE. The REE were remobilized into monazite (and minor allanite, parisite and xenotime) which nucleated as inclusions within apatite or as individual crystals. The monazites have very small ThO2 contents (usually < 1 wt.%), but they occasionally show an inner core of high-Th monazite, with low-Th overgrowth rims. The chemical Th-U-total Pb dating of the high-Th monazites by electron microprobe analysis yields an isochron age of 515 +/- 21 Ma (initial PbO intercept = 68 ppm), or 529 +/- 21 Ma (forced initial PbO = 0), which is contemporaneous with the emplacement of the volcano-plutonic host rocks of the magnetite-apatite mineralization, as well as with widespread sedimentation of Late Proterozoic to Cambrian evaporitic rocks in Central Iran. The monazite age and the mineralogical and geochemical data suggest that the magnetite-apatite deposits are probably related to large-scale brine circulation induced by felsic magmatism during the Cambrian.