Hydrothermal alteration in the Gazestan magnetite-apatite deposit and comparison with other Kiruna-type iron deposits in the Bafq district, Central Iran

The Gazestan magnetite-apatite deposit is hosted within an upper Proterozoic-lower Cambrian volcanic-sedimentary sequence, known as Rizu series, in the Bafq district, Central Iran. The Gazestan deposit occurred in intensely altered felsic-intermediate subvolcanic and volcanic host rocks. Field observations, drill core logging, petrographic studies, as well as geochemical and XRD data are indicative of differences in alterations assemblages and temporal/spatial distribution of the alteration products, compared to other iron oxide-apatite deposits in the Bafq district. Unlike many other Bafq district iron deposits, sodic alteration is only locally developed. Similarly, Ca or actinolitic alteration is poorly developed in Gazestan. Chloritic and sericitic alterations are most closely associated with ore formation in Gazestan. Chlorite commonly associated with magnetite, quartz and calcite in the altered host rocks. The chemical composition of chlorite falls in pycnochlorite and clinochlore fields. Calculated temperature for chlorite formation varies between 324-236 ºC. Sericite occurred both as a proximal alteration in ore zones, and as a distal alteration product in the volcanic and subvolcanic host rocks. Calcic-iron alteration is poorly developed in Gazestan. Potassic alteration marked by development of K- as well as biotite is only locally developed in Gazestan. Boron metasomatism occurs as quartz-tourmaline bands and disseminated grains in altered rocks. The scarcity and local nature of sodic (albitic) and calcic-iron (actinolitic) alterations, and the widespread and proximal chlorite alteration suggest that, compared to most other iron deposits of the Bafq district, Gazestan formed at relatively lower temperatures and possibly shallower depths