Using the chemical analysis of magnetite to constrain various stages in the formation and genesis of the Kiruna-type chadormalu magnetite-apatite deposit, Bafq district, Central Iran

Abstract

Textural and compositional data are presented for different types of magnetite in the Chadormalu iron deposit to discern the genesis of various styles of mineralization. Samples were chosen according to their paragenetic relations to apatite and their host setting: magnetite-apatite veins in the altered host rocks, disseminated magnetite-apatite assemblages in the marginal parts of the main ore body, and massive magnetite associated with irregular apatite veinlets from internal part of the main ore body. Scanning electron microscopy - back scatter electron (SEM-BSE) images reveal that there are three main generations of magnetite in each of the different magnetite-apatite assemblages. Primary magnetite (Mag1) features abundant porosity and a dark appearance. A second generation of magnetite (Mag2) replacing Mag1 shows a lighter appearance with both sharp and gradational contacts with the primary magnetite crystals. The two magnetite types are related to dissolution-precipitation processes due to changing physico-chemical parameters of the ore fluids. A third type of magnetite (Mag3) with a recrystallized appearance and foam-like triple junctions was mostly observed in magnetite-apatite veins in the main ore body and in veins hosted by altered rocks. Electron probe microanalyses (EPMA) were utilized to discriminate the various magnetite generations in the different magnetite-apatite assemblages. Applying published elemental discrimination diagrams shows that most primary magnetites fall into the hydrothermal- and Kiruna-type fields. Primary magnetite contains lower FeO (88.77–93.65 wt.%; average 91.5 wt.%), and higher SiO2 (0.21–2.26 wt.%; ave. 0.32 wt.%), Al2O3 (0.001–0.45 wt.%; ave. 0.053 wt.%), and CaO (0.002–0.48 wt.%; ave. 0.078 wt.%) contents, which might be related to magmatically derived fluids. Secondary magnetites have higher FeO (89.23–93.49 wt.%; ave. 92.11 wt.%), lower SiO2 (0.037–0.189 wt.%; ave. 0.072 wt.%), Al2O3 (0.004–0.072 wt.%; ave. 0.019 wt.%), and CaO (<0.034 wt.%; ave. 0.013 wt.%) possibly showing a lower contribution of magmatic fluids in the formation of Mag2. The magnetite Mag3 contains the highest FeO (91.25–93.8 wt.%; average 92.69 wt.%), low to moderate SiO2 (0.008–1.44 wt.%; ave. 0.13 wt.%), Al2O3 (<0.732 wt.%; ave. 0.059 wt.%), and CaO (<0.503 wt.%; ave. 0.072 wt.%), and appears to have formed by recrystallization of the previous two generations. The different major, minor, and trace element compositions of various magnetite generations might be due to an ore-forming fluid that was initially magmatic-hydrothermal and evolved to moderately brine-dominated meteoric fluids. The involvement of a basinal brine is supported by the occurrence of a late phase 34S-enriched pyrite in the Chadormalu deposit.