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Abstract
Esfordi is a Kiruna-type Fe–P oxide deposit in the Bafq district of Iran. It formed within a predominantly rhyolitic volcanic sequence that formed in a continental margin tectonic regime and is of Cambrian age.
The gently dipping, stratabound ore body is lenticular and displays a well-developed mineralogical zonal pattern. The Fe-oxide rich core contains a central zone of massive magnetite and a more hematitic brecciated rim. The overlying P-rich ore body contains massive and brecciated, apatite-rich variants with accessory hematite and actinolite. A zone of apatite-bearing veins and disseminations envelopes the Fe-oxide and P-rich zones and extends
into overlying volcaniclastics that contain detrital magnetite ± apatite clasts. The main ore zones are surrounded by Ca-rich alteration, dominated by actinolite, extending ~100 m into the more permeable overlying volcaniclastics. Beyond this envelope is widespread development of secondary K-feldspar.
Mesoscopic and microscopic observations reveal a paragenetic sequence containing four generations of apatite. The early stage is a LREE-rich apatite 1 that occurs within the massive and brecciated magnetite core. The second generation is large and brecciated apatite 2, associated with hematite and actinolite. Both apatite 1 and 2 exhibit widespread dissolution
and reprecipitation to form a LREE-poor granular apatite that is generally associated with quartz-carbonate±REE minerals. The final stage involved an overprint of LREE-poor apatite 3-carbonate-quartz-actinolite-chlorite-epidote±bastnaesite±synchesite extending into the host rocks.
Fluid inclusions in apatite 1 have homogenisation temperatures of 375-425oC and
indicate salinities of 14–18 wt. % NaCl. The magnetite displays low δ¹⁸O of -0.1–1.7 ‰, suggesting precipitation from fluids with δ¹⁸O of 7.8–9.6 ‰ at ~400oC, consistent with a magmatic source. Fluid inclusions in apatite 2 homogenise between 195–295oC with indicated salinities of 13–19 wt. % NaCl. The associated hematite displays δ¹⁸O of -0.2–2.3 ‰ which
would be in equilibrium with fluids having a δ¹⁸O of 10.7–13.0 ‰ at ~250oC. Such enriched
isotopic fluids suggest interaction of magmatic fluids with cooler saline fluids that were
probably derived from the underlying carbonate-rich sequences. Fluid inclusions in apatite 3
and quartz homogenise at 145–155oC and, together with a quartz δ¹⁸O of 16.0–17.1 ‰,
suggests precipitation from a fluid with δ¹⁸O of -0.7–2.1 ‰ that is likely to have resulted from
the introduction of a cooler, less saline and isotopically depleted fluid (such as sea water).
The results of this study clearly indicate a significant role for fluids in the evolution of
the Esfordi deposit but do not preclude a role for immiscible Fe-oxide��������P-rich melts in the
initial stages of the mineralising process.