Geochemical characteristics of halogen-bearing hastingsite, scapolite and phlogopite from the Sangan iron skarn deposits, northeastern Iran.

Abstract

Mesozoic shale, siltstone, sandstone and carbonate rocks and Eocene volcanic, volcaniclastic and pyroclastic rocks are intruded by late Eocene granitic batholith and related acidic dikes. As a result of hydrothermal activity associated with the granitoid, two distinct skarn zones of calcic skarn zone and magnesian skarn zone were formed in the carbonate rocks. Cl-bearing amphiboles of ferroactinolite to potassium rich ferrohastingsite composition are restricted to the calcic skarn zone and associating with salite, grandite, magnetite, calcite and sometimes with scapolite. The Cl content of amphiboles shows positive relationships with Fe2+/(Fe2++Mg) and K/(K+Na) and a negative relation with Si/(Si+AlIV). In contrast, F bearing phiogopite is a major mineral in the magnesian skarn. F content exhibits positive relationship with wt% of SiO2 and MgO. The estimated log(fH2O/fHF) equilibrating with the phlogopite ranges from 4.6 to 5.5 at 400°C. Cl rich scapolite is widespread in both calcic and magnesian skarn zones. The scapolite has a compositional range of 23 to 40 mole% meionite in marialite-meionite solid solution series. XC1 of the scapolite ranges from 0.44 to 0.98 (mostly more than 0.75). Scapolite of XC1=0.98 from the hornfels is the most Cl-rich scapolite ever reported. Cl content of the scapolite positively correlate with Na/(Na+Ca+K). Calculated XNaCl values for fluid in equilibrium with scapolite from the Sangan deposits reach up to more than 55 wt% equivalent NaCl. The Cl-rich hastingsite, Cl-rich scapolite, F-rich phiogopite, and fluid inclusions with high salinity (up to 70 wt% eq. NaCl) in granitic rocks and in skarn zones suggest that hydrothermal fluids were derived from the granitoid magma. The ore elements such as Fe and Cu could be transported by solutions as metal-chloride complexes. Large amounts of cations such as Al, Si, and Ca incorporated in the fluids should be derived from the country rocks and from the breakdown products of early skarn minerals.