FLUORINE AND CHLORINE IN BIOTITE FROM THE SARNWOSAR GRANITIC ROCKS, NORTHEASTERN IRAN

The late Eocene-early Oligocene Sarnowsar granitic rocks and many dacitic to rhyolitic dykes intruded in metamorphosed and metasomatized shale, sandstone, calcite and dolomite marbles and volcaniclastic and pyroclastic rocks. As a result several million tons of iron ores formed within carbonate rocks in the Sangan area. The Sarnowsar granitic rocks are mainly granite to granodiorite, metaluminous, I type and high calc-alkaline. Fluorine contents in the biotite range from 0.37 to 4.4 wt. % and Cl contents range from 0.1 to 0.30 wt. %. Fluorine contents in the biotite are negatively correlated with XTi, while Cl contents positively correlated with XMg and XSi. Values of the calculated log (ƒH2O/ƒHF) and log (ƒH2O/ƒHCl) of fluid in equilibrium with the chemical composition of biotite range from 3.38 to 4.43 and 0.57 to 1.2, respectively. The contour lines representing log (ƒH2O/ƒHF) and log (ƒH2O/ƒHCl) values are different with the slope of the trend of biotite composition suggesting that, in addition to chemical structure of biotite, the fluid composition also plays some role in the incorporation of F, and especially Cl in biotite. The F intercept values for biotite in the Sarnowsar granitic rocks are similar to those of igneous rocks and porphyry Cu ore deposits. The Cl intercept values of biotite in the Sarnowsar granitic rocks are similar to those of hydrothermal and ore forming systems. The data suggest that chlorine intercept values for biotite from the Sangan deposit tend to be more Cl rich than comparable values from biotites in common igneous rocks. F-rich biotites and F-poor biotites from the Sangan deposit show similar and narrow ranges of F/Cl intercept values corresponding to Cl-rich and oreforming systems such as porphyry copper deposits. Therefore, the chemical composition of biotite from the Sarnowsar granitic rocks interacted with hydrothermal solutions.