Mineralogy and petrology of iron skarns in western British Columbia, Canada

Abstract

Calcic iron skarns present in a discontinuous belt, extending from Northern California through western British Columbia and into the Alaskan peninsula, were formed in an oceanic island-arc environment and later accreted to continental terrane. These iron skarn deposits are of particular interest because of their large size; minor element suite of Au, Co, Cu, and Zn; igneous protoliths; and common association with primitive intrusions and cogenetic volcanic rocks. A comparative study of six calcic iron skarn deposits in western British Columbia documents several important differences from other major skarn types. Compared to other skarn types, plutons associated with calcic iron skarns have a similar wide range of silica contents but are generally more mafic, and they have similar total alkali contents but much higher Na 2 O/K 2 O ratios (avg = 2.6). The iron content of intrusions is inversely proportional to the iron content of calc-silicate minerals in associated skarn, suggesting a magmatic component of iron in skarn. Fluid inclusion analyses suggest that early, relatively iron-poor calc-silicate minerals formed from hot, relatively dilute fluids (T h = 450 degrees -460 degrees C, 3.3 wt % NaCl), whereas later, iron-rich calc-silicate minerals associated with magnetite formed from cooler, more saline fluids (T h = 370 degrees -460 degrees C, 10-50 wt % NaCl, with local KCl). The correlation between fluid inclusion salinity and skarn composition is consistent with experimental evidence suggesting that the molar ratio of Fe/Al in solution should increase with the cube of the chloride concentration.--Modified journal abstract.