In this study, we established an analytical protocol for matrix-independent and precise Si isotopic analysis using an ultraviolet femtosecond laser ablation system (UV fs LA) connected to a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS, Nu Instruments - Nu Plasma II) in dry plasma mode. We applied then this method to investigate the formation history of microcrystalline quartz in iron ores excavated from the Fortuna Mine (Rhenish Massif, Germany).