Albitite formation by selective pervasive sodic alteration of tonalite plutons in the Cloncurry district, Queensland

Tonalites along the southern margin of the Mt Angelay igneous complex, Cloncurry district, northwest Queensland, intrude amphibolite facies metasediment and metadolerite and pre-date the intrusion of post-1540 Ma potassic granitoids, These tonalites are overprinted by multiple phases of sodic-rich alteration and preserve a systematic textural variation from tonalite to albitite with increasing intensity of albitic alteration. Albite-magnetite occurs as grain boundary alteration, microscopic intergranular veinlets, or as cross-cutting veins. This alteration assemblage is similar to the early sodic alteration observed at many Fe-(Cu-Au) deposits in the district and provides a favourable host for later fracturing and brecciation. Mass-balance calculations indicate that during alteration Na, U, Th (+/- Zr) were added to altered tonalite, while K, Fe, Si, Ca, CI, Zn, Rb, Co, Cr, Sr and H2O were released. The fluids that produced early albite and magnetite, and mobilised quartz during alteration, were saline (> 23wt% NaCl equiv.), Na-rich and were not in equilibrium with plagioclase or K-bearing alkali feldspar. The origin of the Na-rich fluids is as yet unresolved, although they are interpreted to be produced from a deep-seated, possibly magmatic, source.