Albite-rich, silica-depleted metasomatic rocks at Emuford, Northeast Queensland; mineralogical, geochemical, and fluid inclusion constraints on hydrothermal evolution and tin mineralization

Abstract

Late Paleozoic fluorine-rich granites and their host metasediments in the Emuford district of the Herberton tin field contain small zones of intense albitization that host cassiterite mineralization. Sodium metasomatism was accompanied by hydrothermal leaching of granite quartz to form vuggy albite rocks (episyenites). Vugs created by quartz dissolution were filled in by hydrothermal minerals, including albite, K feldspar, and/or muscovite and cassiterite; late phases are dominated by quartz and fluorite. Swarms of parallel veinlets crosscut the albitized zones. These veinlets were formed by repeated fracturing in response to local fluid over-pressures. They contain infill mineralogy similar to the vugs and probably formed at least partly contemporaneously with vug infill.Due to the overprinting at various scales of alteration, vug infill, and vein formation stages, chemical changes associated with episyenite formation are only qualitatively constrained. However, the mineralogical and textural features of the alteration zones suggest that most major and trace elements had substantial mobility during their formation. Major chemical changes associated with episyenite formation include leaching of SiO 2 , K 2 O, Rb, and Ba, with introduction of Na 2 O and minor Sr. During vug-and vein-filling stages there was an introduction of SiO 2 , Al 2 O 3 , Na 2 O, K 2 O, CaO, F, and Sn. The erratic behavior of REE is probably related to the growth of fluorite and minor accessory phases (zircon, monazite, xenotime), with preferential enrichment of heavy REE in rocks with the highest fluorine concentrations.Fluid inclusion study indicates that vug- and vein-filling stages of the hydrothermal evolution involved two types of fluids. A high-salinity fluid (30-50 equiv wt % NaCl) of probable magmatic origin and homogenization temperatures up to 428 degrees C was associated with vug filling in the granite-hosted deposits. A lower salinity fluid (5-15 equiv wt % NaCl), containing variable amounts of CO 2 and having homogenization temperatures between 160 degrees and 420 degrees C, was associated with veinlet formation in the granite-hosted deposits and with vein and cavity filling in the metasediment-hosted deposit. This lower salinity fluid may represent a condensed vapor phase derived from boiling of the magmatic fluid or may be a mixture of magmatic and meteoric components.The close association of albitization and quartz dissolution suggests a genetic link between the two processes. It is proposed that exchange of Na for K in the fluorine-rich hydrothermal fluids caused a substantial increase in quartz solubility and led to quartz dissolution in the albitized rocks.