Cassiterite exsolution with ilmenite lamellae in magnetite from the Huashan metaluminous tin granite in southern China

Abstract

Sn 4+ is generally the dominant form of tin in magnetite-series granites as shown by the presence of cas- siterite or its incorporation into Ti-bearing minerals such as biotite and titanite. Little is known about the behavior of tin in magnetite. The Huashan granite is an oxidized tin granite in the Nanling Range, southern China, where it contains magnetite as the dominant Fe oxide mineral. It is included in biotite as an early phase and also as interstitial grains spa- tially associated with ilmenite, cassiterite, Sn-rich titanite (SnO2 up to 5.9 wt.%), fluorite and apatite. This association indicates that tin enrichment occurred during the late stage of magma crystallization. Ilmenite lamellae display a trellis structure consistent with features of the "oxy-exsolution" process of Sn-bearing titanomagnetite precursor. Micro- inclusions of cassiterite (<1 μm in size) are found only within ilmenite lamellae. This suggests that magnetite with cassiterite inclusions is likely an indicator mineral of oxi- dized tin granites. Although rare in nature, Sn-bearing mag- netite from weathered granites where concentrated in stream sediments, may serve as a strategic tracer for tin exploration in granite districts and in placer deposits, in general.