The Ash Mountain Sn-bearing skarn, located in the Tuya Range of the Cassiar Mountains in northern British Columbia (59°17′50′′N, 130°31′04′′W), was investigated in order to provide the first modern, comprehensive characterization of its mineralogy and zoning and to identify the main skarn-forming processes. The Ash Mountain skarn is unusual with respect to other Sn-bearing skarns because the main Sn-bearing minerals are malayaite and andradite (with 2.33 wt.% SnO2), and no cassiterite has been found to date. The associated intrusive body – the Parallel Creek granite – is a highly fractionated, felsic, A-type, biotite granite that was generated in a within-plate setting. The biotite in the granite is Al-rich annite with elevated Ti (up to 2.77 wt.% TiO2), F (up to 2.04 wt.% F), and Cl (up to 0.33 wt.% Cl); annite in an associated aplite dike contains up to 0.47 wt.% Cl. High F and Cl in annite indicate a high amount of fluxing components and could be used as an indicator of mineralized granites.
The Ash Mountain skarn is an oxidized, calcic tin skarn that was formed through a sequence of metasomatic events that resulted in the crystallization of two distinct skarn zones: a proximal andradite skarn and a distal grossular-diopside-vesuvianite skarn, where the latter formed in the first stage of metasomatic reaction and was later overprinted by an oxidized, Fe,Sn-enriched fluid that formed the andradite skarn. The paragenetic sequence of the skarn was determined to be: (1) Al + Si metasomatism of dolomite-bearing limestone to a grossular-diopside-vesuvianite skarn, (2) overprinting of the grossular-diopside-vesuvianite skarn by a Sn-bearing andradite skarn due to Fe metasomatism, and (3) remobilization of Sn in the andradite skarn by a reducing hydrothermal fluid, less rich in Mg and Fe, to crystallize malayaite. The near endmember composition of the malayaite (maximum of 4.3% titanite component) suggests that this latest stage of skarn formation occurred at low temperatures (ca. <450 °C). The absence of cassiterite can be attributed to prevailing basic conditions during hydrothermal alteration.