Systematic trends in the substitution mechanisms of minor elements in cassiterite

A dataset of 4751 EPMA point analyses on cassiterite crystals from 18 localities spanning a range of different granite-related Sn-bearing mineral systems was investigated and compared with thermodynamic considerations from speciation and lattice strain models. The existence of previously inferred charge balanced coupled substitution mechanisms of Fe³⁺, Fe²⁺, Mn²⁺, Nb⁵⁺, Ta⁵⁺ and W⁶⁺ for Sn⁴⁺ are confirmed. The homo­valent substitution of Sn⁴⁺ by Ti⁴⁺ and Zr⁴⁺ is also confirmed, and the first evidence for homovalent substitution of Nb⁴⁺ and W⁴⁺ in natural cassiterite is presented. The preferred substitution mechanisms for Ti, Fe, Mn, Nb, Ta, W and Zr vary systematically as a function of the mineralising environment. Cassiterite precipitated from silicate liquid dominated systems (such as in pegmatites) primarily display a coupled Fe–(Nb,Ta) substitution mechanism with a 1:2 stoichiometry, whereas cassiterite precipitated in systems dominated by autometasomatic processes (such as greisens) more closely follow a 1:1 stoichiometry, as well as increased levels of Ti. Cassiterite precipitated solely from hydrothermal fluids (such as Sn–(W) quartz vein systems) exhibit the highest Ti concentrations, with lower contents of Fe–(Nb,Ta) coupled substitutions, instead displaying a switch to the incorporation of Fe³⁺ via FeO.OH stoichiometry. Hydrothermally precipitated cassiterite from skarn systems display the highest Fe³⁺ contents, and the lowest Nb, Ta and Ti contents. These systematic changes define a continuous trend in a Ti–(Fe + Mn)–(Nb,Ta) ternary diagram, allowing for classification of the primary source of alluvial cassiterite.