The behavior of tin in granitoid magmas

Abstract

The spatial and temporal relationship between tin mineralization and granitoid rocks is well documented. A great deal of work, including mineralogic, geochemical, fluid inclusion, and stable and radiogenic isotope studies, has been undertaken on numerous cassiterite-bearing deposits in order to resolve the problem of their genesis. Despite this exhaustive research, the association between tin mineralization and felsic magmatism remains poorly understood. Experimental investigations into the behavior of tin in melts have been undertaken to evaluate the physical and chemical constraints on the mobilization of tin from magmas.The concentrations of tin in equilibrium with SnO 2 -saturated, synthetic, peraluminous granitoid melts have been examined as a function of temperature (700 degrees -800 degrees C), pressure (2.0-3.0 kbars), oxygen fugacity (from 0.7 log units more reduced to 1.5 log units more oxidized than the quartz-fayalite-magnetite buffer) and granitoid composition (haplogranite, K feldspar-quartz, and albite-quartz). Melts were found to contain between 400 and 2,500 ppm tin. The solvent capacity of the granitoid melts is increased by decreasing oxygen fugacity and by increasing the alkali/alkali + aluminum ratio (ALK/AL), i.e., K 2 O + Na 2 O/K 2 O + Na 2 O + Al 2 O 3 (Naski and Hess, 1985) and the Na/K ratio of a melt. Changes in temperature or pressure do not exert a significant influence over the concentration of tin in melts. Nuclear gamma resonance analysis of haplogranite glasses indicates that both Sn (super +2) (sub (melt)) and Sn (super +4) (sub (melt)) are stable over the range of oxygen fugacities typically attending the crystallization of granitoids. The coordination and likely speciation of Sn (super +2) and Sn (super +4) in melts is discussed. The basicity of a melt (activity of free oxygens, a (sub O (super -2) ) ), which is often strongly correlated with the ALK/AL of a melt (cf. Hess, 1980; Mysen, 1986), is likely to be very important in influencing the acidity of a coexisting fluid phase. Peraluminous melts (low ALK/AL) will generally display low melt basicities and therefore will buffer the acidity of a magmatic aqueous phase at far higher values than a peralkaline melt.These results indicate that granitoid melts derived from common crustal sources will be markedly SnO 2 undersaturated. Consequently, alternative processes must be sought to explain the elevated tin concentrations in many plutons. The complex distribution of tin in any batholith will be largely controlled by the redox conditions accompanying the crystallization of assemblages capable of accommodating tin and the physical processes which can influence their distribution.