The behaviour of so-called immobile elements in hydrothermally altered rocks associated with volcanogenic submarine-exhalative ore deposits

Evidence is available that some elements, notably Zr, TiO₂, Y, Sc, Ce and Nb are largely immobile during the alteration of volcanic rocks owing to metamorphism, hydrothermal events and weathering (e.g. Floyd and Winchester, 1978). However, it is shown, by reference to analyses of rocks from the environment of five volcanogenic massive sulphide bodies, that while Zr, TiO₂ (and Ce?) are mostly immobile even during intense hydrothermal alteration, Y and particularly Sc and Nb may be extremely mobile. When elements are removed by solution in a hydrothermal fluid it seems that reaction rates are such that these elements are almost totally removed from the rock. Therefore, of the so-called immobile trace elements, only Zr and TiO₂ may be used with any reliability to identify the degree of magmatic differentiation in an hydrothermally altered rock. However, if an element has been mobile it is usually readily identified as having moved.