2013

Extreme fractionation processes responsible for forming HREE-Y-Nb deposits: a review of the petrogenesis and thermal evolution responsible for these deposits throughout North America

Lentz D.R.

University of New Brunswick, Department of Earth Sciences

There as several silica-undersaturated alkalic and alumina-undersaturated alkali complexes in North America that host enormous resources of heavy rare earth elements (HREE), yttrium, niobium, Ta, U, Th, Zn, Sn, and Be. The granitic and syenitic complexes that have melt structures, which enhance the solution capacity of these high technology metals, have many features indicative of extreme fractionation (high incompatible metal abundance, very low K/Rb) reflecting combinations of very selective partial melting of metasomatized mantle through to hyper Rayleigh crystal fractionation and more rarely fluid fractionation. In the cases of selected selective mantle melting and (or) prolonged crystal fractionation, thermal considerations are paramount. In transtensional to extensional settings, magma mixing is not promoted and the residence time of bimodal magmas in the crust is much lower than other geodynamic settings, so heat budget is critical, i.e., the coexistence of mafic magmas with evolving felsic magmas enables protracted selective melting and then also crystallizing processes. Plumes are just one example, many environments that have various types of heat advection create favourable environments for very slow magma cooling. Large Igneous Provinces (LIPS) really illustrate how thermally anomalous regions enhances efficient, crystal fractionation in mafic and felsic systems.  In more viscous felsic systems, in contrast to large mafic ones, boundary layer differentiation via solidification fronts possibly with filter pressing (locally upward into cupolas) is key to extreme fractionation, if high external temperature enhances a very slow rate of cooling. In many mineralized peralkalic systems in worldwide, including North American, there is evidence for these specific processes for the primary extreme concentration of these high-tech metals, i.e., the Nb (meta)tuff (Western Australia), subvolcanic Dubbo trachyte (New South Wales), and numerous other recent volcanic rocks. These are enriched at the magmatic stage, and only modified by fluid-mediated, subsolidus metasomatic reactions, although complex immiscible melts may locally be of importance in mineralizing processes.