Metamorphic controls on ligand and metal distribution in the continental crust

Abstract

This thesis investigates metamorphic controls on the distribution of metals and metal-transporting ligands in the mid to lower continental crust. The metal content of the mid crust is investigated by evaluating the mobility of elements during chlorite devolatilisation in the upper greenschist facies. This interval of metamorphic fluid production was found to coincide with the pyrite-pyrrhotite transition; a reaction that liberates sulfur into the metamorphic fluid and is thought to be important for scavenging gold to form orogenic Au deposits. The pyrite-pyrrhotite transition field was found to extend to higher temperatures (500 – 550 °C) in a natural system than previously described by thermodynamic modelling. Mass balance calculations suggest that S-bearing metamorphic fluids produced during simultaneous chlorite and pyrite breakdown partially extracted Au, As, Bi, Sb, Mn, and W from the rock. Conversely, the uptake of Cu, Pb, Zn, Ni, Co, Ga, Ge, Mo, Tl, V, and Cr by silicate and sulfide minerals acted to retain these metals in the deep crust.At greater crustal depths, biotite devolatilisation causes melting of lower-crustal rocks. Biotite is stabilised to higher temperatures by uptake of the metal-transporting ligand F, which is controlled by Mg#, and factors that affect Mg# in biotite, including bulk composition, coexisting mineralogy, and preferential removal of Fe over Mg during partial melting. Chlorine retention in biotite is inversely correlated to F, such that initial biotite dehydration produces a slightly Cl-enriched, F-poor melt, enriching residual biotite in F. At final dehydration of biotite, especially in MgO- and K2O-rich bulk compositions, F-rich biotite breaks down at lower-crustal conditions to produce a hot, dry, F-rich and Cl-poor granitic melt. The amount of F released into such a melt from only moderately F-enriched biotite is sufficient to form highly F-enriched melt, typical of A-type granites. These metamorphogenicmelts may be important for transport of REE and some HFSE to the upper crust due to the tendency of these elements to be enriched in F-rich melts.An investigation into mobilisation/retention of base metals during ultra-high temperature metamorphism found that element mobility is primarily controlled by the tendency of elements to partition into residual minerals. Sulfide and major silicate minerals in Rogaland metamorphic aureole rocks are sources of metals for uptake into melts produced by biotite dehydration. Copper, Pb, and Zn, are hosted in major silicate minerals at high to ultra-high temperatures, and therefore these are primarily controlled by metamorphic reactions amongst the major silicate minerals. Retention of base and precious metals can occur at high to ultra-high temperatures by shielding in quartz veins, preferential removal of silicate melt, and reaction with residual minerals. These findings suggest that metapelitic rocks in the lower crust may be viable source region for ore deposits.The work presented in this thesis identifies metamorphism as a key driver of metal redistribution in the mid and lower continental crust. Our understanding of the formation of metamorphogenic and crust-derived ore deposits is improved by understanding the relationships between fluid/melt loss, residual mineral growth, and consequent metal partitioning.