Volcanic–plutonic connections and metal fertility of highly evolved magma systems: a case study from the Herberton Sn–W–Mo Mineral Field, Queensland, Australia

Abstract

Understanding the connection between the highly evolved intrusive and extrusive systems is essential to explore the evolution of high silicic magma systems, which plays an important role in discussions of planetary differentiation, the growth of continents, crustal evolution, and the formation of highly evolved magma associated Sn–W–Mo mineral systems. To discern differences between “fertile” and “non-fertile” igneous rocks associated with Sn–W–Mo mineralization and reveal the genetic links between coeval intrusive and extrusive rocks, we integrate whole rock geochemistry, geochronology and Hf isotope signatures of igneous zircons from contemporaneous plutonic and volcanic rocks from the world-class Herberton Mineral Field of Queensland, Australia. The 310–300 Ma intrusive rocks and associated intra-plutonic W–Mo mineralization formed from relatively oxidized magmas after moderate degrees of crystal fractionation. The geochemical and isotopic features of the coeval volcanic succession are best reconciled utilizing the widely-accepted volcanic–plutonic connection model, whereby the volcanic rocks represent fractionated derivatives of the intrusive rocks. Older intrusions emplaced at 335–315 Ma formed from relatively low fO₂ magmas that fractionated extensively to produce highly evolved granites that host Sn mineralization. Coeval volcanic rocks of this suite are compositionally less evolved than the intrusive rocks, thereby requiring a different model to link these plutonic–volcanic sequences. In this case, we propose that the most fractionated magmas were not lost to volcanism, but instead were effectively retained at the plutonic level, which allowed further localized build-up of volatiles and lithophile metals in the plutonic environment. This disconnection to the volcanism and degassing may be a crucial step for forming granite-hosted Sn mineralization. The transition between these two igneous regimes in Herberton region over a ∼30 m.y. period is attributed to a change from an early compressive tectonic environment with a thickened crust, to conditions of crustal thinning and lithospheric extension due to progressive slab rollback. Such tectonic transitions may provide favorable conditions for intrusion-related mineralization. Given the common occurrence of volcanic and plutonic rocks associated with Sn–W–Mo mineralization worldwide, we suggest that a combined understanding of temporal tectonic evolution and plutonic–volcanic connections can assist in assessment of regional-scale mineralization potential, which in turn can aid strategies for future ore deposit exploration.