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Rapid ascent of volatile-charged magma associated with porphyry Cu deposits: Models and crystal textures

Alison Rust 1, Oleg Melnik2, Rustam Abdullin3, Jon Blundy2, Elena Melekhova2

  • Affiliations: 1School of Earth Sciences, University of Bristol, Bristol, UK; 2Department of Earth Sciences, University of Oxford, Oxford, UK; 3Novosibirsk State University, Novosibirsk, Russia

  • Presentation type: Talk

  • Presentation time: Tuesday 15:45 - 16:00, Room S160

  • Programme No: 4.1.5

  • Theme 4 > Session 1

Abstract

Porphyry copper deposits (PCDs) are associated with felsic hypabyssal intrusions with geochemical signatures indicating a water-rich source in the mid- to lower-crust. Such wet felsic magma would be very buoyant, and we explore the hypothesis that its rapid ascent through the crust via dikes promotes PCD generation.  A mathematical model for dike ascent with vesiculation (H2O and CO2), heat transfer, crystallization (including kinetics and evolving crystal size distribution) and latent heat release, demonstrates that extremely fast ascent of volatile-charged magma from PCD-relevant source conditions is feasible. For example, a water-rich dacite can ascend from 30 to a few km depth in days. This is sufficiently fast to prevent substantial loss of exsolved volatiles (and metals) to surrounding rocks during ascent.  However, to form a PCD the volatile-charge magma must not accelerate to the surface and erupt explosively like a kimberlite. The modelling shows that there are conditions for which wet felsic magma that ascended rapidly can decelerate to form an intrusion in the upper crust due to viscosity increase from substantial crystallization induced by water exsolution and cooling. Dike ascent is faster, with more shallow arrest, for wetter, hotter magma and for greater amount of magma entering the dike.  We compare simulated crystal textures with those of natural volcanic and intrusive rocks, and products of experiments replicating magma decompression paths from model runs. We combine the numerical modelling and textural results with conceptual models of sources and deposits to discuss conditions that may be more conducive to PCDs than eruptions or barren intrusions.