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Exploring the volcanic connection in porphyry copper systems

Anna Ellen Freudenstein1 , Dawid Szymanowski1, Lorenzo Tavazzani1, John Dilles2, Cyril Chelle-Michou1

  • Affiliations:  1Department of Earth and Planetary Sciences, ETH Zürich, Zürich, Switzerland; 2College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, USA 

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 22

  • Programme No: 4.1.13

  • Theme 4 > Session 1

Abstract

Volcanic activity is thought to preclude the formation of porphyry copper deposits (PCDs). This is because these deposits require a large effective volume of magma in the shallow crust, which is capable of exsolving large amounts of metal-bearing hydrothermal fluids. It is therefore generally assumed that any coeval or sub-coeval large-scale volcanic activity would terminate ore deposition as the required volatiles and metals are vented to the atmosphere. However, there are a limited number of known mineralized porphyry systems with extant coeval volcanic units where such relationships can be studied. The Jurassic Yerington district, USA, is an exception, with tilting exposing a unique cross section of the volcanic units, deposits, and batholith.  We investigate the relationship between PCDs and volcanic eruptions by examining the petrochronology of zircon in coeval volcanic units associated with the Yerington batholith, using in-situ LA-ICP-MS and high-precision CA-ID-TIMS U-Pb methods. Known as the Fulstone Spring volcanics, these units consist of a thick pile (~1.5 km) of andesitic-dacitic ignimbrites and lavas. Geochronological data shows that these volcanics erupted both pre- and post-mineralisation, but show a <1 Ma gap coinciding with the emplacement of porphyry dikes associated with the copper mineralisation. Zircon trace element compositions and Ti-in-zircon temperatures as well as whole-rock compositions diverge between the pre- and post-mineralisation volcanics and mineralising porphyries, indicating protracted melt evolution and recharge processes in the shallow magma reservoir. We conclude that a resurgence of volcanic activity may have played a role in reducing or terminating copper mineralisation in the Yerington PCD.