Skip to Main Content
Book Chapter

Magmatic Controls on Porphyry Copper Genesis

By
Andreas Audétat
Andreas Audétat
1
Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
Search for other works by this author on:
Adam C. Simon
Adam C. Simon
2
Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154-4010
Search for other works by this author on:
Published:
January 01, 2012

Abstract

This paper deals with the earliest stages of formation of porphyry Cu deposits, starting with the production of partial melts in the mantle and ending where upper crustal magmas reach their solidus and potentially have exsolved a metal-bearing hydrothermal fluid. During all these stages magmatic sulfides exert a major control on the budget of ore-forming metals in the magma. High metal concentrations in mafic arc magmas are favored by low degrees of partial melting in the mantle source region, and by limited removal (or effective redissolution) of magmatic sulfides in the lower crust. Ascending magmas accumulate in large, compositionally stratified magma chambers in the upper crust (5- to 15-km depth), which represent the exsolution source of the mineralizing fluids for the shallower porphyry Cu deposits. Interaction between mafic and felsic magmas in these magma chambers leads to partial mingling/mixing, volatile release, and the formation of magmatic sulfides that incorporate large amounts of Cu and Au, but only little Mo. For porphyry Cu mineralization, it is essential that these magmatic sulfides are subsequently destroyed and thereby release their contained metals to the mineralizing fluids. Evidence from experimental phase equilibria studies and melt inclusions hosted in phenocrysts from plutonic and volcanic rocks in arc environments, combined with fluid inclusion evidence from porphyry Cu deposits, suggest that silicate melts that ultimately give rise to porphyry Cu deposits are likely saturated first with a CO2-rich fluid and later give way to single-phase, low-salinity (typically 5–10 wt % NaCl equiv) aqueous fluids. At the typical fO2 conditions of porphyry Cu-forming magmas (ΔFMQ + 1 to ΔFMQ + 3), sulfur occurs mostly as SO2 in the fluid. Efficient Cu removal from the magma into the overlying porphyry environment is favored by the exsolution of an S-bearing volatile phase that has a low HCl/alkali chloride ratio. The ability of the ore fluid to scavenge and transport Cu increases with increasing fO2 and the concentration of K in the aqueous fluid, and may be maximized at high ratios of SO2/H2S of the fluid. Once formed, efficient focusing of the ore fluid into the upper portions of the magma chamber may be favored by the development of permeable melt channels that act as conduits for the ascent of ore fluid in a pressure gradient through the crystallizing magma. These conduits likely facilitate the contribution of S, Cu, and other metals from mafic silicate melt that ponds at deeper levels of the magma system.

You do not currently have access to this article.

Figures & Tables

Contents

Special Publications of the Society of Economic Geologists

Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe

Jeffrey W. Hedenquist
Jeffrey W. Hedenquist
Search for other works by this author on:
Michael Harris
Michael Harris
Search for other works by this author on:
Francisco Camus
Francisco Camus
Search for other works by this author on:
Society of Economic Geologists
Volume
16
ISBN electronic:
9781629490410
Publication date:
January 01, 2012

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal