SELECT SUPER PORPHYRY ABSTRACTS   BUY  
Back Next Next
Grasberg Cu-Au Deposit, Papua, Indonesia:
1. Magmatic History
 
by
John Paterson  and  Mark Cloos, Department of Geological Sciences, University of Texas at Austin, USA.

in   -   Porter, T.M. (Ed), 2005   -   Super Porphyry Copper & Gold Deposits - A Global Perspective;   PGC Publishing, Adelaide, v. 2, pp 313-329.

ABSTRACT

   The Grasberg Igneous Complex which formed at ~3 Ma is host to one of the largest copper and gold porphyry-type ore deposits discovered on Earth. This study focuses on the magmatic characteristics of the three main phases of intrusion at the level of the open pit mine: the Dalam, subdivided into the Dalam Andesite, Dalam Volcanic and Dalam Fragmental, the Main Grasberg Intrusion (MGI), and the Kali (Early and Late). A sample suite consisting of 225 polished slabs and thin sections shows all units contain plagioclase and biotite as the overwhelmingly dominant phenocryst phases. The Dalam Andesite, MGI and Late Kali contain(ed) hornblende as well. The Late Kali and the MGI also contained minor amounts of clinopyroxene. Apatite is ubiquitous as a trace phase. Magmatic magnetite is identifiable in the Late Kali. The magmatic groundmass in the Kali, MGI, and Dalam Andesite was potassium feldspar, albitic plagioclase, quartz and biotite. A similar groundmass assemblage probably existed in the other Dalam phase rocks, but hydrothermal alteration caused complete replacement. The phenocryst assemblages record no profound changes in magma chemistry over time, but it is probable that the parent chamber was recharged at least twice, and probably several times.
   The porphyritic Dalam Andesite, comparatively equigranular MGI, and porphyritic Kali units are texturally distinct. The Dalam Volcanic and Dalam Fragmental units are, respectively, polymict and monomict matrix-supported breccias, typically containing 5 to 10%, but locally up to 30% clasts. Broken phenocrysts are common in the Dalam phase rocks. Dalam phase magmas only became volatile-saturated and explosive when intruded to near the surface. The monomict fragmental unit formed near the center and the polymict volcanic unit along the edges. The geologic setting is interpreted to be a maar caldera. The MGI passively intruded as a cone shaped plug into the still hot core of the Dalam. The volumetrically minor Early Kali appears to be a small plug into the center of the MGI whereas the Late Kali is a large wedge-shaped dike that cuts all units.

Buy   Top   |      PGC Publishing      |      PorterGeo Home   


This book is NOW AVAILABLE FOR ORDER via the web by selecting the BUY button above.

This abstract was printed from the PGC Publishing website https://portergeo.com.au/publishing.


PGC Logo
Porter GeoConsultancy Pty Ltd
 Ore deposit database
 Conferences & publications
 International Study Tours
     Tour photo albums
 Experience
PGC Publishing
 Our books  &  bookshop
     Iron oxide copper-gold series
     Super-porphyry series
     Porphyry & Hydrothermal Cu-Au
 Ore deposit literature
 
 Contact  
 What's new
 Site map
 FacebookLinkedin