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Physicochemical processes in the magma chamber under the black mountain porphyry Cu-Au deposit, Philippines: insights from mineral chemistry and implications for mineralization

Citation

Cao, MJ and Hollings, P and Cooke, DR and Evans, NJ and McInnes, BIA and Qin, KZ and Li, GM and Sweet, G and Baker, M, Physicochemical processes in the magma chamber under the black mountain porphyry Cu-Au deposit, Philippines: insights from mineral chemistry and implications for mineralization, Economic Geology, 113, (1) pp. 63-82. ISSN 0361-0128 (2018) [Refereed Article]

Copyright Statement

Copyright 2018 Society of Economic Geologists, Inc.

DOI: doi:10.5382/econgeo.2018.4544

Abstract

The Black Mountain porphyry Cu-Au deposit in the Baguio district, Northern Luzon (Philippines) is associated with late Miocene to Pliocene intrusive rocks that retain primary crystallization signatures, providing an excellent natural laboratory to study the details of magma chamber processes. Each igneous phase contains disequilibrium textures, including resorptive textures in plagioclase and distinct core rim textures in amphibole. High crystallization temperatures and pressures in felsic (~880C and ~2.8 kbar) and mafic (~1,030C and ~7.7 kbar) rocks recorded by amphibole phenocrysts reflect crystallization in the magma chamber. Amphibole in felsic rocks aged 6.39 0.49 to 2.83 0.23 Ma have similar crystallization temperatures and pressures, suggesting a long-lived and hot felsic magma chamber under the Black Mountain deposit. Four independent oxybarometers, including amphibole, coexisting magnetite-ilmenite, magmatic anhydrite, and zircon, consistently show a high oxidation state of >NNO+1 for the felsic magma and ~NNO+0.7 for the mafic magma. Element profiles across phenocrystic plagioclase display repeated oscillatory variations of Ca, Fe, and Sr contents with large anorthite variations and a positive correlation between CaO and FeO. The amphibole is classified as group 1 Mg hornblende to tschermakite-hornblende in felsic rocks and group 2 Mg hastingsite in mafic rocks. The group 1 amphiboles show a variety of major and trace element variations from core to rim in disequilibrated grains and transitional compositions to group 2 Mg hastingsite. Both mineral textures and chemistry recorded by phenocrysts of plagioclase and amphibole indicate repeated mafic magma recharge in the felsic magma chamber. The common occurrence of rounded or elongated sulfide inclusions in the megacrystic amphibole from the mafic rocks (2.81 0.15 Ma) suggests a greater enrichment of Cu in mafic rocks relative to the ore-forming porphyries (2.912.83 Ma). This is also consistent with higher Cu contents (3.4 0.9 ppm) in the megacrystic amphibole from the mafic rocks relative to amphibole in felsic rocks (1.0 0.2 ppm) and may indicate up to 3.4 times higher Cu in the mafic magma relative to felsic magma. Therefore, large-scale mafic magma recharge (particularly at ~2.8 Ma), likely introduced ore-forming metals to the long-lived felsic magma chamber, further contributing to the fertility of the felsic rocks and ultimately to Cu-Au mineralization at the Black Mountain deposit.

Item Details

Item Type:Refereed Article
Keywords:porphyry copper gold, Phillipines black mountain, hornblende igneous petrology, geodynamics
Research Division:Earth Sciences
Research Group:Geology
Research Field:Ore Deposit Petrology
Objective Division:Mineral Resources (excl. Energy Resources)
Objective Group:Mineral Exploration
Objective Field:Copper Ore Exploration
UTAS Author:Cooke, DR (Professor David Cooke)
UTAS Author:Baker, M (Dr Michael Baker)
ID Code:124586
Year Published:2018
Funding Support:Australian Research Council (IH130200004)
Web of Science® Times Cited:8
Deposited By:CODES ARC
Deposited On:2018-02-28
Last Modified:2019-03-12
Downloads:0

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