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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

Cao Mingjian, Pete Hollings, David R. Cooke, Noreen J. Evans, Brent I. A. McInnes, Qin Kezhang, Li Guangming, Gabe Sweet and Mike Baker
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 and the Bulletin of the Society of Economic Geologists (February 2018) 113 (1): 63-82

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 ( approximately 880 degrees C and approximately 2.8 kbar) and mafic ( approximately 1,030 degrees C and approximately 7.7 kbar) rocks recorded by amphibole phenocrysts reflect crystallization in the magma chamber. Amphibole in felsic rocks aged 6.39 + or - 0.49 to 2.83 + or - 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 approximately 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 + or - 0.15 Ma) suggests a greater enrichment of Cu in mafic rocks relative to the ore-forming porphyries (2.91-2.83 Ma). This is also consistent with higher Cu contents (3.4 + or - 0.9 ppm) in the megacrystic amphibole from the mafic rocks relative to amphibole in felsic rocks (1.0 + or - 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 approximately 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.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 113
Serial Issue: 1
Title: Physicochemical processes in the magma chamber under the Black Mountain porphyry Cu-Au deposit, Philippines; insights from mineral chemistry and implications for mineralization
Affiliation: Chinese Academy of Sciences, Institute of Geology and Geophysics, Beijing, China
Pages: 63-82
Published: 201802
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 83
Accession Number: 2018-039786
Categories: Economic geology, geology of ore deposits
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. geol. sketch map
N16°21'00" - N16°22'00", E120°34'00" - E120°34'60"
Secondary Affiliation: Lakehead University, CAN, CanadaUniversity of Tasmania, AUS, Australia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2018
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