Physicochemical processes in the magma chamber under the Black Mountain porphyry Cu-Au deposit, Philippines: Insights from mineral chemistry and implications for mineralization
MetadataShow full item record
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 (~880°C and ~2.8 kbar) and mafic (~1,030°C 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 (2.91–2.83 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.
Showing items related by title, author, creator and subject.
Phenocryst zonation in porphyry-related rocks of the Baguio District, Philippines: Evidence for magmatic and metallogenic processesCao, Mingjian; Evans, Noreen; Hollings, P.; Cooke, D.; McInnes, Brent; Qin, K.; Li, G. (2018)© The Author(s) 2018. Published by Oxford University Press. All rights reserved. Mantle-derived mafic magmas may be the source of ore-forming metals in large Cu porphyry systems, but evidence of primary petrogenetic and ...
Volatile variations in magmas related to porphyry Cu-Au deposits: Insights from amphibole geochemistry, Duolong district, central TibetLi, J.; Qin, K.; Li, G.; Evans, Noreen; Zhao, J.; Yue, Y.; Xie, J. (2018)Ore-forming fluid exsolution in a shallow magma chamber is a critical step in the formation of porphyry Cu ± Mo ± Au deposits but one for which direct evidence is rarely found. Here, Cl abundance, major-trace element ...
Micro- A nd nano-scale textural and compositional zonation in plagioclase at the Black Mountain porphyry Cu deposit: Implications for magmatic processesCao, Mingjian ; Evans, Noreen ; Reddy, Steven ; Fougerouse, Denis ; Hollings, P.; Saxey, David ; McInnes, Brent ; Cooke, D.R.; McDonald, B.J.; Qin, K. (2019)Textural and compositional microscale (10-100 μm) and nanoscale (10-100 nm) zoning in a plagioclase phenocryst from a fresh, syn-mineralization diorite porphyry (Black Mountain porphyry Cu-Au deposit, Philippines) was ...