TY - JOUR
T1 - Heterogeneity of mineral chemistry and sulfur isotopic composition of alunite in the Mankayan lithocap, northern Luzon, Philippines
AU - Manalo, Pearlyn
AU - Takahashi, Ryohei
AU - Imai, Akira
AU - Ruth Parcon-Calamohoy, Rhyza
AU - de los Santos, Mervin
AU - Subang, Leo
AU - Christian Alburo, Glenn
N1 - Funding Information:
We thank Mr. Bryan Yap, president of the Lepanto Consolidated Mining Co., for allowing us to publish the results of this study. We are grateful to the exploration geology and mine geology team of Lepanto Consolidated Mining Co. for their logistical and technical support. Financial support was provided by the Akita University New Frontier Leaders for Rare-Metals and Resources program for fieldwork and laboratory analyses. Additional financial support was provided by the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (17K06982, 21K04960, 21KK0089). Part of the data used in this study was acquired when P. Manalo was being supported by a Japanese Monbukagakusho Scholarship. We thank Hinako Sato, Hertz Balmater, Jonathan Macuroy and Enrico Suharjo for the assistance in the laboratory analyses and sample preparation. We also thank Andrea Agangi for software support and his suggestions to the manuscript. We are immensely grateful to Huayong Chen, Alain Chauvet, Isaac Corral and an anonymous reviewer for their constructive reviews that improved the manuscript. Discussions with the members of the Economic Geology Research Groups, Akita University are highly appreciated.
Publisher Copyright:
© 2022 The Author(s)
PY - 2022/7
Y1 - 2022/7
N2 - The lithocap at Mankayan is a contiguous zone of alunite + quartz + pyrite that occur as pervasive alteration of the metavolcanic basement and dacitic pyroclastic rocks. Alunite + quartz + pyrite assemblage also occurs within hydrothermal gold-bearing veins and breccias. Several high-sulfidation epithermal gold orebodies have been previously delineated in the Lepanto Main Enargite orebody, and in the Northwest, Carmen, Florence West and Florence East quartz-pyrite-gold (QPG) veins. This study presents new data of mineral composition and sulfur isotopic ratios of alunites from different zones of the lithocap at Mankayan. Elemental composition maps and point analysis of alunites indicate compositional heterogeneity within a single alunite crystal. Common substituents to the K+ site are Na+ and H3O+, with few Ca2+ substitution in some sites. Limited occurrences of PO43- substitution to SO42- were also documented. Sulfur isotopic ratios (δ34SCDT) of alunite range from + 13 ‰ to + 24 ‰, which are typical of hypogene alunite. Sulfur isotopic ratios of coexisting pyrite are mostly negative, ranging from −5.4 to −1.0 ‰. Few samples of alunite from the Northwest and Florence West quartz-pyrite-gold veins have sulfur isotopic ratios similar to the values of its coexisting pyrite. The estimated temperature of formation using sulfur isotope geothermometry of alunite-pyrite pairs ranges from 197 °C to 364 °C, with most of the samples varying within 220 to 270 °C. The calculated bulk δ34S of the hydrothermal fluid was found to be + 5 to + 6 ‰ for the different mineralization events in Mankayan. The alunite crystals are heterogeneous even in the microscopic scale. Compositional maps show that K and Na concentration vary within single crystals, as well as among different crystals of a sample. The variations of K and Na content mostly follow the crystal growth structure, however, some alunite crystals with irregular variations are also present. Some samples contain aluminum-phosphate-sulfate (APS) minerals intergrown with alunite. Electron probe microanalysis of the alunite crystals showed a generally wide range of composition between the alunite – natroalunite solid solution. Using thermodynamic functions, a relationship between temperature and K+/Na+ of the hydrothermal fluids was determined across the range of alunite-natroalunite solid solution system. The model curves suggest that at temperatures less than ∼ 250 °C a slight change in fluid composition and/or temperature can vary the number of Na atoms per formula unit by 0.3 units. At higher temperatures, a more significant change in the physicochemical conditions is required for a substitution to occur. This explains the wide range of Na content in the alunites from the different parts of the lithocap. The characteristics compiled in this study reflect the fluctuations in temperature and fluid compositions that occurred during the multiple hydrothermal events in the Mankayan District.
AB - The lithocap at Mankayan is a contiguous zone of alunite + quartz + pyrite that occur as pervasive alteration of the metavolcanic basement and dacitic pyroclastic rocks. Alunite + quartz + pyrite assemblage also occurs within hydrothermal gold-bearing veins and breccias. Several high-sulfidation epithermal gold orebodies have been previously delineated in the Lepanto Main Enargite orebody, and in the Northwest, Carmen, Florence West and Florence East quartz-pyrite-gold (QPG) veins. This study presents new data of mineral composition and sulfur isotopic ratios of alunites from different zones of the lithocap at Mankayan. Elemental composition maps and point analysis of alunites indicate compositional heterogeneity within a single alunite crystal. Common substituents to the K+ site are Na+ and H3O+, with few Ca2+ substitution in some sites. Limited occurrences of PO43- substitution to SO42- were also documented. Sulfur isotopic ratios (δ34SCDT) of alunite range from + 13 ‰ to + 24 ‰, which are typical of hypogene alunite. Sulfur isotopic ratios of coexisting pyrite are mostly negative, ranging from −5.4 to −1.0 ‰. Few samples of alunite from the Northwest and Florence West quartz-pyrite-gold veins have sulfur isotopic ratios similar to the values of its coexisting pyrite. The estimated temperature of formation using sulfur isotope geothermometry of alunite-pyrite pairs ranges from 197 °C to 364 °C, with most of the samples varying within 220 to 270 °C. The calculated bulk δ34S of the hydrothermal fluid was found to be + 5 to + 6 ‰ for the different mineralization events in Mankayan. The alunite crystals are heterogeneous even in the microscopic scale. Compositional maps show that K and Na concentration vary within single crystals, as well as among different crystals of a sample. The variations of K and Na content mostly follow the crystal growth structure, however, some alunite crystals with irregular variations are also present. Some samples contain aluminum-phosphate-sulfate (APS) minerals intergrown with alunite. Electron probe microanalysis of the alunite crystals showed a generally wide range of composition between the alunite – natroalunite solid solution. Using thermodynamic functions, a relationship between temperature and K+/Na+ of the hydrothermal fluids was determined across the range of alunite-natroalunite solid solution system. The model curves suggest that at temperatures less than ∼ 250 °C a slight change in fluid composition and/or temperature can vary the number of Na atoms per formula unit by 0.3 units. At higher temperatures, a more significant change in the physicochemical conditions is required for a substitution to occur. This explains the wide range of Na content in the alunites from the different parts of the lithocap. The characteristics compiled in this study reflect the fluctuations in temperature and fluid compositions that occurred during the multiple hydrothermal events in the Mankayan District.
UR - http://www.scopus.com/inward/record.url?scp=85131460320&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85131460320&partnerID=8YFLogxK
U2 - 10.1016/j.oregeorev.2022.104959
DO - 10.1016/j.oregeorev.2022.104959
M3 - Article
AN - SCOPUS:85131460320
VL - 146
JO - Ore Geology Reviews
JF - Ore Geology Reviews
SN - 0169-1368
M1 - 104959
ER -