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[HRE11-P03] Ore Mineral and fluid composition of the Florence Orebody, Lepanto Mine, Mankayan District, Northern Luzon, Philippines
Keywords:Fluid inclusion, Stable isotopes, Enargite, Pyrite, Near-infrared microscopic observation, Fluid inclusions gas analysis
The Mankayan District in Luzon, Philippines hosts several deposits that vary in age and mineralization styles. This study reports the characteristics of the Florence orebody, one of the high-sulfidation epithermal deposits in the district. The Florence orebody occurs within the Cretaceous to the Eocene Lepanto Metavolcanic basement and the Pliocene to Pleistocene Imbanguila Dacite. Two distinct types of veins occurring within 50 m distance of each other were observed: 1) pyrite-rich veins that contain abundant coarse pyrite and quartz, and 2) enargite-rich veins that contain coarse enargite crystals associated with alunite and kaolinite.
In this study, we focus on the geochemical characteristics of ore minerals and their relationship to the mineralization of the Florence orebody. The pyrite-rich veins include sphalerite and galena with trace amounts of bornite, and covellite. Enargite and telluride minerals occur as inclusions within pyrite grains. The enargite-rich veins contain trace amounts of bornite, galena, and bismuthinite that occur within enargite crystals. The alteration halo surrounding the pyrite-rich veins consists of abundant quartz and alunite, while that of the enargite-rich veins consists of abundant alunite and kaolinite. Ar-Ar dating of alunite of the pyrite-rich veins yielded 3.0 Ma, while that of the enargite-rich veins yielded 1.7 Ma.
Pyrite in both the pyrite-rich and enargite-rich veins contains significant amounts of As, Cu, and Zn, and small amounts of Au. The contents of Au and As of pyrite plot both below and above the Au-saturation line, thus suggesting that gold may occur as both nanoparticles and solid solution. Enargite of the enargite-rich veins contains significant amounts of Se, Zn, Te, and Ag.
Microthermometry of aqueous vapor-liquid fluid inclusions in quartz yielded modes of homogenization temperatures and salinity, 190 to 200 ℃ and 2.7 to 6.6 wt % NaCl eq., and 250 to 260 °C and 3.0 to 6.5 wt % NaCl eq. for the pyrite-rich and enargite-rich veins, respectively (Manalo et al., 2020). Bulk gas analyses of the fluid inclusions in quartz of both the pyrite-rich and enargite-rich veins indicates contribution of magmatic fluids. Near-infrared microscopic observations revealed the presence of abundant gas-rich and minor liquid-rich vapor-liquid fluid inclusions hosted in enargite crystals. Bulk gas compositions of the fluid inclusions in enargite show elevated amount of CO2 in addition to H2O.
The δ34SCDT values of pyrite and enargite of the pyrite-rich veins range from -4.6 to +1.0 ‰, while those of the enargite-rich veins range from -6.2 to -0.7 ‰. The δ34SCDT values of alunite of the pyrite-rich veins range from -1.7 to +17.5 ‰, while those of the enargite-rich veins range from -3.2 to +23.3 ‰. The wide range of sulfur isotopic ratios indicate that the environment of formation of alunite may have been different from that of pyrite and enargite, and may have involved magmatic-hydrothermal, magmatic steam or supergene environments. The difference in age, ore mineralogy, pyrite chemistry, fluid inclusion characteristics and sulfur isotopic ratios suggests that at least two different mineralization events occurred within the Florence orebody. This implies the potential existence of other porphyry-type mineralization in the Mankayan District, considering the relationship between high-sulfidation epithermal and porphyry-type systems.
In this study, we focus on the geochemical characteristics of ore minerals and their relationship to the mineralization of the Florence orebody. The pyrite-rich veins include sphalerite and galena with trace amounts of bornite, and covellite. Enargite and telluride minerals occur as inclusions within pyrite grains. The enargite-rich veins contain trace amounts of bornite, galena, and bismuthinite that occur within enargite crystals. The alteration halo surrounding the pyrite-rich veins consists of abundant quartz and alunite, while that of the enargite-rich veins consists of abundant alunite and kaolinite. Ar-Ar dating of alunite of the pyrite-rich veins yielded 3.0 Ma, while that of the enargite-rich veins yielded 1.7 Ma.
Pyrite in both the pyrite-rich and enargite-rich veins contains significant amounts of As, Cu, and Zn, and small amounts of Au. The contents of Au and As of pyrite plot both below and above the Au-saturation line, thus suggesting that gold may occur as both nanoparticles and solid solution. Enargite of the enargite-rich veins contains significant amounts of Se, Zn, Te, and Ag.
Microthermometry of aqueous vapor-liquid fluid inclusions in quartz yielded modes of homogenization temperatures and salinity, 190 to 200 ℃ and 2.7 to 6.6 wt % NaCl eq., and 250 to 260 °C and 3.0 to 6.5 wt % NaCl eq. for the pyrite-rich and enargite-rich veins, respectively (Manalo et al., 2020). Bulk gas analyses of the fluid inclusions in quartz of both the pyrite-rich and enargite-rich veins indicates contribution of magmatic fluids. Near-infrared microscopic observations revealed the presence of abundant gas-rich and minor liquid-rich vapor-liquid fluid inclusions hosted in enargite crystals. Bulk gas compositions of the fluid inclusions in enargite show elevated amount of CO2 in addition to H2O.
The δ34SCDT values of pyrite and enargite of the pyrite-rich veins range from -4.6 to +1.0 ‰, while those of the enargite-rich veins range from -6.2 to -0.7 ‰. The δ34SCDT values of alunite of the pyrite-rich veins range from -1.7 to +17.5 ‰, while those of the enargite-rich veins range from -3.2 to +23.3 ‰. The wide range of sulfur isotopic ratios indicate that the environment of formation of alunite may have been different from that of pyrite and enargite, and may have involved magmatic-hydrothermal, magmatic steam or supergene environments. The difference in age, ore mineralogy, pyrite chemistry, fluid inclusion characteristics and sulfur isotopic ratios suggests that at least two different mineralization events occurred within the Florence orebody. This implies the potential existence of other porphyry-type mineralization in the Mankayan District, considering the relationship between high-sulfidation epithermal and porphyry-type systems.