Zonation of Sulfate and Sulfide Minerals and Isotopic Composition in the Far Southeast Porphyry and Lepanto Epithermal Cu–Au Deposits, Philippines

Abstract

The world-class Far Southeast (FSE) porphyry system, Philippines, includes the FSE Cu-Au porphyry deposit, the Lepanto Cu-Au high-sulfidation deposit and the Victoria-Teresa Au-Ag intermediate-sulfidation veins, centered on the intrusive complex of dioritic composition. The Lepanto and FSE deposits are genetically related and both share an evolution characterized by early stage 1 alteration (deep FSE potassic, shallow Lepanto advanced argillic-silicic, both at~1.4 Ma), followed by stage 2 phyllic alteration (at~1.3 Ma); the dominant ore mineral deposition within the FSE porphyry and the Lepanto epithermal deposits occurred during stage 2. We determined the chemical and S isotopic composition of sulfate and sulfide minerals from Lepanto, including stage 1 alunite (12 to 28 permil), aluminum-phosphate-sulfate (APS) minerals (14 to 21 permil) and pyrite (À4 to 2 permil), stage 2 sulfides (mainly enargite-luzonite and some pyrite, À10 to À1 permil), and late stage 2 sulfates (barite and anhydrite, 21 to 27 permil). The minerals from FSE include stage 2 chalcopyrite (1.6 to 2.6 permil), pyrite (1.1 to 3.4 permil) and anhydrite (13 to 25 permil). The whole-rock S isotopic composition of weakly altered syn-mineral intrusions is 2.0 permil. Stage 1 quartz-alunite-pyrite of the Lepanto lithocap, above about 650 m elevation, formed from acidic condensates of magmatic vapor at the same time as hypersaline liquid formed potassic alteration (biotite) near sea level. The S isotopic composition of stage 1 alunite-pyrite record temperatures of approximately 300-400°C for the vapor condensate directly over the porphyry deposit; this cooled to <250°C as the acidic condensate flowed to the NW along the Lepanto fault where it cut the unconformity at the top of the basement. Stage 1 alunite at the base of the advanced argillic lithocap over FSE contains cores of APS minerals with Sr, Ba and Ca; based on back-scattered electron images and ion microprobe data, these APS minerals show a large degree of chemical and S-isotopic heterogeneity within and between samples. The variation in S isotopic values in these finely banded stage 1 alunite and APS minerals (16 permil range), as well as that of pyrite (6 permil range) was due largely to changes in temperature, and perhaps variation in redox conditions (average~2:1 H 2 S:SO 4 ). Such fluctuations could have been related to fluid pulses caused by injection of mafic melt into the diorite magma chamber, supported by mafic xenoliths hosted in diorite of an earlier intrusion. Resource Geology Vol. 67, bs_bs_banner fluids were relatively oxidized in the Lepanto lithocap, with an H 2 S:SO 4 ratio of about 4. The oxidation resulted from cooling, which was caused by boiling during ascent and then dilution with steam-heated meteoric water in the lithocap. This cooling also resulted in the sulfidation state of minerals increasing from chalcopyrite stability in the porphyry deposit to that of enargite in the lithocap-hosted high-sulfidation deposit. The temperature at the base of the lithoca...

Zircon U-Pb, Molybdenite Re-Os and Quartz Vein Rb-Sr Geochronology of the Luobuzhen Au-Ag and Hongshan Cu Deposits, Tibet, China: Implications for the Oligocene-Miocene Porphyry–Epithermal Metallogenic System

“…The Carius tube was placed into the flask under the freezing conditions at the bottom of the Carius tube. 185 Re and 190 Os mixture diluent, 2 mL of 10 mol/L HCl, and 6 mL of 16 mol/L HNO 3 were placed at the bottom of the Carius tube using a long slim-necked funnel.…”

“…The sample solution was heated up 50 • C until it was dry, then there were added a few drops of concentrated HNO 3 until it evaporated, and we lastly removed the residual Os. The residue was dissolved with several milliliters of 2% diluted HNO 3 . The Re isotope ratio was determined using ICP-MS.…”

Geochemistry and Fluid Inclusions Analysis of Vein Quartz in the Multiple Hydrothermal Systems of Mankayan Mineral District, Philippines

“…The 40 Ar/ 39 Ar age of the alunite from the high sulfidation Northwest quartz–pyrite–gold (NW QPG) deposit is 2.2 ± 0.1 Ma (Manalo et al, ). The 40 Ar/ 39 Ar ages of illite from the Victoria veins, similar mineralization to the older Teresa deposit, are 1.31 ± 0.02 Ma (Sakakibara et al, ), and 1.14 ± 0.02 and 1.16 ± 0.02 Ma (Hedenquist et al, ), respectively. Alunite K–Ar ages of the Lepanto enargite–gold deposit range from 1.56 ± 0.29 to 1.17 ± 0.16 Ma (Arribas Jr et al, ).…”

“…Furthermore, the temperatures of formation of the quartz at Carmen and Florence are also higher than the temperature of formation of the Lepanto enargite main orebody, as indicated by the fluid inclusions in enargite (Mancano & Campbell, 1995; Fig. 11a) and sulfur isotope geothermometry (Hedenquist et al, 2017). The N 2 /Ar ratios of the fluid inclusions in quartz occurring with enargite at Florence are within the same range of the N 2 /Ar ratios of the fluid inclusions in the NW QPG Stage 4 quartz (Manalo et al, 2018), both indicating significant magmatic components.…”

Features of Large Magmatic–Hydrothermal Systems in Japan: Characteristics Similar to the Tops of Porphyry Copper Deposits

“…Based on studies elsewhere, such as at Lepanto, Philippines (Arribas, 1995;Hedenquist et al, 1998Hedenquist et al, , 2017, the magmatic vapor that condenses at shallow depthswhich causes the initial leaching and alterationseparated from a hypersaline liquid at depths of~2-3 km (Henley & McNabb, 1978). The hypersaline liquid at depth formed potassic alteration of biotite and magnetite associated with the Far Southeast porphyry copper deposit.…”