Origin and evolution of mineralizing fluids and exploration of the Cerro Quema Au-Cu deposit (Azuero Peninsula, Panama) from a fluid inclusion and stable isotope perspective.

Abstract

Cerro Quema is a high sulfidation epithermal Au-Cu deposit with a measured, indicated and inferred resource of 35.98 Mt. @ 0.77 g/t Au containing 893,600 oz. Au (including 183,930 oz. Au equiv. of Cu ore). It is characterized by a large hydrothermal alteration zone which is interpreted to represent the lithocap of a porphyry system. The innermost zone of the lithocap is constituted by vuggy quartz with advanced argillic alteration locally developed on its margin, enclosed by a well-developed zone of argillic alteration, grading to an external halo of propylitic alteration. The mineralization occurs in the form of disseminations and microveinlets of pyrite, chalcopyrite, enargite, tennantite, and trace sphalerite, crosscut by quartz, barite, pyrite, chalcopyrite, sphalerite and galena veins. Microthermometric analyses of two phase (L + V) secondary fluid inclusions in igneous quartz phenocrysts in vuggy quartz and advanced argillically altered samples indicate low temperature (140-216 °C) and low salinity (0.5-4.8 wt% NaCl eq.) fluids, with hotter and more saline fluids identified in the east half of the deposit (Cerro Quema area). Stable isotope analyses (S, O, H) were performed on mineralization and alteration minerals, including pyrite, chalcopyrite, enargite, alunite, barite, kaolinite, dickite and vuggy quartz. The range of δ34S of sulfides is from −4.8 to −12.7¿, whereas δ34S of sulfates range from 14.1 to 17.4¿. The estimated δ34SΣS of the hydrothermal fluid is−0.5¿. Within the advanced argillic altered zone the δ34Svaluesof sulfides and sulfates are interpreted to reflect isotopic equilibriumat temperatures of ~240 °C. The δ18O values of vuggy quartz range from9.0 to 17.5¿, and the δ18O values estimated for the vuggy quartz-forming fluid range from−2.3 to 3.0¿, indicating that it precipitated frommixing ofmagmatic fluidswith surficial fluids. The δ18O of kaolinite ranges from12.7 to 18.1¿and δD from−103.3 to −35.2¿, whereas the δ18O of dickite varies between 12.7 and 16.3¿ and δD from−44 to −30. Based on δ18OandδD, two types of kaolinite/dickite can be distinguished, a supergene type and a hypogene type. Combined, the analytical data indicate that the Cerro Quema deposit formed from magmatic-hydrothermal fluids derived from a porphyry copper-like intrusion located at depth likely towards the east of the deposit. The combination of stable isotope geochemistry and fluid inclusion analysis may provide useful exploration vectors for porphyry copper targets in the high sulfidation/lithocap environment.