Reequilibrated fluid inclusions in enargite-luzonite from the Chelopech high-sulfidation Cu-Au epithermal deposit (Bulgaria)

The ores of the Chelopech high-sulfidation Cu-Au deposit are mineralogically and texturally very complex. Enargite is a substantial component of the massive, fine-grained sulfide ore. Weak manifestations of luzonite followed enargite, but high resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction (SAED) patterns unambiguously reveal numerous luzonite layers in enargite structure, as well as abundant pseudomorphs of luzonite after enargite. Scanning electron microscopy (SEM) and infrared (IR) microscopy were utilized for studying the morphology and occurrence of fluid inclusions in enargite. Microthermometiy data, including temperatures of homogenization (T-h) and fluid salinity, were also summarized. Crush-leach techniques, followed by atomic absorption spectrometry (AAS) reveal the major cations participation in inclusion solutes (mol%): Na 86.9, K 6.2, Mg 5.6 and Ca 1.4. Molecular species (CO2, N-2, CH4), liberated on vacuum crushing, were analyzed by mass spectrometry (MS); H2O was determined compressometrically. The T-h-data processed are mainly a compilation of results already published somewhere in the literature. These temperatures (95-245 degrees C) are always below the temperature of enargite stability (T > 270 degrees C). The occurrence and orientation of the fluid inclusions in the enargite crystals, however, as well as the readings of the empirical Na-K-Ca geothermometer (T >= 260 degrees C) imply inclusion trapping during enargite crystallization. The low T-h was ascribed to re-equilibration after deformation (collapse) of the inclusion cavities, caused probably by the polymorphous transition enargite-luzonite on cooling. It is concluded that enargite is deposited at T >= 260 degrees C from solution of low to moderate salinity (ca. 6 wt.%). The stable isotopes (delta D and delta O-1), as well as N-2/Ar ratios point to an ore-forming solution dominated by deeply circulating meteoric water. A pH = 5-6, (in eny case over 4) and slightly reducing environment are assumed.