Phyllosilicates geochemistry and distribution in the Altar Porphyry Cu-(Au) deposit, Andes Cordillera of San Juan, Argentina: applications in exploration, geothermometry, and geometallurgy

MAYDAGÁN, LAURA; FRANCHINI, MARTA; IMPICCINI, AGNES; LENTZ, DAVID

JOURNAL OF GEOCHEMICAL EXPLORATION

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2017

0375-6742

Biotite, chlorite, muscovite, illite, and kaolinite from the Altar porphyry Cu-(Au) deposit of the Andean Main Cordillera of San Juan Province (Argentina) were constrained using X-ray diffraction, electron microprobe, and infrared spectroscopy analyses to map compositional variations. Magmatic biotites from the andesite-dacite mineralized porphyries have higher XMg, K, and F contents and lower Fe/(Fe+Mg) compared to biotites from the andesite-dacite barren porphyries of the district. Hydrothermal biotites from deep levels with potassic alteration and high Cu grades have higher XMg, Mg, and K, and lower Cl contents than equivalent shallower samples. Chlorite crystals associated with Cu mineralization (0.24 to 1.16% Cu) show lower Fe and Mn and higher Mg contents than chlorite from shallow and distal zones. Potassic dioctahedral phyllosilicates are the most abundant phyllosilicates in the Altar deposit, occur in the phyllic and chloritic zones, and are superimposed on potassic alteration. In zones of high copper grades (> 0.8% Cu), potassic dioctahedral phyllosilicates have total Al (apfu) between 2.4 and 2.8 and intermediate compositions between muscovite and illite, whereas those with higher and lower Al contents come from zones with lower Cu grades. The presence of muscovite indicates temperature of the fluids >300°C, whereas the increment of Fe and Mg and decrease in Al in the white micas composition suggest a decline of fluids temperature and fluid/rock ratio. The presence of kaolinite in late veins halos reflects a decrease of the temperature (