The Canadian Mineralogist
Volume 34, pages 547-557 (1996)

THERMODYNAMIC AND TEXTURAL EVIDENCE FOR AT LEAST TWO STAGES OF Au-Pd MINERALIZATION AT THE CAUÊ IRON MINE, ITABIRA DISTRICT, BRAZIL

GEMA RIBEIRO OLIVO and CHRISTOPHER H. GAMMONS
Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7

Abstract

The Cauê mine, in the Itabira District of Brazil, contains a unique precious-metal-bearing mineral assemblage that includes palladian gold, pure gold, native palladium, palladseite, arsenopalladinite and palladium-copper oxide minerals. Thermodynamic calculations indicate that some of these phases are out of equilibrium with each other, and therefore cannot be cogenetic. At least two mineralizing events are required to explain the textures and paragenesis of the Cauê deposits. The main mineralizing event occurred during, or prior to, regional Transamazonian orogenesis, whereas the later event was probably related to more recent weathering of the hypogene ore. Minerals that formed during the main ore-forming episode include palladian gold, palladseite and arsenopalladinite. Textural evidence suggests that this event was synchronous with the D1 deformation. The precious metals were most likely transported as chloride complexes by high- temperature brines, and deposited by changes in pH or f(O2) (or both) accompanying interaction of ore fluids with dolomitic iron-formation. In contrast, thermodynamic and textural relationships involving palladium oxide, pure gold and pure palladium indicate that these minerals could not have survived regional metamorphism, and were more likely formed during recent weathering of the primary hypogene ore. Weathering also resulted in leaching of selenium from the rock, with possible remobilization of palladium and gold as simple hydroxy complexes.