PALLADIUM-BEARING GOLD DEPOSIT HOSTED BY PROTEROZOIC LAKE SUPERIOR-TYPE IRON-FORMATION AT THE CAUE IRON-MINE, ITABIRA DISTRICT, SOUTHERN SAO-FRANCISCO CRATON, BRAZIL - GEOLOGIC AND STRUCTURAL CONTROLS

The Caue iron mine is located ill the Itabira district in the southern Sao Francisco craton. Three geologic units metamorphosed to amphibolite facies have been identified: (1) an Archean volcanosedimentary sequence; (2) a Proterozoic iron-formation unit, and (3) a Proterozoic quartzite unit. The iron-formation unit is cut by amphibolite dikes and tectonically imbricated with talc schists and amphibolites of the volcano-sedimentary sequence. All the rocks were affected by three phases of folding and associated thrusting and boudinage. In the iron-formation unit, the D-1 and D-2 structures have been generated by a progressive simple shear regime with a transport direction consistently east-over-west. D-3 structures are characterized by open folds with all associated crenulation cleavage. The Caue palladium-bearing deposit comprises five orebodies hosted by jacutinga ill the iron-formation unit. Jacutinga is a highly sheared, metamorphosed, hydrothermally altered Lake Superior-type iron-formation composed of hematite, quartz, talc, phlogopite. with minor amounts of tourmaline, apatite, and monazite. In the Corpo Y orebody, the highest gold concentrations occur in quartz veins, whereas in the Corpo X, Central, Aba Leste, and Aba Norte orebodies, the highest gold grades occur in hematite veins with local concentrations of quartz. The gold-rich veins are parallel to S-1 foliation in all orebodies except Aba Norte. In the Aba Norte, the gold-rich boudinaged hematite vein is parallel to S-2. The ore minerals ale palladian gold, gold, palladium, palladium oxide, and palladseite. The main mineralizating event was synchronous with the peak of thermal metamorphism (T = 600 degrees C) and with D-1 shearing and thrusting. The hydrothermal alteration associated with this event is characterized by the formation of talc, phlogopite, and tourmaline, and hematite and quartz veins, generating the jacutinga rock type. Weathering of the jacutinga resulted in the alteration of silicates to kaolin and oxides to goethite. At high temperatures and oxygen fugacities, Pd and Au may have been transported as chloride complexes, and their deposition ri-lay have occurred mainly in response to an increase in pH. This phenomenon may have occurred as a result of mineralizing fluids reacting with the jacutinga, possibly representing an altered dolomitic oxide facies iron-formation (dolomitic itabirite). The most probable source of these precious metals is the Archean volcano-sedimental sequence. The characteristics of the Caue deposit warrant a new type of gold deposit, entitled here ''palladium-bearing gold deposit hosted by highly sheared, metamorphosed Lake Superior-type iron-formation.'' Along the eastern border of the southern part of the Sao Francisco craton, other. palladium-bearing gold occurrences are hosted by jacutinga, suggesting that this geologic environment is exceptionally favorable for this type of deposit.