O Depósito Aurífero Piaba no fragmento ratônico São Luis (NW-Maranhão): petrografia das rochas hospedeiras e fluidos mineralizadores

Abstract

The Piaba gold deposit became, in 2010, the first gold mine in operation in the São Luís Cratonic Fragment, northwestern of the State of Maranhão, Brazil. The tectonic setting comprises the metavolcano-sedimentary rocks of the Aurizona Group and the granitoids of the Tromaí Intrusive Suite, along with minor units. All these rocks formed in island arc setting between 2240 and 2056 Ma. Gold mineralization at Piaba is hosted in a fine-grained granophyric granodiorite (Piaba Granophyre) and in a subvolcanic andesitic rock belonging to the Aurizona Group. The ore bodies are hosted in the Piaba fault, which is an ENE-WSWtrending brittle-ductile structure. These ore bodies consist of stockwork veins and veinlets and their hydrothermal haloes. The petrographic study was undertaken in drill cores of the mineralized zone. This study has shown that the host rocks have been affected by strong hydrothermal alteration that provoked intense mineralogical transformations of the rocks. The most prominent alteration processes have been the sericitization and chloritization of the host rocks, occurring in pervasive and fissure-filling modes. In addition, carbonatization, silicification and sulfidation occurred in response to fluid flow and veining along fractures and small faults associated with the Piaba fault. These latter types of alteration appear to be associated with gold precipitation. Petrographic, microthermometric and Raman microspectroscopic studies have been undertaken in quartz from mineralized veins from a stockwork zone. These studies identified two and three phase aqueous-carbonic fluid inclusions that have been interpreted as produced by heterogeneous trapping during immiscible phase separation. Late aqueous inclusions have also been identified. The mineralizing aqueous-carbonic fluid is composed of CO2 (5 – 24 mol%, density of 0.96-0.99 g/cm3), H2O (74 – 93 mol%), N2 (≤1 mol%), CH4 (≤1mol%), and has 5.5 wt. % NaCl equivalent. The gold ore deposited at 267-302ºC and 1.25-2.08 kbar, corresponding to depths of 4 to 7 km. Fluid composition and the P-T interval of the mineralizing fluid, its relatively reduced character (log ƒO2 -31.3 to -34.3), and the strong sulfidation of the host rocks indicate that gold has been transported as a sulfur complex and was deposited in response to phase separation and lowering of ƒO2 and of the sulfur activity during fluid-rock interaction. These conditions are consistent with the brittle regime and mesozonal to hypozonal conditions. Comparing the geological and fluid P-T-X characteristics of Piaba with those of other gold prospects already investigated in the same region, it is possible to define Piaba as an orogenic gold deposit.