Abstract

Aripuanã is a Paleoproterozoic (1.76-1.75 Ga), stratiform, volcanogenic Zn-Pb-Ag-(Au-Cu) massive sulfide deposit, has an estimated resource of 11.6 million metric tons (Mt) of ore, and is located on the south-southwest border of the Amazon craton (Brazil). Aripuanã resides in a caldera composed of strongly fractionated, felsic metavolcanic rocks, transitional between calc-alkaline and tholeiitic compositions. The entire region was metamorphosed into lower greenschist facies (1.68-1.63 Ga) and the deposit was subsequently thermally metamorphosed and metasomatized (1.56-1.53 Ga).

The mineralized bodies are lenticular and elongated. They are located at the base of a sequence of turbidites, which overlie dacitic and rhyolitic ignimbrites. The rocks of the footwall are silicified, sericitized, and chloritized, and the hanging-wall rocks are carbonate rhythmites and turbidites, hydrothermal marbles, carbonate breccias, and breccia-like rocks, carbonate + talc + tremolite + chlorite rock, talc-, tremolite-, and biotite-rich rocks, and carbonate- and fluorite-bearing cherts.

The deposit was formed during at least four submarine-exhalative hydrothermal cycles in which siliceous marbles and marls represent the initial phase of each cycle, followed by the coprecipitation of pyrite + pyrrhotite + sphalerite + argentiferous galenas ± chalcopyrite, which were disseminated in clay-carbonate breccias or formed massive lenses. This phase was followed by fluoritic cherts and argillites, which were followed by carbonate and (fluorite?) rhythmites, and turbidites to close the cycle.

The variation in total rare earth element (REE) contents and in the Eu and Ce anomalies indicates that the original sediments were mixtures of hydrothermal carbonate and smectitic clay and were deposited below a redoxcline. Isotopic analysis identified δ18OSMOW values between 8 and 13‰ and δ13CPDB values between −7 and −1‰. Modeling isotopic data showed that mineralizing fluids were distinct based on the proportion of magmatic fluid that mixed with the seawater. Carbonate rocks and sulfide minerals precipitated while hydrothermal activity and volcanism evolved from an exhalative to an exhalative-explosive character with temperatures increasing between 100° and 200°C.

The regional metamorphism of the deposit formed carbonate + talc + tremolite + chlorite rocks, tremolite-, biotite-, and talc-rich rocks; it recrystallized the minerals, foliated and folded all of the rocks, and caused limited decarbonation at the sites where carbonate rocks contained silica and smectite. However, the original isotopic signatures were maintained. The thermal metamorphism erased the metamorphic foliation, and the metasomatism generated veinlets of chlorite with sulfides.

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