O Depósito de Ouro Zé Vermelho da Província Aurífera Alta Floresta : uma tipologia de depósito de preenchimento de zona de falha

Abstract

The Ze Vermelho gold deposit is located 23 km north of Paranaita City, northern Mato Grosso State, Brazil, and inserted in the realm of the Alta Floresta Gold Province close to the boundary between the Rio Negro-Juruena and Ventuari-Tapajos provinces of the Amazon Craton. The deposit is hosted in a gently dipping N45W trending thrust fault which overprints high-K calc-alkaline, metaluminous to slightly peraluminous, biotite granodiorite of the Paranaita Intrusive suite of 1,8 Ga. The ore zone consists of two subparallel quartz veins known as Ze Vermelho Main and Upper zones separated by a 40 m thick column of barren biotite granodiorites. The veins show textural and structural features suggestive of an evolution given by multiple injections of a silica-rich fluid where laminated structures developed followed by recrystallization which changes quartz texture and color, from massive and white to fine-grained and grey and, ending up with intense brecciation and opening spaces. The mineralization is gold only and shows direct relationship with the amount of sulphide, mainly pyrite, with chalcopyrite to lesser extent and rare sphalerite. The sulphides occur both as replacing finegrained aggregates of grey quartz and as open-filling spaces and vary from disseminated with less than 2% in volume up to 90% when massive bands and layers are formed, the ore shoots, with thickness varying from less than 2 cm up to 1,60 meters with a mean of 0,50 meters. Based on quartz vein textural variations, presence of free gold, amount of sulphides and gold grade, the deposit is divided into three segments, namely: South, Central and North. The south segment is characterized by veins showing massive quartz with milky aspect, low sulphide amounts and gold grade lower than 5 g/t. Free-gold was only found in this segment and in this case gold grades can reach up to 1 Kg/t. The Central segment shows a more complex evolution and is characterized by grey and recrystallized quartz along with intense brecciation, high sulphide contents and high gold grade up to 500 g/t. The north segment shows narrower grey quartz veins which tend to disappear and give place to gold and sulphide depleted mylonites, suggesting the closing of the mineralized structure. Gold grades in this segment are always uneconomic lower than 1 g/t. Tectonic, structural and textural data integration allows classifying the Ze Vermelho gold deposit as tectonic-controlled of fault-fill vein type with higher gold grades associated higher damage zones where ore shoots are developed. The identification of this gold deposit typology openes up an interesting potential for mineral exploration as ore-shoot repetition both along the strike and dip is a common feature worldwide known in other gold deposits which along with its profitability would allow the development of new gold mines and consequently the economic and social development of the northen Mato Grosso and southern Amazonas and Para regions.