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Location: 338 km W from Cuiabá, Brazil
Faz Ernesto Soares de Carvalho - s/nPontes e LacerdaBrazil78250-000
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The Pau-a-Pique and Ernesto-Lavrinha Deposits are situated in the Middle Proterozoic (ca. 1.0 Ga) Aguapeí belt, a foreland fold and thrust belt that overlies the Early Proterozoic and Middle Proterozoic terrains (Geraldes et al. 2001). The Aguapeí group in the Pau-aPique and Ernesto – Lavrinha areas is structurally marked by reverse faults, isoclinal folds and strong penetrative axial planar cleavage, often crenulated. The Aguapeí Group is composed of conglomerate, sandstone (arenite) and siltstone that are unconformably deposited on the underlying basement in a braided fluvial to marine depositional environment. The metasediments occur within a fold and thrust belt that is deformed under brittle- ductile conditions and are commonly in tectonic contact with the basement. Strong hydrothermal alteration and associated gold mineralization occurs in association with the lower contact of the Aguapeí Group with the underlying basement.In the Ernesto Deposit, the contact zone between the Aguapeí sediments and the underlying basement tonalite consists of a 5 m to 25 m thick magnetite-sericite schist unit, containing lenses and elongated bodies of quartz generally concordant with the foliation, and a 1 m to 3 m thick basal layer of intensely altered, crushed and decomposed rock. The magnetite-sericite schist apparently represents strongly altered and deformed sediment, probably a hydrothermally altered and sheared metapelite (mylonite).The Lavrinha Deposit which is closely linked to the Ernesto Deposit has been interpreted as gold- rich quartz veins and veinlets with coarse grained pyrite occurring along shallow-dipping structure. The main difference with Ernesto is the position of the mineralization in the metasedimentary sequence. Gold mineralization is located along quartz boudins in highly sericitized rock and plunges to the north. The Pau-a-Pique Deposit occurs in close association with the contact of the meta-tonalite basement and the overlying Aguapeí Group metasediments. The tonalite is metamorphosed with a foliated structure but preserving the original igneous texture. The rocks are metamorphosed and deformed under lower green-schist facies conditions. Muscovite schist is developed in the contact between the metatonalites and metasediments and is an important host of mineralization. The muscovite schist has S-C structures and abundant shear bounded sigmoidal veins. The schist has a strong stretching lineation oriented at N20–50W that controls the form of the deposit and sub-surface mineralization.The Ernesto-Lavrinha Deposits consists of gold-rich quartz veins and veinlets occurring along a relatively thick, shallow-dipping structure at the base of the metasedimentary sequence and within altered sulphidic horizons in overlying meta-arenite units. The basal structure is interpreted to be a low-angle detachment fault that has been folded and faulted together with the overlying stratigraphy. Gold mineralization is located along asymmetrical anticlines and synclines that plunge gently to the north and are cut by NW and NE-trending narrow faults. The gold mineralization occurs in three zones: Lower Trap, Middle Trap and Upper Trap. The Lower Trap mineralized zone in Ernesto is widely developed within a mylonitic zone. The mylonitic zone is a deformed version of meta-arenite which was altered and intruded by quartz veining. The mylonitic zone often resembles a healed fault zone that developed along detachment structures. Mineralization in the Lower Trap is 130 m to 210 m wide, with an average thickness of 5 m and is moreor-less continuous for at least 1,000 m along its northern plunge direction. Alteration associated with gold mineralization within the mylonitic unit includes abundant quartz veins and veinlets with coarse-grained euhedral pyrite and medium grained bipyramidal crystalline magnetite. This alteration and mineralization occurs in mylonitic zones near the base of the detachment fault. The Upper Trap, which is widely developed in the Lavrinha Deposit, occurs in metapelitic rocks (hematite sericite schist) in dilation zones of the intensely deformed synclinal troughs. The Upper and Intermediate traps share similar alteration and mineralization suites. The Ernesto-Lavrinha Deposits are described as detachment-style gold deposits, where typically gold mineralization is associated with low-angle to flat detachment faults, generally with a normal (extensional) sense of movement which consistently places younger units over older units.The Pau-a-Pique gold mineralization is associated with intense hydrothermal alteration, and correlates with the occurrence of pyrite, sulphide alteration, quartz veins and sericitization. The envelope of the mineralized zone is approximately 550 m long, maximum of 15 m thick and 400 m deep in the largest extension. In the deeper levels the most common hydrothermal alteration with gold enrichment is strong albite-anorthosic quartz veining associated with chloritization and pyrite. In the shallow levels the most pervasive alteration is silicification, represented by a strong injection of quartz veins and weaker gold enrichment. The albitic alteration probably represents deeper and hot sources of the hydrothermal feeder. The Pau-a-Pique Deposit is developed within brecciated-sheared host rocks which are strongly foliated and moderately metamorphosed and can be described as structurally controlled orogenic gold lode deposit.
