ABSTRACT

The Buraco da Velha copper deposit lies at the northern limit of the Parecis Basin in the Colorado Graben, Rondônia, Brazil. New geophysical data indicate the presence of a magnetic source below and to the north of the Buraco da Velha deposit, where it corresponds with high gamma-ray U, Th, and K counts. We have studied the source of the magnetic anomaly and delineated it through derivative transforms, Euler deconvolution, MaxiMin total magnetization direction analysis, staged inverse modeling, Th/K ratio, and radiometric ternary imaging. The magnetic anomaly is elongated in the east–west direction and measures 23×6  km, and the top of the source is estimated to lie at depths mostly between 50 and 100 m. Based on the magnetic model, we indirectly estimate a Jurassic to Cretaceous age, which is compatible with the 180–80 Ma range dated for the copper mineralization. Gamma-ray data are consistent with the hydrothermal alteration in the sedimentary cover and with the presence of an intrusion in the subsurface. We suggest that the intrusion of the magnetic body generated the necessary thermal energy to mix an already oxidized brine and sulfide-bearing fluids at the border of the Parecis Basin, leading to copper deposition and providing a potential analog for similar environments of mineral deposits elsewhere, such as in the Kupferschiefer deposit (Poland) and the Zambia Copper Belt (Zambia).

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