Summary:
The Boa Esperança deposit crops out as an isolated hill, which is a north–northeast elongated. The topographic high is formed by quartz and magnetite breccias, which cut the Neoarchean biotite granite, the host of the copper mineralization. (2.85 Ga).
The Boa Esperança deposit extends for approximately 850 meters in a WSW-ENE direction, for 640 meters in NNW-SSE direction, and is currently known to a depth of 780 meters below surface through drilling. It has a steep SE plunge and remains open in this direction. The deposit is hosted in a series of breccias cross-cutting a granite intrusion.
Mineralization consists of a series of brecciated zones, which are aligned N60°–70°E and dips to the southeast at 60°–70. An alignment of approximately N40°E can be observed in the field and coincides with the strike direction of the Boa Esperança hill.
The breccias are concentrated around the top of the hill, occupying an area of approximately 450 x 350 m. The copper mineralization, based on drilling to date, has a geometric shape similar to an inverted cone. Currently known mineralization lies between elevations of 350 m to -200 masl. The breccias are distributed throughout the barren pink granite lenses.
The Boa Esperança deposit displays primary and secondary zoning. The primary zoning corresponds to a distal zone, where pyrite (py) dominates, grading towards copper mineralized zones of pyrite–chalcopyrite (py–cpy), chalcopyrite–pyrite (cpy–py) and chalcopyrite (cpy).
The secondary zoning is a supergene alteration and consists of sub-horizontal and discontinuous lenses of a barren leached zone (LIX), a copper oxide zone (OXI) and a mixed zone (MIX) of oxides and primary copper sulfides. The barren leached zone crops out at the hill top, and is composed of hematite, goethite and clay minerals. Despite the large amount of sulfide boxworks found in this area, the leached zone does not contain copper in economic concentrations. The copper oxide zone is located immediately beneath the leached zone and consists of malachite and copper-bearing clays. Below the copper oxide zone is an area of mixed oxides, carbonates, secondary supergene sulfides (chalcocite and covellite) and primary sulfides (pyrite and chalcopyrite).
The bottommost layer, beneath the mixed oxide zone, is a copper-enriched (ENR) zone consisting of sub-horizontal 5–10mthick lenses extending up to 20–30 m and formed by primary and secondary sulfides.
Geochemical associations were identified. Cobalt is concentrated on the surface. However, there is no correlation between copper and cobalt grades in the mineralization. The cobalt is intimately associated with sulfur and iron, suggesting that this element is in the mineral structure of pyrite. Iron is more abundant in the pyrite and chalcopyrite zones, and a higher molybdenum content is found in the chalcopyrite–pyrite mineralization and in the leached zone.
The presence of abundant (>10%) hydrothermally precipitated magnetite with associated copper–iron sulfides in the breccias hosting the mineralization suggests a deposit type such as an iron oxide–copper–gold (IOCG). However, there are features of the Boa Esperança deposit that do not match the proposed IOCG deposit type. Among these are the presence of high sulfur mineral assemblage (chalcopyrite–pyrite), rather than the low sulfur copper sulfide mineral assemblage characteristic of the IOCG deposit type (chalcopyrite–bornite–chalcocite), as well as the high quartz content, the absence of pervasive hydrothermal alteration of the host rock, specifically sodic (albite) alteration, and the absence of gold.
As such, the Boa Esperança copper deposit is interpreted to be a variant of an IOCG deposit type.