Summary:
The Cabaçal and Santa Helena deposits, along with other identified targets, are associated with a Paleoproterozoic volcanogenic massive stringer and disseminated sulphide system hosted within deformed metavolcanic-sedimentary rocks of the Alto Jaurú Greenstone Belt.
Mineralization consists of massive stockwork/breccia-style veins, stringers, and disseminated sulphides, primarily composed of chalcopyrite with lesser amounts of pyrite and sphalerite.
Deposit Type
The mineralization host units comprise volcanic, volcanoclastic, and sedimentary rocks deposited in a marine environment, intruded by basic granitic and plutonic rocks. All units have undergone intense metamorphism and deformation, including folding and local faulting.
The Cu-Au-Ag mineralization zone extends approximately 650 m down-dip and 2,350 m along strike. It trends approximately 135°, dips 10-20° southwest, and has a shallow southeast plunge of 10-20°. At lower grades, the mineralization demonstrates good continuity, with higher-grade gold-copper trends occurring within a broader lowgrade halo. The mineralization spans approximately 200 m in vertical extent, with a true width generally ranging from 10 to 90 m.
The Cabaçal deposit exhibits a range of mineralization styles characteristic of VMS systems, including massive sulphide zones, breccias with sulphide clasts and/or matrix, sulphide stringer zones, and disseminated sulphides. One of the most important styles for the gold mineralization involves a later-stage gold event superimposed on the original VMS stratigraphy. This is exemplified by existing gold-rich quartz-sulphide veins, free gold developed within foliation, and gold-only layers within chloritic alteration zones, detached from the primary VMS mineralization envelope. In addition to drill core observations, photographs from the Cabaçal mine operations effectively illustrate good examples of the mineralization styles.
Mineral species at the Cabaçal mine include pyrite, chalcopyrite, pyrrhotite, sphalerite, galena, and gold, along with bismuth, selenium, and tellurium alloys.
In the uppermost part of the Cabaçal deposit, mineralization has undergone oxidation. Erosion has removed much of the upper clay saprolite, resulting in a relatively shallow depth to fresh rock, generally ranging between 5 and 20 m and averaging between 1 and 15 m below surface.
Significant Mineralized Zones at Cabaçal
Mason & Kerr (1990) carried out a comprehensive reassessment of all geological data and analytical results from boreholes, supported by observations from selected exposures at the Cabaçal mine front.
According to these authors, mineralization at Cabaçal is related to three hydrothermal centres: the Central, East, and South Copper Zones. The main gold zone is a flat stringer zone related to the South Copper Zone (SCZ). In the Central Copper Zone (CCZ), past mining focused on high-grade gold in the core of an intense hydrothermal alteration pipe (ZCl) and, to a much lesser extent, on the massive sulphide lenses. The East Copper Zone (ECZ) exhibits a weaker presence of vein-type copper mineralization, with erratic high-grade gold mineralization of the stringer type.
The attributes of each zone and examples of significant mineralization within them are as follows.
The ECZ is an elongate zone of sulphide stinger zones developed within chloritic CTB. The domain shows some thickening in the basal ZCL at the CTB/TAC contact, which dips at approximately 25° SW, where copper grades can increase to over 1% Cu in SE-plunging shoots. Gold grades tend to be more modest, but high-grade intersections have been identified that warrant further drilling. Examples include: JUSPD119: 5.6 m at 7.7 g/t Au & 1.1% Cu from 45.2 m; JUSPD222: 6.4 m at 60.1 g/t Au, 0.5% Cu & 4.0 g/t Ag from 47.7 m; JUSPD025: 5.0 m at 85.3 g/t Au, 0.7% Cu & 10.0 g/t Ag from 71.5 m. Mason and Kerr (1990) noted that the ECZ corridor was relatively sparsely drilled compared to the other two domains, with the northern, northeastern, and eastern limits of the ECZ not well defined. The mineralization is truncated by the shallow-dipping gabbro intrusion at the SE end of the workings. Limited exploration has been carried out to investigate the potential extension of the deposit east of the gabbro.
The CCZ is cored by a well-developed, intensely chloritized discordant pipe with strong Cu-Au mineralization. It is the only unit that hosts a massive/breccia sulphide layer, in a bed of 5 to 150 cm thick, capped by a ’chert‘ (potentially a silicified tuffaceous horizon). The massive sulphide consists of layers or lenses with the same sulphide assemblage occurring in the stockwork and stringer veins, suggesting that they are related to the VMS feeder system. A zinc-barren window is centred over the pipe in plan view, but the pipe is flanked by a broad zone of elevated zinc grade values (ranging from 0.1% to 5.8% Zn), dispersed at the top of the ZCL and in the Zbi. This represents characteristic VMS zonation outward from a hotter zone of copper in the throat of the pipe, transitioning to cooler, zinc-rich areas at the margins.
The SCZ is also associated with a major alteration pipe, but the early biotite alteration has not been obliterated by the intense chloritic alteration as in the core of the CCZ. The SCZ was interpreted to represent an intermediate stage of development of the VMS system that was more advanced than the ECZ, but not quite so advanced as the CCZ. The SCZ hosts several high-grade copper mineralization zones near the lower limits of the underground development, including intersections such as JUSPD482: 13.4 m at 1.2 g/t Au, 5.5% Cu, 24.7 g/t Ag & 0.5% Zn, JUSPD596: 15 m at 2.7 g/t Au, 5.2% Cu, 9.5 g/t Ag & 1.2% Zn).
The Manoel Leme Formation in the Cabaçal Belt covers an area of about 35 x 2 km (70 km2 ) trending WNW/ESE. This formation is the host to the mineralized sequence at Cabaçal, which is interpreted to be situated on the overturned eastern limb of an east-verging anticline.
Lithology and Structure
The overturned geology of the immediate Cabaçal Mine area can be classified into several principal lithological units (Mason and Kerr, 1990; Monteiro et al., 1988, Osbourne, 1991). A strong structural and alteration overprint can sometimes obscure the original protolith in these schistose rocks. Multielement geochemistry indicates the presence of subdivisions which may reflect variations in detrital input and/or possible intercalation between volcanic and sedimentary units.
Quartz-sericite-chlorite-(biotite) schist (CTB): Predominantly a tuffaceous meta-sediment, this unit hosts high-grade gold and copper mineralization. Data from multielement analyses (e.g. nickel, chromium, titanium and zircon) show variations indicating transitions from felsic to intermediate to mafic compositions.
Chlorite-quartz-biotite-(sulphide) schist (ZCL): This distinctive unit is represented by strong shearing, frequently carrying at least 1-5% sulphide minerals (Chalcopyrite > Pyrite > Pyrrhotite). Locally, remnants of a thin magnetite banded iron formation (BIF) (0.3 – 0.5 m) are found near the top of this unit.
Biotite-quartz-carbonate-chlorite schist (Zbi): This narrow horizon is most prominent where the ZCL is thickest. Although it carries little gold mineralization, it can contain some sphalerite and, locally, galena. Locally, this unit is associated with a thin chert band exhibiting sedimentary structures that indicate the sequence is overturned.
Quartz-sericite-biotite-feldspar schist (TAC): This dacitic metavolcanic/volcaniclastic unit (coded VAP where porphyritic textures are present) occupies the structural footwall of the deposit. It is considered barren, although few holes penetrate to any great depths through the unit.
Post-mineralization gabbro intrusion (Gb): A gabbroic intrusion post-dating mineralization developed east of the mine workings (Gb).