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Location: 20km NE from Canaa dos Carajas, Brazil
Suite 501, 543 Granville StreetVancouverBritish Columbia, CanadaV6C 1X8
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Planalto is interpreted to be a volcanic hosted iron-oxide copper gold (IOCG) deposit, with similarities to other deposits in the Carajás Province and also elsewhere in the world. The key features of this type of mineralisation were defined in the late 1980s and early 1990s based on studies of Proterozoic deposits such as the Olympic Dam (Australia), Kiruna (Sweden) and Missouri (United States of America).The Planalto copper gold deposit has several characteristics that are common with other IOCG deposits in the Carajás region including:• Host rocks are metavolcanics developed in an Archean cratonic continental rift basin;• Mineralisation is hosted in a complex fault system adjacent to the major basin-scale Carajás fault;• Mineralisation is associated with early sodic metasomatic/hydrothermal alteration typified by albite and scapolite formation and followed by potassic alteration (K-feldspar and biotite);• An alkaline granite complex is located immediately adjacent to the mineralization and has similar sodic and potassic alteration;• The main copper-bearing mineral is chalcopyrite. Gold is also present, albeit in low concentration;• Chalcopyrite shows a strong association with magnetite;• Chalcopyrite mineralisation occurs both disseminated and in a stockwork pattern of veins and fine fractures;• Mineralisation contains local concentrations of rare earth elements (hosted in allanite) and patchy molybdenite.There are several key differences from other IOCG deposits in the Carajás region, however, including:• Mineralization is associated with a set of stacked flat to gently west dipping lens-shaped structures;• Absence of major breccia formation commonly described in many of the Carajás deposits;• Overall copper grades at approximately 0.5% Cu are lower than reported for most other Carajás deposits, which range between 0.7% and 2.0% Cu;• Gold concentrations in the chalcopyrite are significantly lower at an average content of 50 ppb Au, compared to 300-500 ppb Au in the majority of the Carajás deposits.Deposit Geology and MineralisationPlanalto copper mineralization is hosted in a thick pile (>700 m) of intensely altered, mafic-intermediate volcanics. The volcanics are generally weathered to a depth typically up to 15 m from surface with only a few outcrops of fresh rocks along the margins of the Cupuzeiro creek.Host volcanics have compositions that vary between andesitic to dacitic. The volcanics have been affected by several episodes of veining. This veining is gently to tightly folded in proximity to mylonite zones.Later veining consists of pinkish K-feldspar, epidote, carbonate and fluorite veins from a few millimetres to several tens of centimetres wide. Magnetite veins, from a few millimetres to several tens of centimetres wide, are common. Disseminated magnetite occurs throughout the volcanics but is most common within the chalcopyrite-rich zones.The earliest hydrothermal alteration affecting the volcanics was a strong sodic alteration forming albite and locally scapolite. This was followed by strong potassic alteration with the formation of K-feldspar and biotite. A later propylitic alteration resulted in intense chloritization of earlier formed mafic minerals.Chalcopyrite and pyrite are the main sulphide minerals observed and occur as disseminations, fracture fill and as irregular aggregates several centimetres in size.Planalto Granite ComplexThe Planalto Granite, occurs at the southern end of the mineralized area. The granite has a core zone of coarse grained strongly foliated biotite and hornblende bearing porphyritic Kfeldspar-rich granite.Mafic volcanics wrap around the western and eastern flanks of this granite.Metasomatic Alteration Surface mapping along the granite contact and core logging have shown that metasomatite zones occur both along this contact and within the mafic volcanics.Drill core logging has identified a sub-horizontal, north-trending pipe-like body of metasomatic alteration infiltrating the volcanics for approximately 350m along the axis of the Homestead deposit at depth of 100 – 150 m. Zones of strong chalcopyrite mineralization are located above and below this metasomatite body.Fracture and breccia zones cut the metasomatite near the outer contact and in places these are partly cemented by chalcopyrite. These are interpreted as part of the fluid pathways that channelled copper-bearing fluids into the volcanic pile. Mineralisation Drilling has identified that copper mineralisation is present along a north-south trend of more than 1500 m within a corridor up to 400 m wide from east to west. This trend is interpreted to cut across the regional NW-SE stratigraphic bedding or layering in the volcanics.Potentially economic mineralisation, at >0.25% Cu, is interpreted to occur in stacked sub-parallel sheet-like structures that persist over the 1500 m north-south strike length along the mineralised zone. Individual zones vary from a few metres to up to several tens of metres thick.Chalcopyrite is the main copper-bearing mineral recognised in the deposit and occurs as coarse millimetre-centimetre-size aggregates and fillings in irregular millimetre to centimetre-wide fractures.Chalcopyrite also occurs as films in hairline fractures and as fillings in fractures in K-feldspar and epidote, quartz, calcite and fluorite veins, and also occurs as fine dissemination in strongly chloritised volcanics.Pyrite is rarely seen in the drill core in the Homestead sector; however, it can constitute up to 5% of the rock in the Cupuzeiro zone and is abundant in the drill core within intervals with more >1% Cu.
