Summary:
The Tucano gold deposits are hosted in shear zones within a number of different Paleoproterozoic, metasedimentary host rocks. This mineralization style is considered to represent an orogenic, structurally controlled gold mineralizing system.
The mineralization at the Tucano Gold Mine occurs in a series of deposits over a sevenkilometre strike length associated with a north-south trending shear zone occurring coincident with a north-south line of topographic ridges. The shear zone is hosted by the Proterozoic aged Vila Nova greenstone belt and occurs near the contact of a large dominantly clastic/ chemical sedimentary package to the southwest and amphibolitic package to the northwest. The greenstone belt has been extensively intruded by younger mafic to felsic intrusive bodies.
From south to north, these deposits have been named TAP A, B, C, and Urucum. TAP D is a separate structure in the west. Higher grades are associated with the more intensely hydrothermally altered iron formations and iron rich carbonate units bounding and intercalated with them.
The texture and mineralogy along the shear zone indicate a high-temperature hydrothermal system. The interaction of favourable rock type, structure, heat, and mineralized solutions has resulted in the deposition of gold bearing, non-refractory sulphides concentrated near or on major lithological contacts. This mineralized zone exhibits intense hydrothermal alteration, particularly silicification and sulfidation, bearing auriferous pyrrhotite, and pyrite. Alteration is most intense in the proximity of reactive meta-sediments such as BIF, followed by amphibolite, carbonate, schist, and to a lesser extent, calc-silicate rocks.
TAP AB AND URUCUM
The TAP AB and Urucum deposits are emplaced within a north-south trending, multiply deformed volcano-sedimentary sequence of rocks that is bounded to the west by the Amapari Granite. The deformation history has resulted in steeply dipping N-S shear zones and the gold mineralization is associated with alteration and sulphide mineralization of various lithologies within the deformed sequence. Mineralization occurs over at least seven kilometres of strike length and has been interpreted to extend to more than 700 m below the surface. A suite of late post-gold pegmatites has been emplaced throughout the sequence in various orientations and is largely barren of gold.
URUCUM EAST
The Urucum East deposit is located east of the Urucum Mine in the northern fold hinge of the Tucano Gold Mine stratigraphy. The deposit consists of a single, east-west striking and flat, (- 30°) north dipping sulphide lode which has an average thickness of seven metres and a strike length of 230 m. Mineralization at Urucum East is hosted in a wedge of carbonate and altered BIF located inside a swarm of parallel, east-west striking and north dipping (-30°) pegmatite dykes/sills which have intruded the host schist unit. The deposit is covered by a blanket of poorly mineralized colluvium up to 10 m thick and the deposit is weathered to a depth of approximately 50 m below the surface.
DUCKHEAD
The Duckhead deposit is located south east of the TAP AB deposit. Mineralization at Duckhead is controlled by the recently interpreted intersection of steep east-west striking shear zones with a BIF lithological contact to form steeply west plunging high grade shoots. The texture and mineralogy along the shear zone indicate high-temperature hydrothermal alteration, particularly silicification and sulfidation, bearing auriferous pyrite.
MINERALIZATION WEATHERING TYPES
Deep weathering is present in most of the deposits. Steeply dipping, high grade mineralization extends to the surface where it is truncated by a layer of colluvium several metres thick. Gold mineralization can be found in the fresh rock at depth, in the saprolite zone created by in- situ weathering of the underlying rocks, and in colluvial deposits which overlie the saprolite mineralization as a blanket, spreading out over the hill slopes. The saprolite and the colluvial mineralization are collectively grouped together as “oxide mineralization”.
Sulphide zones follow shear plane foliation, often crosscutting the BIF and other host metasediments and as bedding parallel lenses dipping either east or west along the limbs of the folded BIF units. Outside the shears and faulted zones, host rocks are poor in sulphide and gold. The accumulation of auriferous massive and/or disseminated sulphides in zones of fractures and folds, and forming plunging mineralized shoots, often crossing lithological contacts, suggests an epigenetic event.
PRIMARY OR SULPHIDE MINERALIZATION
Sulphide mineralization within fresh rock is only found in drill core and does not outcrop at the Tucano Gold Mine. Pyrrhotite and pyrite are the most abundant sulphides. Chalcopyrite, arsenopyrite, sphalerite, and galena occur in trace amounts (less than 1%). At Urucum, the mineralization occurs with intense silicification, and pyrrhotite is the dominant sulphide. At TAP AB, the gold is associated with masses containing 5% to 10% pyrite, and pyrrhotite only occurs in trace amounts.
The individual sulphide masses are several metres thick and can be elongated on strike along north-northwest or north-south orientations. The sulphides extend in depth along a plunge dipping from 10º to 40º at about N10ºW. The dip ranges from almost vertical at Urucum, to 30°W for the western carbonate load at the south of the TAP AB deposit. Studies show that the gold occurs as free gold and not tied into the crystal lattice of the sulphide minerals. Mineralization is not confined to any one lithology, nor is it concordant with lithological contacts.
SAPROLITE MINERALIZATION
Intense tropical weathering reaching down 30 m (Urucum and northern zone) to over 300 m (TAP AB) has caused the formation of saprolite, that is, an in-situ oxidation of the sulphide mineralization and host rocks. The saprolite consists mainly of iron oxides and hydroxides, clay, and silica. Gold mineralized zones within saprolite follow the strike, dip, and plunge of the sulphide mineralization. Semi-decomposed remnants of the sulphide mineralization become more frequent with depth.
Weathering has left much of the saprolite material amenable to free digging, however, some of the saprolite has required blasting prior to excavation. The mineralization in these more competent zones may range from entirely oxidized to silicified with partially or even fresh sulphides.
COLLUVIAL MINERALIZATION
The colluvial deposits occur along north-south trending ridges cut by William Creek. The creek is at about 115 m elevation and the ridges reach 300 m. The top and slopes of the ridges are covered by alluvial and colluvial sediments. It is difficult to separate alluvial and colluvial sediments in the field and therefore they are collectively named colluvium.
All mineralized gold zones in fresh rock are covered by mineralized colluvium, which varies in character according to the subsurface lithology. The grade of mineralization in colluvium tends to reflect the grade of underlying ore shoots, with patches of low grade or barren colluvium usually reflecting low grade or barren underlying saprolite zones. However, zones of mineralization in colluvium tend to be wider than in underlying saprolite due to mechanical transport and development of some secondary mineralization due to variations in surface soil chemistry.
Deep weathering and intense fixation of iron in the upper portions of the soil profile, often create a laterite horizon. The top of the colluvium is usually a layer rarely more than one metre thick composed of silty, clayey, and sandy materials, poor in fragments of limonite. Immediately below there is a variable layer up to 10 m thick containing lateritic fragments rich in iron oxide dispersed in a ferruginous clay-sand matrix which becomes rich in manganese at the base.