Summary:
The Tocantinzinho deposit is best classified as a granite-hosted, intrusion-related gold deposit. Intrusion related gold deposits were defined by Thompson et al. (1998) with the following characteristics:
• Deposits hosted within or zoned proximal to intermediate to felsic granitic rocks.
• Intrusions are typically moderately reduced to moderately oxidized (ilmenite through to magnetite series) I-type granitic rocks.
• The associated pathfinder elements are typically Bi, Te, Mo, W in the core of the intrusion system, zoning outward to distal As, Sb, Pb and Zn.
• The deposits have a range in styles including sheeted, breccia, stockwork, flat-vein and disseminated to greisen that is controlled by proximity to intrusions, depth of emplacement and structural controls on intrusions.
• Mineralization is coeval with the related intrusions demonstrated through zonation with respect to the causative intrusion and mineralized magmatic-hydrothermal transition textures. These may include miarolitic cavities, vein dykes, unidirectional solidification textures, brain rock, and granitefacies control on gold distribution. In addition, age dating in global examples has confirmed synchroneity between intrusions and mineralization.
• Mineralization is typically characterized by reduced, low sulfide (< 2%) ore assemblages including pyrite, pyrrhotite and arsenopyrite with magnetite less common and hematite rare. Proximal gold is typically high fineness and paragenetically related to Bi ± Te.
• Alteration is usually more limited than in typical porphyry environments, and characterized by early, high temperature quartz-feldspar (both potassic and sodic) alteration intimately associated with magmatic-hydrothermal transition textures that evolve to lower temperature white mica / sericite-carbonate-chlorite-quartz alteration and veining typically associated with the main mineralization stage.
• The mineralization formed from H2O-CO2 ± salt magmatic fluids.
• The majority of known deposits are Phanerozoic in age and formed in continental arc to back arc settings.
The Tocantinzinho deposit forms a sub-vertical, northwest-trending elongate body approximately 900 m long by 150 to 200 m wide. It has been drilled to approximately 450 m depth and remains open at depth.
The gold mineralization is bound by two structural zones which mark the contact with the surrounding barren granite and monzonite rocks. The structural corridor represents an outer geological constraint on the mineralization. The andesitic intrusive body close to surface is also largely unmineralized and is therefore an internal constraint on the mineralization.
The grade distribution is similar in the both the Smoky and Salami granitic subunits. The green-grey colour of the Smoky granite is due the alteration of plagioclase to sericite with lesser chlorite, calcite and pyrite particularly in the core of the feldspar. Salami mineralized granites are distinctively bright red due to hematite alteration. Staining for potassium feldspar using cobaltinitrite has revealed that both potassic and sodic feldspar are host to the hematitic dusting in feldspar and therefore the red-pink colour is not indicative of potassic alteration. Anastomosing veinlets are common and similar in scale to those in the Smoky granite but are generally filled with a distinct black chlorite and lesser sericite-calcite-pyrite-quartz. Quartz textures in the Salami and Smoky granites commonly have curvilinear grain boundaries with feldspar. Chlorite-sericite-calcite veins are typically wormy rather than planar and appear to cut feldspar but develop along the margins of the quartz grains suggesting this was a relatively late magmatic-hydrothermal phase. Contacts are diffuse between smoky and salami granites and a complete gradation exists between the two units.
Pyrite is the main sulfide phase at the Tocantinzinho deposit and commonly contains inclusions of chalcopyrite and pyrrhotite. The presence of pyrrhotite is genetically significant because it indicates that the mineralization may have at least formed locally under reduced conditions despite the abundance of hematite in the outer red granite. Gold grains were observed in close association with pyrite along grain boundaries, within fractures and locally as inclusions within pyrite. A white-grey mineral was also observed associated with gold and bismuthinite or bismuth. Multi-element data for samples with anomalous gold (> 0.1 g/t) show a strong correlation pair between gold and bismuth (0.87 correlation coefficient).
Exploration Targets
The Santa Patricia target occurs 2.5 km west of the main Tocantinzinho deposit and is defined as an over 8-km northwest striking copper anomaly (> 20 ppm) based on soil geochemistry that coincides with a regional magnetic lineament.
The Santa Patrícia target geology is mainly comprised of intrusive rocks, including mafic to intermediate and alkali-granitoid plutonic suites, of which are transected by late-stage fine-grained andesite and rhyolite dykes.
The mafic to intermediate suite is composed of medium-grained equigranular to porphyritic granodiorite and quartz monzonite, lesser diorite to monzodiorite and localized gabbro. The alkali-granitoid suite consists of granite to alkali feldspar granite transitional to Salami-textured syenite and quartz syenite. Late phases of this suite include micro-granite, aplite and pegmatite. The contacts between the two suites vary from gradational to crosscutting with the fractionated aplite-pegmatite phases typically as the youngest. Evidence of magma mingling and mixing textures between granite, granodiorite and diorite suggests that the two suites may be coeval.
Copper-molybdenum mineralization occurs within veins that are hosted by all of the above igneous rocks, including the late andesite.
A paragenetic framework has been established that comprises six main vein and alteration stages, including:
• The earliest phase as white quartz ± potassium-feldspar stockwork veining that are typically planar, and less commonly irregular, with associated pink-red potassium-feldspar alteration occurring as vein halos to wider pervasive zones.
• The main copper-molybdenum stage associated with second generation veins composed of magnetite- muscovite-quartz-chalcopyrite-pyrite-molybdenite±hematite within a strong pervasive coarse muscovite alteration, which is greisen-like in places.
• Localized laminated quartz-molybdenite veins with silicification.
• Pyrite-sericite-calcite veinlets and alteration.
• Distal propylitic epidote-calcite-chlorite-pyrite veins and alteration.
• Late K-feldspar-calcite veinlets that crosscut all vein stages.
The Santa Patricia copper molybdenum system is mostly devoid of gold. Where localized gold grades occur (approximately 0.15 g Au/t), the mineralization style is more similar to that within the Salami granite style of Tocantinzinho.
Rock types at the KRB target are comprised of a variety of intrusive rocks from alkali feldspar granite, syenogranite and monzogranite to quartz monzonite, granodiorite and monzonite. These sequences are intruded by microgranite, pegmatite, aplite, as well as late dykes of felsic to intermediate composition. The understanding of mineralization controls at KRB is at a conceptual stage considering the limited exploration campaign carried out at this target to date. Gold appears to be associated with quartz veining and pyrite along the margins of felsic to intermediate dykes.