Summary:
Thin, discontinuous mineralized shear zones at the Toroparu deposit are developed mainly in volcanic rocks. Higher grade, discontinuous shear zone hosted mineralization is narrow and mostly parallel to the schistosity. The main controls on mineralization are the west-northwest striking axial planar schistosity and vein swarms that are well developed in the volcanic rocks, and the folded contact between an intrusive complex and volcanic rocks, particularly the contact of an igneous breccia that forms an important rheological contrast, similar to many other orogenic gold deposits that are strongly controlled by competency contrasts. The Toroparu deposit dips roughly 55° to the west. The Sona Hill deposit has similar controls on mineralization but strikes to the north and dips around 30° to the west.
Two dyke phases are present including hornblende porphyritic andesite dykes and dolerite dykes. Most dykes have an apparent thickness of less than 0.5 m, but some dykes up to 2.5 m thick also occur. The hornblende porphyritic andesite dykes are weakly to intensely deformed with schistosity roughly parallel to that of the host rock. In places, these dykes are folded. Despite being sheared and folded, the dykes are mostly non-mineralized, although some mineralized veins occur along the contacts. Some of these dykes are cut by mineralized shear zones. The dolerite dykes are fine grained, nonsheared, and non-mineralized. They crosscut gold mineralized veins, silicified zones, and shear zones and postdate both the deformation and the gold mineralizing events. The dykes are less abundant and more discontinuous at Sona Hill compared to Toroparu.
A thick, gradational, 10 to 35 m thick layer of saprolite with preserved mineralized quartz veins and veinlets, showing evidence of some gold leaching, is present at the surface at Toroparu and reaches up to 60 m thick at Sona Hill. A weathering profile comprised of overburden, saprolite, and a transition zone has been interpreted for the mineral resource estimate. The overburden has abundant low grade gold mineralization but little high grade.
Deposit types
The interpretation of the deposit type at Toroparu is uncertain, with possibilities including an unusually copper rich orogenic gold deposit on the basis of the host greenschist metamorphic rocks and a strong control of mineralization due to competency contrasts, as well as a metamorphosed porphyry gold-copper deposit. Recent structural interpretation work suggests that Toroparu is not a classic lode type orogenic gold deposit and is unlikely to be a deformed porphyry deposit (Pratt and Smeraglia, 2022). A disseminated or sheeted vein type deposit can be used as a guide for exploration planning purposes.
Mineralization and alteration
Mineralization at the Toroparu deposit estimated as mineral resources in the main zone has a volume of around 1.3 km along strike, around 500 m across strike, and a depth of 550 m. There is a zone of mineral resources approximately 1.1 km to the southeast of the main zone with a volume of around 400 m along strike, 230 m across strike, and a depth of 250 m. There are a few other small zones of mineral resources on the order of 100 m long along strike of and parallel to the main zone. The mineralized shear zones are narrow and discontinuous. Sona Hill has a volume of around 950 m along strike, up to 300 m across strike, and a depth of around 200 m. Sona Hill is characterized by a lower copper content compared to Toroparu. Both deposits are open at depth.
The main body of mineralization at Toroparu is characterized by three different vein assemblages including:
• Gold mineralized quartz and chalcopyrite or bornite veinlets occur both in the volcanic and intrusive rocks and appear to be focused on the boundary between them, particularly within a marginal igneous breccia. Chalcopyrite and quartz are commonly coarse and intergrown. The veinlets are more abundant and thinner in the volcanic rocks, are parallel to the schistosity, and tend to have lower gold grades. The veins are less continuous in the intrusive rocks and igneous breccia but tend to be of higher gold grade and contain molybdenite. Within the intrusive rocks, the veins show an intense chlorite alteration halo. Vein swarms in the volcanic rocks occur in zones up to tens of metres thick with low to medium gold grades, with scattered high grades coinciding with high chalcopyrite content. In places, the veins are folded and boudinaged, with chalcopyrite often concentrated in the boudin necks. Veins range between less than 1 mm up to a few centimetres thick. There are rare 0.4 to 0.5 m thick veins.
• Gold mineralized chalcopyrite only veinlets occur in the volcanic and intrusive rocks. These veinlets are up to a few millimetres thick and are strongly transposed and dismembered parallel to schistosity and are also folded. In places, chalcopyrite veinlets form a scattered network in quartz veins.
• Gold mineralized quartz and molybdenite veins are also present, mostly in the igneous breccia along the intrusivevolcanic contact. These veins are scattered and contain high gold grades.
Local gold mineralized silica alteration occurs as discrete patches or as zones up to tens of metres thick that affect volcanics, intrusives, and igneous breccias. In places the silica is associated with epidote. Silica alteration is characterized by disseminated chalcopyrite and is crosscut by quartz chalcopyrite veins and chalcopyrite veinlets. In contrast with the schistose rocks, the veinlets have an apparent random orientation. Silica alteration halos occur around the chalcopyrite veinlets and have higher gold grades.
Structure
The mineralized veins are folded, sheared, transposed, and boudinaged, and are rotated by shearing parallel to the schistosity in both volcanic and intrusive rocks. Mineralized veins, shear zones, schistosity, and the hornblende porphyritic andesite dykes show a consistent west-northwest to northwest strike and moderate to steep southwest dip, consistent with the regional northwest strike of folds interpreted from the aeromagnetic data. This fractal similarity suggests that the main pit mineralization is part of a major regional fold structure. It is likely that these folds are tight, and most limbs and contacts are parallel to schistosity. Fold areas may be areas of high grade and likely have a shallow plunge and present an exploration target.
Shear zones with a distinctive intense schistosity and sigmoidal shape ranging in thickness from 0.1 m to up to a few metres are common in the Toroparu main pit area, especially in the volcanic rocks. Gold mineralized quartz and chalcopyrite veins are sheared, transposed to parallel and are folded along the shear zones. Some of the shear zones are flooded by silica alteration. Non-mineralized shear zones are characterized by smectic and carbonate, possibly ankerite. Most of the gold mineralized veins, shear zones, non-mineralized shear zones, schistosity, and bedding show a northwest strike and a steep to moderate southwest dip.