Summary:
The Toka Tindung Gold Mine area is located on the eastern margin of the South East Asia Plate within the SulawesiEast Mindanao magmatic arc, a highly mineralised belt of volcanics and high level intrusives of early Miocene to Quaternary age. A series of north-northwest–south-southeast trending faults define structural corridors which host the deposits. The deposits are low-sulfidation epithermal systems.
Low-sulphidation epithermal gold mineralisation within the Project area occurs in shoots that form at dilational sites along quartz vein-hosted structures.
The Toka Tindung deposit lies at the northern end of a series of north-northwest trending faults. These faults define a structural corridor at least 2 km wide and extending for about 15 km south of Toka Tindung Gold Mine. The mineralised system is up to 200 m wide and has been delineated over a 1.2 km strike extent. The gross trend of the stockwork zone and larger veins at Toka show a north strike, with the veins generally near vertical or dipping steeply to either the east or west. Local variations in dip, strike and width occur along individual veins within the stockwork system.
The main structural elements in the Batupangah area are brittle faults and fractures with north, north-northeast and northwest mineralised trends. These structures are interpreted to be block faults and have dips generally steeper than 60°. In summary:
- At Pajajaran, two subvertical, parallel sheeted veins around 2 to 5 m wide and a minor stock occur with a northwestern trend;
- The Blambangan deposit consists of a single vein around 2 to 5 m wide with minor stockwork and a northerly trend;
- The Araren deposit consists of two veins each varying between 1 and 7 m in width with north to northeast trends.
Mineralisation within the concession area is characterised by pervasive silicification and adularia alteration within and around the mineralised vein systems zoning vertically and horizontally. Traces of disseminated pyrite, chlorite, carbonate and hematite accompany this alteration. These rocks are associated with anomalous arsenic, antimony and mercury geochemical signature, with lesser molybdenum and tellurium.
Minor argillic alteration overprints the adularia altered volcanics and comprises variable proportions of smectitite, illite, minor chlorite, carbonate, hematite and pyrite. This overprint is strongest at the contact of the upper andesite.
Haloes around the two mineralisation styles at Toka Tindung Gold Mine provide the basis for a pathfinder alteration model supporting both the Near Mine and Greenfields exploration activities. The two mineralisation styles exhibit similar alteration composition but differ in morphology and spatial distribution of certain trace elements in their haloes. The basic difference between the two mineralisation styles is reflected in the host rocks and how they react to the altering fluids.
In the Toka-style alteration system the volcaniclastic host rocks are permeable, leading to a more extensive alteration halo (with the silica halo at Toka measuring some 3.5 km by 1.3 km).
From an exploration perspective, increasing quartz vein percentages, hydrothermal alteration and elevated gold/silver values, are all interpreted to be indicative of proximal mineralisation.
Weathering and oxidation of the altered volcaniclastics and vein systems is generally weak to moderate and produces mixed assemblages of iron oxides, kaolinite and smectite.
All economic gold and silver mineralisation discovered to date at the Toka Tindung Gold Mine is of low-sulphidation epithermal style comprising quartz–adularia vein and stockwork-hosted gold and silver deposits. This low tonnage, high grade gold mineralisation typically occurs within small shoots that form at dilational sites along the host quartz vein structures. District scale structural controls on mineralisation and include north–south and north-northwest trends.
Within the broader concession area, two broadly northwest-trending structural corridors are interpreted to focus the epithermal mineralisation. Within these interpreted corridors, the shoots are developed at sites of structural complexity where there are strike and/or dip changes, intersecting structures, links and/or jogs developed. Other controls on mineralisation include the stratigraphy, host lithology (volcanics versus volcaniclastics) and proximity to intrusive bodies.
Gold and silver mineralisation occurs within quartz vein structures, which are sulphide and base metal-poor with abundant chalcedonic silica, microcrystalline and fine-grained quartz, adularia and minor calcite. These veins vary from solid vein/lode to breccia, containing both wallrock and quartz vein clasts to stockwork or sheeted veins, to fracture fill, cavity fill or as clasts within a fault. Vein mineralogy includes assemblages of quartz, adularia, gold, clay, carbonate, chalcedony, chlorite, hematite and sulphide. While crystalline quartz is the most common vein phase, other quartz vein textures include bladed, cockade, colloform, comb, drusy, vuggy, crustiform, cryptocrystalline, massive, moss and saccharoidal.
At the Toka deposit, the banded and brecciated quartz-adularia veins range in thickness from <1 up to 10 m and are associated with a zone of pervasive quartz–adularia alteration. In excess of 60 veins have been defined within the deposit, typically grading between 3 and 5 g/t Au, and locally up to 30 g/t Au. Silver grades are generally low, with an Ag:Au ratio in mined ores of 2.3:1. Quartz vein stockwork zones are found between the main veins and have a bulk grade ranging between 0.2and 3 g/t Au. Weathering and oxidation to depths of up to 80 m is localised along the main veins and structures.
The southern pits (i.e. Pajajaran, Blambangan, Kopra, Araren and Alaskar deposits) are much simpler ore systems than at Toka and range from single veins to dual and multiple splayed veins and vein stockworks, with vein widths typically between 1 and 10 m.
The majority of the gold delineated to date (from all deposits including those of the Western Corridor) exists as fine, 5–20 micron (µm), grains of ‘free’ gold or associated with the silver–gold alloy electrum.