The mineralisation is of the ‘orogenic lode gold’ type, comprising mesothermal reefs of quartz containing gold and sulphide minerals including galena, sphalerite and pyrite, hosted by granitic bodies. The reefs are usually one to two metres thick, but have strike lengths of from several hundred metres and up to two kilometres in the Central area. Gold is relatively fine-grained, mostly less than one millimetre, and mineragraphic microscopy shows gold is primarily late-stage. Gold particles are located along grain boundaries, with minor amounts contained within sulphide grains, predominantly pyrite, making it amenable to gold extraction by cyanidation, as the sulphide grains break along grain boundaries and fractures during milling, exposing thin wide gold surfaces to the cyanide solution. The ore is not regarded as refractory, with recoveries usually of 97% to 98%.
The gold-bearing reefs at Charters Towers are typically 0.3 metres to 1.5 metres thick, comprising hydrothermal quartz reefs in granite, tonalite and granodiorite host rocks. There are some 80 major reefs in and around Charters Towers city, of which 22 are included to date in the Company’s resource estimate. The main east-west reef systems are the Brilliant, the Day Dawn, the Mexican, the Queen and the Sunburst, extending over a strike length of five kilometres and cut by NNW trending cross reefs. There is a second E-W system 800 metres to the south comprising the Golden Sunrise, Mary and Clark’s Moonstone line of reefs, and a third system 500 metres further south, comprising the Ruby and Gladstone line of reefs. They are found in extensive sheet-like alteration zones (reefs). The most productive gold-bearing reefs (the Day Dawn, Brilliant and Queen) dip to the north beneath the city of Charters Towers.
The majority of the ore mined in the past was concentrated within a set of fractures over 5 km long East-West, and 500 metres to 1,600 metres down dip in a North-South direction. The mineralised reefs lie in two predominant directions dipping at moderate to shallow angles to the north (main production), and the cross-reefs, which dip to the ENE. The E-W and NNW trends seen at the regional scale are repeated at local scale on the Company’s mineral holdings. The reefs are hydrothermal quartz-gold systems with a gangue of pyrite, galena, sphalerite, carbonate, chlorite and clays. The reefs occur within sericitic hydrothermal alteration, historically known as ‘Formation’.
While the reefs are typically 0.3 to 1.5 metre wide, they range locally up to 6 metres thick, and in isolated cases up to 15 metres. Blatchford (1953) suggested an average width of less than 0.9 metres over most of the field, and this was confirmed by the Company’s modelling of stoped volumes. The ore shoots occur with a periodicity typically in the order of 120 to 300 metres within the reefs, and extend from 200 to 700 metres in the down plunge direction, and are 70 metres to 300 metres wide.
The Company drilled a 2,000m deep diamond-core hole CT5000 in 2008 which confirmed the presence of gold-related alteration and mineralisation beyond previously drilled depths and demonstrated continuation of mineralised structures to at least 1900m on the eastern side of the gold field. Gold mineralisation was now known to persist and extend 700m below the point previously detected and there was potential to significantly increase the total gold resource. Mineralisation was detected in nine structures – the C6 St Patrick, the C5 Brilliant, the C3 Queen West and six other structures that were unable to be correlated with known structures and appear to be previously unknown mineralisation. Lead and zinc values greater than 100 ppm and copper greater than 200ppm usually indicate gold mineralisation.
Charters Towers gold is typically associated with galena and sphalerite in the pyritic sections of the quartz reefs and with associated shearing. Significant gold is not normally present in the disseminated pyrite which occurs in the proximal zone sericitic alteration. For more detailed descriptions of the geology refer to Kreuzer (2003). The ore was deposited at ~400 Ma at depths of between 5 and 14 km, mostly in Palaeozoic granitic rocks (Kreuzer et al, 2007).
This mesothermal, orogenic style gold mineralisation characteristically has great lateral and vertical extent. The ore is typically very high-grade, with past production averaging over 1 ounce per tonne. The Goldfield clearly has excellent potential for high-grade ore beyond the currently identified resources.
Deep drilling by BHP in the 1980s, and more recently in 2008 by the Company under the Queensland Government sponsored Collaborative Drilling Initiative (CDI) program, has demonstrated that the gold mineralised reefs persist to at least 2 km vertically and remains open at depth. At Parcoy-Pataz in Peru (Schreiber et al, 1990) similar quartz-pyrite reefs outcrop and extend to a vertical distance of 1,700 metres.
Most past production was from ore shoots within quartz reefs in remarkably persistent, kilometre scale sheet-like reef structures (faults). In the central area the main producers were the easterly trending reefs, plus subordinate production from the NNW trending cross reefs. The reefs are gently to moderately dipping and are typically 0.1 to 1.2 metres wide, but locally range up to 6 metres thick. The ore shoots occur with a periodicity typically in the order of 200 to 300 metres on the reef structures, and in the city are mostly 200 to 700 m long in the down plunge direction, and 70 to 300 m wide normal to plunge direction. The mined reef structures have statistically hosted ore shoots over 20% to 50% of their area (‘payability’) based on their usual overall cutoff grades of 6dwt (9g/t) gold. These figures are currently being replicated at the Company’s Warrior operations. Time and again new high-grade ore shoots have been found in areas previously discounted as barren. Recent examples of this include the Company’s Washington, Stockholm and Warrior mines.
The structural control of the Charters Towers ore shoots is subtle, often at changes of dip and strike, with some reefs thickening and thinning over short distances with no obvious controlling feature. Shallow plunging ore shoots that do not outcrop at the surface are common. Typical examples of this are the Day Dawn and the recently delineated ore shoots at Warrior. The 1.4 million ounce Brilliant ore shoot in the city was mined to 1.6 km down plunge but did not outcrop.
Due to the heterogeneous distribution of sulphides within the quartz, and the often erratic nature of the gold concentrations within the pyrite, ore grades display an irregular and non-uniform distribution. It is common for poorly mineralised zones of the fissures to pass rapidly along strike into high-grade ore, and vice versa. The ore is locally very rich, with several ore shoots known to average over 2 ounce gold per tonne (e.g. New Queen Cross, Talisman, parts of Brilliant). Although usually coarse-grained, high-grade ore is also found in fine-grained sulphides in shear zones e.g. at Stockholm and Warrior. The continuity of the ore shoots is locally disrupted by minor post-ore faulting which sometimes results in enriched zones of spectacular grade, for example, Day Dawn crushings of 10 ounces of gold to the tonne of ore (Reid, 1917). Structural, petrological and geochemical work is ongoing with the objective of defining vectors to the high-grade ore shoots.
