Summary:
The Chatree Gold Mine is a low sulphidation epithermal gold–silver deposit located in the Loei – Phetchabun volcanic belt in central Thailand. The deposit spans 2.5 by 7.5km and consists of 8 vein zones.
The Chatree low sulphidation epithermal gold–silver deposit occurs as veins, stockworks and minor breccias hosted by a volcanic and volcanogenic sedimentary facies. The main gold–silver mineralisation is characterised by colloform–crustiform banded quartz ± carbonate ± chlorite ± adularia–sulphide–electrum veins. Gold mainly occurs as electrum, both as free grains associated with quartz, carbonate minerals and chlorite, and as inclusions in sulphides, mostly pyrite (Salam et al., 2013).
Oxidation and broad stratigraphic units control the gross distribution of gold and silver mineralisation with specific geological units providing preferred mineralisation hosts. These are most notable at the A Pit where the sedimentary unit hosts the majority of mineralisation.
In the mine area, the host volcanic sequence comprises four main units. The basal unit is dominated by andesite lavas and breccias, containing dacite dome complexes. This is followed by a thick sequence dominated by andesitic breccia. Overlying this is a largely sedimentary package with interfingering rhyolitic breccias. The uppermost unit in the mine area is a lithic rich fiamme breccia. As the lithologies interfinger, the locations of some unit boundaries in core, and especially in chips, are not always clear. All units are cut by dykes, predominantly andesitic, and typically less than 10 metres wide.
Mineralisation
Epithermal gold deposits, such as those at Chatree, are generally formed in areas of active volcanism around the margins of continents, and may be associated with “bonanza” or high grade style mineralisation. Mineralisation in such deposits is deposited from hot fluids which are estimated to range in temperatures from less than 100 degrees centigrade to approximately 300 degrees centigrade. The mineralisation contained in these fluids is deposited as they rise to elevated crustal settings and boil, cool or mix with evolved nearsurface waters which oxidise the fluids to promote high grade gold deposition. The fluids usually travel toward the surface via fractures in the rocks and the resulting mineralisation often occurs at the intersection of these fractures and brittle host rocks in layered volcanic sequences.
The gold mineralisation is closely associated with a variety of types of quartz carbonate veins, some of which contain sulphides. They vary in width from millimetres to tens of metres. Significant amounts of silver and very minor amounts of zinc and lead are also associated with the mineralisation. The deposit is classed as low sulphidation epithermal, and sulphides tend to comprise less than 3% of the ore mass (Cumming, et al). Visible gold is rare within the system. Chlorite is common both in the veins and as alteration in the host rock.
The style of quartz carbonate gold mineralisation found at Chatree is common to a number of the most prolific gold producers in the Pacific rim, including Porgera and Misima in Papua New Guinea, Kelian and Mount Muro in Indonesia (Corbett). The variety of low sulphidation epithermal gold deposits all display controls dominated by structure, competent host rocks and efficient mechanisms of Au deposition.
The gold silver ratios vary significantly at different levels of mineralisation (del Carmen, et al).
The largest prospect in terms of the currently defined mineral resource is A Prospect. Here, the sedimentary unit in the middle of the package of volcanics appears to have acted as a favourable rock unit for mineralisation, with the overlying fiamme breccia acting as a lithological trap to fluid movement, due to low permeability and/or competence contrast. The stockworks and sheeted quartz veins which host gold mineralisation are mainly vertical to steeply west dipping and are best developed in the sedimentary unit which itself dips at 20o to the east. Veining outside this unit tends to be sparser in density, but may still contain gold.
At Q Prospect, in the most northern section of the mining leases, the mineralisation is characterised by brecciation of the host silicified sediments, and has been interpreted as including both strata bound and sub-vertical feeder like zones.
In the southern leases, mineralisation tends to be in fewer discrete veins, with fewer stockworks, and in the C and H Pits area the veins dip northwest at 30 to 70.
Barren andesite dykes and some porphyritic dykes crosscut mineralisation. These occur on a number of orientations and vary in width from less than one metre to several tens of metres thick. There are at least two ages of dyke emplacement (del Carmen, et al). Earlier dykes are often altered and may be associated with elevated gold grades, but the majority of dykes are later, barren and do not affect surrounding mineralised grades. The volume of barren dykes can create significant dilution in some mining areas, being 13% to 14% of drill intersections in the A and Q Prospects. Barren crosscutting massive carbonate veins are another source of dilution.
The mineralisation is considered by Akara to be of Early Triassic Age with dating indicating an age of about 251 million years (del Carmen, et al), close to the age of the host volcanic sequence. Mineralised veining is interpreted to have been emplaced into host rocks which had been previously subjected to intense silicification. The silicification has preferentially altered the andesitic breccia and the epiclastic sedimentary rocks, which tend to host mineralisation in the southern and northern portions of the mine leases respectively.