Kanchanaburi Province - Song Tho South and North, Bo Yai, Bo Noi
Pb Zn Ag
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A series of small lower to middle Ordovician carbonate hosted deposits are situated in the Meklong Highlands, in Kanchanaburi Province of western Thailand along the border with Myanma/Burma, some 300 km north of Bangkok. These include Song Tho North Song Tho South Bi Yai and Bo Noi.
The district encompasses a sequence of at least 3000 m of mainly shallow marine sedimentary rocks deposited between the Cambrian and Jurassic in a basin connected with the "Yunnan-Malaya geosyncline" (Baum et al., 1970). The earliest sequence comprises Cambrian(?) quartzites, slates and biotite schists, overlain by thick developments of Ordovician carbonate rocks was deposited. All known Pb-Zn mineralisation occurs in Ordovician strata. During the Silurian and Devonian, the basin deepened, and several hundred metres of mostly fine clastic rocks (shales and slates) accumulated. During the Carboniferous, marine greywackes (locally turbiditic) and conglomerates were deposited, indicating tectonic instability, although weak. In most parts of the district, marine sedimentation persisted through the Carboniferous. During the Permian, ~300 m of stable platform, strongly dolomitic and bituminous carbonate rocks were deposited. In Triassic times, the environment of depositional became unstable, with sediments only locally deposited in some areas, while in other areas marine conglomerates formed. The most recent marine sedimentary rocks in the area are Lower(?) Jurassic limestones (Diehl and Kern, 1981, and references cited therein).
Two granitic developments are mapped, the smaller Triassic(?) Khwae Yai granite, and the larger, upper Palaeozoic Central granite in the north.
Sulphide mineralisation is stratabound, closely related to reef-like algal crinoidal developments which are incorporated within the thick Ordovician limestones sequence. Fine-grained galena-sphalerite-pyrite represents the dominant ore assemblage, with accessory tetrahedrite and various Sb-Ag sulphosalts in a gangue of barite, silica and carbonate. Sulphides are usually intimately intergrown. Sphalerite has a low Fe content whereas galena is notably argentiferous (Diehl and Kern, 1981).
Sulphide precipitation was predominantly controlled by primary porosity and permeability of the host rocks, although locally, palaeo-karst cavern fillings and collapse breccias are also mineralised (Diehl and Kern, 1981).
Carbonate cementation and ore structure relationships suggest replacement at shallow depths during the Ordovician, with homogenisation temperatures of fluid inclusions from 107 to 174°C. The absence of significant replacement textures and the observation that carbonate gangue and sulphides were coprecipitated indicates the ore-bearing solutions had an almost neutral pH (Diehl and Kern, 1981).
Ore occurs as (after Diehl and Kern, 1981):
Layered galena-sphalerite ore - the most frequent type at Song Tho, with individual layers from a few mm to several tens of cms, which although not conformable on a small scale, are more or less stratabound with the host limestone. The ore occurs more as a sequence of sulphide lenses and layers than banded ores in a strict sense. Sulphides are very fine, with galena dominant forming a matrix that encloses the other sulphides. Two generations of pyrite are evident. Grades are typically 5 to 20% combined Pb-Zn.
Massive sulphide ore - taken as >60% sulphide, occurring as small but compact orebodies several metres thick and 10 to 20 m long. The ore is commonly plastically deformed and folded, locally a small scale of centimetres with fragments of host rocks enclosed within the sulphides. Typically the sulphide minerals are interwoven with authigenic phyllosillicates. The mineralogy is similar to the layered ores.
Disseminated sulphides - found mostly in the vicinity of the main orebodies, but can be traced for several hundred metres along strike within the host limestone bed. The sulphides commonly occupy small fissure faults, bedding fissility and stylolite surfaces. Grades are generally <5% combined Pb-Zn.
Network-like blende ore - found in massive limestone, filling primary porosity such as small caverns, molds, vugs in reef-like carbonate rocks, and comprising mainly sphalerite-wurtzite and crustified iron sulphides to form an irregular network. Colloform pyrite is a major component, while galena is rare.
Varve-like bedded ore sediments - filling fractures and small caverns, with sedimentary structures, such as cut and fill. The dominant sulphide is sphalerite, with accessory boulangerite, pyrargyrite, tetrahedrite and native silver, in a gangue of dolomite, barite and (spherulitic) silica. Gangue and sulphide grains are generally 5 to 8 µm in diameter.
Mineralised collapse breccias - found at Bo Yai, as a zone of >200 m in length with a thickness of up to 50 m. Angular clasts of massive and dark limestone up to 200 mm across are set in a matrix of partly oxidised sulphides and fine grained host rock fragments. Grades vary from a few to 30% Pb-Zn. The clasts also include massive sulphides and other ore types.
Song Tho North
Song Tho North comprises a generally lens shaped orebody which dips at 50 to 80°E and has a north-south strike extent of ~800 m. Mineralisation persists over a width of ~250 m to the limit of economic mineralisation, which reaches a maximum thickness of >20 m. The estimates of mineable (in situ) "reserves" are 1.8 Mt @ 8.7 Pb, 3.8 Zn, 100 g/t Ag). Erosion has removed part of the original deposit, which is largely covered by a thick overburden of laterite, enclosing three outcrops of oxidised mineralisation. The ore occurs as stratabound bands, lenses and disseminated layers of mainly galena, sphalerite and pyrite in units of light bedded limestone. The northern part of the deposit is cut by a major
NW-SE trending shear zone (Diehl and Kern, 1981).
Song Tho South
This smaller lens is <1.5 km south of the Song Tho North. Its northern section is exposed in a steep limestone cliff, where up to 4 m of massive sulphides is exposed. The distribution of the ore is highly irregular. Although generally conformable with the host limestones, the sulphide bands and lenses vary in both thickness and mineralogy. The mineralised zone comprises two separate ore layers over a strike length of about 300 m, with total "reserves" of ~ 0.4 Mt at an unspecified grade. A large gossan covers the line of outcrop line (Diehl and Kern, 1981).
The Bo Yai deposit occurs in a marked karst topography with steep limestone hills and sinkholes, ~18 km south of the Song Tho deposits. The north-south trending zone of mineralisation has a strike a length of >1 km, exposed on the flanks and peaks of several limestone hills. Most of the massive limestone host weak, mainly sphalerite mineralisation as network veining frequently accompanied by small massive sulphide bodies. In many places, these ore enrichments are fractured by major overthrust shear systems, the location of which were influenced by the sulphide accumulations. In small lenses of varve-like bedded ores and mineralised breccia bodies are exposed (Diehl and Kern, 1981).
A north-south-trending, ~1 km long, mineralised outcrop is exposed on the slopes of a hill, 20 km south of Song Tho. The ore zone dips at 20 to 30°E and comprises two layers which have been strongly boudinaged with lensoid inclusions of limestone country rocks within the sulphides adjacent to bedding plane faults.
These Pb-Zn deposits are part of a larger, north-south-trending, elongated belt extending from southern China to the Gulf of Thailand.
The most recent source geological information used to prepare this summary was dated: 1981.
Record last updated: 25/7/2013
This description is a summary from published sources, the chief of which are listed below.
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References to this deposit in the PGC Literature Collection:
Diehl P, Kern H 1981 - Geology, mineralogy and geochemistry of some carbonate-hosted lead-zinc deposits in Kanchanaburi Province, western Thailand: in Econ. Geol. v76 pp 2128-2146|
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