These deposits occur as bulk low grade mines hosted in impure limestone or marls with very fine grained replacement pyrite, and are often distal to a porphyry fluid source. Sediment replacement hosted Au deposits dominate in the Carlin and Battle Mountain Trends of western US, although are mined elsewhere such as at Bau, Mesel, Sepon and Dian-Qian-Gui province in southern China. Exploration for this deposit style occurs throughout the Pacific rim including China, Latin America and NW Australia (Corbett, 2013).
The requirements for the formation of these deposits include:
- Magmatic source for ore fluids which deposit auriferous pyrite which is strongly anomalous in As Sb and Hg, a mineral assemblage typical of quartz-sulphide style Au mineralisation developed in elevated crustal settings from a rapidly cooled ore fluid.
- Extensional structural settings such as the Basin and Range tectonism of western US provides dilatant structures which facilitate the long distance transport of ore fluids.
- Reactive impure carbonate (marl) host rocks are important for the development of the distinctive disseminated Au ores in reactions characterised by the removal of carbonate which provides the necessary open space for ore formation.
Thus, we see two end members as structurally controlled feeders which display higher Au grades such as the 15 g/t Au material from Meikle whereas the lithologically controlled ores from Mesel with stylolites and breccias indicative of carbonate removal host lower Au grades.
The fine grained As pyrite sulphide ores host refractory Au which must often be roasted to liberate the Au and so these can be expensive and environmentally difficult ores to treat. Consequently, oxide ores have initially mined in many locations.
Geology of the Sepon Mineral District
Mineralisation styles recognised with the SMD are: sediment-hosted gold, skarn Cu-±Au and central porphyry Cu-M. Resources published by Oxiana Limited between 2000-2008 reported 83 Mt @ 1.8 g/t Au for 4.75 million ounces of gold in seven separate sediment hosted Au deposits, and supergene copper at three deposits, namely the Khanong (27 Mt @ 4.3 % Cu), Thengkham North (11.4 Mt @ 2.7 % Cu) and Thengkham South (9.8 Mt @ 2.3 % Cu) deposits (Camira, 2010 and Loader, 1999).
On a global scale SMD is significant is a significant Au-Cu district, falling in the upper range of sediment hosted gold deposits containing >100t of contained gold.
The SMD is part of the Truong Son Fold Belt and the Kontum Massif Belts, both of which continues into Vietnam (Loader, 1999). In the Sepon area the Truong Son Fold Belt comprises as series of E-W trending faults truncated against the NW-striking Truong Son Fault. Rhyodacite porphyry stocks, dykes and sills have intruded the Padan-Thengkham district which are genetically associated with the Cu-Au-Mo mineralization. The ages of these intrusions is 330Ma from U-Pb dating (Loader, 1999). Gold is hosted within the Devonian Discovery Formation calcareous shale with lesser amounts in the underlying Devonian Nalou Formation bioclastic dolomite, Silurian-Devonian Kengkeuk Formation calcareous shale and the Ordovician-Silurian Nampa Formation claystone and siltstone sequence. The Nalou Formation bioclastic dolomite units and Kengkeuk Formation calcareous shale predominately host the skarn Cu deposits.
Sulphide mineralogy at SMD includes pyrite, arsenic-rich pyrite, and chalcopyrite with minor sphalerite, galena, bornite and stibnite (Loader, 1999). Alteration types include variable decalcification of carbonate unites, silicification (jasperiod formation) along permeable horizons and faults, locally develop argillisation along dyke contacts, and variable dolomitisation of carbonate units (Loader, 99 and D. Menzies personal observations). Hypogene skarn-style Cu mineralization is characterized by sericite alteration along the intrusion margins as well as: early prograde garnet and pyroxene skarn alteration, cut by later retrograde chlorite-epidote alteration of carbonate units, and hornfels in non-calcareous sedimentary units.
The following are notes on prospects comes from the authors 1996 field notes and Sillitoe 1995.
- Prospects
- Discovery Prospect
- Calc-siltstone with non-calcareous silting and a porphyry sill. Siltstone contacts appear to play roles in gold deposition
- Padan Prospect
- Porphyry Mo system with only minor Cu-Au
- Phu Nga Prospect – quartz-pyrite mineralization with barren jasperiod
- Vang Ngang East Prospect
- Mineralization is in jasperiod and quartz-pyrite develop in steeply dipping beds
- Quartz- pyrite mineralization – quartz is white and pyrite is coarse. Quartz is directly replacing carbonate.
- Quartz-pyrite forms closer to the intrusive source than jasperiod and is considered a low temperature (and pH) equivalent of skarn
- TEM is recommended to detect semi-massive pyrite
- Nalou Prospect
- Dolomitisation and jasperiod development at basal contact of the calc-siltstone with underlying dolomite
- Thengkham Prospect
- Prograde and retrograde skarn
- Thengham stock is made up of an early and inter-mineral porphyry with A-type quartz veining.
- Discovery Prospect
- Intrusives exhibit chlorite-illite-pyrite alteration. Pyrite is replacing mafic minerals and up to 2%.
- Calc-siltstones are black, well laminated, often intensely brecciated and may exhibit up to 10% pyrite.
- Calc-siltstones often exhibit zones of decalcification or silicification or dolomitisation.
- Mudstones and calc-siltstones often contain abundant organic matter and fine bands of diagenetic pyrite.
- Doug Morris’s interpretations (Morris 1997a, Morris 1997b) of the sedimentary sequence, and identification of limestone marker horizons, was fundamental in determining lithological control to mineralization and structural off-sets. This was often done by logging core from the bottom up looking for the deposition sequence to identify marker horizons. Morris also proposed a model with hydrocarbon trap sites using a carbonate sealing unit and the organic rich mudstone as the favourable gold-hosting horizon and principal exploration target.
Paragenetic sequence of SMD mineralisation
The paragenetic sequence proposed by Cromie (2010) for the SMD mineralisation is:
- Early diagenesis of the Sepon Basin with deposition of framboidal pyrite.
- Deposition of gold-poor diagenetic pyrite and pink calcite.
- Deposition of base-metal mineralization in the form of:
- early carbonate-hosted pyrite-galena-dolomite veins, followed by
- low grade Au-bearing intrusion-hosted early retrograde veins with pyrite-sphalerite-galena-quartz-dolomite, then
- low grade Au-bearing intrusion-hosted retrograde veins with quartz-chalcopyrite ± bornite ± molybdenite. Event 3 b) contains low grade Au mineralization ranging from 0.13 to <3 g/t Au mainly in veins containing pyrite and sphalerite. Stage 3c) contains chalcopyrite inclusions and exhibits Au contents ranging from 0.06 to 0.9 g/t Au in the central porphyry and proximal skarns.
- Stage 4 is the main high grade Au in the following sub-types: a) introduction pyrite with Au on rims along fractures or associated with pyrite cutting skarn mineralization, b) Hg-Au telluride filling fractures. Au grades range from >1 g/t to 293 g/t Au.
- Post-high grade assemblage of a) quartz-stibnite-dolomite b) quartz-dolomite and c) calcite-quartz-florite. No Au grades were observed in this stage.
The timing of Cu-Mo has been determined by Re-Os dating of Stage 3 molybdenite to be 287-280Ma at the Thengkham South and Padan Prospects. Broad geological features, mineral assemblages and geochemistry share several broad characteristics of Carlin-style deposits in Nevada and Dian-Qian-Gui province in southern China.