Summary:
The resource area of the Isaac Plains Complex (including Isaac Plains, Isaac Plains East, Isaac Downs and Isaac South) is in the northern part of the Bowen Basin in Central Queensland.
Isaac Plains Complex (IPC) lies within the Permo-Triassic Bowen Basin. The Bowen Basin consists of 10 kilometre (km) thick sequences of volcanic, shallow marine and terrestrial sediments and is categorised back-arc to foreland basin.
The general stratigraphy of IPC includes (oldest to youngest)
- Lower-Permian Reids Dome Beds;
- Lower-Upper Permian Back Creek Group;
- Upper Permian Blackwater Group, and Rewan group.
Coal seams occur within the Rangal Coal Measures which are Late Permian in age. These seams dip gently to the east at approximately 5 degrees.
The coal seams found within the Rangal Coal Measuresare the Leichhardt, Leichhardt Upper and Leichhardt Lower, and Vermont.
The seams have a cumulative thickness of approximately 7-10 m across the deposit.
The Vermont seam was not included in the resource estimate due to the lack of geological information. The results at hand indicate the coal to be of poorer quality.
Isaac Downs
Isaac Downs is located in the northern part of the Permo-Triassic Bowen Basin containing principally fluvial and some marine sediments. The economic seams are contained in the Late Permian Rangal Coal Measures which are approximately 100 m thick. The Rangal Coal Measures are underlain by the Fort Cooper Coal Measures and overlain by the Late Permian to Early Triassic Rewan Group. The transition from the Rangal Coal Measures to the Rewan Formation is 15 to 60 m above the first major seam of the Rangal Coal Measures. The Fort Cooper Coal Measures comprise typically tuffaceous sandstones, siltstones, mudstones and coal seams. The transition between the Rangal Coal Measures and the Fort Cooper Coal Measures is generally clearly marked by the Yarrabee Tuff - a basin-wide marker bed comprised of weak, brown tuffaceous claystone. The first coal seam of the Fort Cooper’s is the Girrah Seam which is 25 metres or more below the Vermont Seam within the Project area.
Alluvium occurs along the Isaac River. The alluvial sediments comprise a fining-upward sequence of unconsolidated sand, silt, clay, gravel, and lithic fragments. The alluvium may extend to a distance up to 750 m from water courses. However, the lateral extent of alluvium may be highly variable.
In the Project area, the Rangal Coal Measures dip to the east to north east (2 to 6 degrees) and include interbedded sandstone, siltstone, and lesser amounts of carbonaceous mudstones, coal, and tuff. A weathered zone extends to 15 m to 30 m below ground level. Typically, the Leichhardt Coal and Vermont Coal Seams occur as a singular package, in very close proximity, with splits in some areas up to 30 m.
The Leichhardt Seam at Isaac Downs is split into five main plies; in stratigraphic order, these are Leichhardt Upper Dilution (“LUD”), Leichhardt Upper (“LU”), Leichhardt Lower 3 (“LL3”), Leichhardt Lower 2 (“LL2”) and Leichhardt Lower 1 (“LL1”). In the west of the deposit, these plies are coalesced and exist as a single, contiguous Leichhardt (“L”) Seam that averages approximately 4.20 m. Mudstone or tuff bands <0.10 m in thickness present as ply boundaries down-dip of the subcrop. However, at approximately 80 m to 100 m depth of cover, more significant seam splits start to develop along a north-northwest orientation, resulting in up to 30 m of interburden existing between the LL-plies, and up to 25 m of interburden between the LU and LL plies. The LUD ply represents a ~0.25 m thick carbonaceous transition between the siltstone overburden and the roof of the coal in the Leichhardt Seam. The LU ply averages 1.30 m in thickness, the LL3 and LL2 plies average 0.85 m thick and the LL1 ply averages 0.80 m thick.
The Vermont seam occurs approximately 0.50 m below the Leichhardt Seam, and is split into two main plies, typically separated by siltstone <0.50 m in thickness. The topmost ply – the VU1 ¬– is the thickest and most consistent ply, averaging 1.30 m across the deposit. The VU2 is next in stratigraphic order, and averages approximately 0.45 m. In the south, a split forms off the base of the VU2, which is named the VU3 and has an average thickness of 0.15 m.
The Girrah Seam is located approximately 50 m below the base of the Vermont Seam at Isaac Downs and has similar characteristics to Isaac Plains East.