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 South has been renamed to Isaac Downs Extension. Any reference to Isaac Plains South is the same as Isaac Downs Extension.
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.
Isaac Downs Extension
Tertiary
Quaternary sediments range in thickness from 2 to 20m (average 7.8m) in the deposit area. Quaternary sediments appear to thicken to the west close to the subcrop of the Vermont seam. Some thicker Quaternary is seen along the banks of the Isaac River in the north and a minor amount along Cherwell Creek to the south.
Weathering
Depth of weathering over the whole deposit ranges from 11 to 36m (average 20.3m). In the seam subcrop zone the depth of weathering averages 19.5m. Deeper weathering zones are generally related to faulting.
Structure and Faulting
In the IP South area, the Rangal Coal Measures dip to the east at 2 to 6 degrees. Dips steepen in the vicinity of major faults. East of the deposit (beyond the limit of drilling) a major thrust system - the Isaac Thrust has been regionally interpreted and identified in the MGCRA seismic line 94-4 to the east of the Isaac River.
North-north-west trending thrust faults and orthogonal transverse faults feature in the resource area. A major thrust fault with a throw up to 30m occurs in the middle of the deposit area. A significant NE trending normal fault with throws of 10-28m occurs in the northern portion of the deposit area.
Coal Seams
The Leichhardt (LHD) and the V1 ply of the Vermont Seam of the Rangal Coal Measures form the principal economic coal resources in the Isaac Plains South Project area. The boundary between the Rangal Coal Measure and the underlying Fort Cooper Coal Measures is the typical a cream to brown tuffaceous claystone band (commonly called the Yarrabee Tuff YT). The YT has been identified immediately below the V1 coal ply in most drill holes in the IP South Project area. Technically the V2 and V3 coal plies are the top of the Fort Cooper Coal Measures.
Leichhardt Seam
The Leichhardt Seam is typically 2.5m thick and splits down dip into the L1 and L2 seams. Once split, the L1 and L2 seams thin progressively downdip to the E and SE. The L1 seam is typically 1.1m thick, and L2 is 0.92m thick. At depth the L2 seam disappears within a strong coarse sandstone sequence. The coalesced LHD seam has some stone bands that are consistent over relatively short distances. The Leichhardt whole seam raw ash averages 21.0%.
Vermont Seams
The Vermont seam lies approximately 25 m below the Leichhardt seam and varies in total thickness between 5-9m (V1 to V32 including parting). There are three plies identified in the Vermont seam (V1, V2 and V3, which is further split into V31 and V32 plies). V1 averages 1.8 metres in thickness, V2 averages 1.4 metres, V31 averages 0.4 metres and V32 averages 0.4 metres.
Coal Quality
Leichhardt and Vermont seam coal in the Isaac Plains SOUTH area may be classified as medium volatile bituminous coal (ASTM Classification) with a reflectance in the order of 1.00%. The LHD seam is generally low in ash and exhibits reasonable washability characteristics. The Vermont seams are higher in ash and exhibit poorer washability characteristics than the LHD seam. The seams can be beneficiated to produce a coking primary and thermal secondary product.
Dimensions
The dimensions of the Isaac Plains South resource are approximately 4.8 km north south (downdip) by 2 km east west. The resource dips to the east at 2 to 6 degrees. The target seams range in depth from 20m to 150m.