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Location: 12 km N from Ashford, New South Wales, Australia
Level 19/10 Eagle StreetBrisbaneQueensland, Australia4000
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Regional Geology The Permian Ashford Coal Measures are expressed as a narrow (< 10km wide) 80 km long basin, stretching from the Queensland border in the north to Inverell in the south, uncomformably overlying a highly deformed Carboniferous age marine sediments (Texas beds) basement. The coal measures may have been deposited in a west dipping half graben. The western margin of the coal measures is marked by a prominent west over east thrust fault– the Severn Thrust resulting in Carboniferous rocks overlying the Permian sediments. Intruding the overthrusted Carboniferous rocks is a leucogranite intrusive of the New England Batholith.Local GeologyQuatenary Quaternary alluvials from the Severn River overlie the Permian and carboniferous sequence. These unconsolidated materials include felsic sand gravel and clay.Weathering Depth of weathering is variable from about 4m to 50m. The average depth of weathering over the open cut area is in the order of 20 to 25m. Very shallow weathering can occur in overthrust granite/aplite rocks. Localised deeper weathering is likely to be related to intense fracturing in faulted zones.Ashford coal measures An upper carbonaceous shale and coal unit is referred to as the Bonshaw seam. Approximately 30-50m below the Bonshaw seam is the Ashford seam. The roof of the Ashford seam is a competent lithic conglomerate with a shaley and clayey matrix. Below the Ashford seam the conglomerates are comprised of rounded pebbles set in a grey shale matrix. Limited studies indicate the source of the Permian sediments appears to be the underlying Carboniferous age formation. The Carboniferous/Permian unconformity interface occurs between 10 to 30 metres below the Ashford Seam.The Bonshaw Seam The Bonshaw seam consists of carbonaceous mudstone and coal. It is erratic in both thickness and location. It is generally lower coal quality than the Ashford seam. This seam has not been included in the resource calculations and mine plan. The Bonshaw seam will be targeted in near-term exploration to ascertain tonnage, raw coal quality and washability.The Ashford Seam The Ashford Seam thickness varies from over 20 metres thickness to less than 3 metres. Mylonite zones are common occurring in the roof and floor of the seam, essentially the weaker coal taking the strain in preference to the stronger conglomeratic roof. The mylonite generally has a high ash due to fault dilution and lower volatile matter due to frictional heating.The seam contains moderately bright coal. Core logs indicate approximately 40% of the coal has >40% bright bands explaining the good coking properties. Partings >0.3m occur in the seam in places particularly in the southern portion of the deposit where the lower plies separate from the main body of the seam. Seam splitting in this southern area appears sedimentary. The sporadic occurrence of parting in the seam elsewhere, however, is likely to be introduced by thrust faulting. In order to exclude thick parting from resource tonnages the seam has been divided into 5 plies for modelling. It should be noted that the large variability in seam thickness makes the allocation of the plies difficult and the plies should only be viewed as useful for the purpose excluding thick parting.Carboniferous Basement The highly deformed Carboniferous age marine sediments (Texas beds) basement consists of white cherts, grey and green claystones, tuffs, and quartzites. The Texas beds are moderately metamorphosed and partly silicified. The sequence has undergone differential weathering providing an irregular surface onto which the Permian coal measures were deposited.Structure and Faulting The principal structural feature in the area is the prominent Severn Thrust which has thrust the Permian Ashford Coal Measures under the older Carboniferous sediments and granite sill. The dip of the thrust is in the order of 20-25 degrees to the west. Orthogonal to the thrust are tear faults which are largely strike slip. Significant strain has been taken up by the weaker coal seams and mylonite zones in the Ashford seam are not uncommon.The Ashford seam strikes approximately 20 degrees from north in the resource area and is essentially parallel to the Severn thrust. Seam dip in the subcrop area is 25 to 30 degrees flattening out to approximately15 degrees downdip under the Severn fault and granite to the west.Intrusives Intrusives and coked coal has been identified in a small number of drill holes. To ensure coked coal and intrusives within the seam are not included in the resource tonnages the thickness of this “waste” has been modelled and debited from the Ashford seam thickness.
