Summary:
The Central Pilbara area extends over an area of 70km E-W by 30km N-S, surrounding the Mining Area C (MAC) processing hub. It comprises three grouped deposits under active mining (namely North Flank, Packsaddle and South Flank) and two exploration stage deposits (namely Jinidi, and Mudlark Well). The North Flank and South Flank deposits are located on MM IF that crops out on the northern and southern limbs of the doubly-plunging Weeli Wolli anticline. The Packsaddle deposit covers BKM IF on the northern limb of the Weeli Wolli anticline, whereas the Jinidi deposit covers BKM IF in the eastern nose of the same anticline. The Mudlark Well deposit is located west of the Weeli Wolli anticline. Mineralisation is hosted by both the BRK IF and the MM IF and is associated with the moderately-dipping limbs and gently W-plunging synclinal keels of a series of regionalscale folds. Mining Area C is located approximately 90km northwest of Newman. BHP’s first MM deposit came into production here in 2003 and the new South Flank mine, immediately to the south, is also developed around a MM resource.
The outcrop pattern is dominated by a series of large-scale, open, upright folds with wavelengths of the order of 20km. These are typically E-W-trending and doubly-plunging, forming a series of domes of which the Weeli Wolli anticline at Mining Area C is a typical example. The cores of domes form low ridges composed of MM IF and shales of the uppermost Jeerinah Formation. The intervening synclines outcrop as ranges of the more resistant BKM IF. The Wittenoom Formation appears to have undergone significant karstic erosion and is rarely exposed in outcrop. It forms the subcrop to a series of E-Wtrending valleys filled with a variety of Mesozoic to Cenozoic sedimentary rocks.
The effects of at least three fold generations are preserved at MAC. In addition to the regionalscale fold generation (Weeli Wolli anticline), an older generation of second-order, meso-scale folds have sinuous hinge-lines and are uniformly north-verging. These folds are overturned to recumbent and a series of sub-horizontal thrusts have developed locally in response to over-tightening of these asymmetric folds (e.g., North Flank and South Flank). The third and youngest generation of folds consists of N-S-trending, open, upright folds with broad wavelengths. The combined effect of the fold generations results in a complex outcrop pattern which reveals a number of smaller domes superimposed on the broader anticline/syncline pattern.
In addition to the sinuous thrusts that thicken fold limbs within the MM IF, a major, steeply Sdipping, normal fault (Neale’s Fault) strikes ENE-WSW through the Packsaddle Range. A break in the eastern part of the Packsaddle Range reflects the position of the NE-trending Weeli Wolli Fault corridor and corresponds with the location of the Weeli Wolli spring and its associated drainage.
Packsaddle – The Packsaddle Range deposits are in production. At Packsaddle, supergene M-G mineralisation is developed in BKM IF over a strike length of almost 50km, with widths of up 1.5km and extending to depths of up to 300m. The Packsaddle Range is located on the northern flank of the regional-scale, EW-trending Weeli Wolli anticline and the Brockman IF stratigraphy dips moderately to gently to the north. Refolded, meso-scale, WNW-trending folds are asymmetric and verge to the north. These play a major role in localising the supergene enrichment. Deep pockets of mineralisation are controlled by a major ENE-WSW-trending normal fault (Neale’s Fault).
The detrital mineralisation at Packsaddle is located at the base of the south-facing scarp of the Packsaddle Range. It consists of scree fans, fed by deeply incised N-S-trending gullies and shedding off the scarp of mineralised BKM IF (Packsaddle Range) to the north.
North Flank – The North Flank series of deposits is in production. Mineralisation is continuous over a strike length of 25km, with widths up to 1km and extending to depths of 270m. North Flank comprises supergene M-G mineralisation hosted by N-dipping members of the MM IF and the BIF-bearing West Angela Member of the Wittenoom Formation. The majority of the Wittenoom Formation has been deeply eroded, particularly in the area immediately adjacent to the North Flank mineralisation, and the EW-trending valley between North Flank and the Packsaddle Range has been infilled with thick sequences of Phanerozoic detrital material.
The thicker intercepts of mineralisation are associated with the thrust-thickened, steeply Ndipping to overturned limbs of north-verging meso-scale folds and with the synclinal keels of these folds, particularly where they lie within 150m of surface.
South Flank – The South Flank series of deposits is in production. South Flank is located on the southern flank of the Weeli Wolli anticline. Supergene M-G mineralisation is hosted by MM IF and the West Angela Member of the Wittenoom Formation. Phanerozoic sediments infill the EW-trending valley, underlain by the dolomitic Wittenoom Formation, between South Flank and the Governor Range to the south (the latter is made of BKM IF).
Bedrock mineralisation extends continuously over a strike length of 27km. Mineralised widths range up to 1.3km and mineralisation extends to 300m vertical depth in places. Although the regional dip of the bedrock is moderately to the south, there are a number of meso-scale folds with sinuous hinge lines which result in a network of synclinal keels and an anastomosing pattern of mineralisation. The synclinal keels tend to be intensely mineralised and typically have thrust-thickened, steep to overturned, N-facing limbs which are also well mineralised, thanks to the combination of steep bedding dip and structurally-enhanced permeability. Some mineralisation is also developed on moderately S-dipping portions of the southern flank of the Weeli Wolli anticline in the absence of meso-scale folding.
Jinidi – The Jinidi deposit is at the exploration stage and will sustain future production at some stage in time. It is located at the eastern end of the doubly-plunging Weeli Wolli anticline. Mineralisation occurs mainly in the Dales Gorge Member, it is of the supergene M-G type and is virtually continuous throughout the entire deposit. Mineralised widths range from 500-1500m and mineralisation extends to depths of 100-250m. It is associated with Eplunging synclines, some of which are asymmetric and N-verging.
Mudlark Well – The Mudlark Well deposits are at the exploration stage and will sustain future production at some stage in time. These are located to the northwest and southwest of the Weeli Wolli anticline and represent sinuous belts of Marra Mamba and Brockman IF cropping out on the flanks of regional-scale, E-plunging folds. The intervening Wittenoom Formation is blanketed by detrital valley fill of various ages.
The deposits located in this area are hosted within BRK IF and MM IF, and all are of the supergene M-G type. Individual orebodies have the following range of dimensions: 2-16km in strike length, 500-2000m in width and extending to depths of 100-250m. The majority of the bedding dips are generally shallower in the north than in the south. Synclinal keels or hinge zones are important ore controls in several deposits.