The Yandicoogina deposits of BHP Billiton and Rio Tinto are located some 90 km north-west of Newman and around 150 km east of Marandoo. They are part of a single, continuous, high grade, low phosphorous pisolitic goethite body which is over 80 km in length within the Marillana palaeochannel network. The deposit averages 500 to 650 m in width and is around 70 m thick in the channel centre.
The deposits infill east-west trending Tertiary palaeo-channels that were incised into shale, dolerite and BIF of the Weeli Wolli Formation in the core of a regional, easterly trending, syncline, which plunges to the east, and exposes broad widths of shallow dipping Brockman Iron Formation on both of its flanks and around the western closure.
The main pisolitic zone is up to 80 m thick and overlies a basal gravel bed. The basal gravel is irregular, and comprises a cemented gravel of 1 to 2 cm hematite pebbles, rimmed by black goethite and siliceous cement, varying from 1 to 2, up to 12 m in thickness. The pisolitic unit is composed in turn of: i). a 0 to 20 m thick basal zone around 300 m wide with 45 to 57% Fe, comprising a pisolitic goethite-ochreous goethite claystone in the channel centres averaging 15 m thick, and a massive goethitic clay unit on the lateral margins; and ii). the overlying main pisolitic zone, or ore zone, which is 40 to 70 m thick and from 400 to 1100 m wide, with average grades of 56 to 59% Fe.
The basal and main ore zones are separated by a 1 to 5 m thick band that varies from clay to clay matrix conglomerate to a massive banded vitreous goethite. At surface, the main ore zone has an up to 12 m thick interval of sub-ore grade (<56% Fe) material, underlain from 12 to 20 m in depth by 56 to 58% Fe which is transitional with the underlying high grade (>58% Fe) ore.
The deposit is basically made up of masses of cemented concretionary iron oxides occurring as irregular, sub-rounded goethitic clasts (up to 3 mm in diameter) separated by either a loose matrix, or a subsequent brown to grey, sub-vitreous to vitreous goethite cement, or are just densely packed. The ore is composed of either i). cemented sub-rounded to rounded iron oxide pisoliths, with some void space; or more commonly ii). pseudo pisoliths of non-iron oxides coated by goethite from iron charged ground waters.
The pisoliths are composed of concentric shells of limonite and vitreous goethite, generally with a core of goethite. Replacement of angular to sub-angular BIF and shale cores by limonite is discernible in some hand specimens. Hematite is subordinate to goethite in these ores.
Together, resources of some 4700 Mt have been indicated. In 2000 proven + probable reserves at the BHP Billiton Yandi deposits totalled 817 Mt @ 58.4% Fe, while Rio Tinto had a proven reserve of 310 Mt @ 58.5% Fe plus a further 870 Mt @ 58% Fe in the indicated and inferred categories.
Reserve and resource figures (Rio Tinto, 2012; BHP Billiton 2012) in 2011 were:
Rio Tinto - Total proved + probable reserves
HG Pisolites, 220 Mt @ 58.7% Fe + Process Ore, 161 Mt @ 58.7% Fe,
Rio Tinto - Total measured + indicated + inferred resource - 471 Mt @ 57.7% Fe (in addition to reserves),
BHP Billiton - Total proved + probable reserves - 867 Mt @ 57.2% Fe, 0.04% P, 5.7% SiO2, 1.5% Al2O3, 10.7% LOI, (included in resource),
BHP Billiton - Total measured + indicated + inferred resource - 2538 Mt @ 55.8% Fe, 0.05% P, 6.4% SiO2, 2.3% Al2O3, 10.9% LOI,
In 2002, the ores assayed approximately 0.05% P, 5% SiO2, 1.4% Al2O3, 10% LOI, with around 65% calcined Fe. BHP Billiton and Rio Tinto were each mining more than 50 Mtpa in 2012 from their respective leases to produce low alumina pisolitic goethite-hematite fines ore. Some 90% of the Rio Tinto reserves are below the water table and require draining before mining.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2012)