The Third Plain zinc silicate (willemite) deposit, located 14 miles south-east of Blinman, has been newly assessed by North Flinders Mines for its economic potential. Originally discovered by Kennecott Explorations in late 1966, and evaluated by them during 1968 with a programme of trenching and shallow percussion drilling, the deposit occurs in a dolomitic limestone unit at the base of the Cambrian sedimentary succession. Kennecott had estimated that an indicated resource of 13,400 tonnes @ 28.5% Zn lies between the surface and 36 feet depth, over a strike length of about 1800 feet, with the mineralisation occurring parallel to bedding planes in bands up to 12 feet thick. Believing that other, larger zinc mineral deposits akin to Electrolytic Zinc's Beltana and Aroona orebodies still remained to be found close to Third Plain, North Flinders Mines undertook an extensive check sampling exercise of known geochemical anomalies in the surrounding area, followed by its own regional stream sediment survey to try to identify possible additional willemite occurrences. In addition it performed appraisal drilling of the Third Plain deposit over three phases, eventually drilling 34 airtrack and rotary percussion holes for a total penetration of more than 5150 feet. Although many significant middle-order zinc anomalies were found and were followed up by detailed ground geological investigations which satisfactorily explained their sources, no potentially economic zinc mineral occurrences were located. However, as a result of this prospecting activity, outcropping phosphatic rock that assayed around 20% P2O5 was discovered in the vicinity of Erina Waters, situated about 20 miles north of the Third Plain deposit. White, sphereolitically radiating crystals of a phosphate mineral (crandallite) were seen in open joints and fractures within a light yellow-brown shale bed that is deposited close to the disconformity between the Parara and Wilkawillina limestones. Grab samples containing 12-20% P2O5 were taken irregularly from 9 outcrops over a strike length of about 1 mile and bed width of up to 70 feet, and it was conjectured that up to 1.4 million tonnes of phosphatic rock could be present down to a depth of 250 feet, given an average grade of 16.5% P2O5. The drilling campaign carried out in 1970 at the Third Plain deposit gave generally disappointing results. Subsurface zinc mineralisation intercepts were erratic and scarce overall, and were much thinner and of much lower grade than expected (only 0.25%-14.2% P2O5), being predominantly found as zinc-manganese carbonate rather than willemite, and hosted by the Parachilna Formation. It appears that the main zone of mineralisation delineated by the latest drilling is lensoidal, with limited depth extent, dipping eastwards at about 40 degrees, with a maximum thickness of 45 feet and an effective length of around 500 feet. The majority of ore consists of ferroan dolomite and manganoan smithsonite combined together in a mixed carbonate, replacement-type relationship, and variably altered to willemite by later supergene silicification. Another smaller, more dispersed mineralised zone occurs 300 feet distant to the morth-east, as a series of small parallel lenses following the ?folded structure of the host sediments (which may also be cross-cut by a set of complex faults). The main zinc orebody at Third Plain has now been adjudged to be of too small a size (approx. 50,000 tonnes), and in the main has an unfavourable structural attitude, to allow for extraction by low cost mining methods. However, North Flinders Mines intends to continue exploring the vicinity of the deposit and other areas along the eastern side of the Blinman Dome to test other zinc geochemical anomalies which may originate in economic mineralisation. A 3-hole drilling traverse completed at the Erina Waters phosphate prospect also gave disappointing results, suggesting that the outcropping phosphatic bed occurs as a thin, gently undulating capping that overlies an irregular erosion surface developed on underlying hard cherty limestone.