Eucla Basin Heavy Minerals - Jacinth, Ambrosia, Tripitaka

South Australia, SA, Australia

Main commodities: Zr Ti
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The palaeo-shoreline of the Eucla Basin in South Australia hosts a string of zircon, ilmenite and rutile heavy mineral accumulations that include the Jacinth, Ambrosia and Tripitaka deposits. Jacinth and Ambrosia are approximately 180 km north-west of Ceduna in western South Australia, and 525 and 750 km respectively north-west of Port Lincoln and Adelaide. Tripitaka is 90 km south-east of Ambrosia.

The onshore segment of the basin covers an area in excess of 0.3 million sq km in south-western South Australia and south-eastern Western Australia, immediately to the north of the Great Australian Bite coastline with the Southern Ocean, extending up to 300 km inland. It has been filled with a sequence up to 300 m thick of Tertiary (Late Paleocene to Holocene) marine, coastal and palaeo-channel sediments.

The northern margin of the basin extends over a distance of approximately 2000 km through Western and South Australia and contains a large onshore province of Tertiary sediments characterised by a number of palaeo-valleys that drained the Precambrian Yilgarn and Gawler Cratons and Musgrave Province.

Over an interval of around 1000 km, the eastern basin margin contains extensive near-shore and onshore facies Tertiary sediments which are characterised by a number of palaeo-valleys extending landward onto the Gawler Craton. Linear coastal landforms are preserved along this margin, include coastal dunes, dune ridges and possible beach ridges, as well as palaeo-valleys, lagoons, estuaries and coastal barriers, although extensive tracts of Quaternary deposits have been superimposed on these remnant Tertiary landscape elements.

The stratigraphy of the eastern Eucla basin margin comprises four third-order eustatic cycles which resulted in four generations of marine and non-marine deposits, reflecting a step-wise evolution of marine and non-marine environments across this region, from the base:
Mid-Middle Eocene calcareous, glauconitic and gritty shallow marine sandstone of the Hampton Sandstone; and limestone of the lower Wilson Bluff Limestone;
Late Middle Eocene marine limestone of the upper part of the lower Wilson Bluff Limestone; lagoonal carbonaceous limestone of the Paling Formation; and marginal marine to fluvial carbonaceous clastics, the Maralinga Member of the Pidinga Formation;
A complex sequence of Late Eocene marine limestone of the upper Wilson Bluff Limestone; marginal marine to fluvial carbonaceous clastics comprising the Anthony Member of the Pidinga Formation; and estuarine clastics of the Khasta Formation;
Middle Miocene - Early Pliocene marine limestone of the Nullarbor Limestone; lacustrine mudstone and dolomitic limestone of the Garford Formation; and marginal marine to estuarine (carbonaceous) clastics, comprising the Kingoonya Member of the Garford Formation, and the Narlaby Formation.

The preserved Tertiary shorelines, which comprise beach, shoreface, barrier, dune, tidal inlet, washover and lagoonal facies, representing multiple higher order high-stands, are associated with numerous palaeo-drainage systems that supplied vast quantities of sediment to the basin from the adjacent cratonic basement. The preserved Tertiary landscape reflects four generations of shorelines, namely: i). mid-Middle Eocene at 41.5 Ma, ii). late Middle Eocene at 39 Ma, iii). Late Eocene at 37 Ma, and iv). Neogene from 15 to 5 Ma.

Sedimentological studies have identified heavy mineral sand (HMS) deposits as:
i). lag deposits along erosional disconformities and/or unconformities;
ii). transgressive deposits at the rear of high-stand (swash-aligned) barriers, including those trapped near the palaeovalley passes;
iii). regressive deposits at the front of prograded barriers; and
iv). aeolian deposits, as low-grade disseminated concentrations in transgressive dunes.

Over the 1000 km eastern Eucla Basin margin, an HMS-bearing barrier dune complex, which extends for more than 800 km, is up to 30 km wide, and over 100 m high. HMS concentrations of detrital zircon, ilmenite, pseudorutile, rutile/anatase, minor leucoxene and monazite, and less commonly aluminosilicates (tourmaline), aggregates of hematite, quartz and clay, monazite and chromite, occur as beach placers in highstand strandlines along the Tertiary shorelines, interpreted to have been deposited as beach placers, mainly by south-east directed longshore drift. Drilling shows that significant placers occur either in the upper part of the thick barrier­dune sand bodies, often 20 m or more below the surface, or close to an erosional bedrock contact.

The Jacinth and Ambrosia deposits are located along a >40 km strike length geological feature and are thought to represent stacked shoreline facies that accumulated during marine transgressions, associated with portion of the Eocene Ooldea coastal barrier in Pidinga Bay. Tripitaka is located 90 km to the south-east of Jacinth and Ambrosia. There is a tendency for the Jacinth and Ambrosia deposits overall to become a continuous and more mineral rich zone towards both the south-east and north-west of their enclosing portion of the Ooldea barrier. HMS appears to have been concentrated as beach placers where the oversteepened shoreface acted as a virtual headland that focused wave-reworking processes during marine transgressions. The most prospective strata are the barrier and associated sands of Tertiary shorelines that were buried by voluminous sand dunes over a period of 40 million years.

The broad outline of the Jacinth deposit is around 1 km wide east-west by approximately 4 km in length comprising a gently west dipping body of 10 to 45 % HM that is up to 6 m thick near the base of a broader envelope up to 30 m thick of 3 to 10% HM, in turn embraced by an envelope that is up to 40 m thick of 1 to 3% HM. This body trends north-south in the south, curving to NNW in its northern half.

The southern tip of Ambrosia is approximately 2 km to the NE of the northern tip of Jacinth. Ambrosia trends NW with a width of 300 to 800 m. It comprises a thinner, up to 3.5 m thick layer of 10 to 45% HM within a more laterally extensive, but vertically comparable halo of 3 to 10% HM, and a broad halo up to 30 m thick of 1 to 3% HM.

At Tripitaka, drilling has delineated a continuous mineralised zone of 1 to 3% HMS approximately 800 m wide and up to 15 m thick with a geological character and setting very similar to that at Jacinth and Ambrosia deposits, containing very high zircon contents of up to 63%.

Iluka Resources (2008) quote reserves and resources in their Eucla Basin deposits at Ambrosia, Jacinth and Tripitaka (at Dec. 31, 2007) as:

    Probable Reserve - 98.4 Mt of ore @ 6.5% HM for 6.42 Mt HM comprising 28% Ilmenite, 50% Zircon, 5% Rutile;
    Measured + Indicated + Inferred Resource - 267.3 Mt ore @ 3.8% HM for 10.19 Mt HM comprising 27% Ilmenite, 49% Zircon, 5% Rutile.

The most recent source geological information used to prepare this summary was dated: 2008.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Hou B, Keeling J, Reid A, Fairclough M, Warland I, Belousova E, Frakes L and Hocking R,  2011 - Heavy Mineral Sands in the Eucla Basin, Southern Australia: Deposition and Province-Scale Prospectivity : in    Econ. Geol.   v.106 pp. 687-712

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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