Abstract of Arnhem Block / Mc Arthur Basin Synthesis

Record 2001/12

Compiled by Lesley Wyborn

• Download the complete chapter


• Executive summary - geology
• Executive summary - metallogenic
• Future work

Download the complete chapter

Download the complete Arnhem Block / Mc Arthur Basin Synthesis report in pdf format [PDF_3.4MB].

[back to top]

Executive summary - geology

The Arnhem Block/McArthur Basin is best known for its world class sediment-hosted HYC Pb/Zn deposit. It is certainly not noted for granite-related Cu-Au deposits. Some Cu mineralisation occurs in the southern McArthur Basin area, and although hosted by mafic volcanics, is more probably derived from copper- and sulphate-rich basinal brines (Wall and Heinrich 1990) than from magmatic fluids (c.f. Knutson et al. 1979).

The regional geology is well summarised by Jackson et al. (1987), Plumb and Roberts (1992), Plumb et al. (1990), Madigan and Rawlings (1994) and Pietsch et al. (1994). The Arnhem Block occurs in northeast Arnhem Land and consists of high-grade metamorphics which were intruded by a suite of restite-rich S-type garnet- and cordierite-bearing granites at around ~1850 Ma (Bradshaw Suite). A suite of younger I-type fayalite-bearing granites (Giddy Suite) intruded the basement at ~1835 Ma. Somewhat similar in composition, the Bickerton Suite of felsic volcanics was extruded at around 1814 Ma.

A series of sedimentary packages belonging to the McArthur Basin was then deposited unconformably on these basement rocks. Throughout the basin five main stratigraphic sequences were deposited, each generally separated by regional unconformities. The lowermost sequence, the Tawallah Group contains most of the igneous suites. The suites are either bimodal, dominantly felsic or dominantly mafic. The volcanics are mostly related to rift phase sedimentation. Some granite plutons are coeval with these volcanic units, but all plutons are relatively small in size and have a limited SiO2 range. Most of the granites are high-level intrusives which grade into their own coeval volcanic ejecta.

[back to top]

Executive summary - metallogenic

There is no known mineralisation associated with the basement suites in Arnhem Land (Bradshaw or Giddy Suites). Heavy mineral sand fractions from Cato River and Wonga Creek were reported as containing 0.25% and 0.27% Sn. Petrological examination indicated the presence of ilmenite, rutile, and minor cassiterite. The actual location of the samples is not known, and the Bukudal Granite, Bradshaw Granite and Latram Granite are all exposed in the general area (Masood Ahmad, pers. comm; Chestnut et al. 1966).

The suites of igneous rocks related to the McArthur Basin are also not considered to have any economic potential. The suites are all too felsic and have high concentrations of high field strength elements, suggesting A-type affinities.

[back to top]

Future work

In view of the limited potential of the granites in the Arnhem Block/McArthur Basin, no further work is recommended. The only potential could lie in the Bukudal Granite with Sn, however, this is considered unlikely to produce significant mineralisation.