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Paleomagnetic dating of weathered regolith at Cobar, NSW B. Pillans, M. Smith and K. McQueen

Paleomagnetic dating of weathered regolith at Cobar, NSW

B. Pillans, M. Smith and K. McQueen

The success of geochemical exploration in areas of thick regolith depends on knowledge of the nature, and history and timing of geochemical dispersion in complex weathered materials.

Paleomagnetic dating of weathered regolith at four sites near Cobar, western NSW, has helped establish a framework for the weathering history of this region. Oriented samples of oxidised saprolite from each site were subjected to stepwise thermal demagnetisation in the laboratory, and remanences were measured on an ScT 2-axis cryogenic magnetometer.

 

Well-defined, stable magnetic remanences were isolated as follows (see Table 1): At Wilga Tank, a deep weathering profile with a ferruginous mottled zone and underlying bleached saprolite, is preserved beneath a dissected basalt flow. A nearby volcanic plug and lava flow yielded K/Ar ages of ca. 15 Ma. The upper part of the sub-basaltic weathering profile yield a pole position that lies close to the 12 Ma (middle Miocene) pole, suggesting a similar age to that of the basalts. Samples from saprolite from a quarry just north of Elura mine indicate two periods of iron mobilisation, one of Middle Miocene (12 Ma) age and the other of Latest Cretaceous to Early Paleocene (60 ± 10 Ma) age. Similar ages were obtained from another site (McKinnons open pit), while at the New Cobar open pit, oxidised saprolite (after Early Devonian shales and siltstones) gives a pole of apparent Jurassic age (ca. 180 Ma).

Similar weathering ages have been determined in the Yilgarn Craton of Western Australia, and also in northern South Australia and southern Queensland. The wide distribution and broad range of geologic and geomorphic settings of these sites are consistent with a paleoclimate-driven weathering episode across southern Australia. Finally, the dispersion patterns for base metals and gold are different for the McKinnons and New Cobar settings. This reflects different chemical and ferruginisation conditions, which probably relate to the different histories of the regolith profiles.

TABLE 1: Summary of paleomagnetic results from weathered saprolite.

 LOCATION    N(+)(1) REMINENCE DIRECTIONS (2) SOUTH POLE

..........o
Decl.

...........o
Incl.

k

...............o
alpha 95

............o
Long.

............o
Lat.

K

............o
A95
Elura mine
Ferruginous nodules 35(35) 206.6 74.4 117 2.25 123.4E 55.3S 43.8 3.71
31(0) 6.3 -59.1 96.2 2.65 118.6E 79.4S 65.3 3.22
New Cobar mine
Oxidised saprolite 42(21) 325.2 -72.2 41.9 3.45 176.5E 54.1S 18.8 5.22
McKinnons mine
Pink saprolite 82(38) 11.9 -61.4 73.2 1.84 109.6E 75.2S 40.2 2.5
Red saprolite 35(35) 205.3 70 297 1.41 114.6E 61.3S 128 2.15
Wilga Tank
Ferruginous mottles 35(15) 2.3 -63.3 52.7 3.37 141.8E 75.4S 25.9 4.86
Perth Basin
Weathering overprint 128(?) 109.9E 82.7S 2.4
Morney profile, Qld
Deep weathering profile 37(17) 17.8 -68.3 2.4 118.5E 59.8S 3.8

Notes: (1) N = number of specimens; (+) = number of specimens with positive inclination
(2) k and K are Fisher precision parameters; a _95 and A95 are semi-angles of 95% confidence.