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Open File Report 37

Lawlers orientation study - Contribution to Field Guide, Eastern Goldfields Trip. 1-2 November 1990

Anand, R.R., Smith, R.E., Churchward, H.M. and Perdrix, J.L.

LAWLERS FIELD STOPS 1 and 2 November 1990
10.00 AM
STOP 1 Brilliant area
An example of regolith in an erosional regime (Unit 2)

  • Low, stony hills
  • stripping of the lateritic profile
  • lag of ferruginous cobbles (iron segregations)
  • red, clay soil
  • pedogenic carbonate
  • pockets of saprolite-bedrock

The ferruginous cobbles occur largely on erosional areas in the Lawlers district and appear to have been derived from the breakdown of iron segregation bodies. The iron segregations, several metres across, commonly occur within the ferruginous saprolite-saprolite horizons in a weathering profile. Ferruginous cobbles are black, generally non-magnetic and many of them are dominated by low Al-substituted goethite and hematite. The internal surfaces of many of the ferruginous cobbles show the goethite pseudomorphs after pyrite. Ferruginous cobbles are high in Fe (71% Fe2O3), Mn (3000 ppm), Zn (380 ppm), Cu (157 ppm), and Co (91 ppm) - mean values of 33 samples.

STOP 2 Agnew-McCaffery area
Comparison of the regolith between erosional (Unit 2) and residual regimes (Unit 1) of the Agnew - McCaffery area. The location comprises two minor valleys.
Residual regime - broad crest, backslope, valley floor (Northern Valley).

  • Gravelly sandy loam to sandy clay loam soil over lateritic residuum.
  • Lateritic gravels with yellowish brown cutans on broad crest and become finer down slope.
  • Down slope, the gravels with dark brown to brownish black surfaces become increasingly more common.
  • Hematite and maghemite are more abundant in lateritic lag gravels on broad crest. This suite of minerals also dominates the backslope and valley floor.
  • Fe2O3 is the most abundant constituent with values greater than 75% in lateritic gravels. The small amounts of Al203 and SiO2 are mainly present as kaolinite.
  • The trace elements V, Mn, Cr, As, Pb and Ga are more abundant in the lateritic gravels than in lag of ferruginous saprolite.
  • Goethites are highly Al-substituted (up to 30 mole%).

Erosional regime - breakaway, pediment slope, valley floor (Southern Valley).

  • Gravelly sandy clay loam soil over saprolite.
  • Subcropping saprolite-ferruginous saprolite (Unit 2) on the breakaway and upper pediments and alluvium-colluvium dominating the valley floor.
  • The pediments below the breakaway are mantled by a coarse lag containing yellowish brown ferruginous saprolite-mottled saprolite.
  • These lags become finer down slope, they are a mixture of clasts with yellowish brown surfaces and others that are black. Cutans are generally not present.
  • Goethite is the dominant mineral. Al-substitution in the goethite from the lags of the erosional regime is systematically lower (13-18 mole%) than goethites from the residual regime. Kaolinite is relatively more abundant in ferruginous saprolite than in lateritic gravels.
  • Fe2O3 averages about 55% in lag of ferruginous saprolitic-mottled saprolite.
  • Cu and Ni are more abundant in lag of ferruginous saprolite-mottled saprolite than in lateritic gravels.

LUNCH 12.45 PM
2.00 PM
STOP 3 Turrett Pit
Colluvial outwash plain (Unit 6), colluvium on lateritic residuum.
Laterite relationships and regolith stratigraphy from mine pit walls.
Units of the weathering profile (Southern half of the Western wall).

  • Gravelly colluvium (hardpanized and containing lateritic detritus).
  • Lateritic residuum with iron segregations.
  • Massive iron segregation bodies (note breakdown of iron segregations into small 5-10 mm nodules); many iron segregations and nodules show goethite pseudomorphs after pyrite).
  • Ferruginous saprolite-collapsed ferruginous saprolite.
  • Saprolite, saprock.


Also (northern half)

  • Colluvium on packed duricrust (black pisoliths in sandy matrix).

1. Colluvium on red clay (some hardpanized).

STOP 4 Agnew gravel pit
Residual regime - backslopes, crest

  • Lateritic nodules, pisoliths.
  • Nodular-pisolitic duricrust (some hardpanized).
  • Pockets of Fe-rich pebbly duricrust.

Erosional regime - breakaway, pediment slopes.

  • Lag of ferruginous saprolite-mottled saprolite, iron segregations.

Lateritic duricrust and lateritic gravels are dominated by hematite, goethite, and maghemite with small amounts of kaolinite, gibbsite, quartz and, anatase. By contrast, iron segregations are dominated by goethite, with small amounts of hematite.

5.00 PM
8.00 AM
STOP 5 Waroonga Pit
Colluvial outwash plain (Unit 6); thick colluvium on lateritic residuum.

  • Red colluvium.
  • Hardpanized red colluvium containing lateritic detritus, with abundant partings (Si rich) and Mn staining.
  • Red clay with some ferruginous granules (note the sharp contact between hardpanized colluvium and red clay).
  • Nodular duricrust (hematite-rich red nodules in a kaolinite-rich matrix).


STOP 6 Brilliant area

  • Fe-rich nodular-oolitic duricrust on crest and breakaway face. - Ferruginous cobbles (iron segregations) on pediments.
  • Local pockets of pedogenic carbonates.
  • Saprolite.

Iron-rich duricrusts are sporadically distributed throughout the Lawlers district and commonly occur on topographically elevated areas. The most characteristic features of Fe-rich duricrusts are:

  • the boundaries between nodules-ooliths and matrix are not well developed in sliced surfaces, despite the pebbly appearance of weathered surfaces;
  • the matrix and nodule compositions are not significantly different and both are Fe-rich;
  • the volume of matrix between the nodules is small;
  • nodules generally lack cutans;
  • weathered ilmenite grains and slightly-ferruginized charcoal fragments occur within pisoliths;
    characterized by high concentration of Fe and very low concentration of Si and Al;
  • dominated by hematite, goethite, and maghemite;
  • large range in values on Mn, Cr, V, Zn, Ni, Co, As, and Ga.


Last updated: Thursday, January 06, 2000 08:22 AM


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