Search CRC LEME :

powered by FreeFind

Publication Policy

Open File Report Series

OFRS Index


Regolith Maps

Annual Reports

Articles & Papers


Minerals Briefs

"Focus on Salt"

Other LEME Reports

Order Form

Open File Report 69

Spectral properties of soil and lag overlying the site of the Beasley Creek Gold Mine, Laverton Region, Western Australia

Cudahy, T.J., Robertson, I.D.M. and Gabell, A.R.

Gold exploration, using spectral remote sensing in the Yilgarn Craton, is difficult for at least two reasons. Firstly, very deep weathering has changed the complex fresh rock mineralogy to a surface mantle, largely consisting of ferric oxides, clays and quartz. Secondly, there is little detailed knowledge of the reflectance properties of these weathered surface materials, of very variable crystallinity, overlying gold mineralisation and in background areas. To address these problems, some of the studies in the P243 project have concentrated on the measurement and analysis of 0.4 to 2.5 µm reflectance spectra of a large range of regolith materials overlying gold deposits at Beasley Creek (this study), Bounty (in the Forrestania region, report 169R) and Panglo (in the Ora Banda region, report 234R). Background areas have included studies at Laverton (report 235R), Lawlers (report 240R) and Ora Banda.

The Beasley Creek gold mine is located in the Laverton area, north-eastern Yilgarn Craton. Before disturbance of the ground by open-pit mining, the Beasley Creek gold deposit was situated under the crest of a small, 3.5 m high rise. Gold mineralisation was related to deeply weathered carbonaceous shale and chert rocks, within an Archaean greenstone sequence. Laboratory reflectance spectra were measured of surface samples collected from two east-west traverses across gold mineralisation and extending into background areas. The samples comprised soil and ferruginous lag overlying subcropping lateritic duricrust, exposed saprolite and mottled zone.

The spectral results show no evidence for sericite or other primary minerals related to the original fresh rock. Instead, the spectra show information related to the products of weathering, namely, ferric oxides (hematite and goethite) and kaolinite.

The spectra of the coarser, ferruginous lag (10-50 mm diameter) show variations in the wavelengths of the ligand-metal charge transfer shoulder, near 0.6 µm, and the iron crystal field absorption minima, near 0.9 µm, indicating hematite or goethite. The relative changes in the wavelength of these parameters are invariant over broad zones (>500 m wide), consistent with hematite-goethite relationships measured by XRD and spatially related to particular, exposed, lateritic units. The goethitic lag is located over saprolite and mottled zone and is interpreted to be the residual product of deflation of the upper, lateritic horizons. The hematitic lag is located over lateritic duricrust.

The soil shows a relatively consistent spectral mineralogy comprising hematite and poorly crystalline kaolinite. The poor kaolinite crystallinity is indicated by the weakly developed absorption doublets at 1.4 and 2.2 µm. Within a 100 m wide zone, over gold mineralisation, there is an 8 nm shift to shorter wavelengths of the charge transfer shoulder, indicating slightly more goethite-rich soil. This shift in wavelength is much less than that shown by the coarse lag.

The soil and lag data indicate a weak relationship between gold and the wavelength of the charge transfer shoulder. This relationship shows that the weathered materials, with more gold, are relatively goethite-rich though, there are too few data to consider this result as significant.

The associated P240 and P241 investigations noted an increase in the overall abundance of iron-rich lag in the vicinity of gold mineralisation. The increase in the total ferric iron content of the soil and lag was examined, using the depth of the crystal field absorption near 0.9 µm. This spectral parameter showed weak correlation with the Fe2O3 content but no relationship to areas of gold mineralisation. However, this does not preclude gold being associated with higher ferric iron content at the surface. An increased amount of ferruginous lag at the surface will increase the total iron content, especially in the context of remote sensing applications.

According to a regolith model (report 243R), passage from saprolite and mottled zone to lateritic duricrust is hypothetically associated with a decrease in the abundance of clays. This relationship was tested for the soil developed over these particular units, using the depth of the AlOH-related 2.2 µm absorption. The depth of the 2.2 µm absorption was found to correlate with the A12O3 content of the soil but there was poor correlation with the position in the regolith. The results showed soil mantling saprolite and mottled zone has relatively shallow 2.2 µm adsorptions (lower clay abundances) compared to the soil covering lateritic duricrust. It is suggested that this anomaly is caused by increased winnowing of the soils, overlying saprolite and mottled zone on the western-side of the low rise, by the prevailing winds.

The associated P240 and P241 studies found that powdery carbonates occur as patches within the soil. However, spectral examination of carbonate-rich soil showed no evidence of the diagnostic carbonate absorption at 2.33 µm, even though scanning electron microscope examinations showed calcium-rich particles were well exposed at the surface of the soil minerals. A related P243 study (report 169R) discovered more than 40% by weight of sub-75 µm carbonate powder was required in a soil before carbonates were spectrally recognisable. This is much greater than the maximum carbonate content of the Beasley Creek soil. The only indication of carbonate in the spectra was an increase in the albedo, particularly the visible brightness.

Spectral analysis of the depth, width and wavelength of the 1.9 µm, water-related absorption, can provide information about the content and site (free, adsorbed, trapped) of water molecules. The soil shows two distinct populations of 1.9 µm adsorptions, one associated with free water (driven off after heating to 100°C), and the other associated with water trapped in quartz as fluid inclusions (unrelated to gold mineralisation).

The coarse, ferruginous lag commonly shows an upward ramp in reflectance from 0.9 and 1.3 µm which can dominate spectral properties in this region and will influence the geometry of the ferric iron absorption at 0.9 µm. Another P243 study (report 244R) found the intensity of this feature is correlated with the depth and width of the 1.9 µm absorption and suggested this water is either adsorbed on the surfaces of the iron oxide crystals, by hydrogen bonding, or is intimately associated with silica and iron oxide and formed after lateritisation (report 243R). This spectral property may be associated with the affects of desert varnish, though the underlying rock mineralogy is clearly evident in the spectra (for example, the hematite-goethite ratio).

The spectral results show that the soil and ferruginous lag overlying the Beasley Creek gold mineralisation are not mineralogically distinct from the background materials. An increased development of goethite (shorter wavelength charge transfer shoulder) appears to be associated with gold at Beasley Creek but goethite is pervasively developed in varying abundances throughout the regolith. Therefore, the significance of ferric oxide development to gold mineralisation needs to be further evaluated, especially in the context of the regolith. The spectral results appear to be useful for the regolith characterisation of the soil and lag and so could be used to help classify regolith materials to assist geochemical gold exploration.

Last updated: Thursday, January 06, 2000 11:42 AM


Cooperative Research Centres Australia

About Us | News & Events | Research
Publications | Education | Staff Only | Links

Contact Us | Disclaimer | Sitemap
© CRC LEME 2004

CRC LEME is established and supported under the Australian Government's Cooperative Research Centres Program. The CRC Program is an Australian Government initiative which brings together research groups with common interests.

CRC LEME Core Parties