Open File Report 53
The distribution of gold and other elements in soils at the Granny
Smith Gold Deposit, Western Australia
Lintern, M.J. and Butt, C.R.M.
The distribution of Au and other elements was examined at the Granny
Smith Au deposit, south of Laverton. The area is extensively covered
by hardpan, which is particularly common in the northern part of
the Yilgarn Craton and inland Australia. Five profiles were studied,
some to 10 m depth, for textural, geochemical and mineralogical
characteristics pertaining to the distribution of Au and other elements.
Laboratory studies were performed to examine the nature and behaviour
of Au in the soil. Selected bedrock samples were analysed for trace
elements in order to determine their potential as pathfinders for
Au mineralization. This study was to complement work previously
performed south of the Menzies Line where a strong association was
found between pedogenic carbonate and Au.
The distribution of Au at Granny Smith appears to be primarily
related to the contact between transported and residual components
of the hardpan and is coincident with a trend towards increasingly
alkaline conditions. South of the Menzies Line, such a change of
pH has no significant effect on the distribution of Au. Segregations
of hardpan material from contact between the transported and residual
components indicates that the matrix does not necessarily contain
all the Au because some is associated with lithorelic fragments
cemented within the matrix. Furthermore, there does not appear to
be any general mineralogical, geochemical or textural associations
of Au with other components within the profiles, although these
do exist within individual profiles. Laboratory experiments indicate
that some Au is associated with specific extractable phases within
the soil e.g., manganese oxides, organic material and soluble silica
but, compared with Au that can be leached using water or iodide
alone, they do not represent a highly significant fraction. Gold
is generally found to be at least as soluble in water and iodide
as in some soils south of the Menzies Line, but mobility in the
surficial environment appears to be severely restricted due to encapsulation
within the hardpan cement. Re-adsorption of dissolved Au by hardpan
is possible, but is weaker than that occurring with ferruginous
soils in the south.
Arsenic, W. Mo and, perhaps, Sb are associated with primary mineralization,
but only As is retained at detectable concentrations in the upper
regolith. Barium, Cu and Zn are weakly enriched in bedrock and are
retained in the upper saprolite; however, Ba may also indicate the
presence of felsic rocks, such as granodiorite, rather than mineralization.
The generally low abundances of Bi, Cd, In, Pb, Se and Ag in primary
mineralization eliminate them as potential pathfinder elements.
The work highlights the problems associated with exploration for
Au in areas dominated by hardpan. Further studies and examples are
required before any recommendations on specific sampling procedures
can be made.
Last updated: Thursday, January 06, 2000 08:59 AM