Open File Report 128
The geochemical discrimination of mineralised and barren ironstones
from the Selwyn Au-Cu Deposit, NW Queensland
J. E. Wildman
Recent exploration for Au-Cu deposits in the Eastern Succession
of the Mt Isa Inlier has concentrated on the type of magnetic ironstones
found at Selwyn. Aerial magnetics are used to find the larger occurrences
of magnetite but they cannot distinguish between mineralised and
barren magnetite bodies. Samples of mineralised and barren ironstones
were used to geochemically differentiate the two types of ironstone.
Comparisons were also made between the unoxidised sulphide ore and
its weathered equivalent found at the surface.
Samples of mineralised and barren ironstones were analysed for
38 elements by XRF and INAA. Photomicrographs and backscattered
electron images were used to describe the mineralogy and textures
of selected samples. Those elements that distinguish mineralised
from barren ironstones have been displayed using several methods
of plotting the data.
The mineralised samples are distinguished by Au and W and the barren
samples by Ba and Mn. Copper is high in the mineralised samples
but there is a large overlap of concentrations with the barren samples.
The SEM investigation shows that Au, W, Sn and Cu can be preserved
in hematite derived from the magnetite. This hematite may then be
a source of local stream sediment anomalies at surface in the case
of Selwyn or at a paleosurface where the ironstones are buried by
Mesozoic or recent sediments.