Silver City Minerals Ltd (ASX:SCI) has the drill spinning to test copper-gold targets identified in geophysical surveys at the Copper Blow project near Broken Hill in far west New South Wales.
The primary aim is to test a large copper-gold target indicated in detailed induced polarisation (IP) survey work.
Magnetic and non-magnetic targets will be tested with initial drilling to comprise two holes, each of which will be about 450 to 500 metres deep.
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One hole will test for deeper extensions to the copper-gold mineralisation outlined by previous drilling with mineralisation hosted in a strongly magnetic ironstone.
A second hole will test a large IP chargeability anomaly about 800 metres southeast of Copper Blow.
The source of this anomaly is unknown but is likely to represent sulphide mineralisation.
Ground magnetic surveys suggest the rocks in the area of this new IP anomaly are non-magnetic and geological mapping indicates the presence of metamorphosed sediments at surface.
The 3D IP model, however, indicates that sulphide-bearing rocks are more than 100 metres below surface.
Two IP configurations
Silver City has used two different IP configurations to assess the project – gradient array and dipole-dipole.
A simplified geophysical map showing the results of dipole-dipole and gradient array surveys.
Gradient array is a rapid, cost-effective technique which enables first-pass mapping of the distribution of sub-surface sulphide.
It is a ‘shallow-looking’ technique and may detect sulphide to perhaps 100 metres below the surface depending on rock types and structures.
Dipole-dipole takes more time to gather data and is consequently more expensive.
It does, however, give higher resolution to the IP data and has the potential to detect significant sulphide to depths of around 300 metres.
This configuration lends itself well to 3D computer modelling.
Work to date at Copper Blow shows gradient array to be an excellent screening tool, while follow-up dipole-dipole enhances and defines targets for drilling, especially at depths greater than 100 metres.
Surveys suggest that sulphide mineralisation occurs in both a magnetic ironstone horizon and non-magnetic meta-sediments.
The initial gradient array survey highlighted areas of known mineralisation and new, as yet undrilled anomalies southeast of Copper Blow.
This was followed up by the high-resolution dipole-dipole survey to produce some outstanding new drill targets which are likely to contain abundant sulphide.
One of these will be drilled as part of the upcoming drill program.
Copper Blow longitudinal section showing grade by thickness plot.
Gradient array surveys extended
On the basis of positive results to date, Silver City decided to extend the gradient array surveys to the northeast of Copper Blow.
Results of expanded geophysical surveys indicate the potential for copper-gold mineralisation in sulphides.
These extend over a strike length over 4 kilometres from the southern to northern targets covering an area of about 2 square kilometres.
This suggests a much larger mineralised complex and potentially more copper, gold and cobalt, than that solely hosted in the magnetic ironstone complex where drilling has been focused.
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Company geologists suggest that not only is the magnetite-bearing shear zone an excellent conduit and trap for mineralisation but the likely source of mineralising fluids may be an adjacent, copper-gold mineralised, magmatic intrusive complex which is not well exposed.
Initial drilling is likely to be followed by a much larger program of 3,000 to 4,000 metres to test the continuation of the magnetic ironstone and new geophysical and geochemical targets.
A soil geochemical program to assist drill targeting has been initiated in the northern target zone.