Hugh Rutter, Flagstaff GeoConsultants
February 2007
In the early 1990’s an exploration programme was commenced to locate the source of the alluvial gold known to have been mined from the Berry Deep Leads near Allendale in Victoria. 52.9 tonnes of gold is recorded to have been taken from the leads.
The area is completely basalt covered and geophysics was considered to provide some direction to exploration. The first requirement was to recognize and define major structure and lithological changes. Airborne magnetics was not appropriate because of the high magnetic properties of the basalt cover and the lack of any obvious magnetic minerals in the Palaeozoic basement.
The alternative, as a regional tool, is the gravity method. In the early work, traditional surveying was used to locate the stations and derive the elevation, but very soon this was replaced with differential GPS. Data collection was simple, relatively rapid and was not influenced by magnetic or electromagnetic effects from man made features.
The Victorian Initiative for Minerals and Petroleum (VIMP) was in its early stages at the time. The VIMP gravity for the larger area, Ballarat - Allendale – Clunes, is shown in Image 2. There are 1550 stations. The main features are the low density granites, but some variations can be seen in the Palaeozoic rocks.
Image 3 shows the distribution of gravity stations collected by Mount Rommel Mining Ltd since the project inception to the present day; data collection has been a continuous process over the years. There are in excess of 15,300 gravity stations almost ten times those of the VIMP compilation.
Image 4 shows the gravity contours produced by this data. The reduction density is 2.67g/cc and all stations have a terrain correction applied. There are many more variations in the Palaeozoic areas, particularly around Allendale, Ballarat and Clunes where the station density is greater. The contour interval is 0.2 milligals (2 gravity units, or 2 :m/sec 2).
Image 5 shows the gravity response in the Allendale area. The contour interval is 0.1 mgl. The large gravity low represents vesicular basalt in the vent of the dormant volcano at Birch’s Bald Hill. But there are many other features that cannot be explained by near surface basalts and must have there origin in the Palaeozoic basement
Image 6 is the same gravity image as the previous one, but some structural features have been added for illustration. There are many others along with probable variations in the constituents of the basements. Density measurements taken from core samples suggest that more shaley rocks have a greater density than the more sandy rocks.
At this stage of the programme there was a need for more information as to the relationship between gravity, geology and structure within a similar gold bearing environment. Ballarat was the obvious choice.
Image 7 shows the gravity response in the Ballarat area. The contour interval is 0.1 mgl. Structural breaks and lithological variations can be interpreted from the data and related to geological features recorded during earlier mining operations. The setting of known zones of gold mineralization can also be seen, e.g. the Ballarat east zone of mineralization lies within a defined gravity low.
Image 8 shows some of the structural features that can be identified in the gravity data; there are many more. The usefulness of the Ballarat gravity data is that it provides a direct relationship between gravity, structure, lithology and gold mineralization that can be used in similar environments such as those at Allendale and Clunes.
The gravity picture from Clunes is shown in Image 9. The Clunes goldfield is close to a granite batholith which is the blue area of low gravity towards the west of the image. Structure with a similar trend to that identified at Allendale and Ballarat can be identified in the area of Palaeozoic rocks.
Rocks from the earlier Western Mining drilling were studied by Binns and Eames (Geochemistry of Wall Rocks at the Clunes Gold Deposit, Victoria. R.A.Binns and J.C.Eames. Economic Geology 1989, Vol. 24, pp 310-319). Their prime interest was the geochemistry, but they also noted:
“Given the otherwise comparable nature of remote and proximal wall rocks, the lower densities of the altered zone at Clunes, averaging 0.10g/cc, might be detectable by gravity survey.”
This gave new meaning to some of the low amplitude gravity anomalies seen at Ballarat, Allendale and Clunes.
In recent months Mount Rommel Mining has been drilling at Clunes. Some drill holes have targeted the areas of low gravity anomaly in view of the results of the Binns and Eames study.
The result of one such hole is shown in Image 12. Highly altered Palaeozoic rocks were encountered along with anomalous gold and arsenic values.
In conclusion, the gravity method can be used to define major structure and lithology below the basalt cover. Care must be taken so as to distinguish low density vesicular basalt from altered basement rocks. Additional electrical methods, such as IP/resistivity will help with the interpretation and provide guidance for test drilling.
Hugh Rutter, Consulting Geophysicist, February 2007
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