Legacy product - no abstract available

These documents have been scanned by the GA Library. Please refer to the document for contents.

These documents have been scanned by the GA Library. Please refer to the document for contents.

Legacy product - no abstract available Never published, see Record 1965/048 instead

The lead-silver-zinc ore deposit of Broken Hill, New South Wales, is among the great ore deposits of the world because of its size, richness, and continuity. To the end of 194.6, approximately £50,000,000 in dividends had been won from recoverable metals worth £210,500,000 gross contained in 63,800,000 tons of ore. The deposit is a hypothermal deposit of Pre-Cambrian age resulting from the selective replacement of two closely adjacent, tightly and complexly folded stratigraphic rock layers. The original sedimentary rocks of the area now consist of tightly folded sillimanite-garnet gneisses with subordinate thin quartzite beds. These contain numerous folded sills of augen gneiss (granite), amphibolite (gabbro), and pegmatite. Post-folding peridotite, granite and pegmatite occur. Probably after considerable uplift and erosion, thin dykes of diabase (dolerite) were intruded, then pegmatite dykes and silicifying solutions, and finally ore-bearing solutions. The folds of the region are tight; steeply inclined, and extremely complex structures resulting from plastic deformation. Individual minor folds were studied in great detail by a method of axial-plane and axial-line analysis. An angular relationship exists between minor and major folds due to strain under torsional stresses. The regional pitch is flatly south. However, sudden reversals of pitch and divergences of pitch in adjacent folds are common. Second-order folds or folded folds exist. Cutting and offsetting the folds are buckles with vertical axes and crush zones of schisted rocks resulting from post-folding but pre-ore faulting movements. The lode occurs in a belt of attenuation between a wide arch on the west and a wide basin on the east. It consists of ma3sive lead-zinc-sulphide replacement orebodies forming (before erosion) a long continuous, irregular, flat, curving pencil of ore, 2,000 to 3,000 feet high and 300 feet thick. In longitudinal section it describes a flat arc pitching downward at each end. The lead lodes resulted from the selective replacement of two closely adjacent, highly folded, favourable beds. Each lode is distinguishable by its gangue mineralogy and metal ratios. No.3 Lens (the lower) has fluoritic rhodonitic gangue and comparatively high Zn:Pb and Ag:Pb ratios. No. 2 Lens, the upper, has calcitic gangue and comparatively low Zn :Pb and Ag :Pb ratios. At least three zinc lodes, with similar mineralogy, occur at higher stratigraphic horizons at the south end of the field. There is little observable zoning. Ore solutions are believed to have migrated up the regional pitch from the south. Intra-mineralization fracturing helped to localize ore shoots within favourable formations.

Benbow crater is the main residual active centre of the large ancient volcano which has formed Ambrim Island. For the eleven months preceding December 1951 it has been the source of extraordinarily intense and prolonged explosive activity. The volume of ash and scoria ejected during this period exceeds 800,000,000 cubic metres. Evidence has been found during the recent inspection of this volcano that suggests that the current phase of explosive activity has ended. The 1950-1951 eruption, possibility of future activity, and effects of the eruption, are discussed in this report.

In Portion 11, Ph. of Stockrington Diamond Drilling near an unnamed creek tributary to Surveyor Creek has disclosed coal continuously for a distance of 7,600 feet south from the northern boundary of the Portion. The seam is split and banded and the coal is inherently high in ash. Proximate analyses of the coal were carried out by the New South Wales Mines Department laboratory in Sydney. All coal ores were forwarded from the field and shale etc. bands of greater thickness than half an inch were discarded by the analysts. Stony coal or carbonaceous shale with S.G. greater than 1.6 was also rejected from the assay samples. Consequently the analyses quoted indicate a composition roughly equivalent to that which might be expected for cleaned or hand-picked coal from this area.

The purpose of a visit to the Cloncurry district, which lasted from 21st August to 30th September, 1951, was to see what aid geophysical work could give in the search for copper deposits in this field which is one of the biggest mineral fields in Australia. While the Cloncurry district in the past produced copper from many small but rich deposits, only a few exceeded 200,000 tons, and the search now is being conducted mainly with the idea of finding and developing huge but relatively low grade copper deposits with at least several hundred thousand tons, or perhaps millions of tons of copper ore containing at least 2.5 to 5 percent copper. It is possible that geophysics might help in this search, and consequently, a number of old copper mines were visited and some geophysical test traverses made. With the rather limited facilities available the tests were confined to self-potential measurements and to a few traverses with a new Atlas gravity meter, especially suitable for ore prospecting because of its lights weight and transportability and its high sensitivity and accuracy. This report gives an account of the survey and its results.

The work done in this investigation was for use in the compilation of a large scale geological map of the A.C.T. The area mapped adjoins that mapped by Flinter and McInnes (1949) and L.C. Noakes (1946). The map accompanying this report includes all of the A.C.T. south of an east-west line through Tharwa. Geological features, including the physiography, stratigraphy, and structural geology of the area, are described in this report.

An area surrounding the magnetic observatory at Toolangi was tested for uniformity of the magnetic field. The three elements measured were declination, horizontal intensity, and vertical intensity. The main tests were carried out over the period, 10th December to 21st December, 1951. Further determinations of horizontal and vertical intensity at different heights above two of the stations were made on the 7th and 9th January, 1952. The methods of observation and results of the survey are discussed in this report.