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The Pamphlet Collection of Sir Robert Stout: Volume 75

Ballarat, Victoria

Ballarat, Victoria.

This district comprises two narrow belts running almost due N. and S., one passing through Ballarat West, and worked pretty continuously for about 5 miles; the other passing through Ballarat East, and worked for about 8 miles. The two belts differ in vein-formation and in vein-materials.

In the western belt the Upper Silurian shales and sandstones, constituting the country-rock, have been much folded, are now almost invariably steeply tilted, and are traversed by true fissures, striking almost always N. and S., filled with quartz, and receiving numerous "leaders" and "droppers" from the country on both sides, the whole sometimes forming a "stockwork" many feet wide.

In the eastern belt the strata have been bent, as at Bendigo, into sharp synclines and anticlines. But in this case, where fracture has occurred along the anticlines, the result has been, not" saddle-reefs," as at Bendigo, but more or less continuous fissures filled with quartz. The most interesting and characteristic feature of the deposits of Ballarat East, however, is not the occurrence of these continuous lodes, but that of the so-called "indicators." These are certain members of a series of thin- page 6 bedded sandstones and slates, which have been traced along the whole length of the belt, so far as the latter has been explored. The indicators, which have exerted an important influence on the gold-bearing character of the quartz-lodes traversing them, are thin beds of dark carbonaceous shale, abounding in pyrite and arsenopyrite, the sulphides in some places completely replacing the shale of the bed. The indicators are conformable with the bounding slates and sandstones, which stand almost vertical, and they share with the whole series in the effect of numerous dip-faults, or minor dislocations on the dip, locally termed cross-courses. Several of them run parallel within a band some 300 feet wide, and one, known as "The Indicator," has been traced N. and S. nearly 8 miles. (See Fig. 1, copied from a drawing by Mr. T. A. Rickard.)

The quartz-bodies in this indicator-belt are not regular reefs, but veins of varying thickness crossing the indicator at all angles; those which present a nearly horizontal crossing, with a strike parallel to the indicator and a slight dip E., having proved most productive. Away from the indicator, the greater part of the vein-quartz is absolutely barren; but at the intersection with the indicator large masses of gold (often more than 100 ounces in one piece) have been obtained, and the greater part of the gold extracted from this belt has come from those parts of the quartz-veins near some one of the indicators.

Mr. E. Lidgey, of the Victoria Geological Survey, in his Special Report on the Ballarat East Gold-Field, gives a list of 15 gold-masses or nuggets found as above described, and varying in weight from 30 to 137 ounces, and in depth from the surface (measured on the indicator) from 160 to 805 feet, the average being 470 feet. In the same report he raises several questions as to the source and distribution of the gold in the indicators, and says that the solution of these problems will require a series of investigations of the country-rock, like that performed by Sandberger in Germany.

The Prince Regent Mine.—Before seeing Mr. Lidgey's report I had taken a set of samples for the purpose suggested, selecting the Prince Regent mine, one of the most productive on the indicator belt, as likely to be most instructive from a genetic standpoint, because it had shown, as the depth of the mine increased, a marked change in the auriferous contents of the page 7 veins crossing the indicators. This mine is about 870 feet deep. Down to the 630-foot level, pay-quartz, sometimes with

Fig. 1.

Fig. 1.

Vertical Section of the indicator, Ballarat-East.

Showing quartz veins.

(After T. A. Rickard.)

rich patches, was worked on the main indicator and on parallel ones; but for the last 240 feet the former has been barren and page 8 the latter have been much less productive than before. Samples were therefore taken on the 630-foot level, where the indicator had carried pay-quartz, and from the 770-foot level, where it had proved almost barren.

The ground near the indicators has been much faulted; and this faulting was found to have an interesting connection with the genesis of the auriferous deposits. The questions raised by Mr. Lidgey comprised, besides the general inquiries,—Was the gold on the indicators leached from the surrounding country-rock? Was it brought from below by ascending, or from above by descending, solutions?—the more specific ones, Why is the gold on these indicators concentrated in patches, and what effect have what are known to the miners as "heads," "headings" and "breasts"? Mr. Lidgey does not explain these terms; but they are probably related to the phenomenon observed by me in the Prince Regent mine, and reported to me as occurring in other mines in the belt, namely, that the chief quartz-veins crossing the indicator end always against a vein of "pug," or soft clay, filling a fault, along which more or less vertical movement has taken place. These slides, cutting off the quartz-veins, have, in my opinion, an important bearing on the questions under consideration.

Samples were taken in the Prince Regent mine: (1) of solid slate and sandstone, both close to, and as far as possible from, the indicator-beds known as "The Indicator" and "The Mundic Slate"; (2) of "pug," clay or broken quartz from the backs or breasts against which the veins crossing the indicator end; (3) of the quartz of the indicator-lodes, both containing and not containing sulphides; (4) of the indicator itself. The results of the analysis of these samples is shown in Tables IV. A and IV. B.

As these tables show, the solid country-rock, containing no sulphides, taken at whatever distance from the lode, is not auriferous; and this was equally true whether the veins on the indicator were productive or barren on the tract from which the sample of country-rock was taken. Moreover, sulphides are found in the country-rock only in close proximity to fault-planes or "backs." And finally, the "pug" filling these "backs" or slight fault-fissures is often highly auriferous, especially when it contains sulphides or an unusual percentage page 9 of organic matter; and it was found to be as highly auriferous at levels where the indicator was barren, as where the indicator carried pay-quartz.

These results point to the following conclusions:
1.The gold on the indicator has not been leached out of the surrounding rock, but has most probably entered the lodes crossing the indicator, from below.
2.The channels that carried the upward currents from which the gold was deposited were probably the fault-planes or "backs," which cut off the indicator-veins.
3.The ascending solutions probably brought with them the gold from below, and gold was deposited along the indicators where the composition of the latter favored precipitation.

I believe therefore that the difference between barren ground and highly auriferous ground on an "indicator-line" is connected with some difference in the character of the indicator itself, at the two levels compared; and, in my opinion, the key to the distribution of gold in this most interesting ore-deposit should be sought in a careful examination of the various indicators which are found to be associated with auriferous and non-auriferous veins of quartz.

The Northern Star Co.'s Mine.—This mine is in Ballarat Vest, and an unusually good typical cross-section of the strata of that belt is afforded by a prospecting cross-cut, driven west at the depth of 1250 feet, and 1264 feet long in 1894 (since extended to about 1700 feet). At 100 feet from the shaft the Guiding Star lode was cut, and a level was driven along it for about 350 feet. The cross-cut. traversed alternate beds of sand stone and slate, including several small reefs of barren quartz. (see k, q, y, Table V.). At 1060 feet from the shaft, an eruptive dike 36 feet wide, and interbedded with the slates and sand-stones, was driven through. It was felsite porphyry, much decomposed, and impregnated, especially near its contact with the country on each side, with much pyrite—the cubes of which were occasionally 18 mm. in diameter. This occurrence is specially interesting, since eruptive dikes are much rarer in the Lower Silurian than in the Upper Silurian auriferous area of Victoria. Samples were taken from this dike and at various distances along the cross-cut. Table V. contains the results of analyses, which are graphically shown in Diagram 3.

page 10

The remarks previously made on Tables I., II. and III. (Bendigo field) apply equally well here; but the analyses of the eruptive dike are specially interesting. The auriferous character of the pyrite in it (see "Dike 1 and 2," Table V,) indicates that an auriferous lode may be associated with the dike not far from its intersection by the cross-cut.