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The Pamphlet Collection of Sir Robert Stout: Volume 75

Walhalla, Victoria

Walhalla, Victoria.

One of the long narrow belts which constitute the auriferous areas of Victoria extends from Jamison on the north to Walhalla on the south, and is notable not only for the richness of some of its reefs, but also as presenting Upper Silurian instead of the Lower Silurian strata, and, above all, because of the peculiar relation between its pay-reefs and dikes of igneous rock—no productive reefs having been found in this belt or in any Upper Silurian area that were not associated with dikes; whereas in the Lower Silurian (Bendigo, Ballarat and Steiglitz) dikes are less common, and when they occur are usually found to be of more recent date than the reefs. (See R. A. F. Murray's "Geology and Physical Geography of Victoria," Melbourne, 1887.) The gold-veins in the Upper Silurian area are sometimes found crossing, more or less horizontally, dikes of great width, and dying out in the slates and sandstones on each side of the dikes. Another type of occurrence is illustrated by I the eruptive dike of Cohen's reef, Walhalla, where the reef accompanies the dike, lying on one or both sides of the dike material, between it and the enclosing country, or occasionally bounded on both sides by the dike. This mine was sampled, as will be observed further on.

Some writers have derived the auriferous contents of these lodes from the dikes.* But heretofore, so far as I know, no observer has actually analyzed samples of the dike to ascertain whether it is auriferous. Two other hypotheses are conceivable. On the lateral-secretion theory, the gold and silver of the quartz (and of the dike, if it carried any) might be derived from the country-rock on both sides; and, on the ascension-theory, the gold and silver might be derived from a deeper source.

The analyses undertaken in order to throw some light on this subject were: (1) of dike-rock containing no sulphides, and not associated with an auriferous quartz-reef; (2) of dike- page 12 rock associated with such reefs; (3) of country-rock from the deep levels at various distances from the lode.

The Long Tunnel Mine on Cohen's Reef.—This mine is entered by a tunnel (see Fig. 2), at the end of which a shaft descends vertically 900 feet. From the bottom of this shaft a level (called the Northwest level in Table VII. A) extends 1800 feet and from the end of this a second or main working-shaft descends about 800 feet. The working-levels from this shaft are 1022 feet, 1122 feet, etc., below the tunnel-level, and about 700 feet more than that below the surface of the hill. The Northwest level cuts through a barren dike and some quartz-veins The dike associated with the main reef is exposed by a cross-cut from this level, and in the deeper workings below this level. Tables VII. A and VII. B give the analyses of samples from the two kinds of dike-material.

On the 900-foot level, a cross-cut 278 feet long has been driven W. to the dike. Samples of country-rock taken in this cross-cut are tabulated in Table VIII.; and Table IX. shows the results of samples from a cross-cut in the 1422-foot level driven 165 feet east from the dike to the boundary of the claim.

Tables VII. A and VII. B show that, where no quartz is associated with the dike, the rock contains little or no sulphides, and is not auriferous; that where the dike is associated with quartz, but the quartz is not auriferous, the dike-rock is likewise barren; that the percentage of sulphides in the dike is greatly increased when it is associated with highly pyritiferous quartz, and that in this case the dike-sulphides are auriferous, though much less so than those of the associated reef.

As all the samples were taken from one dike of each class, it would not be safe to generalize too positively; but the results indicate that, in the Upper Silurian area, the dike-rocks them selves are not auriferous, but that their silver and gold are derived from the accompanying quartz-reefs, and that the agencies which filled with quartz the fissures in the dikes, impregnated the latter with auriferous sulphides—these being however, less highly auriferous than the pyrite of the reefs.

Microscopic examination of samples of both barren and auriferous dike-rock from this mine showed that every sample was more or less decomposed and altered. The principal difference

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Fig. 2.

Fig. 2.

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clearly exhibited is the much greater abundance of pyrite and arsenopyrite in samples associated with an auriferous lode.

Concerning Table VIII., it will be noted that the samples are remarkable for the almost total absence of sulphides. Gold was found in only one sample from this cross-cut, and this sample was taken 97 feet from the dike. In this connection it is perhaps significant that when the dike was cut on this level, it was not associated with auriferous quartz.

The analyses given in Table IX. from the 1422-foot level correspond generally with those obtained from the deep levels of Bendigo; the proportion of sulphides and of gold increasing greatly as the lode is approached. The microscopical examination of thirty thin sections of country-rocks brought out very clearly the increasing development of quartz and pyrite as a lode was approached, but revealed, in most cases, nothing else of definite importance.

* See a paper "On the Formation of Mineral Veins and the Deposits of Metal lic Ores and Minerals in Them," by H. A. Thompson, Trans. Hoy. Soc. of Victoria, viii., pp. 228-249.