ASSAY OF TAILINGS
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ASSAY OF TAILINGS.
From the Appendix to the Jurors' Reports, New Zealand Exhibition, 1865, by DR. HECTOR and MR. WILLIAM SKEY.
Arsenical pyrites in the form of fine dust, from the Waipori Reef, in Otago, was first examined. It had previously been passed through an amalgamating machine, and all the gold had been taken from it, which the most careful management could effect by this means. It was first gently roasted, to get rid of the arsenic and sulphur, and traces of mercury were volatilised during this operation, which had no doubt been left from the mercury used at the reef. After this was effectually performed, the ore was treated with warm hydrochloric acid, and digested until the residue was of a yellow color. The application of mercury easily then effected the separation of the gold from the insignificant amount of quartz and scheelite left, and the weight of the gold in the amalgam, afterwards ascertained. In this case, there was found no less than at the rate of 8ozs. 17dwts. to a ton of tailings, although the presence of gold could not be detected by the eye; every ton therefore of this would contain gold to the value of about £33, which makes it not at all improbable, that when acids are cheaper, these tailings may be profitably worked, by some such system as that just described. A previous roasting of the ore at the mine before amalgamation, might increase the profits of working it very considerably.
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Some mundic tailings, from No. 5 claim, at Coromandel, the workings of Mr. James Ninnis, were also qualitatively analysed for gold.
The mundic is disseminated largely, not only through the reefs at Coromandel, but also in the matrix of the rocks.
Treated according to the system previously indicated, they yielded to two analyses respectively, 32 and 37.35 ozs. of gold to the ton of ore, associated however with a rather large percentage of silver, which, as before observed, appears to be a characteristic feature of the Coromandel gold.
In California, it has been found, that when sand tailings from quartz mills have been lying in a moist state, although they contain no appreciable quantity of gold at first, in a few years it is possible to extract gold from this by washing with a pan or cradle, a circumstance frequently taken advantage of by the Chinese.
Two samples of refuse tailings, from the Kapanga Company's mill, Coromandel, were analysed in 1866, by Mr. W. Skey, of the Geological Survey Department, and gave an average of 3.38 and 4.53 ounces to the ton respectively.
Pyrites, concentrated from tailings in California, gave an analysis from 8 to 12ozs. of gold per ton from the Grass Valley district, and 98ozs. per ton from near Sonora. Simple grinding with mercury to an impalpable powder, but without previous roasting, only extracts about one fourth of the gold contained in it.
