Until now, the loss of gold contained in pyrites has not been taken into account, since it has been assumed that these are saved by concentration if they are valuable, and this topic has been dealt with previously. However, as this gold is classified as non-amalgamable gold, its physical state and the causes of its unwillingness to unite with mercury can be conveniently considered here. In general,pyrite yields only a small proportion of its gold content if passed over the amalgamated plates, and if ground very finely in a mercury pot the extraction rate is better.. Among the ancient processes used to melt gold into pyrites, we may mention treatment in rotating wooden barrels with mercury, as practiced at St. John's on the Hey, and the practice of allowing pyrites to weather before grinding. with mercury. This method of oxidation appears to be decidedly inferior to the alternative plan of roasting sulfides, whereby the oxidation is made more complete and the gold particles agglomerate to some extent. However, the amalgamation of pyrites, even roasted, is far from perfect, with some of the gold still remaining in unsuitable conditions for extraction in this way. The minerals, found in various parts of the world, consist mainly of hydrated limonite or iron oxide, and in most cases are believed to be the result of decomposition of pyritic minerals by atmospheric agents, they are also extremely refractory, which causes the mercury to deteriorate rapidly. , and producing only about the same percentage of gold that can be obtained from unoxidized pyrites.
The most celebrated case of this type is that of the surface ore at Mount Morgan, Queensland, which was an ironstone gossan consisting of siliceous brown iron ore, one opinion derived from the decomposition of pyrites. Although the gold appeared to be free, it could not be amalgamated, yielding only about 30 percent when crushed in batteries and subjected to prolonged grinding in mercury vessels. When the ore was dehydrated by roasting it in reverberation furnaces, the extremely fine gold particles agglomerated and 80 to 90 percent of it could be extracted by amalgamation, with the remainder presumably coated in iron oxides. However, the richness of the ore made even this result unsatisfactory, and in practice a chlorination process was adopted. Mactear noted a similar case in South America, where a limonite ore that yielded only 35 to 40 percent of its gold when treated in Huntington pots was made to produce 85 to 90 percent simply by dehydrating calcination. before amalgamating it. Louis Janin, Jr., mentions another case in minerals from the Southern Cross mine, Deer Lodge County, Montana, consisting of limonite derived from the alteration of pyrites. When analyzing large samples, only one or two gold particles could be seen, even though the ore contained 1 to 2 ounces per ton. This ore yielded only about 40 percent when amalgamated, but more than 90 percent was dissolved by leaching the crude ore with potassium cyanide, and similar results were obtained by chlorination. Here the ore was completely decomposed, but still the gold would not melt much more than if it were still contained in the original pyrites, whereas the chemicals immediately dissolved it.
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The balance of evidence, however, seems to favor the theory that gold exists in pyrites in the metallic state. Although the metal is generally invisible in undecomposed pyrite crystals, it becomes visible when such crystals are oxidized by air and water in nature, or by means of nitric acid, or by combustion or deflagration with nitrate. such decomposition, shiny, lustrous gold particles, angular and irregular in shape, but of considerable size, often become evident. These particles can be separated from the iron oxides by washing, often with nitric acid followed by washing to detect the gold in the pyrites. Also, although it is usually invisible, gold can sometimes be seen in unroasted pyrites. As early as 1874, Richard Daintree and Latta found specimens of cubic pyrites, in which gold could be seen under a microscope, gilding the cleavage planes of the crystals. Again, G. Melville-Attwood, examining gold-bearing pyrite crystals from California in 1881, found that the faces of the crystals were gilded in places, and here and there small specks or drops of gold appeared, partly embedded in the pyrites. . . These films were too thin to be detected by an ordinary lens, so it did not seem surprising that this impalpable material could not be absorbed by mercury. Louis Janin, Jr., more recently discovered pyrite crystals in a porphyritic gangue from the Republic of Colombia, which had gold in small globules on their surfaces. Finally, it has been known for a long time that pyrite crystals are often found attached to an amalgamated plate, the gold particles having been amalgamated on their surfaces. It seems probable, in view of all these facts, that most if not all of the gold is in the metallic state, and its occasional refusal to amalgamate is not very surprising, when we remember how a thin layer of certain sulfur compounds prevents the amalgamation . and how easily hydrogen sulfide would evolve from decaying pyrites. Some authorities have asserted that the metallic gold is disseminated mechanically through the mass of pyrite, but the action of potassium cyanide, in dissolving all the gold in the coarsely crushed pyrites, seems to indicate the correct view that the The interior of the crystal is not auriferous, the gold deposition is superficial, therefore the pyrite enrichment is limited to its crystalline faces and, possibly, but not probable, to its cleavage planes. Caldecott provides strong evidence for the richness of the outer pyrite crystals in the Transvaal Banket ore in his papers on the treatment of Transvaal ore.
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The following details from a microscopic examination by Prof. Morton of the condition in which pyrites remain after being cyanide-leached confirm this view to some extent:
"On the common auriferous iron sulphide, or arsenic pyrites, the solution of potassium cyanide acts rapidly, not dissolving the sulphur, but attacking the gold at its exposed edges, and working its way into the cubes by a slow advance, dissolving the gold as you go. A microscopic examination of the pyrites after removing the gold suggests the method of operation. A sample of very rich pyrites from a mine north of Redding was treated with a weak solution containing less than two-tenths of 1 percent cyanide for 168 hours; the test showed a complete extraction of the gold; Since the sulfides did not show any change in their appearance to the naked eye, some of them were put under the microscope.
“There is no visible change in the shape of the crystals as a whole; along the fractured faces, the mispickel appears clean and undisturbed, showing the silvery-white color and strong refraction of the arsenopyrite. On the faces of the crystals, dark lines appear, short and parallel to each other. In some places they are overcrowded; in other places they are at considerable distances, but always in parallel lines. The lines vary in length from four or five to more than a hundred times their width; the lines are very irregular and often broken. These lines are fissures in the pyrites and run so deep that the microscope does not reveal their depth. When using the higher powers, the walls of one of the fissures looked completely honeycomb, looking a bit like two empty honeycombs placed one in front of the other; evidently the removed mineral has crystallized along its contact walls, at least. As the raw or untreated pyrites do not show such cracking, but instead have a surface marked only by striation lines common to pyrites, I presume that the cracking in the treated sample is caused by the solution acting on some soluble mineral, probably gold. , arranged in plates, appearing in groups, but which, due to its color and isomorphism and the extreme tenuousness of its lines, is indistinguishable from the mass of pyrites that encloses it”.