Gold Ore Deposits

Gold Ore Deposits

The volcanic source of gold deposits is also most clearly indicated in the Cripple Creek district, Colorado, where the deposits of commercial grade are either within a complex volcanic neck or extend for only short distances into the surrounding rocks. It is true that similar mineralization has taken place for considerable distances from the neck, but only in small, scattered deposits, and even those who have been generally opposed to the recognition of magmatic water as a leading agent of ore deposition have had to admit that it was the dominant agent at Cripple Creek.

Even the deposits at Cripple Creek, however, are puzzling to anyone who adheres to the simple idea that vein-forming solutions rise along essentially vertical courses, as many of the veins have terminated downward in barren fissures, some of which contain only a little gouge or finely crushed rock and not even any of the most common gangue minerals of the district. Where developments have been most thorough, however, many of these shallow veins can be traced, either by direct or devious courses, into more persistent veins, one of which, in the Portland mine, has been followed to a depth of 3,100 ft. below the collar of the shaft, or about 5,000 ft. below the original surface of the Cripple Creek Volcano. At that depth the vein became of too low grade to mine, although a rich shoot had been mined from the 2,600-ft. to the 3,000-ft. level. Other pay shoots may exist along it at still greater depths, but cost has discouraged deeper exploration. The structural relations of the vein show that it was formed along a master fissure zone that was repeatedly opened and served as channels for several dike intrusions as well as the different stages of mineral deposition, and was therefore undoubtedly connected with the magmatic reservoir beneath the roots of the volcano.

The ore shoots of the Cresson mine in the same district also afford convincing evidence of the deep-seated volcanic source of the solutions. They have been followed only to a depth of little more than 2,000 ft., but they are located along the margin of a local explosive crater within the main crater of the district. The local crater, known as the “Cresson blowout,” tapers downward into two roots, along one of which ore was followed; but the decreasing size of the ore shoot, in keeping with the downward tapering of the local crater, and the large volume of water discouraged deeper mining. The association of the ore and the roots of the crater, however, point clearly to a common source for the gases that produced the explosive eruption and the solutions, whether gaseous or liquid, that deposited the ore. The rubble mass that contains the small cinnabar deposit described on page 34 is closely associated with the other prong of this local crater. The lack of association of the gold telluride ore and the cinnabar is a detail that adds to the complexity of the local problem of ore genesis. The minerals of the deposits were formed in several stages, and the gold tellurides and the cinnabar both represent very late stages, though which of the two was the later is not evident. The roots of the local Cresson “blowout” and the deepest exposure of the Portland vein are 3,000 ft. and more apart, and overlie different deep-seated channels along which solutions rose, perhaps from distinct local sources; but there are some major structural considerations that suggest that these two deep-seated channels may converge downward towards a common source from which about 80 per cent of the gold mined in the district was derived. Different groups of deposits elsewhere in the district have not been followed so deeply, but the comparatively incomplete evidence regarding them points to derivation from different deep-seated sources beneath the complex or composite crater.

Throughout the exposed vertical range of 3,100 ft. in the Cripple Creek district there has been no systematic change in mineral composition of the veins. There have been a few small indications of rock alteration characteristic of high temperature, expressed by specularite, but these are confined to certain of the older dikes in the neck and have no direct relation to the productive veins. There is, therefore, no indication of downward gradation into deposits characteristic of those formed under mesothermal conditions, and it must be concluded that the vein-forming solutions rose from some reservoir beneath the roots of the volcanic neck and deposited their minerals where structural conditions were favorable, especially in the upper parts of the neck where the increasing number of fissures permitted the spreading and therefore a relatively rapid cooling and supersaturation of the solutions.

As already stated, however, the veins at Cripple Creek, like epithermal veins in many other districts, were formed in successive stages that indicate a progressive change in composition and probably in temperature of solutions. Among the earliest of the vein minerals are quartz and feldspar (adularia), which may be regarded as closely related to and even derived from the highly alkaline rocks of the volcanic neck, and also fluorite, which occurs with quartz and a little pyrite in dense, dikelike masses up to 2 ft. in thickness. The pyrite, where more conspicuous than usual, forms coarsely crystalline masses that resemble in miniature the pyrite in mesothermal veins. Veins composed of the foregoing minerals are productive only where they have been fractured and refilled by a second group of minerals. In this second group there are small amounts of zinc blende, galena, and tetrahedrite, which are also characteristic of many mesothermal deposits, where they commonly follow the bulk of the quartz and pyrite in order of deposition. The tellurides of gold and silver, accompanied by an insignificant amount of free gold, followed this second group. In some places they were deposited close by the base- metal sulphides and associated gangue minerals and in others were formed independently of them, and their distribution was evidently controlled mainly by the unfilled openings or reopened fissures in and adjoining the deposits of earlier minerals. These relations suggest that the tellurides were formed at a somewhat lower temperature than the sulphides, and the veins as a whole suggest deposition through a considerable range in temperature.

The adularia form of potash feldspar is one of the most characteristic minerals of epithermal deposits in volcanic rocks and is everywhere associated with quartz. In many epithermal veins these two minerals are associated with, and to a considerable extent have replaced, a platy form of calcite, which is also a characteristic early mineral of these veins, and is believed, by some at least, to denote deposition at relatively high temperature, although the composition of the solution may also have exerted some control on the crystal form. Whatever its actual temperature of formation, the platy form of calcite is believed to represent deposition at a higher temperature than the scalenohedral (“dogtooth”) form that is a late mineral in the Cripple Creek veins as well as in many of the zinc-lead deposits that are commonly classed as mesothermal.

In short, so far as temperatures of deposition can be inferred from the minerals present, the earlier minerals in epithermal veins appear to have formed at higher temperatures than the later minerals in mesothermal veins, and temperature alone is not the only controlling factor. The original composition of the solution and reactions between it and the volcanic rocks may have had much to do with the minerals of the veins and the order in which they were deposited. Solutions originally of identical composition but rising through rocks of widely different composition may deposit different minerals, especially as gangue; but such elements as sulphur, arsenic, antimony, tellurium, and mercury, and the more common metals, especially if present in great quantity, are most reasonably referred to the original sources of the solutions.

In striking contrast to the majority of epithermal veins in volcanic rocks, the Camp Bird vein in the Ouray district, Colorado, shows a more distinct though by no means ideal relation between epithermal and deeper seated deposits. According to Burbank’s recent work ore deposition at Ouray took place in two distinct periods, the first in late Cretaceous or early Eocene time and the second in late Tertiary time. Generalizations regarding the conditions governing ore deposition, therefore, may be misleading unless deposits of each period are separately considered. The Camp Bird vein, however, belongs to the Miocene group, and has been exposed through a vertical range of 3,000 ft. in volcanic rocks. Its upper part is characterized by base-metal sulphides that represent an early stage of deposition and are cut by gold-bearing quartz typical of epithermal veins. In this part of the vein the gangue minerals are crustified quartz, rhodochrosite, calcite, and fluorite, all typical of the epithermal zone, and the metallic minerals are very fine native gold, a little galena, pyrite, and zinc blende, and some finely distributed tellurides. At a depth of 1,800 ft. below the highest outcrop, however, conspicuous amounts of specular hematite were found, and at the lowest level the vein consists mainly of specularite and quartz with minor amounts of sulphides and precious metal. The sulphides in part are intergrown with or impregnate the specularite and in part form distinct veinlets that cut the specularite and quartz and represent a distinctly later stage of deposition. The most conspicuous sulphide is a pale brown to yellowish zinc blende, similar to that found in some of the outlying deposits of zinc-lead districts in Colorado and in districts remote from any exposed igneous rocks. The gold in this deep level is more closely associated with the sulphides than with the specularite. The deeper part of the Camp Bird vein, therefore, represents at least two distinct stages of deposition, the earlier of which contains minerals characteristic of high-temperature or hypothermal conditions and the later characteristic of lower temperature or epithermal conditions.

The relatively direct, though by no means complete evidence presented by the Cripple Creek veins and the Camp Bird vein favors the inference that as a whole the epithermal veins in volcanic rocks, if they could be profitably followed by mining, could be traced by direct or devious routes to volcanic conduits. Their gold is among the latest minerals deposited, and therefore can normally be expected in greatest quantity at relatively shallow depths, where conditions favoring deposition are most prevalent. Whether other minerals of economic interest occur along these routes in small or considerable quantity depends, as already stated, upon the original composition of the solutions and upon structural and chemical influences along their courses.