USGS Professional Paper 144 – the Copper Deposits of Michigan

If ferric iron was reduced to ferrous iron, there must have been an oxidation of the agent that accomplished the reduction, and it is of interest to know what that agent was. FIGURE 17.—Change in iron content of bleached rock associated with copper Numbers at left refer to table below. FIGURE 18.—Composition of bleached and unbleached rock. (For explanation of manner of constructing the diagram see fig. 5, p. 40.) Among the reducing agents that might have been present are carbon, carbon monoxide, hydrocarbon, hydrogen, and sulphur or some incompletely oxidized form of sulphur. As calcite is plentiful, it is possible that some less oxidized carbon-bearing material acted as the reducing agent and was itself oxidized to the carbonate condition. Lane has, indeed, suggested that the hydrocarbon which R. T. Chamberlin found in rocks of Figure 17 illustrates the change in iron accompanying this district acted as a reducing agent, though Lane mineralization for a number of typical examples. The assumes that it acted on oxidized copper compounds. principal constituents of the unaltered and bleached rock Hydrocarbons are, however, no more plentiful in the are shown in the following analyses: rocks here than in many other regions where no such The general tendency in the alteration that produces peculiar results as deposition of native copper and bleaching is a removal of considerable ferric iron and a destruction of hematite are found; there is no significant somewhat less marked conversion of ferric to ferrous connection between the occurrence of calcite and either iron. The fact that there has been some reduction of deposition of native copper or bleaching, and a the ferric to ferrous iron points to the probability that the iron White Pine mine, where a solid hydrocarbon, most that has been removed was also first reduced to the probably derived from the adjacent carbonaceous shale ferrous state, because under the conditions that is present in fair abundance, the amount of bleaching is probably existed ferrous compounds are more soluble not conspicuously greater than where hydrocarbon than ferric. occurs only in mere traces or not at all, but sulphides are USGS Professional Paper 144 – The Copper Deposits of Michigan – Part 2 – Page 64 of 97 more plentiful there in proportion to copper than at any solution passed through the lode until such a degree of other place in red rocks in the district. concentration was reached that copper precipitation had to begin. It has been suggested that the bleaching around copper resulted from the reaction between the ferric oxide and In places copper is accompanied by only slight if any the metallic copper and therefore occurred after the immediately local bleaching. This is the case, for copper was deposited. Such a reduction of ferric oxide example, in parts of the Pewabic lode, in the upper is readily accomplished and is, indeed, a difficulty in levels of the Calumet & Hecla conglomerate mine, and in chemical analyses when ferric oxide is determined in the the copper-bearing fissures that cut the Kearsarge lode. presence of metallic copper. Moreover, some of the Wells has found in his experiments that the metallic copper ore when brought to the surface shows a thin film copper commonly does not replace the solid reagent that of green copper carbonate or of red oxide of copper causes the precipitation but rather grows out into the surrounding the metal, and this has been suggested as free solution, often not in contact at all with the the oxidized copper compound produced in the reaction precipitant. The laboratory experiments thus appear to cited. Examination and inquiry show, however, that the be in harmony with the natural occurrence. copper when the rock is first broken is not green but bright and metallic or at most covered by a mere film of Copper was deposited in the cross fissures chiefly at cuprite. Moreover, the removal of hematite is not the and near their intersections with thick hematite-rich only effect in the bleached areas. In the amygdaloid lodes. Where in the Ahmeek and Mohawk mines some lodes in particular, the destruction of hematite was of these fissures, notably the Mass fissure, cross the accompanied by an intense miner alogic, chemical, and Kearsarge lode, the destruction of hematite is of a textural breakdown of the rock surrounding the copper— different type from that seen in the bleached rock that so an alteration that would not be likely to result from the commonly surrounds the copper. Along these copper- mere action of the metallic copper on the rock. bearing cross fissures for a width of 10 to several tens of feet the Kearsarge lode is darker and leaner than usual. There is no doubt that the metallic copper was replacing Along the Mass fissure in the Ahmeek mine the dark the bleached rock—that is, that the removal of iron, lean zone extends for 80 to 100 feet on each side of the although everywhere going on at the same time- as the fissure. Determination of iron oxides shows the following deposition of copper near by, was accomplished at any contrast between the dark material and the normal red given point in advance of the precipitation of copper; material near by: bleaching was the front of the alteration wave; replacement by copper was the end. The practical absence of bleached spots without accompanying copper further indicates that deposition of copper and bleaching of rock were intimately associated and that copper has not been removed since its deposition. The exceedingly low copper and iron The changes indicated by these analyses suggest that content of the mine waters likewise proves that reaction the solutions moving generally along the fissure but between iron and copper is not now in progress, or, if at soaking into the Kearsarge lode were at that time and all, to only a very slight extent. place able to destroy but little of the ferric iron but were, The hypothesis that copper sulphide solutions acted as on the contrary, relatively highly charged with ferrous the reducing agent that destroyed the hematite appears iron, which they precipitated. Microscopic examination to fit the facts in a satisfactory way and at once explains of the dark material reveals a difference in two respects both the bleaching of the rock and the deposition of from the normal rock of the Kearsarge lode—(a) much copper in the native state. The power of the metallic chlorite has been introduced throughout and accounts sulphides as reducing agents is shown by the ease with for about all the added ferrous iron; (b) all the finer flakes which they take up oxygen to form sulphates—far more and particles of hematite have been removed and with readily, for instance, than the ferrous iron minerals them the red color, but the larger grains of hematite, succumb to oxidation. In the oxidation of the “sulphide which make up the greatest part, by weight, are still solution” the process has gone so far as to oxidize the present and account for the small decline in ferric iron sulphur, but the copper was deposited as metal. content notwithstanding the marked change in color. The copper occurs generally in irregular masses rather It seems probable that the solutions moved along the than uniformly distributed through the lode. It is intersection of the fissure with the Kearsarge lode; the impossible to assume that the replaced rock itself could main avenue of flow was through the fissure, but there supply enough reactive agent of any kind to cause the was always local penetration into the permeable lode, precipitation of three times its own weight of copper, and in consequence the solutions, before reaching the either in the small particles or in the great masses tons levels now exposed in the mine, were oxidized at the and even hundreds of tons in weight. It is more likely, as expense of the hematite of the lode and became suggested in connection with the saturation hypothesis charged with ferrous iron and uncombined copper. (p. 129), that the reaction was taking place as the Having undergone these reactions, the solutions USGS Professional Paper 144 – The Copper Deposits of Michigan – Part 2 – Page 65 of 97 deposited the constituents with which they had become are either easily soluble or unstable, and so it could not saturated, but the copper was deposited mainly in the be expected that any of these sulphur compounds would fissure itself as masses of native metal while the ferrous now be present, no matter how much SO2 may have iron, which could not be precipitated alone, was formed. deposited as chlorite partly along the fissure and partly In nature oxidized sulphur occurs by far the most in the lode. The lode rock, because of its glassy commonly as sulphate, and when solid sulphides condition, was probably more susceptible to reaction oxidize, sulphates and sulphuric acid are formed from all than the trap that in the main bounds the fissure. or nearly all the sulphur of the sulphides; moreover, as The channel afforded by the fissure and the immediately has been indicated elsewhere,37 the sulphates present adjacent parts of the lode probably constituted an easier as gangue minerals in primary ore deposits have avenue for ascent than that offered by the lode alone. probably been derived by hypogene oxidation of sulphur- The solutions flowing along the fissure, therefore, should bearing solutions. Sulphates are the only oxidized have reached the altitude of the present mine workings compounds of sulphur that are likely to form minerals before those which moved up the lode and and be found in the deposits, but, as only a few of the accomplished the normal mineralization characteristic of sulphates are notably insoluble, it is not to be expected the Kearsarge.

Load more