Materia organiczna, Au, Ni i Co w utworach cechsztynu monokliny przedsudeckiej

Henryk Kucha


Organic matter, Au, Ni And Co in Zechstein rocks of the Fore-Sudetic Monocline (Western Poland)

The Fore-Sudetic monocline is composed of the Precambrian and Lower Palaeozoic crystalline rocks, viz. schists, gneisses, gneissoid granites, phyllites, granodiorites and granites, which are overlain discordantly by sedimentary rocks from the Permian age onwards. They begin with the Rotliegendes appearing in the form of red sandstone; at the top it passes into white sandstone, 1—40 m in thickness. In the SE part of the monocline, a dolomite bed, a dozen or so centimetres thick, overlies the white sandstone. It begins the Lower Zechstein formations that are also represented by dolomitic and clayeybitummous copper-bearing shales, overlain by the Lower and Middle Zechstein limestones and dolomites. The Upper Zechstein is represented by salt-bearing rocks. All these rocks are overlain, in turn, by the Bunter sandstones (Lower Triassic), the Tertiary and Quaternary deposits. Copper has been recorded in the Lower Zechstein limestones, dolomites and shales, as well as in white sandstones that are regarded by some authors as belonging to the Rotliegendes. The principal ore minerals are chalcocite, digenite, bornite, chalcopyrite, pyrite, covellite, galena and sphalerite. Silver, nickel, cobalt and molybdenum minerals have also been found here. The copper minerals are distributed zonally, both vertically and horizontally. Copper mineralization disappears gradually towards the top beds and abruptly towards the bottom in white sandstones.

Clay Minerals from the Copper-bearing Shale
The copper-bearing shale are chiefly dolomitic clay shale. Their main components are dolomite and a clayey-bituminous substance. The basic clay constituent of the copper-bearing shales is illite. It comprises in its structure an essential though most frequently small amount of magnesium as well as iron, in a lesser degree. Illite is accompanied by a slight admixture of kaolinite (Harańczyk, 1964) which is presumably connected with the processes of organic substance transformations. Silica removal caused by the transformation of illite into kaolinite may be sometimes so intense that the occurrence of aluminium hydroxides can be expected (fig. 1 ). Moreover, minerals of the biotite and calcium phosphate groups have been found in the copper-bearing shale (Fig. 1). Ain insignificant admixture of montmorillonite has been also recorded, while the presence of chlorites has been assumed as probable.

Organic Material, Its Nature and Role in the Process of Deposit Formation
Organic material, was extracted from samples etched with HCl and HF, using successively ethanol, xylene, benzene, isopropylic ether, chloroform and pyridine. After the solvent had been evaporized, the X-ray analyses were performed on all the samples. They revealed the presence of sulphur (residuum of sulphides dissolved in HCl and HF), a small amount of copper sulphides and, in some cases, a substantial pyrite content. Their presence is due to the fact that some sulphides form fine grains completely isolated by kerogen from the leaching action of HCl and HF. Samples thus prepared were subjected to the infrared absorption analyses. In consequence, the presence of alkanes with paraffinic and branched chains has been ascertained. In several cases the chain length comprises not less than 4 methylene groups and in some cases even more than 6—7 such groups. In the alkene group, olefins and dienes or higher analogues have been identified. Aromatic compounds with conjugated rings are one of the main components of the organic material. There is also a substantial content of esters and ketones, some of which seem to have the chelated structure. Compounds with nitrogen and sulphur, as well as ethers and —OH groups, have also been recorded. Cu, Fe, V, and occasionally Mo and Co, form a small number of organometallic compounds (Table 1). Humic acids have been detected in the copper-bearing shale (Tables 2, 3; Fig. 3). Their characteristic feature is the prevalence of aliphatic acids with the chain length frequently not less than 6 — 7 methylene groups. They are of the chelated structure and are presumably the derivatives of dicarboxylic acids. They may also have partly the character of hydroxy acids. A certain number of acid groups underwent estrification (Fig. 3). Humic acids with chelated structure constitute only a certain amount of compounds of this type and form presumably organometallic compounds with gold. The organic material played an essential part in the processes of silica migration. This may be inferred from the co-occurrence of quartz and increased concentrations of organic cartoon (Fig. 6). The organic compounds played an equally important part in the reduction of sulphates to sulphides, which is evidenced both by thermodynamic data (Fig. 7) and observations in the deposit. In a sample of copper-bearing shale the co-occurrence of three genetically related laminae has been ascertained, viz. organic material at the bottom, copper sulphides above and carbonates at the top. The reduction of sulphates proceeded presumably at the expense of hydrogen present in hydrocarbons, and partly at the expense of carbon oxidation to CO(2) (Fig. 7). The first type of reduction resulted in an increase in the content of aromatic compounds (carbonization) in the organic material underlying the sulphide veinlet. The other type gave rise to ketones and carbon dioxide, the latter having been bound above the sulphide veinlet as a lamina of carbonates. To assume a strict correlation between the content of heavy metals and the amount Of organic carbon (Table 4) would be an oversimplification of the actual situation in the deposit, since in several cases the copper-bearing shale is barren despite a high content of organic carbon. The situation is still more symptomatic in the case of mineralized sandstone that practically contains no organic substance. Therefore, a correlation should rather be sought between the reduction conditions of biogenic nature and the heavy metals content. According to this approach, in the diagenetic stage the zone of copper-bearing shales could have acted as an „exporter” of the reducing environment upon the adjoining rocks. This could result in the displacement of ores. The number of organometallic compounds is limited. They are formed by Cu, V, Fe and presumably sometimes by Mo and Co. There is a close correlation only between organic carbon and the sulphur content (Harańczyk, 1972); it is independent of the heavy metals content. This may suggest that sulphur was partly transported in complexes not connected with heavy metals, maybe in the form of dissociated sulphates of potassium and calcium earth metals. The sulphur excess appeared as organic compounds with (SO), (SO(2)), (—HSO(3)?), (—S—S). The reduction processes proceeded at the expense of hydrogen present in the organic material or through oxidation of carbon from hydrocarbons either to compounds containing the carbonyl and carboxyl groups or to CO(2), in the case when the reaction was run out. A great number of observations have been collected so far concerning the contribution of the organic matter to metal concentration in the Zechstein deposits (Harańczyk, 1961, 1972). In this process an equally important part was played by the sorption properties of clay substance and of polar organic compounds in relation to polar metal complexes (the formation of Lewis’s acids). During the diagenesis several metals were reconcentrated; the process was followed by formation of minerals even by those elements that occur in a very small amount. Moreover, the organic material was transformed due, on the one hand, to the processes connected with metal reduction and, on the other, to the catalyzing action of clay minerals.

Gold in the Zechstein Copper-bearing Rocks
Gold distribution in the Zechstein copper-bearing rocks has been determined by means of microscopic examinations in reflected light, by emissive spectrography and X-ray microanalysis. The resultant data have proved the occurrence of organic gold compounds that form scanty, narrow and isolated secretions in the copper-bearing shale (Kucha, 1973). In the extreme case the gold content in organic concentrations is as high as 3000 ppm. Most frequently, however, it is much lower and is detectable during the X-ray microanalysis only because of a very low background in the matrix composed of organic compounds (Figs. 12, 10, 11). In many samples no gold at all has been recorded despite the fact that the scanning profile on the microprobe was up to 1 mm and that the analyses were repeated two or three times in different parts of the sample. Infrared absorption analyses indicate that humic acids derived from the copper-bearing shale have a structure capable of forming organic compounds with gold. Presumedly, some humic acids form compounds with gold, which have the structure suggested by Radtke and Scheiner (1970). A certain amount of gold occurs very likely in the form of aurothio compounds (Fig. 10). During the diagenesis the organic material together with organic gold compounds underwent transformations. In several cases these processes resulted in the release of gold and its later binding in the recrystallizing ores, mainly in native silver. In the extreme cases even native gold appeared. It has been found in the boundary dolomite in the form of inclusions in pararammelsbergite that is associated with minerals of the cobaltite-gersdorffite series. An analysis in microvolume has shown that it is e-lectrum containing about 71.5% Au, 25% Ag, 0.7% As, 1.2% Ni. An insignificant admixture of Cu, Bi and Fe has also been recorded. Both the high arsenic content in electrum and the co-occurrence with nickel diarsenides and the cobaltite — gersdorffite series indicate that gold migrated in complexes associated with arsenic. Gold also appears as an admixture in ore minerals (Table 7), its content being most substantial in native silver. It seems that native silver from the copper-bearing shale contains more gold than its counterpart from carbonate rooks. Frequently, irregular gold distribution in silver, correlated with the arsenic content, may be observed. Many a time the secondary nickel and cobalt minerals from the boundary dolomite have the gold content amounting up to several thousand ppm. A similar gold content has been recorded in violet bornite — a rare variety of bornite. The other ore minerals such as silver minerals, galena, pyrite, bornite, chalcocite, chalcopyrite and sphalerite have characteristic insignificant gold contents that generally do not exceed a dozen or several dozen ppm. It seems that ore minerals from the copper-bearing shale and the boundary dolomite contain a higher Au admixture than their counterparts from sandstone and carbonate rocks.

Nickel and Cobalt in the Zechstein Copper-bearing Rocks
The highest nickel and cobalt contents have been recorded in the copper-bearing shale. Sometimes this regularity is somewhat obliterated because of the secondary displacement of these elements to the zone of boundary dolomite or to the top part of sandstone. Most frequently, the cobalt and nickel minerals form fine grains disseminated in the body of the copper-bearing shale. In several cases the grains are so fine that they are imperceptible under the microscope in reflected light. The X-ray microprobe analyses have shown that the concentration curves of cobalt and nickel in the copper-bearing shale frequently do not coincide (Figs. 15, 16). This indicates that the precipitation of nickel does not coincide precisely in time with the crystallization of cobalt compounds. The finely disseminated nickel-cobalt mineralization corresponds in its chemical composition to the linneite — siegenite and the cobaltite — ferrocobaltite — cobalt-bearing pyrite series (Figs. 15, 16). Secretions corresponding in their chemical composition to julukulite are also fairly common. The nickel and cobalt minerals often form inclusions in bornite and chalcocite. The increased contents of cobalt, nickel and arsenic are then observed in copper minerals that contain such inclusions (Tabl. 1, Fig. 1, 2, 3, 4, Fig. 17, Table 8 ). These inclusions belong to the cobaltite— gersdorffite series. Their composition is inot homogeneous (Table 8 ). It is interesting to note the high copper content in one of the analysed inclusions (Table 8, no. 5; Table 9, no. 17/11), which has not been recorded so far in the minerals of the cobaltite — gersdorffite series. The mineral in question resembles cobaltite except its weaker tendency to idiomorphism; it is also cream-white in colour. It may be isostructural with cobaltite. The nickel-cobalt minerals present in the boundary dolomite have a somewhat different chemical composition (Tables 10, 11). Pararammelsbergite occurring in paragenesis with the cobaltite — gersdorffite series should be mentioned here. Both pararammelsbergite and the accompany ing minerals of the (Ni, Co) AsS type contain inclusions of native gold (electrum). Pararammelsbergite should be assigned to the nickel — cobalt — arsenic — gold paragenesis. This type of genetic association is presumably a low-temperature variety of the typical cobalt — nickel — silver — arsenic paragenesis described by Ramdohr (1960). In the vicinity of the discussed mineral association chalcocite, stromeyerite and native silver have been found. These minerals are probably earlier than pararammelsbergite and the accompanying minerals. Moreover, an inappreciable gold and arsenic content in native silver indicates that they belong to the separate paragenesis Cu-Ag-S. In the course of microscopic and X-ray microprobe examinations it has been found out that no typical cobaltite and gersdorffite appear in the Zechstein copper deposits (Tables 9, 10, 1 1 ). The data obtained allow a presumption that there is a continuous cobaltite — gersdorffite series in these deposits. This series sometimes show a remote association with the smaltite — chloantite series, which manifests itself in an increase in the arsenic content in disfavour of sulphur. Besides, in the boundary dolomite a mineral has been recorded which, considering its physico-chemical properties different from those quoted in the literature, has been defined as mineral „X ” (Table 11, tabl. 2 , fig. 1, 2, 3, 4, 5). The mineral in question is paragenetic with skutterudite, the cobaltite — gersdorffite series, sphalerite and chalcopyrite. The mineral ,,X” reveals the greatest similarity to safflorite and rammelsbergite as far as both its chemical (Table 11) and physical features are concerned. The colour of anisotropic effects corresponds to safflorite, while the other optical features resemble those of nickel diarsenides. However, the mineral ,,X” differs markedly in the chemical composition (Table 11) and much lower hardness from the known nickel and cobalt diarsenides. It seems, therefore, to be a new component of the safflorite — rammelsbergite series. In the copper-bearing rocks on the Fore-Sudetic monocline siegenite (Nguy en Van Nhan, 1970) as well as niocollite, maucherite and vaesite (Harańczyk, 1967) have been also recorded. The X-ray microprobe analyses permitted to ascertain the presence of cobalt- and nickel-bearing pyrite (tabl. 1, fig. 5, 6 , Fig. 18). It sometimes contains up to 7% of cobalt and 2% of nickel. In most cases, however, the concentration of these two elements in pyrite is lower (Table 12). The Ni and Co content in pyrite in the vertical profile of the deposit is variable (Fig. 19), being the highest, in the pyrites from the top of white sandstone, from the boundary dolomite and the copper-bearing shale. In the latter case we are rather concerned with the occurrence of finely disseminated nickel-cobalt mineralization instead of pyrite than with an increase of the Co and Ni contents in pyrite. Fairly substantial Ni and Co contents have been also noted in pyrite from the bottom of the copper-bearing dolomite. In some profiles in carbonate rocks, at a certain height above the copper-bearing shale, an increase in the Ni and Co content may be sometimes observed. Among the copper minerals, the highest nickel and cobalt contents have been noted in bornite (up to 5% Co, 0.6% Ni and 3.6% As — Table 8 ), chalcocite (up to 1% Co, 0.2% Ni and covellite (up to 2% of both Ni and Co). Visibly higher Ni and Co contents have been ascertained in those copper minerals that have bivalent cations of this element. This is probably -connected with the similarity of ionic radii in the case of Cu, + 2 (Fe+2) and Ni+2 and Co+2 (Smith 1963).

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