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The Proterozoic Pahtohavare Cu-Au deposit in the northernmost part of Sweden within the Fennoscandian shield consists of a syngenetic stratiform sulphide-magnetite mineralisation (East Ore) which is uneconomic and three stratabound to discordant epigenetic Cu-Au mineralisations (Central, South-East and South Ores) hosted by the Viscaria formation. These epigenetic deposits are hosted by fine-grained albite felsite formed by alteration of graphitic schist while the East Ore is hosted by tuffite. The black graphitic schist have acted as a chemical trap for the mineralising fluids explaining the decomposition of the graphite within the schist proximal to mineralised zones and altering it into albite felsite. The past tectonic events made the Kiruna area having a favorable permeability for epigenetic solutions like saline hydrothermal fluids. This favourable permeability is one of the main important characteristic which explains the formation of Pahtohavare ores. A scapolite-biotite alteration is enveloping the albite-altered mineralised zone and occurs in all stratigraphic units. One albite alteration of the tuffite is related to the intrusion of the footwall mafic sill and the other one is an additional ore-related mineralised albitization which is distinguishable by the lack of spatial relationship with the mafic sill and the occurrence of disseminated Ferro-dolomite. Chlorite has been formed by replacement of biotite and amphibole. A negative correlation between Mg and Cl contents of amphiboles is distinguishable which indicates that Mg-Cl avoidance mechanisms can control the incorporation of halogen in the amphibole structure. Scapolite from scapolite-biotite alteration surrounding the ore-bearing albite felsites and ore veins have a dominantly marialitic composition which indicates that the alteration must have been due to highly saline fluids. The occurrence of dipyre in Pahtohavare can be explained by the fact that the formation of the deposit happened in a Na-Cl rich environment. The main ore minerals are chalcopyrite and pyrite occurring disseminated, as veinlet, or filling breccias, and they are often associated with quartz and carbonate. Pyrrhotite is locally significant. Accessory minerals such as sphalerite, galena, millerite, native gold, tellurobismuthite, altaite, molybdenite, tellurides, and native gold in epigenetic ores occur as inclusions in sulphides and quartz.Alteration zones surrounding the Pahtohavare ores have chemical and mineralogical zonations similar to Rakkurijärvi. The Mg-content of biotites decreases toward the ores of the South, East and South East zone. Addition of potassium and depletion of calcium and manganese are characteristic of the biotite-scapolite alteration zone. The ore-bearing albite carbonate alteration zone shows a depletion of K2O and an addition of Na2O, CaO and MnO. The relation between the depletion of Na and scapolitisation-albitisation is close as it is likely due to Na-Ca exchange reactions. Co-variation diagrams are good geochemical discriminants which can be used as exploration tool. The alteration characteristics of the Pahothavare deposit share similar features with other iron oxide and sulphide deposits such as e.g. Bidjovagge, Norway. The sodic alteration is the most important one because of its association to the mineralisation. There are two generations of ore-forming fluids at Pahtohavare. The main physicochemical parameters that controlled hydrothermal alteration and gold mineralisation are pH, ƒO2 and temperature. The decrease of ƒO2 triggered the replacement of pyrite by pyrrhotite and its occurrence is spatially related to the mineralisation. The magnetite-pyrite and hematite-pyrite assemblages may have buffered the pH increase and ƒO2 decrease of the ore fluids. The chloride complexes of copper and gold are the most important one concerning the transport. The destabilization of gold chloride complexes is the main mechanism of gold deposition. This destabilization is due to an increase of pH from CO2 loss, cooling and dilution of the solution. The high salinity of the fluid can be explained by the metasomatic hydration of biotite and amphibole formation. Salinity is an important factor which determines the precipitation of metals from chloride complexes. Pahtohavare is considered as a copper deposit because of the low concentrations of gold due to the low initial concentrations of gases in the ore fluid and the fluids have not reach the native gold solubility 103 ppb limit for Cl complexes but has crossed the copper 100 ppm limit for Cl complexes. The copper content of the hypersaline brines at Pahtohavare have a range of 100-500 ppm which is comparable to saline magmatic fluids of the Cloncurry district in Australia. Validerat; 20140325 (global_studentproject_submitter) |
