Olof Martinsson

Early proterozoic Cu(Au) and Fe ore deposits associated with regional NaCl metasomatism in northern Fennoscandia

Scapolite is widely distributed in 1.9-2.5 Ga volcano-sedimentary rocks and 1.77-2.2 Ga igneous rocks over several hundred square kilometres in northern Fennoscandia, comprising northern Sweden, no ...

Magmatic-hydrothermal fluids in the Pahtohavare Cu-Au deposit in greenstone at Kiruna, Sweden

The Proterozoic Pahtohavare Cu-Au deposit is located in the greenstone belt near Kiruna, northern Sweden. The greenstone consists of mafic volcanic rocks with pillow lavas, mafic sills and albitized rocks, including tuffites, black schists and mafic sills, together with carbonates and mineralized zones. Mineralization occurs as impregnations, epigenetic quartz-rich breccias and fracture fillings with pyrite, chalcopyrite, pyrrhotite and gold in a complex tectonic environment. Fluid inclusions indicate an early formation of quartz and pyrite at temperatures initially near 500°C and a pressure of 2–2.4 kbar from a supersaturated aqueous solution of magmatic origin. In addition to halite cubes, daughter minerals of sylvite, calcite, hematite, graphite and two unknown phases are found. The main stage of chalcopyrite and gold deposition is characterized by aqueous fluids of variable salinity (up to 30 eq. wt.% NaCl including CaCl2), at temperatures below 350°C and pressures between 1 and 2 kbar. A minor CO2 phase with some N, accompanies this stage. Gold was transported as a chloride complex which destabilized due to an increase in pH (as a consequence of the CO2 loss) as well as cooling and dilution of the solution. The ore deposition occurred as a result of mixing with a low salinity aqueous solution during tectonic fracturing with pressure fluctuations and CO2 unmixing. Late oxidation of ores was caused by low to moderately saline (3 to 13 eq. wt.% NaCl) low temperature aqueous solutions.

Age, petrology and geochemistry of the porphyritic Aitik intrusion, and its relation to the disseminated Aitik Cu-Au-Ag deposit, northern Sweden

Abstract The Aitik Cu-Au-Ag deposit in northern Sweden is hosted by strongly altered and deformed 1.9 Ga old Svecofennian volcaniclastic rocks. A porphyritic quartz monzodiorite intrusion of subvolcanic origin is situated in the structural footwall to the ore. U–Pb TIMS zircon dating of the quartz monzodiorite yielded an age of 1887±8 Ma, which coincides with the age obtained for the subduction-related Haparanda suite of granitoids in Norrbotten. It is intruded by minor, comagmatic phases, including units of finer grained quartz monzodiorite and diorite. The finer grained intrusive phase, which can be traced into the ore zone of the Aitik deposit, is believed to represent apophyses protruding from the upper part of the quartz monzodiorite. The Aitik intrusion, comprising the quartz monzodiorite and its comagmatic phases, is affected by regional metamorphism, deformation, and hydrothermal alteration. Potassic alteration is most evident, and expressed by the growth of secondary biotite and K-feldspar. The sub-economic Cu-Au-Ag mineralization hosted by the Aitik intrusion mainly consists of chalcopyrite, pyrite, and magnetite of dominantly magmatic-hydrothermal origin, and is present in four forms: disseminated, as veinlets, in quartz-feldspar veins, and in biotite-amphibole veins. This mineralization extends in economic grades into the adjacent volcaniclastic rocks in the roof of the intrusion. The Aitik intrusion is similar in many respects to porphyry copper generating intrusions regarding tectonic setting, petrography and chemical composition. The intrusion-hosted sub-economic mineralization might form part of a porphyry system with its major part represented by the main mineralization in the overlying volcaniclastic rocks.

Stratigraphy and tectonic setting of the host rocks to the Tjårrojåkka Fe-oxide Cu-Au deposits, Kiruna area, northern Sweden

Abstract The Tjårrojåkka area is located about 50 km WSW of Kiruna, northern Sweden, and hosts one of the best examples of spatially and possibly genetically related Fe-oxide and Cu-Au occurrences in the area. The bedrock is dominated by intermediate and basic extrusive and intrusive rocks. An andesite constrains the ages of these rocks with a U-Pb LA-ICPMS age of 1878±7 Ma. They are cut by dolerites, which acted as feeder dykes for the overlying basalts. Based on geochemistry and the obtained age the andesites and basaltic andesites can be correlated with the 1.9 Ga intermediate volcanic rocks of the Svecofennian Porphyrite Group in northern Sweden. They formed during subduction-related magmatism in a volcanic arc environment on the Archaean continental margin above the Kiruna Greenstone Group. Chemically the basalts and associated dolerites have the same signature, but cannot directly be related to any known basaltic unit in northern Sweden. The basalts show only minor contamination of continental crust and may represent a local extensional event in a subaquatic back arc setting with extrusion of mantle derived magma. The intrusive rocks range from gabbro to quartz-monzodiorite in composition. The area is metamorphosed at epidote-amphibolite facies and has been affected by scapolite, K-feldspar, epidote, and albite alteration that is more intense in the vicinity of deformation zones and mineral deposits. Three events of deformation have been distinguished in the area. D1 brittle-ductile deformation created NE-SW-striking steep foliation corresponding with the strike of the Tjårrojåkka-Fe and Cu deposits and was followed by the development of an E-W deformation zone (D2). A compressional event (D3), possible involving thrusting from the SW, produced folds in the central part of the area and a NNW-SSE striking deformation zone in NE.

Scapolite: a tracer for the initial lead isotopic composition in sulfide deposits with later additions of radiogenic lead

The initial lead isotopic composition of metamorphosed and tectonically reworked sulfide deposits is not always preserved, as sulfides easily change their lead isotopic composition through incorporation of lead derived from external fluids or redistribution and recrystallization of the deposit. Sulfide trace-lead and in cases even galena-lead from such deposits may show exceedingly radiogenic lead isotopic compositions. Thus, the initial lead isotopic composition has to be estimated from other minerals. Scapolite, which is a common phase in alteration haloes associated with epigenetic sulfide deposits in northern Sweden, has very low uranium-contents. Therefore, its trace-lead contents could preserve the initial isotopic composition of the ore-forming fluids. As scapolite is more resistant to recrystallization, it is more likely to reflect the original lead isotope signature of the deposit. This is illustrated using scapolite and sulfides from the Pahtohavare Cu-Au deposit in northern Sweden, which is hosted by Palaeoproterozoic mafic tuffites and graphitic schists and was affected by a mild thermal metamorphism during the Caledonian orogeny.

Gold mineralogy at the Aitik Cu–Au–Ag deposit, Gällivare area, northern Sweden

The low-grade Aitik Cu–Au–Ag deposit is a deformed and metamorphosed porphyry-type deposit, and as such it belongs to the group of ores that require detailed mineralogical investigations of precious metal occurrences to assist in determining the recovery processes. The character of gold in the Aitik deposit varies substantially. Gold alloys display highly variable Au/(Au+Ag) ratios, and Hg is commonly a constituent. A change from dominantly sulphide-associated to groundmass-associated gold as mining progresses towards depth is accompanied by a change in the chemical composition of gold. Towards depth, the gold content in electrum and amalgam decreases (from c. 66 to 22% in electrum and c. 23 to 4% in amalgam), and the amount of native gold grains increases. The most common mineral assemblage associated with gold at deep levels (600 m and below) is K-feldspar, biotite, plagioclase, quartz, chalcopyrite and pyrite. This study demonstrates that magmatic–hydrothermal and metamorphic processes responsible for the diversity in copper mineralisation styles within the Aitik ore body probably have also played a role in the variable character of gold observed at Aitik today.

Post-1.9 Ga metamorphic, mineralization and hydrothermal events in northern Sweden

The bedrock of northern Sweden comprises a suite of rocks including Archaean (2.8–2.5 Ga), Karelian (2.4–2.0 Ga) and Svecofennian (1.96–1.78 Ga) successions. Several ore-types occur in northern Sweden, including e.g. the Fe-apatite ores and epigenetic Fe oxide-Cu-Au deposits in the Norrbotten county and the VMS deposits in the Skellefte district further to the south. The metamorphic and deformational history is complex, and the oreforming processes appear to have taken place during different stages. In recent years a number of geochronological studies have been initiated in order to understand the geological evolution and the timing of magmatic events. This contribution is focused on the post-1.9 Ga evolution in the Norrbotten county, the geology of which has been described in recently published SGU county maps with description (Bergman et al. 2001).

Timing of plutonism in the Gällivare area: implications for Proterozoic crustal development in the northern Norrbotten ore district, Sweden

Zircon ion probe (secondary-ion mass spectrometry or SIMS) data from a set of intrusive rocks emplaced in the vicinity of major ore bodies, as well as from large igneous intrusions in the Gallivare ...

Svecofennian Ore-Forming Environments

The Fennoscandian or Baltic Shield (both names occur in the literature) occupies the northern part of Europe. Pre-cambrian areas are exposed in Norway, Sweden, Finland, and Russia and their continuation beneath the platform cover sequences to the east and south have been better understood through studies within the European “Euro-probe” project. The craton of which the exposed Fenno-scandian Shield forms a part is bordered to the west by the Caledonian orogenic belt. Precambrian rocks of the same craton outcrop again in the Ukrainian Shield and the Voronezh Massif (cf. Gee and Zeyen, 1996). The Fennoscandian Shield is composed of Archean to Neoproterozoic rocks (Fig. 1). It is beyond the scope of this guidebook to describe all the different settings in detail, but adjoining areas that both predate and postdate the Svecofennian rocks that are the main interest of this field trip will be briefly described. The term Svecokarelian is used for the orogeny that occurred between 1900 and 1800 Ma (i.e., emphasizing deformation and metamorphism as defining the orogeny), while the term Svecofennian is used for the supracrustal rocks that were emplaced during c. 1.95 Ga to 1.85 Ga. To the reader unfamiliar with literature on the Fennoscandian Shield it is important to remember that these terms are not used consistently in the literature. The pre-Svecokarelian crustal growth can be subdivided into Archean and Paleoproterozoic. During the Archean, greenstone belts and tonalite-trondhjemite-granodiorite (TTG) terranes formed, while crustal growth during the Paleoproterozoic involved rifting of the Archean basement with the formation of rift-fill sequences of sedimentary and igneous rocks, and addition of juvenile Paleoproterozoic crust by accretionary processes along the margin of the Archean continent. The oldest known rocks in the Shield are c. 3.1 Ga in age and were generated during the Saamian Orogeny (3.1—2.9 Ga). Rocks of the age 3.2 to 2.7 Ga are present in the Archean nuclei of the shield and are composed of tonalitic gneisses and migmatites. The oldest documented magmatic and metamorphic event took place at c. 2.84 Ga (Nurmi and Sorjonen-Ward, 1996). Rift-related greenstones, subductiongenerated calc-alkaline volcanic rocks and tonalitictrondhjemitic igneous rocks were formed during the Lopian Orogeny (2.9—2.6 Ga). These greenstone belts form a prominent part of the Finnish and Russian bedrock, but are minor in Sweden. The Hattu schist belt in the southwestern part of the Archean of Finland seems to record a collisional arc setting