Pär Weihed

Global comparisons of volcanic-associated massive sulphide districts

Abstract Although volcanic-associated massive sulphide (VMS) deposits have been studied extensively, the geodynamic processes that control their genesis, location and timing remain poorly understood. Comparisons among major VMS districts, based on the same criteria, have been commenced in order to ascertain which are the key geological events that result in high-value deposits. The initial phase of this global project elicited information in a common format and brought together research teams to assess the critical factors and identify questions requiring further research. Some general conclusions have emerged. (1) All major VMS districts relate to major crustal extension resulting in graben subsidence, local or widespread deep marine conditions, and injection of mantle-derived mafic magma into the crust, commonly near convergent plate margins in a general back-arc setting. (2) Most of the world-class VMS districts have significant volumes of felsic volcanic rocks and are attributed to extension associated with evolved island arcs, island arcs with continental basement, continental margins, or thickened oceanic crust. (3) They occur in a part of the extensional province where peak extension was dramatic but short-lived (failed rifts). In almost all VMS districts, the time span for development of the major ore deposits is less than a few million years, regardless of the time span of the enclosing volcanic succession. (4) All of the major VMS districts show a coincidence of felsic and mafic volcanic rocks in the stratigraphic intervals that host the major ore deposits. However, it is not possible to generalize that specific magma compositions or affinities are preferentially related to major VMS deposits world-wide. (5) The main VMS ores are concentrated near the top of the major syn-rift felsic volcanic unit. They are commonly followed by a significant change in the pattern, composition and intensity of volcanism and sedimentation. (6) Most major VMS deposits are associated with proximal (near-vent) rhyolitic facies associations. In each district, deposits are often preferentially associated with a late stage in the evolution of a particular style of rhyolite volcano. (7) The chemistry of the footwall rocks appears to be the biggest control on the mineralogy of the ore deposits, although there may be some contribution from magmatic fluids. (8) Exhalites mark the ore horizon in some districts, but there is uncertainty about how to distinguish exhalites related to VMS from other exhalites and altered, bedded, fine grained tuffaceous rocks. (9) Most VMS districts have suffered fold-thrust belt type deformation, because they formed in short-lived extensional basins near plate margins, which become inverted and deformed during inevitable basin closure. (10) The specific timing and volcanic setting of many VMS deposits, suggest that either the felsic magmatic-hydrothermal cycle creates and focuses an important part of the ore solution, or that specific types of volcanism control when and where a metal-bearing geothermal solution can be focused and expelled to the sea floor, or both. This and other questions remain to be addressed in the next phase of the project. This will include in-depth accounts of VMS deposits and their regional setting and will focus on an integrated multi-disciplinary approach to determine how mineralisation, volcanic evolution and extensional tectonic evolution are interrelated in a number of world-class VMS districts.

Age of porphyry-type deposits in the Skellefte District, northern sweden

Abstract The Skellefte District in northern Sweden consists of metamorphosed Lower Proterozoic submarine volcanic, sedimentary, and intrusive rocks. Several massive sulphide deposits occur in the volcanics, and several small porphyry-type deposits exist in the oldest granitoids, which are coeval with the volcanics. The volcanic rocks have been dated previously at 1.88 Ga while the oldest granitoids have an age of 1.89 Ga. The aim of this study was to establish the timing of the porphyry systems. For this purpose, an intrusive quartz-feldspar porphyry, associated with the Tallberg porphyry-type deposit, has been dated. The U-Pb zircon age is 1886+15 -95 Ma which is within the error limits of both the host tonalites (the oldest Jorn granitoids) and the lower Skellefte volcanic rocks, hosting the massive sulphides. While the massive sulphide deposits were formed on the 1.88–1.89 Ga old, early Proterozoic seafloor, the porphyry-type deposits formed farther north inside the marginal arc, both types of deposit ...

The Tallberg porphyry copper deposit in northern Sweden: A preliminary report

Abstract This paper summarizes the present knowledge of an early Proterozoic porphyry type mineralization at Tallberg in the Skellefte district, northern Sweden. Ore potential and implications on exploration are also briefly discussed. The Tallberg mineralization is situated in the c. 1890 Ma Svecokarelian Jorn granitoid complex. The setting in early orogenic granitoids comagmatic with island are volcanics, the type of mineralization (disseminated and vein type Cu, Mo, and Au), and the propylitic and phyllic alteration around subvolcanic quartz-feldspar porphyritic stocks resemble those of Phanerozoic porphyry copper deposits. The gold is interpreted to have been partly remobilized into shear zones, corresponding to lines of weakness set up by early stress fields.

Post-deformation, sulphide-quartz vein hosted gold ore in the footwall alteration zone of the Palaeoproterozoic Långdal VHMS deposit, Skellefte District, northern Sweden

Abstract The Palaeoproterozoic, c. 1.88 Ga old Långdal VHMS deposit is situated in the eastern part of the Skellefte District, northern Sweden. In the stratigraphic footwall to the VHMS ore a sulphide-quartz vein system with high gold grades was mined in the second half of the 1990′s. The Långdal VHMS ore is hosted by the uppermost part of the Skellefte Group volcanic rocks, close to the contact with an overlying fine-grained sedimentary unit. Regional structural studies indicate that bedding surfaces in volcanic rocks are parallel to the contact between the volcanic and the sedimentary rocks. Based on the differences in structural style on each side, the contact is interpreted as a major structural break. The Långdal ore is situated close to this break that may have focussed fluid flow during metamorphism and deformation. The orientation of the contact indicates that it either is a D2 structure or that it was at least active during D2. The structural development in the altered footwall rocks to the Långdal VHMS ore indicates that gold-bearing sulphide and sulphide-quartz veins both pre- and post-date the first deformation. Gold associated with the vein system can thus not only be attributed to syngenetic exhalative or replacement processes. The close spatial relationship with the massive sulphide deposits suggests, however, that the gold was remobilized from these syngenetic systems. It is concluded that sulphides were introduced at several stages during the geological evolution of the area as: a) syngenetic disseminations of sulphide and folded, pre-S1 stringer sulphide±quartz veins in the footwall related to the syngenetic VHMS deposit, b) syn-S1 sulphide veins in the footwall gold ore, c) main, post-S1, sulphide-quartz veins associated with the gold ore in the footwall rocks to the Långdal VHMS deposit, and d) post-S1 to pre-S2 galena and sphalerite rich veins post-dating the main, post-S1, sulphide-quartz vein system in the footwall to the Långdal ore. From these relationships it is suggested that gold was re-mobilized from the sulphide rich parts of the VHMS system into post-D1 structures during or slightly after the peak metamorphism. The timing of this event is poorly constrained to post-date the syngenetic ore emplacement by 20–80 m.y.

Some aspects on the subdivision of the Haparanda and Jörn intrusive suites in northern Sweden

Abstract The geographical subdivision between the Haparanda and the Jörn suites of intrusive rocks in northern Sweden has not been very well defined. Early stratigraphical schemes placed these two granitoid suites in two separate orogenic cycles, where the Jörn belonged to the older cycle and Haparanda to the younger. Our present knowledge regarding the isotopic ages of these rocks in northern Sweden has changed this view, but has also made the distinction between the two suites less clear. Based on recent Sm–Nd isotopic work combined with geochemistry and some new U–Pb zircon data, we point out some similarities as well as some differences between the Jörn and Haparanda suites of rocks. Two U–Pb zircon age determinations performed give upper intercept ages of 1891±32 Ma and 1861±19 Ma which are interpreted as maximum ages. The two samples are taken from the Luleå area, on each side of the Archaean–Proterozoic boundary, as defined by Sm–Nd isotopic analyses of c.1.9 Ga old intrusive rocks combined with the southern limit of outcropping Archaean rocks. On the basis of new results together with results from previous studies of areas north and south of the Archaean–Proterozoic boundary, we also suggest how to separate the Haparanda and Jörn suites of rocks due to their geochemical, and isotope geochemical, characteristics. The Haparanda suite generally has negative ϵNd(t) values and was formed within or in marginal parts of the Archaean craton. The Jörn suite was formed in an juvenile, island-arc terrane, that was accreted to the Archaean craton during the later, collisional stages of the Svecokarelian orogeny. In a similar way, we connect the Haparanda suite of rocks with the Archaean craton, and the Jörn suite of rocks with Svecofennian juvenile crust.

Source of metals and age constraints for epigenetic gold deposits in the Skellefte and Pohjanmaa Districts, central part of the Fennoscandian Shield

The lead isotopic compositions of galena in Early Proterozoic gold deposits have been determined for three districts in northern Sweden and central Finland. The deposits are hosted by a variety of ≈1870–1890 Ma Svecofennian host rocks including the volcanosedimentary succession within the Skellefte District island arc in Sweden as well as I-type tonalites at Jörn (Sweden) and Pohjanmaa (Finland). The deposits are epigenetic in relation to these Svecofennian rocks and are part of a goldbearing metallogenetic belt, which can be followed for 600 km parallel to the southwestern margin of the Archaean Domain. In spite of these epigenetic relationships, the lead isotopic data indicate that the deposits are not dramatically younger than the ≈1870–1890 Ma Svecofennian host rocks (probably not exceeding 10–20 million years). Two principal lead sources were activated when the gold deposits were formed. The most significant source is represented by the I-type tonalites, which constitute a relatively primitive (μ = 9.3) and widely distributed source in the entire metallogenic belt. In addition, the volcanic components in the westernmost part of the Skellefte District constitute an extremely primitive (μ <9.0) source, which only locally was an important contributor to the epigenetic deposits in this metallogenetic belt. The significantly different lead isotopic composition estimated for these sources indicates that the volcanic rocks in the western part of the Skellefte District were not comagmatic with the I-type tonalites recognized at Jörn and central Finland.

A stable isotope study of the Palaeoproterozoic Tallberg porphyry-type deposit, northern Sweden

The Tallberg deposit is situated in the Skellefte District in northern Sweden. It is a Palaeoproterozoic equivalent of Phanerozoic poryphyry-type deposits. The mineralization is situated within the Jörn granitoid complex and is associated with intrusive quartz-feldspar porphyries. The granitoids are coeval with mainly felsic volcanic rocks hosting several massive sulphide deposits. The alteration is generally of a mixed phyllic-propylitic type, but areas or zones associated with high gold grades exhibit phyllic alteration. Ore minerals are pyrite, chalcopyrite, sphalerite, magnetite, and trace amounts of molybdenite. In this stable isotope study, quartz, sericite, and chlorite from the alteration zones were sampled. The magmatic quartz has a ∂18O composition of + 6.2 to +6.7‰ whereas the quartz in the hydrothermal alteration zones have values ranging from +7.5 to +10.6‰. The calculated temperatures for this fractionation range from 430° to 520°C. The sericites have ∂18O ranging from +4.6 to +8.2‰ (average +6.6‰) and ∂D -31 to -54‰ (average -41‰). Chlorites range from ∂18O +4.2 to +7.7‰ and ∂D from −34 to −44‰. The range of ∂34S of 11 pyrite samples is +3.8 to +5.5‰ with an average of +4.6 ± 0.5‰, suggesting a relatively homogeneous sulphur source, probably of magmatic origin. Modelling waters in equilibrium with the minerals indicates early magmatic fluids with ∂18O of ≈ 6.5‰. This fluid mixed with a low ∂18O and high ∂D fluid, which is tentatively identified as seawater. The ∂18O signature of sericite and chlorite also indicates significant water-rock exchange, explaining the positive ∂18O values for the waters in equilibrium with the hydrated minerals.

Regional implications of an age determination of a gneissose granitoid south of the Skellefte district, northern Sweden

Abstract A gneissose granitoid, the Siktrask dome, situated c. 20 km south of the central parts of the Skellefte District has been dated at 1859±3 Ma. This age excludes the possibility that this gneiss dome is a basement to the Svecofennian metamorphosed supracrustral rocks surrounding the intrusive. The age further suggests that this dome and other similar deformed granitoids south of the Skellefte District could be interpreted as synkinematic Jorn-type intrusives, but situated in an amphibolite facies environment, contrary to the undeformed Jorn granitoids which are situated in a greenschist facies environment.

Fluid chemistry of the Palaeoproterozoic Fäboliden hypozonal orogenic gold deposit, northern Sweden: evidence from fluid inclusions

Abstract A new ore province, the Gold Line, southwest of the Skellefte District, northern Sweden, is currently under exploration. The largest known deposit in the Gold Line is the hypozonal Fäboliden orogenic gold deposit. The mineralization is hosted by arsenopyrite-bearing quartz veins, within a steep shear zone in amphibolite facies metagreywacke host rocks. Gold occur in fractures and as intergrowths in arsenopyrite-löllingite, and as free grains in the silicate matrix of the host rock. The hydrothermal mineral assemblage in the proximal alteration zone is diopside, calcic amphibole, biotite, and minor andalusite and tourmaline. Primary fluid inclusions in the Fäboliden quartz veins show a CO2-CH4 or a H2S (±CH4) composition (the latter recognized for the first time in a Swedish ore deposit). The primary fluid inclusions are associated with arsenopyrite-löllingite (+gold) and the CO2-CH4 fluid was also involved in precipitation of graphite. A prevalence of carbonic over aqueous fluid inclusions is characteristic for a number of hypozonal high-temperature orogenic gold deposits. The Fäboliden deposit, thus, shows fluid compositions similar to other hypozonal orogenic gold deposits. The proposed main mechanism for precipitation of gold from the fluids, is a mixing between H2S-rich and H2O?-CO2±CH4 fluids. Fluid inclusion data indicate arsenopyrite-löllingite and graphite deposition at a pressure condition of about 4 kbar. Graphite thermometry indicates maximum temperatures of 520-560°C for the hydrothermal alteration at Fäboliden, suggesting that at least the late stages of the mineralizing event took place shortly after peak-metamorphism in the area, i.e. at c. 1.80 Ga.

The Jokkmokk granitoid, an example of 1.88 Ga juvenile magmatism at the Archaean-Proterozoic border in northern Sweden

Abstract The Jokkmokk granitoid is exposed in a large plutonic massif northwest of Jokkmokk in northern Sweden. It is light grey to white, fine-grained, with megacrysts of feldspar and glomeroporphyritic hornblende and biotite. Small enclaves of mafic rocks and synplutonic mafic dykes are products of mingling with a coeval and possibly cogenetic mafic magma. The Jokkmokk granitoid was previously considered to belong to the c. 1.8 Ga Lina S-type intrusive suite, but the Jokkmokk granitoid has a unique calc-alkaline to alkali-calcic, metaluminous to weakly peraluminous, character with a moderate LREE enrichment and a flat HREE pattern, and a flat to slightly positive Eu-anomaly. U-Pb TIMS zircon dating of the Jokkmokk granitoid gives an age of 1883±15 Ma which is coeval with the emplacement of the Haparanda suite, but contrary to the Haparanda suite it displays a positive εNd(t) value of 2.8, indicating a more juvenile Palaeoproterozoic character similar to the Jörn suite in the Skellefte district. This type of magma seems to be restricted to the palaeoboundary between the Archaean craton in the north and Palaeoproterozoic juvenile crust in the south. Spatial correlation with low angle, south dipping, WNW-trending shear zones and NNE-trending subvertical shear zones, highlight the possibility that this unique magma type is related to transtension in the overriding plate and partial melting in a sub-arc mantle wedge during NE-directed subduction processes related to the early stages of the Svecokarelian orogen. This type of setting has been advocated as the potentially most favourable tectonic setting for porphyry copper formation.