SGU Rapporter och meddelanden 113 Rapporter och meddelanden 113 Economic geology research and documentation Volume 2 2001–2002 Magnus Ripa, editor Economic geology research and documentation, Volume 2 2001–2002 Rapporter och meddelanden 113 Economic geology research and documentation Volume 2 2001–2002 Magnus Ripa, editor Sveriges geologiska undersökning 2003 ISSN 0349-2176 ISBN 91-7158-684-9 Cover: The Exportfältet ore field at Grängesberg. Photo Christer Åkerman. © Sveriges geologiska undersökning Geological Survey of Sweden Layout: Agneta Ek, SGU Print: Elanders Tofters, Östervåla 2003 2 Preface The current volume is the second to report on results from the work within the ore documentation programme at the Geological Survey of Sweden (SGU). The first volume reported on results from work performed during 1999 and 2000 (Economic geology research. Volume 1. 1999–2000. P. Weihed (ed.). Sveriges geologiska undersökning C 833, 136 pp.). The aim is to publish various papers of interest to exploration and mining companies as well as the layman. Most papers result from compilation of data from published papers and from prospecting reports in the archives of SGU. In some cases additional field work and analysing of samples have been performed. Uppsala, June 2003 Magnus Ripa Contents Styles of hydrothermal alteration and accompanying chemical changes in the Sången formation, Bergslagen, Sweden, and surrounding areas . 4 A. Hallberg The Morön silver deposit . 36 A. Hallberg The Nyborg Zn-Ag mineralisation, Skellefte district, Sweden . 42 J.-A. Perdahl Apatite-bearing iron ores in the Bergslagen region of south-central Sweden . 49 M. Ripa & L. Kübler Nickel exploration in northern Sweden by the Outokumpu Oyj company . 55 C. Åkerman 3 Styles of hydrothermal alteration and accompanying chemical changes in the Sången formation, Bergslagen, Sweden, and adjacent areas Anders Hallberg Anders Hallberg, 2003: Styles of hydrothermal alteration and ing sequences, combined with heat from the waning volcanism accompanying chemical changes in the Sången formation, and channelled by fractures formed by the ongoing extension, Bergslagen, Sweden, and adjacent areas. In Ripa, M. (ed.): provided the fundamental conditions for a long-lived environ- Economic Geology Research. Vol. 2, 2001–2002. Uppsala 2003. ment in which rocks could be completely Na-altered. Sveriges geologiska undersökning Rapporter och meddelanden 113, K alteration proper is not found in the Sången formation, pp. 4–35. ISBN 91-7158-684-9. but is more common in the overlying Älgen formation. A more intense form of alteration, probably regional in character, is the Mg alteration observed at several localities in Abstract the Sången formation. It is characterised by substantial deple- tion in alkali – almost all of the Na is leached from the rock and The Bergslagen ore district in south-central Sweden is mainly K is strongly depleted – and an addition of MgO to the rock. hosted by a belt of Palaeoproterozoic metavolcanic and meta- The feldspars have been destroyed and mica has been formed. sedimentary rocks. In western Bergslagen, the supracrustal rocks Several samples from different localities show a similar style of have been divided into three principal formations, representing Mg alteration indicating that the alteration has reached comple- different phases in the volcanic evolution of the area. The lower- tion and that the Mg-altered rock was in equilibrium with the most and oldest supracrustal formation, the Sången formation, hydrothermal fluids. consists of poorly sorted volcanic sandstones and represents an “intense volcanic stage” characterised by eruptions of enormous Anders Hallberg, Geological Survey of Sweden, Box 670, volumes of mainly rhyolitic volcanic rocks over large areas. SE-751 28 Uppsala, Sweden. E-mail: [email protected] Overlying formations, the Älgen and Torrvarpen formations, represent progressively calmer volcanic activity. The Zr/TiO2 ratios of rocks from the Sången formation Introduction and from mafic rocks and early orogenic granite intruding the Sången formation divide these rocks into two distinct groups, This paper describes a lithogeochemical approach to hy- a mafic group with low ratios and a felsic group, including the drothermal alteration in western Bergslagen. The aim is metavolcanic rocks and the early orogenic granites, with high to further the understanding of alteration in Bergslagen ratios. This bimodal distribution is in agreement with previous and to provide a tool to chemically describe and classify observations based on different methods (e.g. Allen et al. 1996). different styles of alteration. To this end, 35 rock samples The Zr/TiO2 ratios of the early orogenic intrusions in the area, some of which are suggested to be subvolcanic intrusions, over- were collected from outcropping metavolcanic rocks in lap with the ratios for some of the volcanic rocks suggesting that the Sången formation (Lundström 1995), from rocks in- they could be co-magmatic. truding the Sången formation, and from rocks overlying A large portion of the rocks in the Sången formation have it. Additional chemical data from published and unpub- been affected by one or more episodes of alteration. Using lished papers were recorded in databases and where pos- whole-rock geochemistry and a number of classification dia- sible the location from which each sample was taken was grams, it is possible to discern three principal types of regional given co-ordinates in the Swedish National grid (RT 90). alteration in the Sången formation and adjacent areas: Na altera- The well-documented geology of the area (Lundström tion, K alteration, and Mg (mica) alteration. These types are in 1995, Allen et al. 1996 and references therein), with a general agreement with previous studies in the region. Na al- large number of published works on the geology in gen- teration affected both early orogenic granites and felsic volcanic eral and altered rocks in particular and a large volume of rocks. It mainly affects the alkali content, with minor effects on published whole-rock geochemistry for less altered as well the other major elements. It is characterised by the replacement as altered rocks, makes the area highly suitable for a study of K2O by Na2O. Na alteration was not restricted to the lower volcanic sequences, the Sången formation, but continued over of alteration geochemistry. The area has long been consid- formation boundaries into the overlying Älgen formation. Some ered a type locality for Bergslagen volcanic-sedimentary of the Na-altered rock shows a tendency towards Mg alteration. stratigraphy (Sundius 1923, Allen et al. 1996). The Na-altered rocks had probably reached equilibrium with the hydrothermal fluids, since identical alteration is found at several localities which represent large areas. It is suggested that high- salinity fluids occurring as groundwater under evaporite-form- 4 Geology 1979). In the western part of the region deformation and metamorphism were locally less intense than in other ar- Regional geology eas of Bergslagen and the supracrustal rocks are unusually well preserved. This fact has attracted scientists for more The Bergslagen region in south central Sweden consists of than a century. a belt of Palaeoproterozoic volcanic and sedimentary rocks The bedrock of the most closely studied areas of intruded by large early- to post-orogenic intrusions. The western Bergslagen has recently been published on supracrustal rocks and older intrusions have been folded 1:50 000 scale maps: 11E Filipstad SO and NO (Lund- and faulted in several epochs, metamorphosed from green- ström 1995), 11F Lindesberg SV (Lundström 1983), 11F schist to amphibolite grade, and affected by hydrothermal Lindesberg NV (Lundström 1985), and 11E Filipstad NV alteration (e.g. Lundström & Papunen 1986, Lundqvist (Björk 1986). The geological map shown in Figure 1, is 6650000 Formations Usken fm. 1425000 Älgen fm. Sikfors mb. Älgen fm. Bredsjö mb. Vassland fm. Sången fm. 1475000 6600000 Legend Younger granite Early orogenic intrusion, felsic Early orogenic intrusion, mafic Marble Metasedimentary rock Metavolcanic rocks, felsic Fig. 1. Geological map of the central parts of western Bergslagen, compiled from Stephens et al. (in prep). Inset map and thick grey lines on the geological map show the extent of different formations (Lundström 1983, 1995). Coordinates given on inset map accord- ing to the Swedish grid system RT 90. 5 simplified from the digital version of the recently com- 1994, Beunk et al. 1985, de Groot et al. 1988) argue for pleted map of Bergslagen and surrounding areas. the existence of an older basement in order to explain the Lundström (1995) subdivided the lithostratigraphy large amount of felsic volcanic rocks and the upper crustal of the central parts of western Bergslagen into three lo- signature indicated by the isotopic work. Tentative cor- cal formations, each of which can be correlated to an relations with the Utö area in eastern Bergslagen, where older stratigraphical division of Bergslagen, e.g. Oen et older volcanic rocks rest on quartzitic greywackes, suggest al (1982) and Sundius (1923). The stratigraphically low- that Palaeoproterozoic metasedimentary rocks could form ermost formation is the Sången formation, consisting of a basement to the Sången formation. The Vassland forma- poorly sorted volcanic sandstones. This is overlain by the tion, occuring south-east of the Sången formation, con- Älgen formation, which mainly consists of better-sorted sists of rocks