©2013 Society of Economic Geologists, Inc. Economic Geology, v. 108, pp. 309–335
Evolution of the Paleoproterozoic Volcanic-Limestone-Hydrothermal Sediment Succession and Zn-Pb-Ag and Iron Oxide Deposits at Stollberg, Bergslagen Region, Sweden* NILS F. JANSSON,1,†,** FABIAN ERISMANN, 2,*** ERIK LUNDSTAM,2 AND RODNEY L. ALLEN1,2 1 Division
of Geosciences and Environmental Engineering, Luleå University of Technology, 971 87 Luleå, Sweden 2 Boliden
Mineral AB, Exploration Department, 776 98 Garpenberg, Sweden
Abstract The Stollberg Zn-Pb-Ag and magnetite mining field is located in the Bergslagen region of the Fennoscandian Shield. The main Stollberg ore deposits comprise a chain of orebodies that occur discontinuously for 5 km along a prominent marble and skarn horizon. Orebodies mainly contain magnetite and combinations of sphalerite, galena, pyrrhotite, and lesser pyrite and chalcopyrite within marble and skarn. Previously, the two main limestone (marble) units in the Stollberg area were regarded as structural repetitions of one single horizon. Based on sedimentary and volcanic facies and structural analysis, the mineralized Stollberg limestone is now shown to be the uppermost of two different limestone units within a ca. 3-km-thick Paleoproterozoic (~1.9 Ga) volcanosedimentary succession. Approximately 2 km of preserved footwall stratigraphy is recognized below the Stollberg limestone, as opposed to ca. 500 m in previous structural models. This new interpretation has allowed the stratigraphic evolution prior to the mineralizing event and extent of the Stollberg hydrothermal system to be investigated in detail. After formation of the Staren limestone ca. 1 km below Stollberg, the depositional basin subsided to below wave base, while adjacent areas were uplifted and eroded. This led to the deposition of a ca. 600-m-thick, shallowing-upward sedimentary sequence in which normal-graded subaqueous mass flow deposits pass upward to polymict limestone-volcanic breccia-conglomerates. This sequence is attributed to progradation of a fan delta depositional system. The breccia-conglomerates are overlain by ca. 500 m of juvenile rhyolitic pumice breccia that is interpreted as a major pyroclastic deposit. Conformably above is the Stollberg ore host, which comprises planar-stratified, rhyolitic ash-siltstone interbedded with Fe-Mn–rich hydrothermal sedimentary rocks and limestone, all deposited below wave base. This ore host package is extensively altered to skarn and mica schist. The thickness, extent, and homogeneous composition of the rhyolitic pumice breccia below the ore host suggest that volcanism was accompanied by caldera subsidence and that the Stollberg ore deposits formed within the caldera structure. The ore host is overlain by planar-stratified, rhyolitic ash-siltstone and subordinate sedimentary breccias deposited below wave base from turbidity currents and suspension. Skarns in the Stollberg ore host unit are interpreted as metamorphosed mixtures of variably altered rhyolite, limestone, and hydrothermal sediments. Whole-rock contents of Al, Ti, Zr, Hf, Nb, Sc, Th, Ta, U, and heavy rare-earth elements are highly correlated in skarns, limestone, magnetite mineralization, and variably altered rhyolites in the Stollberg succession, suggesting that these elements were supplied by a felsic volcaniclastic component and were immobile during alteration. The felsic volcaniclastic component is calc-alkaline and characterized by negative Eu anomalies and light rare-earth element enrichment. Strong positive Eu anomalies are only observed in limestone, skarn, and iron ore in the Stollberg ore host, i.e., in samples rich in Mn, Ca, and Fe. The Stollberg ore deposits are interpreted as metamorphosed, hydrothermal-exhalative and carbonate replacement-type mineralization. The hydrothermal-exhalative component formed first by accumulation of sediments rich in Mn and Fe, coeval with limestone formation during waning volcanism. Burial of the hydrothermal system by sediments of the stratigraphic hanging wall led to a gradual shift to more reducing conditions. At this stage, the Stollberg limestone interacted with more sulfur rich hydrothermal fluids below the sea floor, producing strata-bound, replacement-type Zn-Pb-Ag sulfide and additional iron oxide mineralization.
Introduction THE BERGSLAGEN region in the Fennoscandian Shield (Fig. 1) contains inliers of Paleoproterozoic (ca. 1.91–1.89 Ga) metavolcanic and metasedimentary rocks that are enclosed by † Corresponding author: e-mail,
[email protected] *A digital supplement to this paper is available at http://economicgeology.org/ and at http://econgeol.geoscienceworld.org/. **Present address: Boliden Mineral AB, Exploration Department, 936 81 Boliden, Sweden. ***Present address: Earth Resource Investment Group, Gotthardstrasse 27, 6300 Zug, Switzerland.
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synvolcanic to early Svecokarelian (ca. 1.9–1.8 Ga), mainly granitoid intrusions. The metavolcanic rocks are dominantly calc-alkaline rhyolite (Lagerblad, 1988; Stephens et al., 2009) that formed in a subaqueous, back-arc basin developed on continental crust (Allen et al., 1996). Over 6,000 mineral deposits and prospects are known in the Bergslagen region (Fig. 1). Most are hosted by marble units within the metavolcanic succession. The supracrustal succession, early intrusions, and mineral deposits underwent ductile deformation and, generally, amphibolite-facies regional metamorphism, and were intruded by further granitoids during the Svecokarelian Orogeny (Stephens et al., 2009).
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Submitted: July 14, 2011 Accepted: June 29, 2012
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Late- and post-Svecokarelian intrusive rock (1.87-1.67 Ga)
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Svecofennian metavolcanic rock (1.91-1.89 Ga)
Iron oxide deposit in skarn or marble
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Svecofennian metasedimentary rock
FIG. 1. Geologic map of Bergslagen region modified after Allen et al. (1996) and Stephens et al. (2009), showing location of the Stollberg area (So). Also shown are locations of the Falun (F), Ställdalen (Se), Garpenberg (G), Dannemora (D), and Zinkgruvan (Z) deposits. Green dots denote Mn-rich iron oxide deposits whereas red squares denote sulfide showings and deposits, all from the database of the Geological Survey of Sweden. Inset shows simplified geologic setting of the Bergslagen region (BR) in northern Europe. BIF = banded iron formation. 0361-0128/98/000/000-00 $6.00
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The most common mineral deposits are metamorphosed iron oxides that include strata-bound marble- and/or skarn1hosted magnetite deposits, strata-bound apatite-bearing magnetite-hematite deposits, and stratifrom hematite-magnetite banded iron formation. The first type has traditionally been divided into Mn-rich (Mn > 1 wt %) and Mn-poor (Mn < 1 wt %) types (Geijer and Magnusson, 1944). Western Bergslagen also hosts several metamorphosed, syngenetic FeMn oxide deposits (e.g., Holtstam and Mansfeld, 2001), characterized by abundant manganese oxides, in contrast to the Mn-rich iron oxide deposits in which Mn mostly resides in carbonate and/or silicate gangue (Geijer and Magnusson, 1944). Strata-bound Zn-Pb-Ag-(Cu-Au) and stratiform Zn-Pb-Ag(Cu) massive sulfide deposits are less abundant than the Fe ores, but include numerous small- to medium-sized deposits and three large deposits: Falun (closed mine; 28 Mt production), Zinkgruvan (operating mine; ca. 37 Mt production), and Garpenberg (operating mine; ca. 38 Mt production) (Allen et al., 1996, and unpub. data). These polymetallic sulfide deposits in Bergslagen are difficult to classify in terms of the major, internationally recognized genetic classes of ore deposits. Similar to volcanogenic massive sulfide (VMS) deposits, many of the Bergslagen deposits consist of massive to semimassive sulfides hosted in marine felsic volcanic successions and have K-Mg-Fe alteration zones (now mica schists) present in footwall strata (Allen et al., 1996). However, unlike typical VMS deposits, most of the deposits in the Bergslagen region (e.g., Garpenberg) are strata-bound rather than stratiform, hosted mainly by metalimestone units within the volcanic successions, and spatially associated with amphibole, pyroxene, and garnet skarn (Allen et al., 1996, 2008). The original limestones most recently have been interpreted as mainly marine stromatolite carbonate reefs that formed in water depths of 5 to 50 m (Allen et al., 2003). Sulfide deposits within these limestones are strata-bound replacements, essentially synvolcanic in timing, and without demonstrated genetic links to igneous intrusions, as might be expected if the deposits were typical intrusion-related skarn ores (Allen et al., 2003, 2008; Jansson and Allen, 2011a, b). The stratiform polymetallic sulfide deposits in Bergslagen are mainly sheet-like (e.g., Zinkgruvan). In geometry, structure, and metal assemblage, they resemble typical sedimentary-exhalative (sedex) Zn-Pb-Ag deposits. However, they occur in successions of mainly felsic volcaniclastic rocks and limestone rather than in siliciclastic sedimentary rocks. The Stollberg mining field is one of Sweden’s oldest mining areas. Early written accounts mention mining in 1354 A.D.; the last mine closed in 1982 after ca. 6.65 Mt of ore had been mined (Ripa, 1996). The mining field comprises a ca. 5-km-long, NS–trending belt of ca. 25 workings (Fig. 2), traditionally divided into the Svartberg field in the north and the Stollberg field in the south (Geijer and Magnusson, 1944). The mined deposits comprised steeply dipping, tabular-lenticular bodies of sphalerite-argentiferous galena and Mn-rich magnetite, ranging 1 In this paper, “skarn” is used purely descriptively and in a nongenetic sense in a fashion similar to Törnebohm’s (1875). The term refers to stratabound or stratiform calc-silicate rocks dominated by clinopyroxene, garnet, and/or clinoamphibole without any necessary spatial or genetic link to igneous intrusions.
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from impregnations to massive orebodies. Both are hosted mainly by a steeply dipping unit of interbedded marble, skarn, and metarhyolite within a succession of felsic metavolcanic rocks belonging to the 1.91 to 1.89 Ga Svecofennian rock suite (Ripa, 1988, 1994; Stephens et al., 2009). The Stollberg deposits have similarities both to Mn-rich iron oxide deposits in Bergslagen, such as the 6.5-Mt Ställberg and 6.24-Mt Basttjärn deposits (Geijer and Magnusson, 1944), and to strata-bound, carbonate-replacement Zn-Pb-Ag-(CuAu) sulfide deposits (Geijer, 1917; Allen et al., 1996; Ripa, 1996) such as Falun and Garpenberg. These iron oxide and polymetallic sulfide deposit types are commonly spatially separated, but, locally, such as at Stollberg and at the 66.8-Mt Dannemora deposit, iron oxide and sulfide deposits have been mined together (Tegengren, 1924; Lager, 2001). Understanding the relationships between these two deposit types is of fundamental interest to base metal exploration in the region. Relationships of the Stollberg deposits to semiregional geochemical alteration patterns and petrologically exotic wall rocks formed by amphibolite-facies metamorphism of hydrothermally altered, mainly rhyolitic rocks were addressed in earlier studies (Geijer and Magnusson, 1944; Arvanitidis and Rickard, 1981; Selinus, 1982, 1983; Ripa, 1988, 1994, 1996; Beetsma, 1992). Arvanitidis and Rickard (1981) and Ripa (1988, 1994, 1996) concluded that the mineralization was syngenetic and formed during the evolution of a large subaqueous hydrothermal system in a felsic volcanic setting. Ripa (1988, 1994, 1996) attributed the Mn-rich iron oxide deposits to an exhalative stage, whereas the sulfides were interpreted to have formed later by replacement of the ore host in conjunction with Mg alteration. Alteration and replacement occurred prior to ductile deformation and amphibolite-facies metamorphism during the Svecokarelian Orogeny (Ripa, 1988, 1994; Stephens et al., 2009). Beeson (1990) regarded Dammberget in the Stollberg field as a Broken Hill-type deposit. This interpretation was based in part on metamorphic grade, occurrence of a mainly oxidized stratigraphic sequence, and the Mn-bearing, garnet-rich chemical halo surrounding the deposit. On the basis of fluid inclusions and textures, Beetsma (1992) suggested a very different model, in which sulfides precipitated immediately after peak metamorphism (560°–600°C at 2.0–3.5 kbar). Little work has been done to characterize the facies architecture, stratigraphy, and structure of the Stollberg mining field on a regional scale. The present study is the result of collaboration between Luleå University of Technology and Boliden Mineral AB in order to produce a new geologic map of the Stollberg area (Fig. 2), show the distribution of volcanic and sedimentary facies, and investigate the origin of the ore deposits. Geologic cross sections (Figs. 3, 4) and local stratigraphic columns across the ore-bearing interval are produced to show the setting of the deposits. In addition, major, trace, and rare-earth element (REE) lithogeochemistry of the stratigraphic succession and its relationship to the mineralized interval are evaluated. Based on the results, the volcanosedimentary evolution of the Stollberg area as well as the timing, setting, and mode of iron oxide and sulfide mineralization are discussed. This paper does not provide a detailed description and interpretation of the alteration system and mineralization at Stollberg, which will be the focus of future contributions.
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Post-Svecokarelian dolerite Undifferentiated granitoid Amphibolite after mafic intrusion or volcanic rock Coherent Fsp-porphyritic rhyolite Pelite and graywacke Planar-bedded, rhyolitic siltsandstone, subordinate breccia
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Cordierite-rich alteration zone (in stratigraphic hanging-wall)
Rhyolitic siltstone, sandstone and breccia (locally normal graded) Massive polymict breccia-conglomerate (locally with limestone clasts) Limestone / Mn-rich dolomite or limestone Strongly Qz+Fsp-phyric, massive rhyolitic sandstone
Skarn Gedrite-rich alteration zone
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Fe deposit >100 kt / 7 km (Fig. 2; Hjelmqvist, 1966). East of the marble, a poorly exposed succession of felsic metavolcanic rocks hosts several Mn-poor magnetite skarn deposits (Geijer and Magnusson, 1944) and is bordered farther eastward by Svecokarelian metagranitoids (Fig. 2). Two large, N-S–striking amphibolite units occur, one west of the Stollberg mining field and one west of the Staren marble. Ripa (1988) suggested an extrusive origin for the amphibolite west of Stollberg. The area is transected by NNW-trending, nonporphyritic dolerite dikes and, in the northern part, by NE-trending dolerites with phenocrysts of plagioclase and augite (Fig. 2). The NE-trending dikes likely belong to the anorogenic, ca. 1.48 to 1.47 Ga Tuna dolerite suite, whereas the NNW-trending dikes likely belong to the 0.98 to 0.95 Ga Sveconorwegian dolerite suite of Stephens et al. (2009). A few small pegmatites of unknown age that cut tectonic foliations are observed. During recent mineral exploration, a reversal in stratigraphic younging direction has been identified between the Gränsgruvan and Svartberg areas (Fig. 2), confirming that these deposits occupy the same stratigraphic level on opposite limbs of the Stollberg syncline. Grip (1983) and Ripa (1988) also interpreted the Staren marble as a fold repetition of the Stollberg ore host marble, with the altered metavolcanic rocks east of Stollberg occupying the core of an anticline. However, the present study shows that the Staren and Stollberg marbles are different stratigraphic units, and hence that a continuous, generally westward-younging stratigraphic sequence exists from the Staren area to the center of the Stollberg syncline (Fig. 3). Metamorphic and Structural History Folds in the Stollberg area are generally tight to isoclinal, N-S–striking, S-symmetric, upright structures with steeply S plunging but locally N plunging fold axes (Fig. 4). They are
FIG. 2. Geologic map of the Stollberg area, showing the location of major iron oxide and sulfide deposits (1 = Gränsgruvan, 2 = Cedercreutz, 3 = Hag-Mygg-Ryss, 4 = Kogruvan, 5 = Dammberget, 6 = Stollgruvan, 7 = Brusgruvan, 8 = Floberg, 9 = Gubbo, 10 = Kärrgruvan, 11 = Nyberg, 12 = Marnäs and Lustigkulla, 13 = “Limonite ore,” 14 = Baklängan). Modified after Ohlsson (1979). Grid is Swedish National Grid RT90. Profiles constructed during ongoing research and exploration are indicated. Note that premetamorphic rock names are used for all rock types, excluding those for the hydrothermally altered rocks and metamorphosed mafic rocks. Fsp = feldspar, Qz = quartz. 0361-0128/98/000/000-00 $6.00
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Skarn Massive polymict breccia-conglomerate (locally with limestone and mafic clasts)
Amphibolite after mafic intrusion or volcanic rock Coherent Fsp-porphyritic rhyolite
Undifferentiated granitoid
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Section Northern Stollberg-Staren Area (6677000)
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FIG. 3. Geologic cross section over the Stollberg-Staren area along grid coordinate 6677000, shown on Fig. 2. Note that premetamorphic rock names are used for all rock types, excluding the hydrothermally altered rocks and metamorphosed mafic rocks. Bt = biotite, grt = garnet, fsp = feldspar, qz = quartz.
Fault Moderately-strongly garnetporphyroblastic rock
Undifferentiated felsic volcanic rocks Cordierite-rich alteration zone (in stratigraphic hanging-wall) Fsp+Qz-phyric rhyolitic pumice breccia-sandstone Normal graded, rhyolitic, lithic breccias, crystal-rich sandstone and siltstone
Polymetallic Pb-Zn-Ag sulphide deposit
Drillhole Gedrite-rich alteration zone Rhyolitic sandstone Post-Svecokarelian dolerite
Moderate pervasive Bt-Qz +/- Grt alteration
Younging direction Silicification
Limestone / Mn-rich dolomite or limestone Strongly Qz+Fsp-phyric, massive rhyolitic sandstone Planar-bedded, rhyolitic siltsandstone, subordinate breccia Quaternary overburden
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JANSSON ET AL.
Svecokarelian structures that fold an earlier foliation (S1), which is subparallel to bedding (S0), and are thus here designated as F2 folds. An S2 foliation and local N-S–trending, D2 shear zones parallel the axial surface of the F2 folds. F2 fold hinges are generally parallel to a strong, S-dipping, subvertical stretching lineation. Svecokarelian regional metamorphism at Stollberg was interpreted by Ripa (1994) to have attained medium grade (amphibolite facies) at temperatures of 550°C below 3.5 kbar. Beetsma (1992) suggested higher temperatures of 560° to 600°C at 2 to 3.5 kbar. Ripa (1996) based his pressure-temperature (P-T) estimate on the absence of sillimanite, yet both andalusite and sillimanite were identified in the hydrothermal alteration envelope during the current study. Thus, the P-T estimate of Beetsma (1992) is probably more accurate. Both estimates are based on mineral assemblages in metamorphosed, hydrothermally altered rocks and are in agreement with other P-T estimates for northern Bergslagen as summarized by Stephens et al. (2009). Garnet porphyroblasts in the altered wall rocks overgrow S1 that constitutes a strong quartz grain-shape fabric. Garnet commonly forms elongate laminae and bands alternating with quartz-biotite. The biotite crystals are aligned parallel or subparallel with the garnet bands. The garnet bands show symmetric boudinage within S1 accompanied by pressure shadows of biotite, suggesting that D1 deformation outlasted formation of the garnet bands. Magnesian hedenbergite in the ore zone forms coarse granoblastic aggregates that overgrow garnet. Sphalerite, galena, pyrrhotite, magnetite, arsenopyrite, chalcopyrite, and pyrite form inclusions within single crystals of magnesian hedenbergite and garnet, and are also common in veinlets cutting both minerals. Gahnite-hercynite (ZnAl2O4-FeAl2O4) porphyroblasts occur in the stratigraphic footwall of the Stollberg ore deposits and grew parallel to a crenulated foliation interpreted as S1 (Fig. 5). The S1 foliation and boudinaged garnet bands and their pressure shadows are folded about S2. A distinct S2/S1 crenulation foliation is developed in the quartz + biotite assemblages surrounding the garnet boudins and porphyroblasts. The S1/S2 intersection lineation plunges ca. 70° south, parallel to the predominant plunge of F2 folds. Locally, asymmetric pressure shadows with bundles of fibrolitic sillimanite have formed around the garnets. Assuming that sillimanite growth marks the metamorphic peak in the area, this must have occurred after D1 boudinage of the garnet bands. Some porphyroblasts of gedrite, anthophyllite, manganogrunerite, and muscovite in the footwall alteration zone and ore zone lack evidence of ductile deformation. Based on these observations, regional metamorphism under amphibolite-facies conditions is interpreted to have outlasted penetrative ductile deformation during D2. Gedrite porphyroblasts have undergone retrograde alteration to chlorite and serpentine, whereas biotite was chloritized (Ferrow and Ripa, 1991). Gahnite-hercynite porphyroblasts have undergone retrograde alteration to assemblages of diaspore + sphalerite + pyrrhotite and sericite + sphalerite + pyrrhotite. The F2 folds are cut by numerous brittle NE-trending faults and shears and lesser NW-trending faults. A predominant strike-slip movement with sinistral displacement is 314
EVOLUTION OF THE SEDIMENT SUCCESSION AND Zn-Pb-Ag AND IRON OXIDE DEPOSITS AT STOLLBERG, SWEDEN
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FIG. 4. Geologic cross section of the Baklängan deposit. Profile 4 on Figure 2. Interpreted premetamorphic protoliths are shown in parentheses. Small, unnamed mineralization above fault is interpreted as a tectonic repetition of the Baklängan deposit. Grid is Swedish National Grid RT90. Abbreviations: AB = albite, AMP = orthoamphibole, BT = biotite, CAL = calcite, CAM = calcic clinoamphibole, CPX = calcic clinopyroxene, FL = fluorite, FSP = feldspar, GED = gedrite, GHN = gahnite, GRT = garnet, HBL = hornblende, MC = microcline, QZ = quartz, ZO = zoisite. 0361-0128/98/000/000-00 $6.00
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Explanation Stratabound Zn-Pb-Ag sulfide and/or magnetite Grt > Ghn-Amp-Crd porphyroblasts
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Planar-stratified rhyolitic silt-sandstone Fe-rich chemical sediments Fsp>Qz-phyric rhyolitic pumice breccia-sandstone Matrix-supported mafic polymict conglomerate
inferred for the NE-trending faults based on exploration drilling, interpretation of geophysical data, and offsets of lithologic contacts (e.g., Fig. 4).
Massive polymict conglomerate (black: limestone clast / cross: mafic clast) Normal graded, rhyolitic, lithic breccia
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STAREN
Stratigraphy Allen et al. (1996) demonstrated that a facies-oriented approach can be applied to the study of metavolcanic and metasedimentary rocks in the Bergslagen region, even in amphibolite-facies areas like Stollberg. In these situations, any interpretation first requires careful consideration of the effects of postdepositional diagenesis, alteration, metamorphism, and deformation by detailed textural analysis. This approach was routinely applied in the current study with emphasis on the texturally best preserved rocks, and led to the recognition of several distinct volcanic and sedimentary facies (see Digital Supplement 1). Many lithologic contacts are sheared and, in these cases, the stratigraphic relations are difficult to determine. The effects of folding were routinely monitored by documentation of stratigraphic younging indicators and fold symmetry. Results suggest relatively consistent westward younging from Staren to the center of the Stollberg syncline, and predominant S symmetry of F2 folds in the entire area. Thus, the Staren marble and Stollberg ore deposits are both situated on the same limb of the Stollberg syncline (Fig. 2). A generalized stratigraphic column (Fig. 6) is divided into two successions that are separated by a large rhyolite intrusion. For brevity and emphasis of primary features, interpreted premetamorphic rock names are used in the following descriptions where possible. Post-Svecofennian rocks are not discussed. The Staren succession is described with a focus on the northern part of the study area, which shows the most complete succession, although similar rock types are observed throughout the area. Volcanic rocks are named using the terminology of McPhie et al. (1993) and Allen et al. (1996). In this paper, “porphyritic” is restricted to lavas and intrusions, whereas the term “phyric” is used for volcaniclastic rocks with
STOLLBERG
Mafic intrusion Coherent Fsp-porphyritic rhyolite
Crystal-rich sandstone and polymict conglomerates directly above Staren limestone
400
200
FIG. 6. Schematic lithostratigraphic column of the Stollberg-Staren area. Abbreviations are after Figure 4.
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scattered, larger phenocrysts in a finer-grained matrix. For the latter, sedimentary size terms (e.g., sandstone) are used to describe grain size.
Staren limestone: The stratigraphic contact between the strongly quartz + feldspar-phyric rhyolitic sandstone and the Staren limestone is obscured by a fault (Fig. 7), which has juxtaposed a fine-grained, rhyolitic siltstone between these two rock units. Preservation of primary textures is poor in the limestone due to metamorphic recrystallization. The resulting marble is coarse grained (ca. 2–3 mm) and massive. It is commonly very pure, which has allowed extraction of industrial quality marble (Fig. 2; Hjelmqvist, 1966). Minor skarn composed of epidote, actinolite, and magnesian hedenbergite is observed locally; the marble grades laterally into massive skarn at some locations (Ohlsson, 1979). In addition, the limestone contains several interbeds of massive rhyolitic siltstone and sandstone.
The Staren succession Strongly quartz + feldspar-phyric, massive rhyolitic sandstone: Rocks stratigraphically below the Staren limestone (eastern part of Fig. 3) consist of homogeneous, moderately sorted, massive, crystal-rich, rhyolitic sandstone. The unit contains abundant 1- to 4-mm angular-subangular quartz and feldspar crystals (ca. 20 vol % each) and crystal fragments in a massive, granular, sandy matrix. The unit is estimated to be over 200 m thick; however, the lower contact has not been observed (Fig. 7).
SS F15
SS S S F-1 4 F13
1472000 Lake Mellsjön
SS F-
16
W
E
1472500
Z- 0 m
v v
v v
v v
v
v
v v
v v v
v
v v
v v
v
v
QUATERNARY OVERBURDEN
v
POST-SVECOKARELIAN DOLERITE DYKE v
AMPHIBOLITE (Basalt or mafic intrusion)
v
METARHYOLITE (Coherent Fsp-porphyritic rhyolite / chilled margin of porphyry) v
METAMORPOHSED MAG-RICH CONGLOMERATE (Matrix-supported mafic polymict conglomerate)
v
METAMORPHOSED BRECCIA (Limestone-matrix breccia and interbedded rhyolite-limestone)
v v
Z-500m METAMORPHOSED BRECCIA-CONGLOMERATE (Massive polymict limestone-volcanic breccia-conglomerate, blue denotes abundant limestone clast) METARHYOLITE METARHYOLITE (Normal graded, rhyolitic, lithic breccias) (Strongly Qz>Fspar-phyric, massive rhyolitic sandstone) METARHYOLITE (Normal graded, rhyolitic siltsandstone +/- skarn
WAY-UP INDICATION BORE HOLE WITH LITHOGEOCHEMISTRY SAMPLE LOCATIONS
MARBLE (Limestone)
FAULT
METARHYOLITE (Massive, rhyolitic siltstone)
MODERATE-STRONG PERVASIVE AB-QZ-GED ALTERATION
Staren North - profile along 6676775.
FIG. 7. Geologic cross section of the northern Staren area (profile 3 on Fig. 2). Interpreted premetamorphic protoliths are given in brackets. Grid is Swedish National Grid RT90. Mineral abbreviations are after Figure 4. 0361-0128/98/000/000-00 $6.00
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The Staren limestone is directly overlain by a crystal-rich sandstone with ca. 40 vol % 1- to 2-mm feldspar crystals in a dark gray groundmass. The sandstone passes upward into a ca. 5-m-thick marble with a faint relict texture reminiscent of clast-supported conglomerate. This clastic marble is sharply overlain by a polymict conglomerate with rounded, clast-supported, weakly quartz + feldspar- and strongly feldspar-phyric felsic volcanic clasts >10 cm in diameter, in a chloritized matrix. Whereas some clasts are composed of rock types similar to those of the stratigraphically underlying units, the source of the feldspar-phyric clasts is unknown. Normal-graded, rhyolitic siltstone and crystal-rich sandstone with limestone interbeds: Conglomerates in the upper part of the Staren limestone unit are directly overlain by a succession of rhyolitic siltstone-sandstone with subordinate interbeds of limestone- and/or banded actinolite-rich skarn. The rhyolitic beds are dominated by fine-grained, planar-bedded, silty material, but locally include thin beds of massive sandstone rich in quartz + feldspar crystals and crystal shards. These coarser beds commonly have sharp bases and display normal grading. Rarely, feldspar-phyric clasts occur in the sandstones. Normal-graded, rhyolitic, lithic breccias: The normal-graded, siltstone-sandstone unit is sharply overlain by several normalgraded breccia units with 5 to 50 vol % monomict, subangular, 0.5- to 2-cm clasts of fine-grained rhyolite in a matrix containing 3 to 5 vol % 1- to 2-mm quartz + feldspar phenocrysts. Each graded breccia unit is 1 to 20 m thick. Normal grading in the upper meter of each unit is defined by upward decreases in clast abundance and size and in grain size of the matrix, and by transition to massive or coarsely banded sandstone, followed by laminated siltstone (Fig. 8A). Angular blocky silicic clasts occur near the base, whereas more ragged silicic clasts with diffuse boundaries are common in the center of each unit. The former are interpreted as lithic clasts whereas the latter are of uncertain origin; they may be altered and compacted pumice fragments. The uppermost graded breccia bed is notably polymict with the appearance of limestone clasts and has an interval of interbedded limestone and rhyolitic siltstone at the top, marking the transition to the overlying unit. Massive polymict limestone-volcanic breccia-conglomerate: A ca. 100-m-thick succession of poorly sorted, massive, generally polymict breccias and conglomerates overlies the normal-graded, rhyolitic lithic breccias. The breccia-conglomerate unit contains clasts of limestone and subangular to well-rounded clasts of fine-grained, weakly feldspar-phyric rhyolite ranging in size from 0.5 cm to over 10 cm (Fig. 8B). Subangular rhyolitic clasts are commonly finely laminated and locally have curviplanar margins. Less competent limestone clasts are strongly and plastically deformed such that their original shapes cannot be determined. Ragged, locally feldspar + quartz-phyric, laminated clasts with a matrix of biotite or calcic clinoamphibole occur in places. A repeated variation in the size and abundance of clasts appears to define at least three sedimentation cycles. These cycles each involve the transition from a basal, clast-rich (locally clast supported) breccia-conglomerate dominated by rhyolite ± limestone clasts to a matrix-supported breccia having clasts of the ragged type, to an increase of the latter at the expense 0361-0128/98/000/000-00 $6.00
of the former. Little systematic grading in grain size of the matrix was observed in these cycles, although irregular finingupward trends occur locally. Limestone-matrix breccia and interbedded rhyolite-limestone: The breccia-conglomerate unit is overlain by a carbonate unit characterized by large, commonly >10 cm angular clasts of aphyric siliceous rhyolite in a calcite matrix with subordinate ca. 1-mm grains of quartz and feldspar. A decrease in the abundance and size of the larger clasts, a decrease in abundance of matrix quartz and feldspar grains, and a gradation to interbedded rhyolitic siltstone and marble are observed stratigraphically upward. Matrix-supported, polymict mafic conglomerate: The limestone-matrix breccia is sharply overlain by a massive, matrixsupported conglomerate. This conglomerate contains ca. 25 vol % 0.5- to 1-cm clasts of aphyric mafic volcanic rock and subordinate felsic volcanic clasts in a weakly feldspar-phyric matrix. The conglomerate is altered to an assemblage of actinolite-epidote-magnetite and the mafic clasts are very magnetite rich. Clast-supported, polymict, rhyolite-mafic conglomerate: This conglomerate is the uppermost known clastic unit of the Staren succession. The unit is poorly sorted, clast supported, and polymict with subangular, weakly feldspar-phyric, pink and gray felsic volcanic clasts and rounded mafic volcanic clasts. The matrix is granular with sparse 2- to 3-mm feldspar crystals. The upper depositional boundary of the unit has not been observed as it is truncated by the base of the rhyolite porphyry, which is described below. This truncation is the reason the stratigraphy is here divided into the Staren and Stollberg successions. We emphasize, however, that this is largely an arbitrary division, essentially corresponding to the lower and upper part of one succession, and is not meant to imply that the Staren and Stollberg successions are two unrelated stratigraphic successions. The Stollberg succession Feldspar + quartz-phyric, rhyolitic pumice breccia-sandstone: The Staren succession is directly overlain by the ca. 500-m-thick stratigraphic footwall of the Stollberg ore host, which is strongly modified mineralogically, chemically, and texturally by pervasive hydrothermal alteration (Ripa, 1988, 1996). However, in places, texturally well preserved zones reveal two main facies comprising feldspar + quartz-phyric rhyolitic sandstone and monomict breccias with large ragged, micaceous, aphyric to feldspar-phyric fragments ranging in size from 10 cm (Fig. 8C). The fragments commonly display fiamme-like shapes in a siliceous matrix and are interpreted as former pumice clasts that have been altered, compacted, and metamorphosed; they are aligned parallel to S1 and are folded around S2. Feldspar crystals range from 1 to 10 vol % and up to 4 mm in size. Variations in grain size and clast abundance are common, yet systematic normal grading has only been recognized locally, possibly due to the strong overprint by alteration, metamorphism, and ductile deformation. Planar-bedded rhyolitic silt-sandstones with local magnetite-rich beds: The uppermost 20 to 30 m of the footwall to the Stollberg limestone mainly comprises intensely altered and metamorphosed banded rocks. These rocks contain biotite, almandine-spessartine garnet, calcic clinoamphibole,
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EVOLUTION OF THE SEDIMENT SUCCESSION AND Zn-Pb-Ag AND IRON OXIDE DEPOSITS AT STOLLBERG, SWEDEN
FIG. 8. A. 80-cm section through normal-graded bed in the normal-graded, rhyolitic lithic breccias unit, showing gradation from monomict breccia to finely laminated siltstone. B. Example of the massive polymict breccia-conglomerate unit, containing dark gray (1) and light gray (2) rhyolitic siliceous clasts and irregular limestone clasts (3). C. Monomict breccia with abundant irregular clasts, interpreted as phyllosilicate-altered and metamorphosed pumice fragments. Feldspar + quartz-phyric rhyolitic pumice breccia-sandstone unit. D. Interbedded laminated rhyolitic siltstone, magnetite, and grossular/spessartine-rich garnet. Planar-bedded rhyolitic silt-sandstones with local iron oxide beds unit. E. Stollberg limestone, here brown weathering and weakly dolomitized with scattered porphyroblasts of magnetite (not seen). F. Planar-bedded, rhyolitic, silt-sandstone unit; center of image shows a normal-graded unit, grading from feldspar + quartz-phyric sandstone to aphyric siltstone. G. Feldspar-porphyritic core facies of coherent feldspar-porphyritic rhyolite. H. Coarsely banded facies from marginal zone of coherent feldspar-porphyritic rhyolite; banding may represent relict flow-banding. 0361-0128/98/000/000-00 $6.00
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cummingtonite, manganogrunerite, anthophyllite, gahnitehercynite, andalusite, sillimanite, and, locally, knebelite (Fig. 4). Distal to zones of intense hydrothermal alteration (Fig. 2), the unit comprises K-rich, planar-bedded siltstones (ca. 1-cm bed thickness) interbedded with thin laminae of magnetite and grossular-spessartine garnet (Fig. 8D). These K-rich rocks pass gradationally upward into skarn and then gradationally into the Stollberg limestone. The Stollberg limestone: The Stollberg limestone commonly is strongly altered adjacent to mineralized zones, and no unambiguous primary textures were observed. Least-altered sections are in the southern part of the mining field as well as locally in the western, stratigraphically uppermost parts (Fig. 2). Here the unit comprises dolomitic and Mn-rich limestone (Fig. 8E) interbedded with up to ca. 50 vol % thin (10–100 cm) beds or boudins of massive to planar-bedded rhyolitic siltstone, or skarn. The massive and banded marble/skarn facies grade laterally into each other along strike within the ore host (Fig. 4). The style of mineralization in the Stollberg limestone is described in more detail in Ripa (1988, 1994, 1996) and Beetsma (1992). Planar-bedded, rhyolitic silt-sandstone: The Stollberg limestone and ore zone are directly overlain by ca. 0.5 to 1 m of amphibole-rich skarns, which pass upward into planar-bedded rhyolitic siltstone with local normal-graded sandstonesiltstone beds (Fig. 8F). This siltstone-sandstone succession continues to the core of the Stollberg syncline and is at least 700 m thick. The graded beds display Bouma sequences A to D and, locally, E (Bouma, 1962), characteristic of low-density turbidites. Lower parts of the beds typically are coarsegrained, moderately to strongly quartz- and/or feldspar-phyric sandstone. The bed bases are locally erosional and may display load casts and cross-bedding. Tops are commonly planarlaminated, rhyolitic siltstone, but, locally, cm-thin, mica-rich beds define the tops. Small-scale convolute folding and truncation of the uppermost, finely laminated siltstone is observed directly below the base of some sandstone beds (Fig. 8F). Matrix-supported, biotite-altered pumice fragments similar to those within the feldspar + quartz-phyric rhyolitic pumice breccia-sandstone occur locally in the middle of the graded sandstone beds. In places, >1-m-thick beds of matrix-supported lithic breccia and monomict pumice breccia occur within the siltstonesandstone succession. The lithic breccias contain ca. 10 vol % 0.5-cm angular rhyolitic lithic clasts in a 5 to 10 vol % 1- to 2mm quartz-phyric sandy matrix. Coherent Rocks Massive, medium- to coarse-grained amphibolites Whereas Ripa (1988) interpreted the Stollberg amphibolite (Fig. 2) as extrusive, no evidence for such an origin was observed in the current study. The unit comprises massive, medium-grained amphibolite with little textural variation from core to contacts. The upper and lower contacts are commonly highly strained biotite/phlogopite schists, likely resulting from alteration and shearing. Consequently, the nature of the primary contacts is unknown. Overprint by preto syn-S1 hydrothermal alteration and tectonic fabrics suggest a pre- to syn-D1 emplacement. The unit is considered to be 0361-0128/98/000/000-00 $6.00
most likely a sill; a similar origin is postulated for the major amphibolite west of the Staren limestone (Fig. 2). At Gränsgruvan, on the western limb of the Stollberg syncline, fragmental textures reminiscent of hyaloclastite have been observed in the amphibolite. Coherent feldspar-porphyritic rhyolite A large body of feldspar-porphyritic rhyolite occurs between the Staren and Stollberg limestones in the northern part of the area (Figs. 2, 8G). The stratigraphic lower contact has a wide, fine-grained, chilled margin with local flowbanded (Fig. 8H) and relict spherulitic and perlitic textures. A progressive increase in phenocryst size and groundmass grain size is observed toward the interior of the unit, where feldspars are up to 4 mm in diameter. The rhyolite is overprinted by albite + quartz + gedrite-rich alteration, similar to that observed in surrounding volcaniclastic rocks. The stratigraphic upper contact has not been observed. Interpretation of Depositional Processes and Environments Due to the paucity of outcrops in the Staren area, observations of bed forms and facies associations are restricted mainly to exploration drill cores. The abundance of angular to subangular quartz and feldspar crystals and crystal fragments in the strongly quartz + feldspar-phyric rhyolitic sandstone suggests a juvenile pyroclastic origin and minimal transport prior to deposition. However, the very crystal rich character and relatively good sorting suggest that the pyroclastic debris was significantly reworked. The unit may represent a shallow, subaqueous, rhyolitic pyroclastic deposit from which the finer-grained ash fraction has been winnowed out by wave action. Nevertheless, the presence of angular crystals and lack of stratification make this interpretation unlikely. Alternatively, the unit represents a strongly fines- and pumice-depleted, crystal-rich, pyroclastic flow deposit (cf. Walker, 1972; McPhie et al., 1993). Billström et al. (1985) determined carbon and oxygen isotopes in one sample of Staren limestone. The δ13C value of –0.1‰ overlaps those of marine stromatolitic limestones in Bergslagen, yet the δ18O value of 10.9‰ is distinctly lower than most Proterozoic marine carbonates. Based on their entire data set for 40 samples, Billström et al. (1985) concluded that most limestones in Bergslagen, including the Staren limestone, initially had a normal marine signature, but that δ18O was commonly shifted to lower values by alteration. Similar results were reported by Allen et al. (2003), who obtained similar δ13C and δ18O values for regional dolomitized limestones with relict stromatolitic textures. Based on similarity in stratigraphic setting and stable isotopes to variably altered stromatolitic limestone elsewhere in Bergslagen, the Staren limestone likely formed by stromatolite growth in the photic zone, forming a small limestone platform or reef (cf. Allen et al., 2003). Purity of the limestone over a strike length of over 7 km and restriction of detrital material to distinct felsic interbeds suggest an environment starved of volcaniclastic material with episodic deposition of felsic interbeds. These events may correspond to discrete distal volcanic eruptions or to minor regressions and progradation of reworked volcaniclastic sediments onto the limestone platform.
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The sandstone-conglomerate sequence that overlies the limestone is interpreted to reflect a period of faulting and erosion. Occurrence of rounded clasts in the conglomerates having compositions unlike those in the directly underlying units suggests that uplift occurred elsewhere, laterally along strike, and that debris was transported from the uplifted block(s) into the basin. The normal-graded, rhyolitic siltstone and crystal-rich sandstone with limestone interbeds and the normal-graded, rhyolitic, lithic breccias are interpreted to have formed by a combination of suspension settling and subaqueous flow sedimentation below wave base. These units record transgression and deepening of the depositional basin. A stratigraphically upward increase in coarse-grained, normal-graded mass flow deposits and a decrease of suspensionsettled, fine-grained material stratigraphically upward suggest that these units are increasingly proximal mass flow deposits in a prograding sediment fan. The increase in degree of rounding upward in the conglomerate succession and change to more massive, poorly sorted beds, without finer-grained silty sections, are attributed to the transition to a high-energy, probably shallow-marine environment. The polymict nature of the clast assemblage and variable rounding, including rounded rhyolitic, mafic, and limestone clasts, occurrence of subangular, laminated clasts reminiscent of flow-banded rhyolite, and wispy, pumice-like clasts, suggest a mixed provenance, comprising both epiclastic material of variable composition and relatively juvenile, rhyolitic volcaniclastic material. The associations and succession of facies described above may be attributed to the progradation of a sedimentation system, such as a sediment fan or fan delta, from an active fault scarp into a rapidly subsiding basin. Paucity of bed-form observations prevents detailed characterization of this depositional system. However, it is conceivable that the normalgraded, rhyolitic siltstone and crystal-rich sandstone with limestone interbeds are bottomset turbidites, formed near the lower flanks of a fan, whereas the massive, polymict breccia-conglomerates are high-energy deposits, formed more proximal to or within channels of the fan, presumably in a shallow-water or even subaerial environment. Given the similarity between the massive polymict breccia-conglomerates and the conglomerates directly overlying the Staren limestone, it is likely that they were deposited by similar processes in a similar environment. As such, the latter probably were deposited during the earliest stage of basin subsidence, prior to subsidence to below wave base. The top of the Staren succession, where intersected by drill holes, is defined by the lower contact of the coherent feldspar porphyritic rhyolite. The upper contact of the rhyolite and transition to the Stollberg succession has not been observed directly. The homogeneous composition, abundance of monomict feldspar-phyric pumice clasts, poor stratification, and thickness of the feldspar + quartz-phyric rhyolitic pumice breccia-sandstone suggest that this unit represents a rapidly emplaced, large-volume, juvenile pyroclastic deposit (cf. Allen et al., 1996). The unit is interpreted as having been deposited from several syneruptive, pumiceous mass flows. Few observations of normal grading suggest that at least part of the unit was deposited subaqueously, yet the generally massive character, in addition to the strong overprint of alteration and deformation, precludes an unambiguous interpretation of 0361-0128/98/000/000-00 $6.00
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depositional environment for most of the unit. Gradation upward to the planar-bedded, rhyolitic silt-sandstones with local iron oxide beds does, however, indicate a subaqueous environment immediately following, and possibly during, deposition of the pyroclastic debris. The planar-bedded, siltstonesandstone facies is attributed to suspension settling of fine-grained rhyolitic material below the effective storm wave base. For wave-exposed oceanic margins, effective storm wave base is at most 150 m (Immenhauser, 2009). However, the continental rift (Oen et al., 1986) or submerged continental back-arc (Allen et al., 1996) settings proposed for Bergslagen suggest that estimates for smaller epeiric-neritic basins or shallow siliciclastic basins are probably more appropriate. Effective storm wave base within these environments is, on average, less than 40 m (Immenhauser, 2009). The interbeds of garnetiferous magnetite are interpreted as metamorphosed Fe-Mn–rich chemical sediments that episodically co-settled with the fine-grained rhyolitic sediment. Billström et al. (1985) reported δ18O and δ13C values ranging from 9.5 to 11.2‰ and –2.3 to –0.8‰, respectively, for four samples of the Stollberg limestone. However, due to pervasive alteration, lack of primary textures, and geochemical data (presented below), it is uncertain whether the Stollberg limestone formed by stromatolite growth in a shallow subaqueous environment (less than 200 m water depth), or from accumulation of Ca-Mn ± Fe carbonates on the sea floor in a deeper environment. Bed forms in the overlying planar-bedded, rhyolitic siltsandstone are consistent with deposition from a combination of suspension settling and low-density turbidity currents below wave base (cf. Allen et al., 1996). The presence of angularsubangular feldspar crystals and local pumice fragments suggests that these are essentially syneruptive but distal resedimented volcaniclastic deposits (cf. McPhie et al., 1993). Alternatively, as suggested by strong lithogeochemical similarity to the footwall rhyolitic rocks, the hanging-wall rhyolitic siltstone-sandstone unit may have been derived by reworking of unlithified pyroclastic deposits in the footwall succession, where originally exposed along strike or at basin margins. Except for the subordinate coarser-grained breccia beds, the Stollberg hanging-wall succession displays a general fining upward, culminating with the pelitic sequence in the core of the Stollberg syncline. The nature of the hanging-wall succession is thus indicative of basin subsidence, transgression, and burial of the Stollberg limestone by low-density, rhyolitic mass flow deposits and suspension-settled, fine-grained rhyolitic material. Lithogeochemistry (presented below) of the rhyolitic volcaniclastic rocks of the Stollberg succession suggests they may be comagmatic with the coherent feldspar porphyritic rhyolite. The coherent rhyolite most likely is a high-level intrusion that was emplaced into the juvenile volcaniclastic pile shortly after or during the pyroclastic eruptions, and could represent a magma that is the same as or compositionally similar to that which fed the eruptions. However, as the upper contact of the rhyolite has not been observed, the exact timing and mode of its emplacement are uncertain. The rhyolite is overprinted by gedrite-rich alteration similar to that in the surrounding volcaniclastic rocks. Consequently, the rhyolite is either a lava dome or a shallow intrusion that was emplaced prior to hydrothermal alteration of the footwall rocks at Stollberg.
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Ore Deposits Most sulfide mineralization at Stollberg occurs as disseminations in the Stollberg limestone and within the upper part of the footwall alteration envelope. The mineralization is most commonly low grade and dominated by sphalerite, pyrrhotite, chalcopyrite, and arsenopyrite. It grades irregularly into semimassive and locally into massive sulfide bodies, which are considerably richer in galena (Table 1). Ripa (1996) noted a broad-scale zonation in sulfide mineralogy at Stollberg. Chalcopyrite and pyrrhotite are more abundant in the altered footwall volcanic rocks than in the Stollberg limestone, which, in contrast, carries more sphalerite and galena. All of the Stollberg ore deposits except Brusgruvan (Fig. 2) are characterized by greater abundance of pyrrhotite over pyrite as well as locally high contents of
arsenic, hosted by arsenopyrite and minor loellingite. The style of sulfide mineralization ranges from impregnations in limestone and massive magnetite, impregnations with abundant magnetite in limestone, and intergrown sphalerite-calcic clinopyroxene or amphibole (e.g., Baklängan) to thin veinlets and “ball-ores” with sharp tectonic boundaries. The last, commonly developed at lithologic contacts, is interpreted as veins of syntectonic, remobilized mineralization. At Stollgruvan (Fig. 2), sphalerite accompanied by pyrrhotite and tremolite forms irregular veins that replace the Stollberg limestone. The style of magnetite mineralization varies greatly, even in single deposits. At all deposits, excluding Brusgruvan, magnetite occurs disseminated with pyrrhotite, sphalerite, galena, knebelite (Mn fayalite), and local garnet in serpentine-spotted marble. The serpentinized marble grades into stratabound massive magnetite orebodies, such as at Stollgruvan,
TABLE 1. Characteristics of Selected Stollberg-Svartberg Deposits* Brusgruvan
Stollgruvan
Dammberget
Baklängan
KogruvanMyggruvan
Lustigkulla-Marnäs
Zn (wt %) Pb (wt %) Ag (ppm) Zn/(Zn + Pb) Fe (wt %) Mn (wt %) As (ppm)
3.21 15.61 3201 0.171
