Geol. Mag. 152 (3), 2015, pp. 537–556. c Cambridge University Press 2014 537 doi:10.1017/S001675681400048X U–Pb geochronology of the syn-orogenic Knaben molybdenum deposits, Sveconorwegian Orogen, Norway ∗ BERNARD BINGEN †, FERNANDO CORFU‡§, HOLLY J. STEIN§¶ & MARTIN J. WHITEHOUSEࢱ ∗ Geological Survey of Norway, 7491 Trondheim, Norway ‡Department of Geosciences, University of Oslo, 0316 Oslo, Norway §Centre for Earth Evolution and Dynamics, University of Oslo, 0316 Oslo, Norway ¶AIRIE Program, Colorado State University, Fort Collins, CO 80523-1482, USA ࢱSwedish Museum of Natural History, 104 05 Stockholm, Sweden (Received 29 March 2014; accepted 4 August 2014; first published online 11 November 2014) Abstract – Paired isotope dilution – thermal ionization mass spectrometry (ID-TIMS) and secondary ion mass spectrometry (SIMS) zircon U–Pb data elucidate geochronological relations in the historically important Knaben molybdenum mining district, Sveconorwegian Orogen, south Norway. This poly- phase district provided c. 8.5 Mt of ore with a grade of 0.2 %. It consists of mineralized quartz veins, silica-rich gneiss, pegmatites and aplites associated with a heterogeneous, locally sulphide-bearing, amphibolites facies gneiss called Knaben Gneiss, and hosted in a regional-scale monotonous, com- monly weakly foliated, granitic gneiss. An augen gneiss at the Knaben I deposit yields a 1257 ± 6Ma magmatic zircon age, dating the pre-Sveconorwegian protolith of the Knaben Gneiss. Mineralized and non-mineralized granitic gneiss samples at the Knaben II and Kvina deposits contain some 1488– 1164 Ma inherited zircon and yield consistent intrusion ages of 1032 ± 4, 1034 ± 6 and 1036 ± 6 Ma. This age links magmatism in the district to the regional 1050–1020 Ma Sirdal I-type granite suite, corresponding to voluminous crustal melting during the Sveconorwegian orogeny. A high-U, low-Th/U zircon rim is present in all samples. It defines several age clusters between 1039 ± 6 and 1009 ± 7 Ma, peaking at c. 1016 Ma and overlapping with a monazite age of 1013 ± 5 Ma. The rim records protracted hydrothermal activity, which started during the main magmatic event and outlasted it. This process was coeval with regional high-grade Sveconorwegian metamorphism. Molybdenum deposition probably started during this event when silica-rich mineralizing fluids or hydrous magmas were released from granite magma batches. An analogy between the Knaben district and shallow, short-lived porphyry Mo deposits is inappropriate. Keywords: Mesoproterozoic, Sveconorwegian Orogeny, molybdenum deposit, zircon, U–Pb, hydro- thermal zircon, granite magmatism. 1. Introduction lated to Sveconorwegian orogenic processes. However, Large-scale redistribution of mass takes place in the the metallogeny and geochronology of this resource are continental crust during orogeny, helped by tectonic unexplored. transport, aqueous fluid flow and silicate melt migration In this paper, we report new zircon U–Pb geochrono- (Jamieson et al. 2007). This redistribution also leads to logical data on granitoid rocks associated with miner- redistribution of economically important metals (e.g. alization in the three main deposits of the Knaben dis- Stein, 2006). The south-westernmost lithotectonic do- trict. The challenge is that magmatic, metamorphic and main of the Sveconorwegian Orogen in south Nor- metasomatic events have closely followed each other. way (Fig. 1), called Rogaland-Vest Agder, is a well- To resolve these events, we choose to combine the mi- defined molybdenum province (Bugge, 1963; Stein & croanalytical strength of the secondary ion mass spec- Bingen, 2002; Bingen et al. 2006; Sandstad, 2012). The trometry (SIMS) and the precision of isotope dilution geochemical anomaly is mainly expressed by common – thermal ionization mass spectrometry (ID-TIMS) small syn-orogenic vein-type Mo deposits, mined at the methods for selected samples. The objectives are to beginning of the 20th century. The Knaben district is the establish the time of magmatic and metasomatic events most important among them and, mined until 1973, is in the district, to relate these events to geological events one of the historically important Mo mining districts in recorded at a regional scale and to provide a first-order Europe. A total of c. 8.5 Mt of ore was extracted with an interpretation of the genesis of this district. Molybden- average grade of c. 0.2 % (Bugge, 1963). There is little ite Re–Os geochronology on the Knaben district will doubt that the genesis of the Knaben district was re- be presented elsewhere. †Author for correspondance: [email protected] Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.126, on 25 Sep 2021 at 04:06:18, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S001675681400048X 538 B. BINGEN AND OTHERS a Terrane boundary Other fault or shear zone Opx-in isograd BBergenergen FFlålå + Opx-in Oslo rift + + Caledonian nappes Hardangervidda Telemark HHønefossønefoss 980-920 Ma magmatic suites 66000 supracrustals 66000 1050-1020 Ma Sæsvatn-Valldal OOsloslo Sirdal suite Suldal supracrustals 1050 Ma Feda suite Telemark Langvatn-Kobbernuten 1170-1140 Ma bimodal Dalen magmatic suites BBykleykle 1220-1180 Ma bimodal magmatic suites 1280-1250 Ma bimodal magmatic suites 110 Knaben Undifferentiated Opx-in Svofjell Setesdalen Ivesdalen + Opx-in Ørsdalen + EEgersundgersund Fennefoss AArendalrendal Flottorp-Vårdal Gursli Sira b 0 5 Kvinesdal 0 0 KKristiansandristiansand 9 5588 Telemarkia Rogaland-Vest Agder MMandalandal 50 km Bamble-Kongsberg Molybdenum deposits Idefjorden Localities and units Eastern Segment Figure 1. (a) Sketch map of the Sveconorwegian Orogen in south Norway, following Koistinen et al. (2001), showing the distribution of Mesoproterozoic magmatic rocks and localities described in the text. The Sirdal suite is from Slagstad et al. (2013). Molybdenum deposits are marked in red. (b) Inset map with the distribution of the main lithotectonic domains in the Sveconorwegian Orogen. 2. Geological context The Telemarkia Terrane was formed during a short- lived accretionary event between 1520 and 1480 Ma, 2.a. The Sveconorwegian Orogen called Telemarkian (Bingen et al. 2008a; Roberts et al. The c. 600 km wide Sveconorwegian orogenic belt 2013). Voluminousplutonic and volcanic suites formed consists of late Palaeoproterozoic – Mesoproterozoic in this time interval probably in arc and back-arc crustal domains assembled at the margin of (proto-) geotectonic settings. Their isotopic signatures are com- Baltica during the Sveconorwegian–Grenvillian Oro- paratively juvenile, though they require contributions geny (Falkum, 1985; Andersen, Andresen & Sylvester, of older, Palaeoproterozoic crust (Andersen, Andresen 2001; Andersson, Möller & Johansson, 2002; Åhäll & Sylvester, 2001; Bolle, Demaiffe & Duchesne, 2003; & Connelly, 2008; Bingen, Nordgulen & Viola, 2008; Roberts et al. 2013). Several younger Mesoprotero- Bogdanova et al. 2008; Cawood et al. 2010; Roberts zoic magmatic suites are known. These include a small et al. 2013). The orogen can be divided into four volume of mafic magmatism at 1347 ± 4 Ma (Corfu & main lithotectonic domains, separated by approxim- Laajoki, 2008) and three significant events of bimodal ately orogen-parallel Sveconorwegian shear zones. magmatism, associated with sediments, peaking at These are, from east to west, the parautochthon- c. 1285–1250 Ma, 1220–1180 Ma and 1170–1145 Ma, ous Eastern Segment and three transported domains and variably interpreted in back-arc, continental rift commonly called “terranes”: the Idefjorden, Bamble- or basin-and-range setting (Heaman & Smalley, 1994; Kongsberg and Telemarkia terranes (Fig. 1b). The tim- Bingen et al. 2002, 2003; Brewer et al. 2002, 2004; ing of the main crust-forming magmatic events de- Laajoki, Corfu & Andersen, 2002; Andersen, Griffin creased towards the west, from c. 1690 Ma in the East- & Sylvester, 2007; Pedersen et al. 2009; Roberts et al. ern Segment to c. 1500 Ma in the Telemarkia Terrane. 2011). The westernmost Telemarkia Terrane is further divided The main Sveconorwegian orogenic phase, called into several crustal sectors with distinct lithological and the Agder phase, started at 1050 Ma (Bingen, Nordgu- metamorphic properties: the Telemark, Rogaland-Vest len & Viola, 2008). In the Rogaland-Vest Agder sector, Agder, Suldal and Hardangervidda sectors (Fig. 1a). the intrusion of syn-orogenic, K-feldspar megacrystic Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.126, on 25 Sep 2021 at 04:06:18, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S001675681400048X Knaben Mo deposits, Norway 539 Opx 930-920 Ma + Rogaland anorthosite complex 930-920 Ma Osm charnockite pluton + Ørsdalen Knaben 970-930 Ma Pig Sirdal Hbl-Bt granite pluton + 1050-1020 Ma granitic gneiss Cpx Evje Sirdal suite + 1050 Ma augen gneiss Flottorp 0' Feda suite Sira 58 30 Vårdal Granitic gneiss undifferentiated Egersund Fennefoss Banded gneiss Gursli Rafoss + undifferentiated gneiss N Shear zone Kvinesdal Isograd Pig: pigeonite-in 25 km Osm: osumilite-in Feda Opx: Opx-in in felsic gneiss Cpx: Cpx-in in Hbl augen gneiss Mo deposits Other localities Sveconorwegian Mandal 580' 00 Orogen 70 80 Figure 2. Geological map of Rogaland-Vest Agder showing the location of Mo deposits and regional isograds. The map follows Falkum (1982) and the Sirdal suite follows Slagstad et