The Tectonostratigraphic Setting of Stratabound Sulphide Deposits in the southern Trondheim Region, Central Norwegian Caledonides

ODD NILSEN

Nilsen, O. 1988: The tectonostratigraphic setting 01 stratabound sulphide deposits in the southern Trondheim region, Central Norwegian Caledonides. Nor. geol. under. Buff. 412. 55-66.

The large number 01 mainly massive, polyrnetallic, stratabound sulphide deposits within the sout­ hern Trondheim region are spatially conlined to the different nappe units 01the central Scandinavi­ an Caledonides. The rocks 01 the parautochthon and Lower Allochthon comprise metasandstones and arkoses which are virtually barren 01 any sulphide mineralizations, except lor a lew showings 01 "sandstone lead' disseminat ions. The middle and lowermost parts 01 the Upper Allochthon are dominated by sediment-ho sted chalcopyr itelpyrrhotite assemblage s within calcareous mixed pelitic and psammitic rocks with minor malic metavolcanic and plutonic intercalations. The structurally higher Gula and St0renlMerAker units 01 the Trondheim Nappe Complex 01 the Upper Allochthon contain chiefly pyrite-dominated assemblages in malic and mixed malicllels ic volcanogenic settings and have highly variable Cu, Zn and Pb contents. The stratabound sulphides 01 the Trondheim region were deposited during successive stages and within restricted sedimentary and volcano­ sedimentary environment s related to the development 01 the lapetus Ocean. Deposition started in basin sediments during Late Protero zoic rilt ing, but was mainly the result 01 early subduction and the development 01 an active island-arc magmatism in the Early Ordovician. The structural and strat igraphical sequences were subsequently affected by Caledonian lolding and metamorphism prior to and during eastward transport as composite nappe units in Silurian times. The post­ depositional tectoni c and metamorphic events have affected the different ores and their wallrocks to various degree as revealed by apparent tectonic control and the internal metamorphic tabric 01 the deposits.

Odd Nilsen, Institutt for Geotoql, Universitetet i Oslo. P.OBox 1047. Blindern, 0316 Oslo 3. Norway.

Introduction Recent research has led to significant advan­ The Trondheim region const itutes one of the ces in our general understand ing of the structu­ most important mining districts in the Scandina­ ral geometry and the tectonostratigraphical vian Caledonides . For more than 350 years the relationships between the different major tecto­ district has served as a main producer of nic units of the Scandinavian Caledonides. In coppe r and pyrite, and the region can still be particular, within the context of IGCP Project reagarded as one of the most important copper no. 60 (CCSS) the palaeogeographic and pa­ districts in Norway. laeotectonic environments of deposition of the The geology of the southern part of the massive sulphide deposits of the orogen have region covered by 1:250.000 map-sheet "R0­ been reexamined and redefined (Zachrisson ros ' has recently been comp iled (Nilsen & 1977,1980,1986, Bjerlykke et al. 1980, Grenne Wolff, in press), and the present account covers et al. 1980, Yokes 1980, Stephens et al. 1984, the geolog ical setting of the stratabound sulphi­ Grenne & Lagerblad 1985). However, in compa­ de mineral deposits of this map area, with rison with similar deposits in the Caledonides special reference to the tectonostratigraphical of Sweden and the western and north-sentral framework of the region. Earlier reviews on the parts of the Norwegian Caledonides, limited stratabound sulphide depos its of the region attention has been paid over the last decade have been given by Helland (1873), Holmsen to the setting and nature of these deposits in (1919), Foslie (1925, 1926), Wolff (1967), Yokes the south-sentral part of the Trondheim region (1976), Rui (1977), Bugge (1978) and Nilsen (Fig. 1). (1978). 56 Odd Nilsen NGU- BULL.4 12,19 88

TECTONOSTRATIGRAPHY OF THE SOUTHERN TRONDHEIM REGION

UPPER ALLOCHTHON

Storen Meroker Nappe Nappe

Gula Gula Nappe Nappe

BlOho Essonchjo·0yfjell Nappe Nappe

MI DDLE ALLOCHTHON

Seetrc Remsklepp Risberget Nappe Nap pe

Amotsdal Kvitvola Nap pe Noppe

LOW ER AllOC HTHO N Osen­ Roe Nappe Fig. 1. Tectonostrat igraphic map of the southern Trondheim Region, centra l Norway (on facing page). In the key map (above) the Caledonian auocntnon is stippled. In the legend (this page), nappe names to the left are for the western part of the region, and those to the right for eastern areas. PARAUTOCHTHON/ AUTOCHTHON

...... Precombrion ...... ····· crystalline ...... The southern Trond heim region comprises ...... CB...... basement a sequence of nappes emplaced above an autochthonous or parautochthonous Precam­ • - Sulphide ore deposit brian crystalline basement with a thin cover of Vendian to Lower Palaeozoic sediments. The allochthonous complexes are derived from four groups of allochthonous comp lexes: the av wide variety of environments and include Lower, Middle, Upper and Uppermost Alloch­ elements of the Precambrian basement. Syn­ thons, the last-mentioned not being present theses of the tectonostratigraphic development in the area under discussion. A correlation of the Trondheim region Caledonides have of the different formal and informal nappe been given by Roberts (1978), Wolff (1979), units of the region in the framework of this Wolff & Roberts (1980) and Roberts & Wolff tectonostratigraphy is presented in Table 1, (1981 ), and a descr iption of the central-sout­ and the setting of the strata bound mineralizati­ hern part of the Scandinavian Caledonides has ons will be described accord ing to these sub­ been presented by Gee et al. (1985b). Ro­ divisions (Table 1, Fig. 1). berts & Wolff (1981) distinguished three major allochthonous complexes within the Trondheim region; the Basal, Lower and Upper AIIoch­ Tectonostratigraphyand thons. More recently, a compilation of the tecto­ mineralizations nostratigraphy of the Scandinavian Caledoni­ Lower Allochthon des has been published in a 1:2 M map by The Lower Allochthon is developed in the Gee et al. (1985a). They distinguish between eastern and southern part of the region (Fig. NGU- BULL. 412, 1988 The Tectonostratigraphic Setting ofStratabound Sulphide Deposits 57 58 OddNilsen NGU- 8U LL. 412 ,1988

TEGONOSTRATIGRAPH Y WESTERN SUCCESSION EASTERN SUCC ESSION Robe , ', & Wolf! Gee et 01. (19811 (1985)

Hovin G p . ST0REN MERAKER Sulcimo G p . UPPER NAPPE Stc ren G p. AL LOC HT HO N NAPPE Fund sjo O p .

Undol Fm. G ULA (Trond heim G ULA Asli Fm. G ulo Gp No ppe Comp lex) G ulo G p . NAPPE UPPER NAPPE Singse s f m. Sing so s Fm. A LLO CHTHO N ESSANDSJ0- Rosjo Fm, BLAH 0 0 YFJEll Aursund G p. NAPPE NAPPE LOWER Stug USjD Fm.

S,URA- AL LO CH THO N Tynset G p . RISBERGET REMSKLEPP N APPE NAPPE A ugen gne iu M IDDLE ""nit ALLOCHT HON AMOTSDAL KVITVOL A NAPP E BASAL NAPPE ALLOCHTHON LOWER OSEN-R0A A LLOCHTH O N NAP PE

PARAUTOCH THO N

AUTOC HTHO N

PRECAMBR IAN CRYSTALLINEBASEM EN T

Table 1. Tectonostratigraphy of the southern Trondheim region. The main tectonic units according to Roberts & Wolff (1981) and Gee et al. (1985a, b) are shown in the centre of the diagram. They are flanked by their corresponding sub-units of the western and eastern parts of the region which are mentioned in the text. The nomenclature of major units adopted in this account is that of Gee et al. (1985a, b).

1), and includes part of the Oseri-Re« Nappe 1981). No economic sandstone lead deposits Camp/ex (Nystuen 1981). It comprises Late are known within the region, but two minor Proterozoic sandsto nes, arkoses and quart­ pyritic lead/zinc pro spects in Tufsingdalen are zites of the so-ca lled "Sparagmite basin". In the confin ed to an intercalated black phyllite unit. western part of the region major repetitions of parautochthonous basement gneisses appe­ ar as slices within the Lower and Middle Alloch­ Middle AI/ochthon thons which, in this district, were more strongly The Middle Allochthon compr ises a comp lex recumbently folded and refolded during the of several separate nappe units above the Caledonian orogeny than correlatives in the Lower Allochthon. In its lower part the Kvitvole eastern and southern areas. and Amotsda/ Nappes consist of barren arko­ The Lower Allochthon is virtually barren of ses and feldspathic quartzites, generally devel­ any sulphide mineralization of economic signifi­ ped as flagstones. A conspicuous zone of ance. However , several low-grade lead minera­ mylonites and phyllonites usually separates the lizations have recently been discove red within Kvitvola and Amotsdal Nappes from the overly­ the autochthonous and parautoc hthonous ing Remsklepp Nappe. sandst one/black shale sequences close to the The Remsk/epp Nappe (Wolff 1979) compri­ contact to the Precambrian basement granites ses an augen-gn eiss unit which has been in the Femund area in the southeastern part corr elated with the Tannas Augen Gneiss of the region (Krause & Bakke 1986). There. Napp e in southern Jarntlano in Sweden (Ask­ galena occurs as disseminations within the lund 1960, Stromberq 1962). The major part Late Proterozo ic to Cambrian sandstones, and of the Remsklepp Nappe consists of a series apparently represents the class of "sandstone of strong ly imbricated, mostly flat-lying sheets lead' ores of the Laisvall-type which extent of feldspathic quartzites with intercalations of along the Caledonian fron t zone through quartz phyllites, crysta lline limestones, mafic Sweden and Norway (Bjerlykke & Sangster metavolcanites and black phyllites. The entire NGU-BULL.412,1988 The Tectonostratigraphic Setting ofStratabound Sulphide Deposits 59

unit will here be referred to informally as the no apparent spatial relationship with the thin Tynset group. This unit is especially well deve­ horizons of mafic metavolcanites and metado­ loped in the Grimsdalen-A\\Idal region (Heim lerites in the area. A weak dissemination of 1968, 1972, Kleine-Hering 1969) and in the chalcopyrite is commonly present within the Tynset-Hummelfjell district (the "Hummelfjell adjacent feldspathic quartzites. The affiliation formation' of Rui (1972, 1981a, Rui & Bakke of the ores with adjacent stratabound dlssernl­ 1975)). nations of magnetite and hematite within the The quartzitic sandstone sequence in the lithic sandstones of the Tynset group has not Hummelfjell and Tron area is intruded by a yet been investigated, but a spatial relations­ dolerite dyke swarm of tholeiitic composition hip with Upper Proterozoic red-beds cannot which is believed to mark the first episode of be excluded. Hence, cupriferous red-bed con­ ritting associated with the opening of the tape­ glomerates are reported from the parautoch­ tus Ocean. A correlation with the Ottfjall dlaba­ thon in the Brydal area (Krause & Bakke ses of the Sarv Nappe in Sweden (Stromberg 1986). Hematite and magnetite disseminations 1962), which were intruded around 650 to 700 occur sporadically as distinct strata within the m.y. ago (ctaeseon 1976), seems evident, as Oseri-Bea Nappe as well as within the Tyn­ suggested earlier by Tornebohrn (1896), Ask­ set group (Kleine-Hering 1969) and the Sarv lund (1961)and Point et al. (1976). A correlation Nappe in Sweden (Stromberg 1962). of the Tynset group of this area with the In the S01na deposit in Alvdal, chalcopyrite­ Tossastiallet Group of the Sarv Nappe (Kumpu­ pyrrhotite disseminations occur in strongly lainen 1980) appears more likely than with sheared metadolerites (epidote-amphibolites) units of the Essandsj0 Nappe as indicated by within quartzites of the Tynset group. The Guezou (1981) and Roberts & Wolff (1981) or deposit reveals a geological setting similar to with parts of the high-grade Seve Nappes and that of the solitary tunderren deposit in the equivalent units of the Upper Allochthon by Sarv Nappe in Jarnttand (Vogt 1887, Tegen­ Gee et al. (1985a), Stephens & Gee (1985), gren 1924). A weak pyrrhotite dissemination Stephens (1986)and Zachrisson (1986). is also present in prospects within a black At Oppdal, in the western part of the region, schist horizon in the uppermost part of the dolerite-intruded meta-arenites (the Seetrs Remsklepp Nappe. Nappe). tectonically overlying the Risberget No sulphide mineralizations have been recor­ Augen Gneiss Unit (Krill 19~O}i·are likely corre­ ded in the western (sestra) equivalent of the latives of the RemskleppNappe complex. The Remsklepp Nappe. metadolerite dykes here have yielded a Rb/Sr isochron age of 745±37 m.y. (Krill 1980). Based on lithology and tectonostratigraphic position of the Sretra, Sarv and Rems~lep? Upper Allochthon units and the chemistry and geochronology of The Essandsj0-0yfjell and Blah0 Nappe the associated dolerites, a correlation of these The Essandsj0-0yfjell Nappe unit constitutes nappe units appears likely (Gee 1980). The the lowermost part of the Upper Allochthon in feldspathic quartzites of the Sretra Nappe are the eastern part of the region. The rock-types strongly tectonized and are quarried as flagsto­ in this tectonic unit are generally in a higher nes in the Oppdal district (KriIl1986). tectonic and metamorphic grade as compared A few minor stratabound sulphide deposits with the Lower Allochthon, and comprise a are associated with the metasandstones and series of calcareous metapelites and metagrey­ metapelites of the Tynset group in the Alvdal wackes of the Aursund Group (Rui 1981 a, area (Fig. 1). There, the Baugsberg and Tron Bakke 1978) which was formerly designated deposits constitute the most important of the as "Reros schlsts', In northeastern areas they stratabound sulphide deposits of the district. are generally developed as garnetiferous gar­ In general the ores are of a chalcopyrite­ ben schists (Bryn 1959, Bakke 1978),but in the pyrrhotite type with a minor sphalerite con­ R0ros area muscovite-biotite-chlorite schists tent. Pyrite and galena generally occur as dominate. An upper unit of the Aursund group accessory minerals. The ores occur as flat­ comprises greenish-grey metagreywackes with lying, E-W-trending ruler-shaped lenses, isocli­ tuffitic intercalations and subordinate horizons nally folded within calcareous chlorite-sericite­ of mafic metavolcanites (the R0Sj0 formation quartz phyllites of the Tynset group, and have of Rui &Bakke (1975)and Rui (1981 a)). 60 Odd Nilsen NGU-BULL.412.1988

A distinction between two separate nappe was, at the time of its closure in 1977, the units, a lower Essandsj0 and a higher 0yfjell oldest operating mining company in Norway. Nappe, has been made further north in the Over the years a great number of mines were Essandsjl?Jen area by Wolff (1979).These were operated by the company and the total produc­ correlated with the Seve and Koli Nappes, tion was of the order of 120.000tons of copper. respectively, in the Hetaq-Tanntors area of As with the majority of the Caledonian strata­ western Jamttand on the Swedish side of the bound sulphide deposits, the ore bodies of the border (Roberts & Wolff 1981, Sjostrom 1983). R0ros district have a lensoid or ruler-shaped Due to a pervasive imbricate tectonic style, and morphology, generally with their long axes an extensive retrogressive alteration of garnet parallel to prominent tectonic linear elements and hornblende to chlorite and biotite, a distinc­ (e.g. the Mugg, Storwartz and Kongen depo­ tion between the Seve and Koli Nappe equiva­ sits). The general conformability between linear lents has been difficult to establish in the aeros structures and massive sulphide ores in the area. A transitional boundary between the Seve Norwegian Caledonides was demonstrated and Koli units has also been recognized in the earlier by Vogt (1952) and later by Rui (1973 western Helaq-Tanntors region (Sjostrom a.b) and Nilsen (1978)from the southern Trend­ 1983). In the present account the Essandsje heim region. and 0yfjell units are informally combined into As with the deposits in the Alvdal district the one - the Essandsjl?J-0yfjell Nappe. ores of the R0ros district have no apparent The Essandsj0-0yfjell Nappe is characteri­ spatial relationship with metavolcanites and zed by a great number of rootless, lensoid and must be considered to be of a sediment-hosted concordant sill-like bodies of metagabbro and type, concordantly intercalated within the cal­ amphibolite within the metasediments of the careous muscovite-biotite-chlorite schists of Aursund Group. Close to the strongly tectoni­ the Aursund Group and generally in close zed boundary to the subjacent Remsklepp proximity to gabbro bodies. In general, the Nappe there are a large number of serpentini­ deposits of the structurally lowermost StugUSj0 zed, chromite-bearing, ultramafic bodies, from Formation of the Aursund Group are characteri­ the R0ros area to Gudbrandsdalen in the zed by the assemblage of chalcopyrite and south, a distance of more than 150 km (Kjerulf pyrrhotite with subordinate or accessory pyrite 1879, Carstens 1920, Rui 1972, Stigh 1979). and sphalerite. Galena is virtually absent in In view of the structural position, character and most of the ores in question; where present, host lithologies of these ultramafic rocks the galena may be accompanied by rare minerals Essandsj0-0yfjell Nappe is thus comparable such as dyscrasite, native Bi and naumannite to units of the Seve Nappes, although in (Jesanq 1964). Usually the ores display a Sweden the serpentinized ultramafic bodies in through plastic "durchbewegt' texture which the Seve generally lie close to the contact with reflects the pervasively sheared and brittle the overlying Koli Nappes. tectonic style of the nappe unit, as revealed In the western part of the map area, the by the great number of adjacent, flat-lying Blah0 unit (Hansen 1971, Krill 1980) is charac­ sheets of ultramylonite ("hardart') within the terized by strongly deformed and metamorpho­ Aursund Group meta-arenites. sed garnetiferous biotite-hornblende gneisses The R0Sj0 Formation occupies the western and amphibolites with intercalations of meta­ part of the Aursund Group and hosts a great gabbros and with pods and small bodies of number of pyritic ores (Lieungh 1973, Rui & chromite-bearing serpentinites close to the Bakke 1975). There, sphalerite may constitute tectonically underlying Sretra Nappe. On ac­ an important component of the ores, e.g. at count of the characteristic lithologies, meta­ the Lergruvbakken deposit, which was the last morphic development and tectonostratigraphi­ mine in operation with ore grading 0.74% Cu cal position, a correlation between the Blah0 and 7.8% Zn (Bugge 1978). unit and the Essandsj0-0yfjell Nappe (Gee et To the south and southwest the Essandsj0­ al. 1985a)seems evident. 0yfjell Nappe wedges out and only a few, In the R0ros district a great number of strata­ minor, pyritic sulphide deposits are associated bound cupriferous sulphide deposits are con­ with the Aursund Group. In the Tynset area a fined to the Aursund Group of the Essandsj0­ thin «3m), stratabound manganiferous horizon 0yfjell Nappe. The R0ros Mining Company of the Aursund Group (e.g. the Storassn pros­ (R0roS Kobberverk) was founded in 1644 and pect) is composed of bedded spessartite, NGU - BULL. 412. 1988 The Tectonostratigraphic Setting of Stratabound Sulphide Deposits 61

w _ No.of deposits _ E 60 40 20 0 20 40 60 80 I I I I I I I I I I I I I I

Mer~ ke r Storen I I

Gula I IGula

Essand sjo ­ BIAho I IOyfje ll

IRemsk lepp

Cl Parautochthon I I I I I I I 1 I 1 I I I

Fig. 2. The distribution of stratabound sulphide deposits within the correlatable tectonostratigraphic units in the western (W) and eastern (E) parts of the southern Trondhe im region (map-sheet "R0ros' 1:250,000).

Mn-ankerite and braunite, and may represent The Gula Nappe occupies the central part the southernmost distal facies of the sediment­ of the region in the core of a complex antifor­ hosted Aursund Group deposits . mal structure. The MerAker and ste ren Nappes In the western BIAh0 unit correlative, a few were apparently emplaced upon the Gula rocks stratabound sulphide deposits are present (Fig. (Furnes et al. 1980) prior to the emplacement 2). They were exploited for pyrite and copper of the entire Trondheim Nappe Complex above on a limited scale at the turn of the century, the Essandsj0-0yfjell and BIAh0 units. In this but the reserves were small and not easily model, the eastern part of the area of the accessible. The deposits occur as thin «1 m) MerAker Nappe occurs in an inverted position but massive pyritic strata intercalated within the above the Essandsj0-0yfjell Nappe (Fig. 3), but strongly contorted BIAh0 garben-gneisses in there is still an uncertainty over this question close proximity to garnet amphibolites (e.g. the as structural and textural evidence of thrust VArstigen prospects ), or as heavy, stratabound , faults is sparse and inconsistent. chalcopyrite-pyrrhotite disseminations in the In the southern Trondheim region the Gula latter (e.g. the Tilset prospects ). As with the Nappe comprises two principal units. A lower, R0sj0 deposits in the eastern part of the region calcareous, high-grade metapsammitic unit, the (Rui & Bakke 1975), a possible volcanogenic Sings;}s Formation, contains intercalations of affiliation may be attributed to the deposits in black schists, mafic metavolcanites and bitumi­ question, but due to the pervasive metamorphic nous ribbon quartzites. The SingsAs Formation and tectonic imprint of the ores and wallrocks is flanked by dark, metapelitic units (the Un­ of the BIAh0 deposits, this question remains dal and Asli Formations, respectively, in the unsettled. west and the east) with subordinate horizons of quartzite conglomerate and crystalline limes­ tone (Nilsen 1978). The age of the Gula rocks is uncertain, but Tremadocian fossils in the The Trondheim Nappe Complex Asli Formation suggest that the greater part In the Trondheim region the upper part of the of the Gula unit may be of Cambrian age (Star­ Upper Allochthon is generally referred to as mer 1941, Gee 1981). the Trondheim Nappe (Wolff 1967) or Trond­ Thin horizons of mafic metavolcanites, in tieim Nappe Complex (Guezou 1978, Roberts places with ultramafic inclusions, occur within & Wolff 1981). This nappe complex comprises the Gula Nappe, in particular along the bounda­ three principal nappe units, from west to east ry between the SingsAs and the Asli Formations the Steren, Gula and Mer;}ker Nappes . (Nilsen 1974). A great number of pyritic, cuprife- 62 OddNilsen NGU - BULL. 412, 1988

TECTONOSTRATIGRAPHY AND ORE DEPOS ITS OF TH E SOU THERN

T RO NDHEIM R EGI ON , NORWEGIA N CALEDONIDES

Legend :

G re enstone - J Tuff

~ Qu ar tz ­ kera toph yre Conglomerate

Psammite II Lime stone

Pelite '-- -_ ----'1 Mylonite

Serpentinite I I loo 0 0 0 0001 Augen gneiss

l: u ~ n n : n ~ :::J Gabb ro

f:::::::::: :::::I Granitoid s

WESTERN SUCCESSION I

Singstis GULA

Undal

STOREN

AOSjo ESSANDSJO ­

OYFJ ELL BLAHO StugUSjO

REMS KLEPP

RISBERGET

AMOTSDAL

OSEN -R OA OSEN -RO A Parau tochton ...~ Parautoc ht hon Autoc hthon =Zi ~ i ~ i ~ i ~ i;;;;;;: : ; ~ Autoch t hon ...... ' ...... ' ".. ::::~ ...... - . P€ B asement

IO. N;h on- 87)

. Fig. 3. Tectonostratigraphy and ore depos its of the southern Trondheim region, Norweg ian Caledonides. NGU-BULL.412,1988 The Tectonostratigraphic Setting ofStratabound Sulphide Deposits 63 rous, stratabound sulphide deposits are con­ especially in western areas (Roberts et al. fined to these Gula amphibolites which are 1984). capped by, and laterally wedge out into, thin, A great number of stratabound sulphide cherty iron-formations (Nilsen 1978). The sulp­ deposits are associated with the metavolcani­ hide deposits are pyritic, with chalcopyrite and tes of the steren and Fundsj0 Groups. In the pyrrhotite as minor constituents. Sphalerite southern Trondheim Region the deposits are occurs locally, and galena is absent from most clustered in the Alen, Folldal and Innset­ of the Gula ores. The orebodies are small, Rennebu districts, but several deposits also with tonnages well below 1 million tons, and occur within the volcanic horizons from Gaulda­ with grades of from 1 to 3% Cu and less than len to Gudbrandsdalen.The only sulphide mine 1% Zn. The associated iron-formations are in the region in operation today is confined to mostly developed as magnetite-bearingferrugi­ the metavolcanites of the steren Nappe, viz. nous grunerite-garnet quartzites. A few nickeli­ the Tverrfjellet mine. The Tverrfjellet deposit, ferous ore deposits are associated with the which was put into production in 1968, repre­ ultramafic and gabbroic inclusions within the sents the next largest sulphide orebody in Gula amphibolites. Exploration work at the Norway with a tonnage of 16 mill. tons of ore Vakkerlien prospect during the late 1970's with 0.86% Cu and 1.07% Zn (Motys 1977, revealed an ore body of 400.000 tons of ore Krupp & Krupp 1985). Formerly, the Folldal with 1% Ni and 0.4%Cu (Thompsonet al. 1980). area was next to the R0ros area the main A large number of prospects are confined copper district in the southern Trondheim to sulphide-disseminated black schists and region (Foslie 1926, Aasgaard 1935, Page ribbon-chert intercalations within the Gula 1964). Several mines in this area produced a Nappe. The mineralizations are chiefly pyrr­ total of 4.5 mill. tons of ore with 1-2% Cu hotitic with subordinate pyrite and with a neglib­ during the years from 1748 to 1969 (Bugge le content of copper. 1978). The Meraker and steren Nappes constitute In general, the massive, stratabound sulphi­ the main volcano-sedimentary units within the des of the steren and Fundsj0 Groups are southern Trondheim region, and form the pyritic with chalcopyrite, pyrrhotite and sphale­ uppermost nappe units of the region - the rite as minor constituent minerals. The copper steren Nappe in the west and the MerAker contents vary between 0.5 and 2% and the Nappe in the east. These include pillowed zinc content varies greatly from less than 1% tholeiitic greenstone units in their stratigraphi­ up to 5% in the Killingdal mine (Rui 1973). cally lowermost parts which are metamorpho­ The lead content is generally very low, from sed in low to medium grade. Intercalations of nil to 1% Pb. In general, the ores associated quartz-keratophyre are prominent in the ensi­ with the island-arc related Fundsj0 Group reve­ matic island-arc related Fundsj0 Group meta­ al a minor, but significantly higher Pb and Zn volcanites in the east (Wolff 1967, Nilsen 1978, content in comparison with the ocean-floor Grenne & Lagerblad 1985), while a thick pile related steren Group deposits (Vokes 1976, of greenstones of ocean-floor tholeiitic charac­ Bj0rlykke et al. 1980). However, within both ter (Gale & Roberts 1974, Grenne et al. 1980) settings, lateral and vertical variations with with associated local volcanoclastics and rib­ respect to the relative proportions between the bon-cherts characterize the western Steren three main base metals (Cu:Zn:Pb) reflect the Group unit. Both these basal metavolcanites specific stratigraphical and along-strike siting are followed unconformably upwards by low­ of the massive sulphide deposits. Thus, in the grade shallow-marine sediments which are Fundsj0 Group the copper content of the ores succeeded by thick sequences of mainly tur­ decreases southwards from the basalt-hosted bidite-facies metagreywackes and siltstones deposits (Py+Cpy± Sf)of Alen (Killingdal)along with polymict conglomerate horizons in both strike to mafic tUffite/quartz-keratophyre set­ the western (Hovin and Horg Groups) (Vogt tings (Py+SI+Mt) with bituminous pelitic inter­ 1945, Chaloupsky 1970, Walsh 1986) and the calations in the Tolga area. eastern (Sulamo, Kj01haugen and SIAgAn The vertical zonation of the deposits has Groups) parts of the region (Wolff 1967, 1979, been difficult to reconstruct due to the general Hardenby 1986).Thick units of greenstone and pervasive deformation of ores and wallrocks, local andesites and rhyolite tuffs also occur andto the general inaccessibilityof the abando­ within these mainly Ordovician sequences, ned mines. However, at Killingdal, Birkeland 64 Odd Nilsen NGU-BULL412.1988

(1986) has recently deduced a possible feeder penetrative foliation and metamorphism, and zone as revelaedby chalcopyrite and pyrrhotite in many areas produced inverted metamorphic disseminations in chloritic schists, stratigraphi­ zonations (e.g. in the Essandsj0-0yfjell and cally underlying the massive, sphaleritic pyrite SIAh0 Nappes). The fact that ore provinces ore which is capped by ferruginous cherts. within the Caledonides apparently coincide Minor cherty iron-oxide facies rocks are with the majortectonic units has been emphasi­ commonly intercalated with some orebodies zed by Zachrisson (1977). (e.g. at Tverrfjellet mine), or occur as separate As shown in Fig. 2, the complexes of the iron-ore strata, capping or adjacent to the Upper Allochthon reveal the greatest sulphide volcanogenic massive sulphide orebodies. ore potentials of the southern Trondheim regi­ Only a few of these have been exploited on. In a broad sense there is an apparent commercially during the last century (e.g. St. change from Cu-rich, sediment-hosted depo­ Olaf mine, Rennebu(Nilsen1978». sits in the lower allochthonous units towards The orebodies are generally lens- og ruler­ pyritic, Cu/Zn deposits of volcanogenic affiliati­ shaped, and their present morphology and on in the upper allochthonous units (Fig. 3). setting is evidently tectonically controlled. A similar trend has been encountered in the Wallrock alternation is commonly revealed by stratabound sulphide deposits within the Cale­ pervasive chloritization, silicification and serici­ donian allochthon of Sweden (Zachrisson tization. Evidence of post-ore deformational 1980). The sediment-hosted ore deposits of events associated with sulphide remobilization the allochthonous units below the Trondheim is revealed in several of the ores by rotation Nappe Complex show no apparent spatial and brittle deformation of pyrite porphyroblasts relationship with the early Caledonian mafic which are engulfed by plastically deformed magmatism, which may have served as an pyrrhotite and base-metal sulphides. The un­ energy source in the development of convecti­ conformably overlying metasediments of the ve hydrothermal metal-bearing brines in the Hovin and suiamo Groups of the MerAker and initital stages of the orogen development. With steren Nappes are barren with respect to ore the onset of submarine volcanism in the Up­ deposits. per Allochthon, stratabound and laterally zoned exhalative deposits were formed as revealed by the Gula and St0ren/MerAker deposits. Conclusions Here, pyrite+chalcopyrite± sphalerite and pyri­ The stratabound polymetallicsulphideminerali­ te+sphaferite+magnetite paragenesesseem to zations of the southern Trondheim region were be confined to proximal centres of tholeiitic deposited in successive stages and within volcanlsrn and distal mixed sedimentary/vol­ restricted sedimentary andvolcano-sedimenta­ canoclastic facies, respectively. A major task ry environments during the development of the in the future will be, by means of detailed Japetus Ocean. They were deposited from the petrological, structural and geochemical stu­ time of early ritting, as revealed by the partly dies around the deposits in question, to try dolerite-intruded basin sediments of the to establish the ore-forming environments of Remsklepp Nappe, through the early subduc­ the different units in this part of the Trond­ tion phase with the development of the first heim region. ensimatic island-arc magmatismin the Fundsj0 Group. . In pre-Middle Arenig time, obduction of ocean-floor (Steren Group) assemblages and Acknowledgements the Fundsj0 arc rocks upon the rocks of the The present review developed from a cooperation with the Geological Survey of Norway in the compilation of the Gula Nappe took place, with a subsequent 1:250.000 map-sheet "R0ros'. The mapping and investigati­ uplitt and erosion of the volcanic pile before ons of massive sulphide deposits of this region were initia­ Ordovician sedimentation commenced. The ted by the late Prof. JAW. Bugge at the Department of Geology, University of Oslo, in the early 1970's, as a contri­ structural and stratigraphic sequences were bution to the "R0ros Project' (Bugge & Rui 1970). The writer subsequently affected by Caledonian folding is indebted to Ingolf J. Rui for his cooperation in the investi­ gation, compilation of data from the eastem part of the map and metamorphism prior to and during an area and for many stimulating discussions. An outline of the eastward transport as composite nappe units present investigation was presented at the VII IAGOD symposium (Nilsen 1986),and constructive criticism of a first in Middle to Late Silurian times. The stacking draft of the manuscript by two anonymous referees is grate­ of the nappes post-dates the isoclinal folding, fully acknowledged. NGU· BULL412, 1988 The Tectonostratigraphic Setting ofStratabound Sulphide Deposits 65

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