geology of the rØros district, south-eastern trondheim ... · the røros district is situated in...

GEOLOGY OF THE RØROS DISTRICT,

SOUTH-EASTERN TRONDHEIM REGION

WITH A SPECIAL STUDY OF THE

KJØLISKARVENE-HOLTSJØEN AREA*

INGOLF J. RUI

Rui, I. J.: Geology of the Røros district, south-eastern Trondheim region with a special study of the KjØliskarvene-HoltsjØen area. Norsk Geologisk Tidsskrift, Vol. 52, pp. 1-21. Oslo 1972. The Røros district is a part of the Trondheim region which belongs to the Central Norwegian Caledonides. Based on some scattered fossil occurrences and correlation with neighbouring areas, the contii:mous lithostratigraphical succession of metamorphosed, eugeosynclinal supracrustal rocks in the district :s believed to range from Cambrian to Silurian in age.

The stratabound character of the serpentinite belt, located in the upper part of the Røros schists close to the border of the Hummelfjell formation, suggests shallow intrusive or extrusive emplacement of the ultrabasic bodies.

Detailed investigations of tectonic and primary sedimentary structures in the KjØliskarvene-HoltsjØen area indicate that the whole lithostratigraphical succession within the Røros district is inverted. The Dictyonema-bearing black shale at Nordaunevoll is located at the border between the Gula schists and the HersjØ formation, which are therefore dated to Cambrian and Lower Ordovician respectively. The HersjØ formation has usually been correlated with the StØren Greenstone in the western Trondheim region.

l. J. Rui, Institutt for Geologi, Universitetet i Oslo, Oslo, Norway.

Introduction The Røros district is situated in the south-eastem Trondheim region not far from the border between Norway and Sweden. The geological map of the district (Fig. 1) is based partly on a number of unpublished theses by Færden (1949), Birkeland (1967), Rui (1968), FlatebØ (1968), Nilsen (1969), and Bøe (1971), as well as my own investigations carried out between 1964 and 1970 with the help of assistants. This later work includes a more detailed remapping of parts of the areas investigated earlier. In addition the area north of lake Langen - Kjøli Mine and the area between Mugg Mine - iake Sætersjøen have been mapped by l. Bakke and B. Lieungh, respectively.

Unpublished geological maps and notes by the late Professor Th. Vogt and Johs. Færden, together with their assistants and students, and unpublished data from Ø. Pettersen, who is presently the geologist at Røros Kobberverk, were of great help during the later remapping of the area.

• Publication No. 2 in the 'Røros Project' of the Institute of Geology, University of Oslo.

2 l. J. RUI

� Devonian sedimentary rocks

The Cambro- Silurian stratigraphy

Greenschists (Hummeltjell area)

Grey to greenish mica schists

Hummeltjell tormation - grey feltspathic metasandstones and minor quartz'ites

Roros schists - calcareous grey and greenish schists. Marble. quartzite conglomerate, serpentinites and graphitic schists in the upper parts

Mainly darl< phyllitic schists

Sætersjo formation - alternating polygenic c.onglomerates, grey to greenish metasandstones and schists

Kjurudal format ion - mainly dark phyllitic schists

Hersjo format ion - greenstone/greenschist wi th beds of quartzkeratophyres

Gula schists - dark phyllitic schists in the south. Al-silicate bearing mica schists in the north. Marble, quartzite conglomerate ·

Greenschists, sometimes associated with Fe-Mg-skarn

Calcareous grey psammitic schists with interbedd ed graphitic schists l graphitic quartzltes

Caledonian lntrusives

Metagabbro/amphibolite Serpentinite

Quartzdiorite (granite)

Caledonian augengneiss

Fig. l. Geological map of the Røros district.

Eocambrian sparagmites

Basement Complex

Gra ni te

Porphyry

Thrust fault

' ' Sulphide ore

N Nordaunevoll fossil locality

N

Il

GEOLOGY OF THE KJØLISKARVENE-HOLTSJØEN AREA 3

4 l. J. RUI

The areas covered by Eocambrian and Precambrian rocks to the east and southeast of the major thrust wne have been investigated by G. Holmsen (1935), Falck-Muus (1936), P. Holmsen (1943), Holmsen & Holmsen(1950),

and Schaar (1962). The restricted Kjøliskarvene-Holtsjøen area (Fig. 3, marked out on Fig. 1)

is a part of the results of fieldwork in the Alen district, which was subrnitted for the cand. real. degree at the University of Oslo in 1968 (Rui 1968).

The geology of the Røros district was included by Wolff (1967) in a wider

discussion of the stratigraphy and tectonic structures of the Trondheim region. The main purpose of this paper is to give a more detailed description

of the geology and stratigraphy of the district and add further evidence for the large inverted nappe structures proposed by Bugge (1954) and later by Wolff (1967).

The geology of the Røros district

Stratigraphy

The HersjØ formation, a thick sequence of green metavolcanics, separates the supposed older Gula schists of Kjerulf (1871) from the younger sedimentary rocks found further to the east. As will be seen later, the Gula schists and the Hersjø formation probably belong to the Cambrian and Lower Ordovician, respectively, which means that the whole rock sequence is tectonically inverted with successively younger rocks towards the east. This is in agreement with Bugge's (1954) general view and with the conclusions reached later by Wolff (1967) based on investigations in the Meråker area (Wolff et al. 1967).

So far three lithological units have been recognized within the Gula schists. In the west, a thick sequence of mainly grey calcareous psamrnitic to sernipelitic schists occurs, sometimes containing well-preserved metasandstones with primary bedding ranging from a few cm to several dm in thickness. Graphite-bearing mica schists and graphitic quartzites are comrnonly found along the margin of the formation, but are also interbedded with the calcareous schists.

Then follows a thin 'wedging out' horizon of greenschists, aften associated with Fe-Mg-skam stained by brownish iron mcide (the main rninerals are garnet, cumrningtonite and quartz, and sometimes appreciable amounts of magnetite and pyrrhotite). The wide lateral extension in the same strati

graphical position strongly supports a volcanic origin. The youngest member of the Gula schists consists mainly of dark, in part

graphite-bearing, pelitic to semipelitic schists, generally low in carbonates. However, horiwns of pure marble and metamarl occur interbedded with the

schists. Small pebbled quartzite conglomerates are usually observed. The


formation is continuous with the Al-silicate-bearing Drøia schists in the Alen

and Haltdalen areas (Vogt 1941) and probably also with the Selbu m.illstone

horizon further to the north. Andalusite-bearing schists are found as far

south as Hersjøfjell to the south of lake Øyungen (Tørnebohm 1896, p. 96). A description of the upper Gauldalen district was given by Vogt (1941)

when a closer study was made of the Dictyonema fossil locality found in

1888 by J. H. L. Vogt (1889). The fossils are found south of the Hyllingen gabbro complex close to Nordaunevoll, marked N on the maps.

The Hersjø formation consists mainly of greenschistsfgreenstones with

intercalated beds of quartzkeratophyres; the latter gradually increase in

frequency southwards from the Vangrøfta river. Beds of greenstone conglomerates or agglomerates occur together with beds of lava with primary

lava structures as pillows and variolites (Bugge 1910, p. 23; G. Holmsen 1915, p. 21). These rather weakly deformed volcanic rocks have been observed in the valley of Vangrøfta and in the HersjØfjell mountain to the west of the Hersjøen lake.

Stratigraphically above the green metavolcanics follow the Kjurudal

formation, which is composed mainly of dark, sometimes grey phyllitic

schists. According to Birkeland (1967) conglomerate occurs dose to the

greenschist border to the northeast of Hersjøen.

The Sætersjø formation, named from type localities to the south of Sætersjøen, consists of alternating polymict conglomerates (Tørnebohm 1896, p. 94), grey feldspathic metasandstones and sernipelitic schists. Northwards

the conglomeratic layers gradually disappear. Close to the southern limit of the map, marble is exposed in a small

brook not very far from the base of the Sætersjø formation. The Røros schists (Kjerulf 1871, p. 43; Tørnebohm 1896, p. 71) occur in

a stratigraphical position between the Sætersjø and the Hummelfjell forma

tions. At the base there are dark, in part graphite-bearing, phyllitic schists, which, in the southern areas, are followed by alternating grey and greenish schists which continue northwards to the Røros area. From here they are traced roughly eastwards to Brekken and further north towards lake StugusjØ where they correlate with the biotiteporphyroblast-bearing 'Stuedalsskifer' of Reusch (1891, p. 31).

The Røros schists are generally found to be comparatively rich in carbonate minerals and plagioclase feldspar. The green colour is due to the presence of chlorite.

Because of an increase in metamorphism towards the northwest as shown by Goldschmidt (1915), the chlorites of the Røros schists become partly

replaced by garnet and hornblende through reaction with quartz and carbonates. This causes the colour of the rocks to become more grey. The dark

porphyroblasts of hornblende can be up to several cm long and are some

times arranged in a fan-shaped pattern (garbenschist). These rocks are usually developed within parts of the north-eastern areas and north of the western

end of lake Aursunden. In the dome-shaped structure to the southwest of

6 I. J. RUI

Killingdal Mine beautiful garbenschists rest on greyish metasandstones belonging to the Hummelfjell formation.

Conglomerates, marbles serpentinite bodies and alternating grey psam

mitic schists and graphitic schists are commonly found doser to the Hummelfjell formation.

Carstens (1929), Bryn (1959), Schaar (1962) and Kisch (1962) have de

scribed the petrography of the Røros schists. Altemating beds of grey feldspathic metasandstone and minor quartzites

make up the Hummelfjell formation. Rather large fragments of clastic plagioclase feldspar and sometimes bluish quartz are seen with the naked eye. The wide distribution of these rocks in the Hummelfjell mountain is

because of the double folding. Tectonically below the metasandstones in Hummelfjell, altemating grey

to greenish grey mica schists occur. Further down in the valleys of Hodalen and Nørendalen, greenschists of undoubtedly volcanic origin probably re

present the youngest member of the Cambro-Silurian stratigraphy so far mapped in the Røros district.

The Devonian sedimentary rocks found to the east of Røros have been described by Goldschmidt (1913). Notes on some tectonic structures occurring in these rocks have been given recently by P. Holmsen (1963).

Numerous bodies of intrusive rocks are found in the district. The acidic, mainly trondhjemitic types usually occur as small sill-like bodies which are restricted to the older divisions. They are occasionally found to the east of

the Hersjø formation. The gabbroic intrusives are more widely distributed and seem to be present in all the lithological units. They range in size from larger differentiated complexes to small sills and dikes which are sometimes verf closely spaced as in the Alen area and Hummelfjell mountain. Petrographical descriptions of the different intrusives are given by Goldschmidt (1916), Carstens (1920, 1929), P. Holmsen (1943), Holmsen & Holmsen (1950), Vogt (1941), and Kisch (1962).

The serpentinite belt occurring along the south-eastern margin of the Trondheim region, already shown very accurately on Tømebohm's (1896) map, is a subject of controversy. The serpentinite bodies are found in the Røros schists, and not very far from the base of the Hummelfjell formation. This belt can be well demonstrated southwards from Tolga to Tynset and Alvdal and further to the southwest through Folldal and down into Gudbrandsdalen, a distance of more than 150 km. The stratabound character has been pointed out by Kjerulf (1879, p. 177), Tømebohm (1896, p. 78)

and Carstens (192{}, p. 99). More complete sections through the transitional zone between the typical

Røros schists and the Hummelfjell formation further south in the Tynset area include polygenic conglomerate, quartzite conglomerate, marble, serpentinite bodies and altemating graphitic and psammitic schists. In places pure serpentinite or tale conglomerates occur next to the serpentinite bodies. Within the present area one single exposure of quartzite conglomerate is located


o

MERAKER AREA RO R O S D l S TR I C T

( F. Chr. Woltt et al. 1967) (Present pa per l

?

l Greenschists

c · :? � ? 0 1 � Mica schists

·- ( ��"" �S�l�å�g�å_n __ G_r_o_u _p ______ k�� :�f-��.�: �� H-u_ m_ m_e_l_t j _e_ll ________ �

(silurian graptolite __ .-1 . . : : formation fauna) -- � ?, ��--�----�--�--� - - l - - ( h't h. t l

_ _ ( - _, - grop 1 e se 1s s

Kjolhaugen

Group

• • • l -__.:::-_.:::- l? �

• •

- --

Roros schists

��------�� - -�

(Metabasites) v� "'"',.","""_

Sulåmo Group

Fundsjo Group

Sonvatn Group

o o -;ro . -- c-- ro 7o -.-:0z.0-:_0 :rr------------1 o•o o•o • o o o o Sætersjo tormation �: oo_

o O ooooo no..9. �o o� �� --------�

'"'-' � l.) "'v 1"\.J

Kjuru dal formation

L.

o Hersjo tormation

- -- � • • 1'(Dictyonema shale)

- --

- -• • ... • • • •

- - c -

- o - Gula schists - ·-? L.

.o

E o

u

Fig . 2. Correlation between the lithostratigraphy of the Røros district and the Meråker area.

8 I. J. RUI

to the southwest of Røros. Marble is observed in some localities near Tolga while the graphitic schists are usually recorded adjacent to the Hummelfjell forrnation.

Wolff (1967, p. 138) proposed an emplacement of the serpentinite bodies in connection with the development of larger thrusts in the area, arguing for a late period of intrusion (Middle to Late Devonian) into schists of different ages. These latter statements are not supported by the present work. The serpentinite bodies do not occur in schists of different ages, but within or very dose to a stratigraphical horizon. To explain the formation of the serpentinite or tale conglomerates, the serpentinite bodies must have been exposed to erosion after their emplacement.

The serpentinites most probably represent shallow intrusions or may even be ultrabasic lavas which poured out on to the ocean floor during some tectonic events in the geosyndine (cf. Kulling 1933, Zachrisson 1969, and also Oftedahl 1969, Strand 1970). The sudden change of the sedimentary environment during the deposition of the upper parts of the Røros schists with forrnation of conglomerates, Iimestones and later accumulations of the rather coarse-grained Hummelfjell sandstones, reflects the instability of the geosyndine at that time.

Correlation between the stratigraphy of the Røros district and the well investigated Meråker area (Wolff et al. 1967) suggests that the border between the Røros schists and the Humrnelfjell forrnation is dose in age to the transition between Ordovician and Silurian and a Middle to Upper Ordo� vician age for the Serpentinite belt (Fig. 2). The fossiliferous Silurian Slågån group in Meråker may correspond to the altemating graphitic and psarnmitic sclilits below and dose to the base of the Hummelfjell formation. The ultrabasic rocks are actually absent in the Meråker area, but reappear further north (Tømebohm 1896, p. 77).

An unequivocal chronostratigraphical set-up within the Røros district, however, still involves certain difficulties. The fossil bearing Otta Serpentinite Conglomerate from Ottadalen in Gudbrandsdalen (Strand 1951, p. 18) has been correlated with similar conglomerates from the Røros district by Vogt (1945, p. 507). The fossil fauna of this conglomerate (HedstrØm 1930,

Yochelson 1963) suggests a Lower Ordovician age for the serpentinite belt.

T ectonic structures

A major thrust fault, drawn fairly accurately by Wolff (1967) separates the Carnbro-Silurian rocks from the underlying grey, arkosic sandstones which belong to the Eocambrian sparagmite basin of southem Norway. The sparagmites rest on Archean basement granites and porphyries exposed in the 'Riksgrense antidine'. The geology of these areas has been described by G. Holmsen (1935), Falck-Muus (1936), P. Holmsen (1943), Holmsen &

Holmsen (1950) and Schaar (1962).

The western thrust fault earlier recognized northwards from Kjøli Mine by Færden (1949) seems to be of more local importance and it gradually dies


out southwards. The thrust boundaries are characterized by mylonitic and

ultramylonitic rocks. The eastern major thrust fault is also commonly associated with augengneisses.

Two generations of deformation seem to be responsible for the main tectonic features of the district. The early folds (Fl) are tight to isoclinal and occur on all scales. The F l folds are best demonstrated by the lithological borders within the Gula schists. The regional schistosity is parallel to the axial plane of these folds. The trend of the F l fold axes was originally

about NNE-SSW, but in places is largely scattered by a later episode of

folding (F 2) with axial trends of a bo ut NW -SE. The F 2 folds are usually more open in style.

The area considered can be divided into two distinctly different tectonic units by the western thrust fault and further southwards approximately along the river Glomma.

The complex structural pattern within the western part can be explained

as a result of the interference of two fold phases with comparatively steep dips on the bedding surfaces and the parallel or subparallel regional schistosity, but also locally by the inhomogeneities in the stress field created

by the comparatively large and rigid gabbroic intrusives. The eastem area, which is characterized by broad arching structures gently

dipping in various directions, is probably more complex than it appears to be. The early F l folds with practically horizontal bedding and axial planes

are not so easily recognized on the regional scale. The later F 2 folds, however, are well developed in places.

The thrust faults of the area are probably related to a late stage of the early F l folds. The thrust planes have been deformed later by the F 2 folds. This seems to be very clear in the area to the southeast of Røros. North of lake

Rambergsjøen the thrust plane strikes east-west with a northerly dip while

it strikes north-south and dips westwards to the west of the lake. The Lower Devonian sedimentary rocks which seem to have been origin

ally deposited on mylonites and augengneisses in the area to the north of lake Feragen probably postdate the large thrust movements.

The Kjøliskarvene-Holtsjøen area The Kjøliskarvene-Holtsjøen area (Fig. 3) clearly demonstrates the inverted lithological succession in the Røros district. The geology of this small area

is therefore worthy of detailed description.

Sedimentary rocks belonging to the upper Gula schists In this mountainous area downfolded remnants of the upper parts of the

Gula schists occur together with green homblende schists belonging to the

Hersjø formation.

10 I. J. RUI

-�8 .!!� > �� o -�

"

o,·B :�

�� ·;:::: ·---· .o

;D <.J • •

lv�� +

N

Sea le

o 2km

RCiros schists

Dark schist s (Kjurudal formation)

Gr&&n hornbl&nd& schists (HersjCi formation)

Black ·sch i sts ( HollsjCihCigda)

Band&d rock s&qu&nc& with quartzlt& conglo-m&rat& and marble/skarnifi&d marbt&.

M&tas&dim&nts with prlmary s&dim&ntary structur&s (Upper Gula schists)

M&tagabbro (with ullrabasic segr&gations to th& E of Guldal mine)

Quartzdiorlt&

Holtsjoskarven

/ -

--

40" go• -;;' ;(

y�

\ N ."

Thrust fault f Ax&s of

fl' &longa !lon of p&bblu

_I Fold ax&s ( "\ ...__,u Strik&/dip (" -> --

Strlke/dip X X facing normal or inv&rt&d det&rmlned

Nordaun&vou- Dlctyon&ma locality

Fig. 3. Geological map of the KjØliskarvene-HoltsjØen area.

Sulphide ore d&posit

Magn&tit& ore

Localitles wlth dllf&r&nt types o prlmary sedimen tary structur&s

Staurollte


--

_o

o

Fig. 4. Stratified metasediments from the upper parts of the Gula schists. (A) Alternating massive, fine-grained and graded beds. Locality I, south-eastern KjØliskarvene. (B) Banded and laminated metasediments. Intrusive porphyritic amphibolite to the left.

About one km SSW of lake MeinatjØnn.

The metasediments are mainly pelitic to semipelitic in composition but they grade into psammites by an increase in quartz content. The rocks are often schistose and appear to be ordinary mica schists. In places in the Kjøliskarvene area, however, altemating beds of massive metamorphosed mudstones, gritty mudstones and muddy sandstones having graded bedding are present. The individual beds can be from one half to one metre in thickness (Fig. 4A), but more often a rapid altemation of thin fine-grained beds or even weak lamination occurs. This primary sedimentary banding gives rise to a characteristic ribbon pattem on weathering (Fig. 4B).

The thicker massive beds are very hard and often have a conchoidal fracture. The colour of the finer-grained rocks is darkish grey while the coarse-grained types are brownish grey. W eathering gives rise to a yellowish

brown colour. Close to the base of the interlayered graded beds, sharp-edged clastic

fragments of quartz are common. Various well preserved primary sedimentary structures occur in these weakly deformed and recrystallized rocks.

12 l. J. RUI

Carstens (1920, p. 43) first mentioned these rock types and suggested that they were of Ordovician age.

The metasediments are generally low in lime and other Ca-rich minerals are only occasionally observed. Plagioclase feldspars are sparsely distributed.

The coarser-grained parts of the graded units found in Kjøliskarvene consist of relatively large angular and sliverlike quartz fragments, cemented by a very fine-grained matrix (Fig. 5). Occasionally rock fragments and rarely plagioclases are found scattered within the same cementing matrix. The matrix itself consists mainly of quartz, biotite, and amphibole with minor amounts of rutile, zircon, and occasionally chlorite and ores.

The ferromagnesian silicates are all light in colour as is evident from the pale yellowish brown colour of biotite and nearly colourless nature of the tiny amphibole needles. Sometimes the amphibole shows pleochronism in shades of pale greenish colours. Radiating aggregates and lammellar twins after (010) are commonly found. The amphiboles show either straight or oblique extinction ( about 15 to 17 degrees) and low to medium first order interference colours; it is probably a mixture of anthophyllite and cummingtonite.

According to Kisch (1962, pp. 46, 53) anthophyllite-biotite-cordierite schists occur together with kyanite-bearing staurolite-garnet-biotite schists in his 'Heimvola zone', some 12 to 15 km towards the north in Tydal. Undoubtedly these rocks are found in the same stratigraphical position as the sedimentary rocks in the Kjøliskarvene and the Holtsjøen areas.

Fig. 5. Photomicrograph of subgraywacke from the base of a graded bed. Clastic quartz (q) and rock fragments (r) are cemented in a very fine-grained matrix. Locality Il, central KjØliskarvene area. Crossed nicols, 35X.

r 01mm.


Fig. 6. Photomicrograph of subgraywacke (same as shown in Fig. 5). Tiny amphibole needles penetrate the Jarger clastic quartz grains. Crossed nicols, 135X.

During recrystallization of the primary matrix, tiny amphibole needles

have corroded and penetrated the larger quartz grains giving the border

zones a fringed look (Fig. 6).

Volumetric analysis of the relatively coarse-grained basal part of a graded

unit (shown in Figs. 5 and 6) gives:

Quartz fragments of sand size (0.06 mm) 40% Feldspar fragments of sand size 1% Matrix and rock fragments 59%

The rock fragments constitute about 5% of the matrix.

According to Pettijohn's (1949) diagram this modal composttlon corresponds to a subgraywacke. The composition of the fine-grained massive

beds is similar to the matrix of the coarser-grained types.

The sedimentary rocks occurring further to the north in the river Synner

hold and the areas surrounding Holtsjøhøgda to the west of the Holtsjøen lake

strongly resemble the rocks from the central Kjøliskarvene area. The rocks,

however, are generally more schistose and well recrystallized. Alternation of

more or less pelitic and psammitic metasediments are still recognizable.

Primary sedimentary structures have not been found, but clastic fragments

of quartz have been observed with the naked eye in some relatively coarse

grained psammitic beds close to the western shore of Holtsjøen.

The chief constituents of a brownish grey schist from the area of Synner-

14 l. J. RUI

hold are quartz, brownish biotite, sericite, chlorite and some small garnet porphyroblasts; tourmaline, epidote, and ores occur in minor amounts.

Close to the southwest border of the Holtsjøen lake staurolite-bearing brown pelitic schists altemate with psammitic schists. Staurolite occurs together with quartz, brown biotite, chlorite and garnets with accessory amounts of tourmaline and ore minerals. Smaller garnet porphyroblasts are sometimes partly included in the staurolites.

According to Vogt's (1941, p. 173) map, andalusite-bearing schists occur just to the northeast of Holtsjøen. Tørnebohm (1896, p. 96) pointed out the great similarities between the brown gamet and staurolite-bearing mica schists from the Holtsjøskarven mountain to the north of Holtsjøen and the Selbu schists (Selbu millstone horizon), well known from old quarries some 40 to 50 km towards NNW. The petrography of these latter rocks has been studied by Carstens (1929, p. 25) who also analysed them chemically.

The heterogeneous banded rock sequence

The occurrence of these rocks is more or less transitional between the Gula schists and the younger metavolcanics (greenschists).

This banded rock sequence consists of pelitic to semipelitic schists (similar to the schists described above) interlayered with beds of dark green amphibolite, other thin greenish grey beds, conglomerate and marble.

The dark green amphibolites are a few cm thick and probably represent metamorphosed basaltic tuffs heralding the ensuing volcanic activity of Lower Ordovician age.

The colour of the thin greenish grey beds is due to the high content of epidote and actinolite, together with quartz and carbonate. The rocks probably correspond to primary marls.

The quartzitic pebbles and boulders of the conglomerate horizon are strongly elongated. The cementing matrix is green coloured and consists of quartz, epidote, chlorite and some hornblende.

A thin horizon of marble is found just below the metaconglomerate. This is usually not more than one metre thick but locally an apparent thickness of 6 to 8 metres is known in the western area where the rocks are strongly folded. The impure layers of the marble have been transformed into a dense brownish skarn which is mainly composed of gamet (optically anomalous) and diopside with minor amounts of quartz, calcite and epidote. Later mineralizing solutions have penetrated the marble and deposited ore minerals such as pyrrhotite, magnetite, chalcopyrite and sphalerite. Another green skam occurs in connection with this mineralization. Epidote is very common here and sometimes occurs as well-developed crystals 2-3 cm in size.

The greenschists in the strongly folded anticlinal structure in Holtsjøhøgda are rimed by the dark graphite-bearing schists which are also found locally

in the same stratigraphical position in Kjøliskarvene. The quartzite conglomerate is observed at one locality. The marble horizon seems to be replaced

by carbonate rich schists.


Less than l km to the NW of the Holtsjøen lake, black graptolite-bearing shales were discovered by J. H. L. Vogt (1889) in a little gully named

Blåhenbekken (the fossil locality is known as the Nordaunevoll locality). A detailed description of this locality has been given by Vogt (1941, p. 171).

Not very far from the fossil locality he also describes (p. 176): ' ... a wholly massive bright brown biotite schist with a peculiar yellowish red coat of weathering. The rock contains a very bright brownish coloured biotite, the flakes being placed without markedly parallel arrangement, quartz, plagioclase, clinozoisite, a little almost colourless horn blende .. .'. It is noteworthy that Vogt 's description of the rock agrees well with the massive sedimentary rocks from KjØliskarvene.

Størmer (1941, pp. 162-163) described Dictyonema flabelliforme sociale

from the black graptolite shales which indicate their Tremadocian age. The area between HoltsjØhØgda and the fossil locality is very poorly ex

posed, but the Dictyonema shale most probably occurs in a stratigraphical position similar to the black schists in HoltsjØhØgda and close to the greenschist contact.

The green metavolcanics of the Hersjø formation

These rocks are rather homogeneously basaltic in composition, and are finegrained and schistose with a dark green colour. The most common minerals are bluish green hornblende and Na-rich plagioclase together with rninor epidote minerals, chlorite, carbonate and sometimes small amounts of biotite.

Occasionally thin layers of fine-grained white to greyish quartz-albite schists are interbedded with the greenschists. These layers sometimes exhibit banding due to alternating light and green coloured laminae. The almost white bands consist of quartz and albite while the green contain

additional hornblende, clinozoisite, chlorite and some biotite. These banded acidic and tuffaceous schists are characteristic of the area

just below the marble horizon in Kjøliskarvene which is stratigraphically at the base of the greenschists.

Sedimentary rocks younger than the green metavolcanics

The greenschists in the SE slope of Kjøliskarvene rest on strongly deformed dark phyllitic schists and a tectonic break separates both from the underlying and supposed younger Røros schists. The Røros schists in this small area have not been differentiated or studied in detail.

The thrust zone is characterized by strongly mylonitized and brecciated, dense and dark coloured rocks.

lntrusives

The supracrustals above the thrust are heavily intruded by smaller bodies of porphyritic and equigranular amphibolites of basaltic composition. They generally occur as sills and occasionally as dikes (Fig. 7). Vogt (1941,

16 I. J. RUI

Fig. 7. Massive grad ed beds with load casts younging to the right, cut by dark amphibolitic dikes. Locality l,

south-eastern Kjøliskarvene.

pp. 174-175) and Kisch (1962, pp. 13-14, 28-33) described the petrography

of these rocks from Nordaunevoll and the Tydal district, respectively.

East of Guldal Mine, lens-shaped bodies of serpentinite, a few metres wide

and some tens of metres long occur as segregations within the metagabbro.

Thin sections of the serpentinite show beautiful pseudomorphs of serpentine

after olivine. A rather large body of quartzdiorite occurs in the south-eastem part of

the mapped area. Moreover, sills of quartzdiorite are occasionally found in Kjøliskarvene and to the west of HoltsjØhØgda.

Sedimentary and tectonic structures

As mentioned above various well-preserved primary sedimentary structures

occur in the Kjøliskarvene area. Graded bedding is by far the most common

and widespread, but other types of structures, including load casts, erosion

furrows, pseudonodules, slumps and small scale cross bedding, are found at

two localities (Figs. 8 & 9). The localities are numbered I and Il on the

geological map of Kjøliskarvene (Fig. 3). The beds show poor sorting and

good grading and are probably turbidites.

The sedimentary structures show that the metasediments must be older

than the underlying greenschists and thus, the section at least down to the

thrust plane must be inverted.

The sedimentary structures described are given different terms and inter

pretations by different authors. It is, however, beyond the scope of this

paper to discuss any further the origin and nature of the structures involved.


Fig. 8. (A), (B) and (C) Erosion furrows, accentuated by load casting. (D) Graded bed containing bent and twisted 'clay clasts' Locality Il, central Kjøliskarvene.

Other interpretations would hardly affect the stated age relationship between

the different forrnations, i.e. the metasediments belonging to the Gula

schists must be older than the structurally underlying green metavolcanics

(HersjØ forrnation).

18 I. J. RUI

. . . . . . . . .. . . . .

. . . . . . . . . . . . .

Fig. 9. Pseudo-nodule developed at the junction between finegrained and graded beds. Small scale crossbedding is seen at the top of the section. Locality l, south-eastern KjØliskarvene.

As shown by the geological maps the district has suffered strong folding during the Caledonian orogeny. The two episodes of folds are responsible for the diverging trends of the megascopic fold axes of the area (about N-S in HoltsjøhØgda and about NE-SW in Kjøliskarvene).

The trend of the mesoscopic fold axes (similar type) of the Kjøliskarvene area shows a very dose resemblance with the megascopic fold axes (Fig. lOB). The orientations of the rod-shaped quartzite pebbles of the metaconglomerate in the same area are likewise parallel to the trends of the fold axes (Fig. lOA).

. ..

....

..

A B

Fig. 10. Stereographic projections of structural data. Schmidt's net, lower hemisphere. (A) Elongation lineation of quartzite pebbles, 132 observations (1-10%, 10-20%, 20-30%, more than 30% with max. 59%). Quartzite conglomerate from the KjØliskarvene area. (B) Fold axes.

NW

Roros schists

K j urudal formation


,, ,,

/'l ,' l(

l

�� Hersjo formation

� Black schists

SE

Guldal Mine

o 500 1000

m

Quartzite conglome

rate and marble/skar

nlfied marble

U,pper Gula schists

m

1100

700

Fig. 11. The overtumed and folded rock sequence in the KjØliskarvene-Holtsjøen area. Note the probable position of the Dictyonema horizon at the border between the Gula schists and the HersjØ formation.

The link between Kjøliskarvene and Holtsjøhøgda is shown in Fig. 11

which illustrates the strongly folded, overturned and thrusted beds in Kjøli

skarvene.

Conclusions

Sedimentary structures in the Kjøliskarvene area have shown that the Gula

schists are older than the green metavolcanics belonging to the Hersjø

formation. The occurrence of Dictyonema flabelliforme sociale in shales near

Nordaunevoll (Stønner 1941, Vogt 1941) is located close to the base of the Hersjø formation, which means that the main parts of the Gula schists belong to the Cambrian, while the base of the Hersjø formation is Lower Ordovician (Tremadocian).

The Hersjø formation has usually been correlated with the Lower Ordovician Støren Greenstone (Vogt 1945) in the western Trondheim region, and most recently by Wolff (1967). With the Dictyonema Shale at Nordaunevoll next to the base of the Hersjø formation and the fossiliferous Bogo Shale

(Blake 1962) closely above the Støren Greenstone, the age of this volcanic

period is fairly well established as Lower Ordovician (Tremadocian-Aren

igian) between the equivalent stages 2 ea and 3 by of the Oslo region.

The chronostratigraphy of the Røros district is in part still based on some

what conflicting evidence, but the Iithostratigraphical correlation with the

Meråker area (Wolff et al. 1967) seems fairly reliable.

The general dips of the bedding in the Røros district are towards the west

while the top senses of sedimentary structures in the supracrustals seem to

face eastwards from the areas occupied by the Gula schists. This necessarily

20 I. J. RUI

means that the whole rock sequence is overturned, which is in agreement

with the general view of Bugge (1954) and Wolff (1967).

To the north of the Feragen lake the Lower Devonian sedimentary rocks,

which seem to be partly deposited on mylonites and augengneisses, postdate

the large orogenic thrust movements.

Acknowledgements. -I am grateful to Professor J. A. W. Bugge for his constant and encouraging discussions during the investigations. I also wish to express my thanks to J. Færden, Ø. Pettersen (A}S Røros Kobberverk), T. Birkeland, R. FlatebØ, O. Nilsen, P. Bøe, B. Lieungh and I. Bakke for allowing me to use their unpublished geological maps for compilation.

Professor T. Strand and Dr. A. Mukherjee most kindly commented on the manuscript, and Dr. D. L. Bruton and Dr. W. L. Griffin corrected and improved the English text.

This work is part of the 'Røros Project' in the Central Norwegian Caledonides -a series of investigations undertaken by the Institute of Geology at the University of Oslo and directed by Professor J. A. W. Bugge. The 'Project' is made possible through the cooperation of the mining concerns in the area, namely A/S Killingdal Grubeselskab, A/S Røros Kobberverk and A/S Folldal Verk.

December 1970

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geology of the rØros district, south-eastern trondheim ... · the røros district is situated in...

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