N GU - BU L L 433 , 1997 - PA GE 10

Ophiolitic ultramafites in the Folldal-Reros tract, and their Cr-{PGE) mineralisation

LARS P. NILSSON',BRIAN A.STURT' & DONALD M. RAMSAY'

,Geological Survey ofNorway,RO.Box 3006-Lade, N-7002 Trondheim, Norway. ' 27MarleeRoad, Broughty Ferry, DundeeDOS3EY, Scotland, UK.

Tectonostratigraphic setting Remapping of the Vaga-Roros tract has shown the existence The largest of the massifs is at Feragen (15 km'),with ma­ of a major dismembe red ophiolite (Fig.l) - the Vagamo jor Cr-mineralisation . The main rock types are variably ser­ Ophiolite.This wasthrust in above rocksof the Heidal Group, pentinised dun ite and peridot ite (varying from cpx-bearing uplifted and deeply eroded prior to th e deposition of the se­ harzbu rgite to cpx-po or lherzolite) and dun ite. Moda l laye­ dimentary/ volcanic Sel Group. A number of other massifs, ring and mant le tecto nite fabrics strike NW-SE to E-W with mainly ultramafic, are know n in the region bet ween Folldal steep dips.The body has a pseudostratigraphic th ickness of and Feragen (Fig.l), though no clear collect ive origin has approxima tely 4-5 km. Dunite is abundant in the north previously been propo sed.These massifs are mainly concen­ (>50%), wh ere the peridotites are seen as detached rafts in trated at the Sel-Heidal boundary (Fig.l).The massifs along dunite, wherea s peridot ite dominates in the south. Cr-ores th e Sel-Heidal boundary are floored by a thrust plane,above are exclusively hosted in the dun ite. Typical pod iform ores the Heidal unit, and overlain by rocks of the Sel Group. Such predominate, though serni-stratiforrn ore bod ies are also relationships are seen at Fasteen, Stenkletten, Ra ud­ present. These are common features in the upper mant le hammeren and Feragen. The unconform ity is also exposed section s of op hiolites. Ore-types include fine-gra ined imp­ at a num ber of other locations (Sturt et al. 1995, also this regnations and banded, schlieren/ve in, nodular (grape, pi­

volu me). Serpentinite cong lom erate, derived from the ultra­ que, leopard) and massive (usually>55% Cr20 3) types. The mafics,occurs in the lower part of the Sel Group.Many of the Cr/Cr+AI rati os in chrom itite vary from 0.74 (in the north­ conglomerates were originally identified as ultramafi c bod ies. east) to 0.86 (in the south west) which may indicate way-up We consider that th e ultramafic/ mafic massifs occurring to the northeast. Generally, the chrom iti tes are PPGE deple­ along the bou ndary of the Sel and Heidal Groups have a ted (especially Pt,average c.l ppb) and have low PGEgrades common orig in and represent the lower part of a once cont ­ (100-200 pp b).The highest grades are in th e south west with inu ous op hiol ite sheet. PGE values up to 1.6 ppm (Geitsjogruva). In a belt fro m Middagshaugen (2.5 km south of Reros) to The massifs of Raudhammeren (5 km'), Klettene, Sten­ Feragen in th e east, the tectonostratigraphic relation s of th e kletten, Fasteen and Raudk letten have very similar Cr-ore massifs are complicated by Scandian deformation. Collect­ types to those at Feragen,though their degree of serpent ini­ ively, they have a fairly complete, though dismemb ered, sati on is higher. They again have low PG E values, tho ugh op hiolite pseudo stratigraphy. In addition to upp er mant le Fasteen is slight ly anomalous with values up to 1.1 ppm . ult ramafics, layered and vari-textured gab bros,gabb ros with The Osthammeren serpentinite body (0.05 krn') isdistinct­ dykes and even dykes-in-dykes have all been identified and ly anomalous in terms of its PGE content and very high mapped. The inset map (Fig.l ) shows that the oph iolit ic Cr/Cr+AI ratios (0.90).Thesefeatures can be taken to indicate a rocks occur in two E-W belts.The south ern belt of rnafic/u l­ lower position in the upper mantle section than is the casefor tramafic rocks is underlain by a thrust and overlain by rocks the bodies described above. The Cr-ores are mainly of a rich of the Sel Group. The northern oph iolit ic belt, however, is schlieren/vein type.The chromitites are Os, lr, Ru,(Rh) and Pt both underlain and overlain by rocks of the Sel Group. enriched,whereas Pd and Au valuesare low.The PGEfractiona­ Where exposed,e.g.at Raudhammeren,the base of the ultra­ tion pattern has an uncommon shape with a marked Pt high mafite is marked by a zone of ultra mafi c mylo nites directly (up to 9 ppm).The average Pt enrichment is 200-300 timeshig­ above phyllites of the Sel Group.This we consider is th e re­ her than for the chromitites of Feragen and Raudhammeren. sult of Scandian th rusting, thus duplicating tecto nostrati­ PGEgrades in the Osthammeren chrom itites vary from 1 to 11 graph ic relationships. ppm with an average of 1.5ppm (n=12).The host serpenti nite shows an average grade of 40 ppb (n= l O).The platinum group The Cr-(PGE) mineralisation mineralogy isvery complex. The northeast ern part of th e Folldal- Rerostract has been by References far the most important Cr-m ining district in Norway,accoun­ Sturt, BA, Bee, R., Ramsay, D. M. & Bjerkqar d,T.1995: 5tratigraphy of the ting for more than 90 % of th e country's total prod uction 40, Ott a-Vaga tract and regional stratigraphi c im pli catio ns.(Extended 000 tons of chromium ore dur ing the period 1820-1940. abstract).Norgesgeologiske undersokelse Bulletin 427, 25-28. ."

R0ROS Raudhammeren Reros • N S A

Gr1\bergel KVERNDAlEN

GAMMEl- \ C ~ _____ \ VANGEN / "0 ~ \ 0 : Devonian I- cl S: Sel Group 6 ~60 ~ Ophiolite VESlEAsEN N 0 : H: Heidal Group .( • ! ynset 13.... 0 25 km gn: Precambrian ~ ~ FAsTEEN ~ I A augen gneiss 0 2 3 5 km cC - STYGGBERGET 45 / STRAUMSAs LBREKKEBEKKDAlEN --=- 41 53/ 5ii .b' 31 Alvdal

I) x....,"":~:!!:o:!!:::=;~~==='7""=~""':-'"'--' STEINBAKKEN LEGEND ~ A RT I N KO llE N / HOlBERGET STEINFlOEN " KlEBERBERGET RAUDKlETTEN DOLTJORNSIETRI Ultramafic fragments

• Ultramaf ic body; serpentinite (+ chrom itite). • variably altered harzburgite / Iherzolite and chrornmte-bearinq dun ite (in the Reros ­ Feragen area both ultramafic and mafic rocks are included in a common colour code).

Basal thrust JOTUN NAPPE • Layered c1 inopyroxenite - dun ite z C) (Jotun Nappe) c c . Serpentinite conglomerate, cc SELGROUP rarely with serpentinite substrate C r­ r- Fault -I:> vAGAMO OPHIOLlTE w '-,. Younging direction based on sedimentary w structures HEIDAL / GULA / HOMMELFJELL 45 Fold axis with plunge (45' towa rds NW)

PRECAMBRIAN AUGEN GNEISS "U Basa l thrust 33<, Mineral and stretching lineation with plunge » (ou a Nappe) _ LA-V_..&.:l_ (33' towards NW) C) m