Ophiolitic Ultramafites in the Folldal-Reros T Ract, and Their Cr
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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. ." <P ' 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.