Mo and Zn Minerallzations RELATED to GRANITES IN
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Revista Brasileira de Geociências 17(4):571-5 77, dezembro de 1987 Mo AND Zn MINERALlZATIONS RE LATED TO GRANITES IN CALDERA ROOTS IN TH EELSJAE-AREA, OSLOPALEOR IFT, NORWAY SVEIN OLERUD* ABS T ~ AC T Th e Oslo Paleorift evolved in th e Pro terozoic basement of southeastern Norway and the Skagerak Sea in the Permian ; it form s a par t of a larger system of taphrogenic elements occurring at th e margin of th e Balt ic Sh ield . It consists of alkaline igneous rocks and Carnbro-Silurian sedim ents. Th e mo st prominent epigenetic mineralization ty pes are: A. intramagmatic Mo-depo sits; B. contact metasom atic c eposits (Zn , Pb , Fe etc.); C. rift margin deposits (Ag etc). Th e Elsjae area shows good examples of the A and B types. The Elsjae area is situated in th e northern part of the paleorift at the point of intersection be tween tw o dee ply erode d cald era struc tures. Cambro-Ordovician shales and Iimestones form a 2 krn 2 inlier enclosed in Permian ín tru sives. The sedime nts are co ntact metamorphosed and altered by met asomatic processes. Th e sphalerite mineralization is found in skarnaltered Iimestone layers an d lenses at severa l leveis in th e Cambro-Ordovician hornfelsed sedim ents. The Elsjae are a is unde rlain b y Permian granites which are hydrothermally altered and brecciated and carry low-grade molybdenite min erali zation in zones dominated by quartz-sericite-pyrite alte ration . Field evid en ces indicat e that th e Zn-skarn mineralization are related to the Storasyungen gran ite , which was intruded earlier th an the ring d yk e syste rn of th e calderas, while the Mo-mineralization was formed at a lat e stage in th e development of th e calderas. RESUMO O Paleo-rifte de Oslo , desen volvido no emba samento proterozóico na regrao Sud este da Noruega e no Permiano do mar Skag erak, representa uma das unidades do ex te nso sistema de eleme ntos tafrogênicos qu e ocorrem na marg em do Escudo Báltico. Compõe-se de ro cha s íg neas alca linas de idade perm ian a e de rocha s sedime ntares cambro-silurianas.Os t ipos de min er alizações epigené ticas mais proeminentes são : A. depósito de Mo intramagmático ; B. depósitos metassomá ticos de contato (Zn , Pb , Fe etc .); e C. depó sitos em margem de riftes (Ag etc.) . A área de Elsjre apresenta bo ns exe mplos dos tipos A e B. A região de Elsjee situa-se na porção norte do paleo-r ift e, na zona de in terseção entre duas estru turas de caldeiras pro fun daménte ero didas . Cerca de 2 km 2 de folhelhos e calcários cambro-ordovicianos jazem encaixados na s intrusivas perm ian as. Os sedime ntos mo stram metamorfismo de contato e alteraçã o po r pro cessos metassom ático s. A mineraliz ação de esfalerita enco ntra-se em camadas e lentes car bonáticas escarn itizadas , e em vários níveis do s sed imen tos cambro-ordovicianos transfor mad os em hornfelses. Subjacentes à área de Elsjee há granito s permiano s hidroterminalizados e brechados, e portam mlneralízação de baixo teor de molibdenita nas zonas com predomínio da alte ração quartzo-seri cita-pirita. Evidê ncias de cam po indi cam qu e os escarnitos mineralizados co m Zn se relacionam ao Granito Stor teyungen , o qu al é mais antigo que o sistema de diq ues anelares das cald eiras, enquanto a min eralização de Mo se formou no s estádios tardios do dese nvo lvime nt o das caldeiras. IN T RODUCT ION This paper reports the results of a Cambro-Ordovician sedimentary rocks which occur as a 2 stu dy of the hydrothermal molybdenum and zink km 2 large rift in Permi an int rusives (Olerud 1984) (F ig. mine ralization within the Elsjee area in the Oslo Paleorift I). The area is located on the eastern margin of the Stryken in Sou th Nor way. ring complex, interpreted as a cauldron root by Scott The Oslo Paleorift evolved in the Proterozoic basement (1979), where syenite porphyry and felsites make up a of southeastern Norway and the Skagerak Sea duríng the system ofring dykes. To the north , a circular alkali granite Permian, and forms part of a larger system of intrusion occurs with remnants ofporphyric syen ite along taphrogenic elements occurring at the margin ofthe Baltic th e borders forming an incorn plete circular structu re Shield in northwest Europe (Fig . 1). It mainly consists of (Scott 1980). No supracrustal Permian rocks are prese rved alkaline Permian igneous rocks and Cambro-Silurian inside this ring structure, as it is in the Stryken ring sedimentary rocks. complex. The alkali granite is ínterpreted as a late A great variety of mineralization types, bo th intrusion in the central, deeper part of the caldera, where orthomagmatic and epigenetic, is related to the magm a it has completely destroyed the sup racrustal and tism of the Oslo Paleorift. The most prominent epigenetic subvolcanic rocks found elsewhere in the Oslo Rift mineralization ty pes are: A. intramagmatic Mo-deposits; calderas. The Elsjre area is situated at the point of B. contact metasomatic deposits (Zn , Pb, Fe etc.); C. rift intersection ofthese two main ring structures (Fig. 1). The margin deposits (Ag etc.) (Ihlen 1986a). The Elsjee area Carnbro-Ordovician rocks are in contact with five shows good examples of the A and B types. The different intrusives (Scott 1979 , Olerud 198 2a) (Fig . 2) metallogeny of the Oslo Paleorift is rece ntly described in and are underlain by various Perm ian granites, which have detail in a volume edited by Olerud & Ihlen (1986). produced extensive contact metamorphism and skarn The Elsje area is situate d in the deeply eroded northern alteration in the limes tone s. part of th e paleorift. It con sists of contact-rnetarnorphic * Geological Survey ofNorway. Box 3006 , N-7002 Trondheim, Norway 572 Revista Brasileira de Geociências. Volume 17, 1987 NORWAY EL5J0 AREA FIG.2 i i , , i I i LEGEND O 5 10km PERMIAN CAMBRO- SILURIAN XX Alkali gran ite(ekerite) ~~-~~ 5edimentary rocks,contact metamorphosed ++ Granite and granite porphyry (biotite granite) --- near the mtrusives 5yenite (nordmarkite. grefsen-syenite.etc.! PRECAM BRIAN Monzonite (larvikite. kjelsãsite ,etc) c=J Gneiss Gabbro ("Oslo-essexite'; etc) 5ubvolcanic rocks (syenite por phyry, felsite etc) - Main faults Volcanic rocks (rhomb porphyry, basalts. ek.l Figure 1 - Geologieal map of the northern part of the Oslo Region eompiled and simplif ied from Broegger& Seh etelig (1923), Saether (1962), Larsen (1978), Seott (1980), and Olerud (19 82a). Index map shows the loeation of the Oslo Graben and the eontinuation to the south in the Skagerak Sea. Local rock-type nam es in parentheses. Sphalerite mineralization occurs in skarn horizones nodules to discoid-formed lenses with maximum sizes of2 within the Cambro-Ordovician sequence of carbonaceous x 40 x 50 m (Ihlen 1980). The zinc mineralizations were and mar1y shales and limestones. The limestone beds and described by Goldschrnidt (1911) and recent1y in more fault zones are part1y replaced by zink bearing skarn. detail by Ihlen (1980). Sphalerite occurs in a medium Skarn alteration with associated Zn·W mineralization aiso grained skarn rock dominated by hedenbergite, garnet, occur along fault zones in the argillaceous hornfelses. and small amounts of calcite, amphibole sulphides, and Scattered molybdenite mineralization is found related to scheelite. Between the skarn rock and the surrounding hydrothermal alteration zones in the under1ying Permian Cambrian graphitic shales there is always a zone of granites. This molybdenite mineralization is found in one garnet-graphite rock. This characteristic zonation is found deep (475 m) drill hole (DDH2, Fig. 2), and the all over the Elsjae area. The zinc content of the skarn description of the mineralization is based on data from lenses seldom exceeds 10%, and the average is estimated to this drill hole. to be 5%(Mathiesenetal. 1976). Underground mapping and drilling show that only a SPHALERITE SKARN MINERALlZATIONS Uno small fraction of the limestone lenses is altered to skarn. derground mapping in the abandoned mines by Ihlen The Zn-mineralized skarn lenses are to widespread and low (1980) and surface mapping by Olerud (1984) show that grade to be recoverable. the most important zinc mineralizations in the western Other sphalerite occurences in the eastern part of the part of the Elsje area are related to limestone lenses at a Elsjee area are described as mineralízed fault zones and as single stratigraphic levei in the Cambrian graphitic shales. thin skarn-altered limestone beds (Goldschmidt 1911 , The limestones lenses have irregular shapes from small Mathiesen et ai. 1976, Ihlen 1978). Revista Brasileira de Geociências. Volume 17, 1987 573 -~''-'\'''''-~ ' \-' '-' ~- ' ~- ' '-' :- , -;:- ,,-'~ ',.. \ ..\ .. ', ... \ x x x x x X l..'c' !5.:!-\;,/~":!-~!-':~~~:~-\;.!_~-!._'\l,.. X ',~~:,'.!f,'J:·;:;tk XX ~~ ~~ " : " : " ! " : , ~ ;;':;,-':;;':;,-.:;} J. -..." -': ' -':" -..""/ '" ,":},~-',::),~_j,J DIAMOND DRILL HOLE 2 X 'J .. I \ '-!._\/:~,!~_'.!~~'" X ':f_~:f~:'~_~~_;;~~> X X X x~ \'..."'-:\','..','.!,'--:"'-"-'~"", .. -,:},~-'.,:~,.~~,~-,,"'"",",,..,,,)~ ~ X XX X \'~~::~-~:..~,~:t:~ ·\ - ~ ... , "'~_~~_'\;~5:f5:f5::y' X X o XX: : X1~~!~tt~i~wi~i 't.#J)~' i;fl~E~tf*~~~l~1~,X ~ ., ~ ,_'-----">~ X X " " " ,~ , ." ." ." ,,, .,, .,, ., ,,,, ,,, .,, ,, , ,, , ,, , ,, , ,,, .,, ,, , ,, ,, x "" ~ i õ · I" 100 L- --' --l'lkm PERMIAN ROCKS CAMBRO-ORDOVICIAN ROCKS Diabasedyke Hornfelsed graphitic shales Alkali granite and shales with some Syenite marble layers Granite porphyry II Phyllic alteration of the Stor évunqen granite rocks Granite porphyry I Sphalerite-bearing skarn Granite I mineralization Syenite porphyry Molybdenite. scattered mineralization in altered intrusives Faults Figure 2 - Geological map ofthe Elsja:area.from O/erud (1984) PERMIAN ROCKS.