Cu-Zn-Pb ores in the Aijala-Orijarvi area, southwest Finland

Abstract

The Aijala Cu-Zn ore deposit, the Metsaemonttu Zn-Pb ore deposit, and the Orijaervi Zn-Cu ore deposit are located in the leptite zone of southwest Finland. The sulfide ores of central Sweden belong to the same metallogenic province. The zone can be interpreted as a mid-Precambrian island-arc structure.The volcanic rocks in the Aijala-Orijaervi area are predominantly volcanoclastic, although units of lava origin are also encountered. In chemical composition they are predominantly calc-alkalic, although basaltic volcanite layers of tholeiitic composition are also met with. The lower section of the volcanite formation is composed of a silicic volcanic group whose composition varies from rhyolite to dacite. The chemical composition of the intermediate-basic and basic volcanite groups in the upper section varies from basaltic to andesitic. They often show primary volcanic structures, such as pillow lavas, volcanoclastic structures, and phenocrysts. The volcanites are overlain by argillaceous and arenitic weathering sediments that exhibit primary sedimentary structures such as graded bedding and slumping.The rocks in the area were metamorphosed in the lower-pressure amphibolite facies. In the Aijala-Orijaervi zone the rocks were folded in two phases, F 1 and F 2 . The general regional folding, F 1 , is often isoclinal with a gently plunging fold axis. The younger crossfolding, F 2 , is more local and shows subvertical fold axes.The Cu-Zn deposit of Aijala and the Zn-Pb deposit of Metsaemontttu are both sulfide disseminations or breccias in the upper part of the acid volcanite group in a pyroclastic unit with quartz and plagioclase phenocrysts. The Orijaervi Zn-Cu ore deposit consists of sulfide disseminations, breccias, and veins of massive sulfides in a zone of cordierite-sericite and cordierite-anthophyllite rocks. The wall rocks of the ores include chlorite-bearing skarns, dolomitic limestones, quartz rocks, and cordierite-anthophyllite rocks.The mineralization took place after the decline of intense acid pyroclastic volcanism. All three ore deposits are severely deformed and partly remobilized. They are composed of several minor orebodies which in detail cut and brecciate the wall rock but as a whole are stratabound in relation to the country rock.Each ore deposit has alteration zones whose rocks exhibit dolomitization, silicification, sericitization, and magnesium-iron metasomatism. Associated with the ore deposits at Metsaemonttu and Aijala there are alteration pipes of cordierite-anthophyllite rocks and a blanket type of alteration zone. As a rule the alteration around the ores has depleted the rocks in sodium and increased the abundances of magnesium, iron, manganese and titanium. The behavior of silicon, aluminum, calcium, and potassium varies when compared with the presumed primary composition.Even though the proportion on massive sulfides is low, the Aijala Cu-Zn, Metsaemonttu Zn-Pb, and Orijaervi Zn-Cu ores may be allied with massive Precambrian volcanic-exhalative sulfide ores as their proximal ore types.