Origin of black shales and the serpentinite-associated Cu-Zn-Co ores at Outokumpu, Finland

• DOI: 10.2113/gsecongeo.94.7.1007
• Publication Date: 2009-04-03T13:38:06Z
• Authors: Kirsti Loukola-Ru...
• Keywords: Tourmaline

The Outokumpu Cu-Zn-Co deposits are associated with serpentinites, interpreted as ophiolitic. serpentinites surrounded by dolomite-rich rocks, calc-silicate fine-grained quartz metamorphosed black shales (black schists), and mica schists. ore is hosted mainly but also rocks Calc-silicate anomalously enriched respect to Ni Cr. original reserves in the area were 50 million tonnes (Mt) sulfide grading 1.2 3.8 percent Cu, 0.6 1 Zn, 0.1 0.2 Co, Ni, 0.8 g/t Au, 9 Ag.Black schists serpentinite-associated prospects mines Kainuu-Outokumpu compared studying more than 100 drill cores over 800 samples. entire exhibit many features common: an average of 7 S graphitic C, formation thicknesses commonly exceeding m, textures such laminae rich spheroidal pyrite locally veined sulfides. uniform distribution heavy metals thick widespread shale formations provides evidence for influx hydrothermal fluids seawater during sedimentation organic-rich mud.Carbon isotope delta 13 C values grade from -30 -19 per mil. Values about -20 mil probably result exchange reactions between organic carbonate metamorphic processes. Fine-grained ( 34 -12.7 -6.4 mil, those coarse-grained ranging -5.9 +1.4 ores comparable These suggest bacterial reduction sulfate, addition S. Nickel concentrations pyrite, high 0.7 percent, occur Talvivaara schist-hosted occurrence (300 Mt, 0.26% 0.14% 0.53% Zn). Cobalt (0.3% maximum) (average 1.1%) differ. vary widely pyrites. This suggests that composition ore-forming was not constant; rather, Zn differed successive fluids.The (>20 m) deposited basins where bottom waters fluids. According genetic model now presented, formed a cap, enabling pulses Cu-Zn-Co-rich precipitate beneath cap. Subsequently, tectonic metaperidotites intruded shale-ore association. Alteration serpentinite released Ca- Si-rich material precipitated serpentinite. During processes, continued concentration silica-rich host resulted massive ore-bodies. Serpentinite-associated rare because close association coincidental.The feasibility using characteristics exploration tool tested Hammaslahti Cu-Zn-Au Vihanti Zn-Cu-Pb provinces Finland, well area. At both Vihanti, could have served cap rock under which precipitated.