Mineralization in the Karmoy ophiolite, southwest Norway

Scott, James L. (1992) Mineralization in the Karmoy ophiolite, southwest Norway. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Karmoy ophiolite is a remnant of Early Ordovician oceanic crust which is thought to have formed in a supra-subduction zone environment, and undergone development in a back-arc basin before its final emplacement on the Baltic craton. The uppermost sections of this ophiolite are exposed in the study area; the rock types are gabbros. sheeted dykes, plagiogranites, and lesser amounts of pillow lavas, diorites, volcaniclastic sediments, and breccias. -- Contained within these rocks are two sulfide deposits, the Visnes massive l-c-Cu /.n sulfide deposit and the Feoy massive Fe-Cu-Ni-PGB sulfide deposit. While they arc spatially quite closely related, these deposits record two distinctly different periods of mineralization during the evolution of the ophiolite complex. -- The Visnes deposit formed as a result of sub-seafloor alteration of basaltic rocks of the Visnes High Level Complex. Rocks of this complex are altered to spilites on a regional scale. Geochemically, alteration of the sheeted dykes is defined by: a relative loss of K2O, CaO, MnO, A1O203, MgO, Cr, Ni, Cu, Zn, Pb and Sr; a relative gain of TiO2, Fe2O3, Na2O and Zr, and; variable changes in P2O5 and SiO2 relative to unaltered rocks from similar tectonic environments. -- More local zones of intense alteration and mineralization within the sheeted dykes are called "epidosite zones", and these represent flowthrough zones for highly corrosive, magma-influenced hydrothermal fluids. "Epidotites", or deuterically formed rocks composed of nearly 100% coarse crystalline epidote, are closely associated with such zones, and their presence suggests that the hydrothermal fluids responsible for formation of epidosite zones may well have had a direct magmatic water input. However, oxygen isotope evidence from epidotites and plagiogranites suggest that the magmatic fluids may ultimately have been derived from magma-chamber assimilation of seawater altered roof rocks. -- The Feoy deposit is a small, orthomagmatic Ni-sulfide body which appears to have very few known analogues elsewhere in the world. Petrogenetic modelling using Platinum Group Element (PGE) abundances and sulfur isotopes indicates that the sulfides precipitated from an already-depleted partial melt during its ascent from a mantle source. The deposit is predominantly a massive pyrrhotite/chalcopyrite deposit with minor pyrite and pentlandite. It is unique in its high concentrations of Platinum Group Minerals (PGM's); this is a reflection of the PGE-enriched nature of the residual sulfide fraction present in an already-depleted (previously melted) mantle parent. -- The co-magmatic silicate host rocks for the Feoy deposit are high-Mg basalt dykes, and these contain Ni-sulfides interstitial to silicate minerals. The host rocks can be petrogenetically modelled using PGE's, and as expected show PGE patterns indicative of formation from a second stage melt, or from partial melting of an already-depleted mantle. -- The Feoy deposit formed later than the Visnes deposit; rocks associated with the Visnes deposit are spilitized, and are dated al ca. 493ma (using Pb/Pb Zircon dating techniques); rocks associated with the Feoy deposit (high-Mg dykes. Type 2 plagiogranites and clinopyroxene-phyric intrusions in the Feoy area) are not spilitized, and are dated at ca. 470ma. -- All rocks in the study area are variably altered and deformed. It appears that such deformation post-dates formation of both sulfide deposits as well, since they have both been mobilized along shear zones. Also, volcaniclastic sediments of the Torvastad Group, which are compositionally and genetically related to late clinopyroxene-phyric intrusions in the area, and which represent a period of arc-basin formation following arc-volcanism, are themselves quite deformed and cut by shear zones. -- The occurrence of tourmaline-bearing breccias in the Feoy area is an intriguing addition lo the study of late deformation features. Breccia, containing rounded Ni-sulfide and tourmalinite fragments in a hydrothermal cement matrix, is found in the Feoy area. Textural, isotopic and chemical analysis of the breccia and tourmaline strongly suggests that it formed as a result of the forceful intrusion of a hydrous granitic body into the surrounding country rocks. This may well represent part of the nearby West Karmoy Igneous Complex, which is a large granitic pluton that intrudes all rocks of the Karmoy ophiolite. Indeed, shear zone formation in the study area may also be related to this late, post-emplacement event.

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