Geochemical variations in a Proterozoic hydrothermal mafic breccia dyke related to Ni-Cu-Fe skarn mineralization at Annehill, Bergslagen, Sweden

Abstract

Nickel sulphide breccias form an uncommon type of Ni mineralization. The development of a Proterozoic mafic breccia dyke and Ni-Cu-Fe skarn in felsic metavolcanic wall rocks at Annehill, Bergslagen, Sweden, is genetically related to the emplacement of a continental tholeiite dyke. Hydrothermal alteration prior to and during brecciation distinguishes this mineralization from intramagmatic Ni mineralizations. Breccia development can be divided into two stages: An initial pre-brecciation stage of pervasive alteration of the felsic metavolcanics produced minerals in the paragenetic order hydromuscovite-tourmaline-phlogopite-amphibole. Microprobe data shows this corresponds to B, F and Cl metasomatism respectively, and can be correlated with major element variations. High Ti mobility is demonstrated by the development of sphene poikiloblasts in the metavolcanics. A second stage of mechanical fracturing developed a 1 km long, up to 20m wide, breccia zone, with fragments of more or less altered metavolcanics in an amphibole matrix. Amphibole compositions become more Fe-rich closer to the tholeiitic dyke, which itself has undergone autometamorphism, with no magmatic minerals present. Fe and V provide the best indicators of chemical variation in the altered rocks, breccia dyke and tholeiite. Highest B contents coincide with highest K, Rb, Li and Co in the altered rocks. There is a steady decrease in Fe, V, Ti, Mn, Co, and P from the tholeiite through the breccia and pervasively altered metavolcanic to least altered metavolcanic. With decreasing Fe there is an increase in Ca, Mg, and K, and Fe also shows a positive correlation with Be, Pb, Sc and Eu. REE contents of pervasively altered and brecciated samples generally lie between those of the least altered metavolcanic and the tholeiitic dyke. This supports a model for contamination of the felsic metavolcanics with material derived from the tholeiite dyke. The skarn ore comprises pyrrhotite with minor chalcopyrite and pyrite, and is located along the eastern margin of the breccia. Ni and V contents increase to 0.7%. Emplacement of the tholeiitic dyke, along a pre-existing fracture, initiated a hydrothermal cell in the upper crust. Volatiles derived mainly from the wall rocks were responsible for the alteration of the felsic metavolcanics. At the same time a Nibearing hydrothermal phase developed in the crystallizing magma. Brecciation could have been triggered by fault movements or pressure build up in the hydrothermal cell. Pressure release through brecciation allowed injection of the Ni-bearing hydrothermal phase into the felsic wall rocks to form the skarn mineralization along one side of the breccia dyke.