Geology and stable isotope study of Arthrath mafic intrusion and Ni-Cu mineralization, northeast Scotland

The Ordovician Arthrath mafic intrusion is synorogenic and comprises orthopyroxene-rich xenolithic rocks and maflc-ultramafic cumulates with low-grade Ni-Cu sulphide mineralization; the geology and mineralogy are described. The sulphide mineralogy consists mainly of pyrrhotite, chalcopyrite and pentlandite displaying dominantly magmatic textures with some later remobilization by shearing and hydrothermal activity. The role of country-rock assimilation in controlling the mineralization has been assessed through a study of oxygen and sulphur isotopes in rocks and minerals.

delta(34)S values for sulphides, mainly pyrrhotite, from igneous rocks are -1.2 +/- 1.0 parts per thousand (1 sigma), from xenolithic igneous rocks -0.9 + 0.5 parts per thousand (1 sigma) and from metasediment hornfels -10.0 +/- 2.5 parts per thousand (1 sigma). delta(18)O values for igneous rocks range from 4.5 to 8.7 parts per thousand, for biotite are 5.1 and 5.2% and for orthopyroxene range from 6.9 to 7.8 parts per thousand. It is concluded that, despite much isotopic and geological evidence for substantial assimilation, the mineralization is dominated by magmatic sulphur. Whole-rock and mineral delta(18)O values record a post-magmatic interaction with a surface-derived fluid at high temperature.

A model is envisaged whereby a mafic magma, close to sulphur saturation on emplacement, assimilated siliceous country rocks and this triggered the separation of a Ni-Cu-rich sulphide Liquid. The model predicts that Ordovician mafic intrusions in the Grampian region that contain Lower Zone (olivine-rich) rocks acid evidence of extensive assimilation are prospective for mineralization.