The Shamrocke ore body is a stratiform deposit of disseminated copper-iron sulphides found within lenses of calcareous meta-arkose occurring in Lomagundi Group graphitic schist (Late Precambrian) at Karoi, Rhodesia. Both sulphides and sediments were subjected to high grades of regional metamorphism.
Argon/argon isochron ages indicate a major metamorphic event at 550 m.y. (Damaran orogeny) with later minor argon losses. Ore sulphide 8 a~ S values range from + 3.0 to + 14.8~ CDT and a general decrease from footwall to hanging-wall reflects an original sedimentary environment where sulphides formed about the sea bed from hydrothermal fluids progressively mixing with sea water. Isotopically lighter sulphides formed syngenetieally in the host rocks from bacterial reduction of sulphate. The pyrrhotite was probably formed from pyrite during metamorphism, and owing to reducing conditions maintained similar 8S~S values to the original pyrite. Oxygen and carbon isotopic analyses of mineralised and unmineralised carbonate lenses are consistent with deposition as marine limestones in an evaporitic environment and/ or near hot spring vents. * I. N. S. Contribution No 734
The deposit was first studied by Kyle (1972) who concluded that the mineralisation is epigenetic but possibly rectasomatically relocated during metamorphism.
Thole (1976), however, considers the Shamrocke deposit to be syngenetic in origin. Dechow and Jensen (1965) in a sulphur isotope study of some central African sulphide deposits, measured ten samples from the Mangula copper mine of the Deweras Group and five from the Chipinga Badza Prospect in a pyroxenite. The negative 6 s~ S values obtained for the former deposit led the authors to suggest a bacteriogenic origin for the sulphides whilst values close to 0 %o for the former deposit were consider ed consistent with a magmatic origin.