Paper No. 10
Presentation Time: 10:35 AM
STABLE ISOTOPIC CONTRAINTS ON THE PROCESSES OF COUNTRY ROCK CONTAMINATION AND POST-MAGMATIC ALTERATION OF THE KABANGA NI-CU SULFIDE DEPOSITS, TANZANIA
The Kabanga Complex of Tanzania is located in the Kibaran orogenic belt that extends from Zambia through the Dominican Republic of Congo, Burundi, Rwanda and Tanzania in a generally NE-SW direction. Massive and disseminated sulfides occur in mafic and ultramafic rocks of the Kabanga North, Kabanga Main and Main Intrusion. Rock types include melanorite, pyroxenite, olivine pyroxenite, harzburgite and gabbronorite. Hydrothermal alteration of the intrusions is common, with serpentine, chlorite and amphiboles present as secondary minerals. Country rocks are predominantly metasedimentary sequences that locally contain disseminated and lensoidal pyrite and pyrrhotite. Sulfur isotopic values of samples from the Kabanga North, Kabanga Main and Main Intrusion vary from 12 to 18, 10 to 23 and 10.5 to 17.5, respectively, whereas the d34S values of the spatially associated country rocks vary from 9 to 17, 12 to 21 and 7 to 20, respectively. The d18O values of olivine and pyroxene from the pyroxenite and peridotite bodies of the Kabanga North Intrusion range between 6.3 and 7.0 and between 6.9 and 7.4, respectively. d18O values of pyroxene and plagioclase from the peridotite, pyroxenite and gabbro units of the Kabanga Main Intrusion range from 5.3 to 7.5 and 6.9 to 7.1, respectively. Metasedimentary country rocks are characterized by d18O values between 9.5 and 12.5. The elevated d18O values of olivine and pyroxene from the Kabanga North intrusion are suggestive of relatively high (up to 35%) assimilation of country rocks. The range of values found in the Main and Kabanga Main intrusions suggest that the elevated values are the result of low temperature hydrothermal alteration, rather than assimilation. We suggest that extensive country rocks assimilation occurred only locally, but that country rock sulfur was more uniformly transferred to all magmas, probably via a vapor phase.