MULTI-STAGE MINERALIZATION IN THE ZAMBIAN COPPERBELT BASED ON RE-OS ISOTOPE CONSTRAINTS

The Zambian Copperbelt (ZCB) is the world’s second largest sediment-hosted stratiform copper district, with about 100 Mt of copper production and reserves. The deposits are hosted in Lower Roan Group siliciclastic rocks of the Neoproterozoic Katangan Supergroup. The age of the Roan sequence is poorly constrained between 877 ± 11 Ma, the age of a basement granite, and 750 Ma, the age of overlying diamictite and volcanic rocks. The Katangan sequence was deformed and metamorphosed during the Lufilian orogeny, between about 590 and 500 Ma. Proposed models for the deposits’ genesis range from syngenetic to epigenetic, however, previously obtained mineralization ages all suggested a late metamorphic timing. Here we use Re-Os isotopes to demonstrate that mineralization occurred in multiple stages over the evolution of the Katangan basin.

Preliminary data on the Konkola deposit suggests a mineralization event at ~800 Ma. This provides a new minimum age for Roan sedimentation, and confirms an episode of diagenetic mineralization occurred in the ZCB. A five point isochron with analyses from the Nkana, Chibuluma and Nchanga deposits yields an age of 583 ± 24 Ma, and a ¹⁸⁷Os/¹⁸⁸Os initial value of 1.22 ± 0.13 (MSWD=0.67). The sulfides (chalcopyrite, carrolite, bornite) have Os concentrations ranging between 24-75 ppt and Re concentrations between 0.6 to 5.8 ppb. This age overlaps ages of earliest biotite and monazite (~580-592 Ma) in the ZCB, as well as a synorogenic granite in the southern Lufilian Arc (~560 Ma). A Re-Os molybdenite age of 525.7 ± 3.4 Ma was determined for the Nkana deposit. Re and Os concentrations for the molybdenite are 1789 ppb and 325 ppm, respectively. This mineralization formed during peak metamorphism, as recorded by ages of ~530 Ma on whiteschists throughout northern Zambia. Previous dates on mineralization elsewhere in the ZCB returned ages of 500 to 512 Ma.