Holwell_etal_Munali OGR accepted.pdf (3.88 MB)
The Munali Ni sulfide deposit, southern Zambia: a multi-stage, mafic-ultramafic, magmatic sulfide-magnetite-apatite-carbonate megabreccia
journal contributionposted on 2017-03-23, 14:51 authored by David A. Holwell, Chloe L. Mitchell, Grace A. Howe, David M. Evans, Laura A. Ward, Richard Friedman
The Munali Intrusive Complex (MIC) is a flattened tube-shaped, mafic-ultramafic intrusion located close to the southern Congo Craton margin in the Zambezi belt of southern Zambia. It is made up of a Central Gabbro Unit (CGU) core, surrounded by a Marginal Ultramafic-mafic Breccia Unit (MUBU), which contains magmatic Ni sulfide mineralisation. The MIC was emplaced into a sequence of metamorphosed Neoproterozoic rift sediments and is entirely hosted within a unit of marble. Munali has many of the characteristics of craton-margin, conduit-style, dyke-sill complex-hosted magmatic sulfide deposits. Three-dimensional modelling of the MUBU on the southern side of the MIC, where the Munali Nickel Mine is located, reveals a laterally discontinuous body located at the boundary between footwall CGU and hangingwall metasediments. Mapping of underground faces demonstrates the MUBU to have intruded after the CGU and be a highly complex, multi stage megabreccia made up of atypical ultramafic rocks (olivinites, olivine-magnetite rocks, and phoscorites), poikilitic gabbro and olivine basalt/dolerite dykes, brecciated on a millimetre to metre scale by magmatic sulfide. The breccia matrix is largely made up of a sulfide assemblage of pyrrhotite-pentlandite-chalcopyrite-pyrite with varying amounts of magnetite, apatite and carbonate. The sulfides become more massive towards the footwall contact. Late stage, high temperature sulfide-carbonate-magnetite veins cut the rest of the MUBU. The strong carbonate signature is likely due, in part, to contamination from the surrounding marbles, but may also be linked to a carbonatite melt related to the phoscorites. Ductile deformation and shear fabrics are displayed by talc-carbonate altered ultramafic clasts that may represent gas streaming textures by CO2-rich fluids. High precision U-Pb geochronology on zircons give ages of 862.39 ± 0.84 Ma for the poikilitic gabbro and 857.9 ± 1.9 Ma for the ultramafics, highlighting the multi-stage emplacement but placing both mafic and later ultramafic magma emplacement within the Neoproterozoic rifting of the Zambezi Ocean, most likely as sills or sheet-like bodies. Sulfide mineralisation is associated with brecciation of the ultramafics and so is constrained to a maximum age of 858 Ma. The Ni- and Fe-rich nature of the sulfides reflect either early stage sulfide saturation by contamination, or the presence of a fractionated sulfide body with Cu-rich sulfide elsewhere in the system. Munali is an example of a complex conduit-style Ni sulfide deposit affected by multiple stages and sources of magmatism during rifting at a craton margin, subsequent deformation; and where mafic and carbonatitic melts have interacted along deep seated crustal fault systems to produce a mineralogically unusual deposit.
The management of Consolidated Nickel Mines, especially Simon Purkiss, are thanked for their assistance with setting up the fieldwork, providing a research funding for this project, including the MGeol projects of CLM, GAH and LAW; and for granting permission to publish. The staff of Mabiza Resources at the Munali Nickel Mine, including Danny Musemeka, Peter Munkondya, and Matt Banda, are thanked for logistical support on site in Zambia and for facilitating underground work. Reviews by Steve Beresford and John Hronsky are acknowledged for providing positive and helpful suggestions in improving the manuscript. The geochronology was funded by a Society of Economic Geologists Student Research Grant awarded to GAH. Discussions in the field with Darryl Mapleson, Daryl Blanks and Rob Moore are acknowledged. Clare Kelly is thanked for technical support with Micromine and Shaun Graham for assistance with Mineralogic mapping at Zeiss and support for LAW’s MGeol project.
CitationOre Geology Reviews, 2017
Author affiliation/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Geology
- AM (Accepted Manuscript)