Fluid evolution and characterisation of mineralising solutions in the Central African Copperbelt
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Date
2009-07-08T08:12:58Z
Authors
Greyling, Lynnette Natasha
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Abstract
The reconstruction of the fluid evolution of the Zambian Copperbelt is presented,
where extensive stratiform copper-cobalt mineralisation is hosted in the metasediments of
the Katanga Supergroup located in the Lufilian Arc in the Democratic Republic of the
Congo and Zambia. Fluid inclusions are characterised from three different tectonic
settings at selected deposits within the Zambian Copperbelt in terms of basinal, early
orogenic, and late orogenic settings.
Fluid inclusion microthermometry, Raman microspectroscopy, laser ablation
inductively coupled plasma mass spectrometry, and stable isotope analysis are used to
investigate the nature of basinal, early orogenic, and late orogenic fluids.
NaCl-CaCl2, MgCl2-KCl brines are found to have circulated the sedimentary pile
immediately after stratiform copper mineralisation and are the earliest fluids documented
from the Copperbelt. MgCl2-KCl brines were also trapped together with CO2-CH4-N2 fluids
during these times. Basinal fluids were trapped under minimum pressure temperature
conditions of 350 bar and 120°C.
Four distinct early orogenic fluids are found at the Chambishi, Nchanga, and Nkana
deposits and consist of NaCl-CaCl2-MgCl2-KCl, CaCl2-KCl, NaCl-CO2-CH4-N2, and pure
carbonic fluids. It is seen that these fluids at pressures of 550-800 bar and temperatures
of 153-260°C which corresponds to depths of ~5.6-8.2 km.
Late orogenic fluids from the Mufulira and Nkana deposits are the hottest and most
complex fluids documented during this study and contain mixtures of hypersaline NaCl-
CaCl2-MgCl2 ± KCl and NaCl-CaCO3-saturated ±N2 brines. Fluids were trapped at
minimum temperatures of 240°C and 1750 bar pressure. In addition to these fluids, a low
salinity CaCl2-MgCl2-CO2-CH4 fluid is also present during late orogenesis. Minimum depth
estimates for late orogenic fluids are around 6.61 km. The highest salinities of all three
fluid types are found within these late orogenic fluids in addition to the highest levels of
Pb, Zn, Cu, Co and Sr. This indicates the prolonged presence and equilibration of these
fluids with the host rock. Moreover, late orogenic fluids are also more oxidised compared
to relatively more reduced basinal fluids.
A key to the widespread and paragenetically complex nature of the Copperbelt ores
may be related to the longevity of fluid flow and the progressive ability of fluids to
transport metals. The marked increase in Pb and Zn in later fluids may be relevant to the
formation of regional epigenetic Pb-Zn deposits further afield of the Copperbelt, such as ii
at Kabwe and Kipushi. The late orogenic fluids seen in the Copperbelt compare
favourably with mineralising fluids of the epigenetic Pb-Zn Kabwe deposit in terms of
metal composition, temperature and moderate salinities.
It is envisaged that early stratiform mineralisation is followed by continuing fluid
circulation in the basin whereby metals are scavenged by hotter fluids which continue to
act as mineralising agents.