The geology of the Nkana-Mindola sediment-hosted Copper Cobalt deposit, Zambian Copperbelt, Zambia
Creators: name: Croaker, M
Publisher: name: University of Tasmania
Year: 2023

description: The Nkana-Mindola (NKM) Cu-Co deposit is located in the southeast portion of the Zambian Copperbelt, and has seen nearly sixty years of continuous mining. All economic mineralisation is currently confined to a narrow, argillaceous interval, known colloquially as the Ore Shale. Highest grade intervals occur on the NE limb of the NW plunging Nkana Syncline, and constitute a stratabound orebody with strike extent of ~16km. Including lower grade mineralisation, positioned on the SW limb of the fold, increases the strike length to ~35 km. The deposit has an estimated global resource of in excess of 670mT@ 2.2% Cu and 550 Mt @ 0.1% Co, making it one of the larger Cu deposits in the Zambian Copperbelt.The host strata of NKM deposit are Neoproterozoic meta-sedimentary rocks of the Lower Roan Group. This ~400 m thick package records the early stages of rift development, and can be broadly subdivided into two mappable units: the basal fluviatile-dominated Mindola Clastic Formation (MCF), and overlying marginal marine-dominated Kitwe Formation (KF). In detail, five depositional stages are defined, constituting packages of member status, which reflect cyclical shoreward facies shifts. These include 1) the Basal Sandstone Member; a sub-aerial alluvial fan and braided fluvial assemblage, 2) the Kafue Arenite Member; a sub-aerial to subaqueous alluvial fan, fan delta to braided deltaic assemblage, 3) the Copperbelt Orebody and Rokana Evaporite Members; an upward-shoaling marginal marine argillite-sandstone-carbonate package deposited under local evaporitic . conditions, the 4) Nchanga Quartzite Member; a sandstone-argillite assemblage deposited in braided deltaic environment; and 5) the upper Chambishi Dolomite Member; a carbonate-evaporite assemblage deposited in a shallow marine to deltaic environment.The fine-grained siliciclastic-carbonate-bearing KF has, in the main, laterally persistent beds and facies‚ to the kilometre scale. However local facies, if not thickness, variations occur in its basal ore-bearing unit, the Copperbelt Orebody Member (or so called Ore Shale). These facies variations at the level of ore coincide with more pronounced thickness anomalies in footwall MCF strata. Specifically, lateral transitions from richly mineralised argillaceous facies of the Copperbelt Orebody Member to poorly endowed arenaceous or carbonate facies correspond to abrupt 'pinch-outs' of the underlying MCE The overall change in stratal architecture across the MCF-KF interface records an expansion in depocentre dimension and accommodation development, during the transition from rift initiation to rift climax.Lower Roan Group strata and their contained stratabound ores were deformed during the ~560Ma to 520Ma Lufilian Orogeny. This resulted in regional scale folding of the sedimentary and malic volcanic rocks of the Katangan Supergroup, forming northwest-southeast trending structural fabric and penetrative cleavage throughout the southeastern portion of the Lufilian Fold Belt The 2nd order Nkana-Syncline is one of the dominant structures in the southeast corner of the ZCB and the penetrative cleavage is related to this event. Folds formed during the Lufilian Orogeny are open to isoclinal and predominandy plunge northwest with local plunge reversals. Numerous 3rd and 4th order parasitic folds to the Nkana-Syncline are recognised within the NKM deposit. D3 folding dominates at the NKM deposit.An analysis of the fold patterns and geometries at NKM show that the nucleation sites wavelengths and attitudes of macroscopic folds are significantly influenced by the original basin architecture particularly the position location and morphology of MCF depocentres. With progressive amplification and tightening of folds rheological contrast between the MCF and KF increasingly influenced the mechanics of folding and the style of deformation partitioned along the contact of these two units. While localised veining and more regional decoupling are both characteristic of this stratal interface there is little evidence exists for significant displacement.Different vein generations at NKM are distinguished by cross-cutting relationships with the main foliation. Bedding parallel quartz-carbonate-sulphide veins are typically formed in carbonaceous argillite facies and have a restricted distribution to the base of the Copperbelt Orebody Member. Foliation parallel and sub-parallel quartz-carbonate-mica-sulphide veins are also confined to the argillaceous units. However these are observed only in the highest strain parts of the deposit. Paragenetically late stage veins which cross-cut foliation are rare and host sulphides.The copper sulphide minerals are laterally zoned across the NKM deposit with a bornite-chalcopyrite dominant assemblage in the north and a chalcopyrite-pyrite assemblage in the south. The zonation coincides with a change from the northern dolomite-argillite facies association to the carbonaceous-carbonate facies association in the south. Vertical zonation is variable however a uniform stratabound pyritic hangingwall to the copper orebody occurs across the deposit. High grade Cu intervals correspond to fringes of MCF depocentres and local lateral facies boundaries in the Copperbelt Orebody Member. The relationships suggest that mineralising fluids were directed along the base of the host horizon through tapering MCF packages. Relatively high permeabilities within the MCF both primary and reaction-enhanced coupled with fault- and facies controlled cross-stratal fluid pathways and partitioning of in situ sulphur sources (anhydtite±pyrite) within the Copperbelt Orebody Member all contributed to the distribution of primary Cu-Co ore.Sulphide remobilisation occurred during deformation/metamorphism resulting in the localised concentration of sulphides aligned parallel to cleavage in fold hinges and syn-tectonic veins. Late stage syn-tectonic pyrite and vein hosted pyrite remains predominately confined within the Cu ore envelope and the immediate hanging wall. The macroscopically stratabound folded geometry of the orebody is taken to indicate that the majority of the majority of Cu sulphides pre-date the development of S<sub>3 </sub>cleavage and peak metamorphism.Potassic and sodic alteration is recorded mineralogically by antigenic K-feldspar sericite albite and metamorphic phlogopite. These phases form part of an alteration assemblage which includes quartz dolomite/calcite and varying amounts of tourmaline and rutile. Phlogopite (Mg#&gt;0.7) has consistently high K<sub>2</sub>O/Al<sub>2</sub>O<sub>3</sub> values (&gt;0.68) a feature interpreted to record metamorphism of argillaceous strata affected by pre-existing potassic alteration. There is broad variation in the intensity of potassic alteration throughout the deposit with weakly mineralised strata positioned on the SW limb of the Nkana-Syncline possessing significantly lower levels of K<sub>2</sub>O compared to the NE limb. Carbonaceous-carbonate argillite bas TOC values varying between 0.01 to 3.54%. Ag Bi Cu Co and Mo are significantly enriched relative to 'average shale' abundances.Barren gap massive carbonates possess C-O isotopic signatures (δ<sup>13</sup>C values ~+3.2‰o and δ18O ~+23.4‰) that are largely indicative of precipitation from Neoproterozoic seawater. There is a trend towards depleted δ13C and δ18O values (average values of δ13C = -14.5‰ and δ18O = + 13.8‰) in strata closely associated with argillite indicating additional contribution of organic carbon. All mineralised syn- and post-cleavage vein sets (VII and VIII) have C-O isotopic values that are indistinguishable from their host strata indicating that no new reservoirs of carbon or oxygen were introduced during orogenesis. Sulphur isotopes of various anhydrite phases indicate a consistent seawater source for sulphur contained in sulphate phases at least <sup>87</sup>Sr/<sup>86</sup>Sr values (0.7183 to 0.7413) of carbonates are more radiogenic compared Neoproterozoic carbonate.Sedimentology structural paragenetic and isotopic data suggest that Cu mineralisation at the NKM deposit occurred in a multi-stage basinal process throughout diagenesis and orogenesis. The circulation of Mg- K- and Na- rich fluids formed large scale basin wide alteration patterns that extended beyond the NKM deposit It is proposed that the transportation of the metals now hosted within the Copperbelt Orebody Member are best explained by moderate temerature (~140 to 180°C), oxidized, evaporite derived basinal brines circulating through the broader basin. Copper precipitation resulted from these fluids reacting with organic material and/ or mixing with sulphides or sulphide solutions.