Structural controls, 3D geometry and the evolution of copper mineralization at Frontier Mine, Zambian Copperbelt

Date
2024-12
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Publisher
Stellenbosch University
Abstract
The Frontier Mine was discovered by a regional soil sampling campaign and defines a structurally-controlled vein-hosted Cu deposit within deformed Neoproterozoic sediments of the Katangan Supergroup. This sequence was deposited in a series of intra-cratonic rift basins, hosting Cu-Co deposits defining the Central African Copperbelt and deformed by the Lufilian Orogeny between ~590 and 500 Ma. The deposit is located on the northeastern flank of the Kafue Anticline, a Paleoproterozoic basement dome, within the Zambian Copperbelt, renowned for its sediment-hosted, stratiform copper deposits. The greenschist-facies, tectonostratigraphic package of Frontier Mine has been duplicated by a regional-scale isoclinal recumbent fold which was coaxially refolded by a set of open to closed, upright folds, defining a type three refolded fold pattern (F1/F2). The Frontier Mine differs from other deposits as Cu occurs predominantly as coarse-grained chalcopyrite in quartz-carbonate veins, hosted predominantly within the Mwashya Subgroup of the Katangan Supergroup, and shows a close association to sodic (albite) alteration, where at least three mineralized vein sets are recognized in the pit. Early bedding/S1-parallel veins (Vein Set 1) formed during layer-parallel shear and the onset of recumbent folding (F1). This early vein generation is relatively poorly mineralized, but was associated with a near-pervasive, bedding-parallel albitization and replacement of the mainly metapelitic wall rocks. The reaction hardening process promoted brittle deformation during the subsequent upright refolding (F2) of the rocks and the formation of a second generation of highly-mineralized, saddle-reef type veins (Vein Set 2) in the hinges of particularly upright F2 folds (F1/F2). The controls of Vein Set 2 by fold hinges defines the overall shoot-like geometry of the orebody, parallel to the SE-plunging folds. The subhorizontal orientation and brecciated textures of a subsequent Vein Set 3 underline the formation of veins through continued northeast-southwest shortening of the brittle wall rocks during the lock up of the refolded first-order fold. Successively emplaced vein sets document the close interplay between the progressive structural evolution of the deposit, recording northeast-southwest shortening, and associated hydrothermal fluid flow and alteration (albitization). This close association of progressive deformation, hydrothermal fluid flow and alteration is further constrained by an implicit 3D model, allowing for the integration of outcrop-scale structural data collected in the field, describing the controls of the mineralization (bedding and formation contacts, F1/F2 fold hinges and cross-cutting vein sets), with drillhole data recording downhole lithologies and grade intersections, where a few of these drillholes contain downhole structural logging. The implicit 3D model allows for the visualization of the deposit-scale geometry of the lithologies, Cu mineralization and alteration, and the interchange of these features. Moreover, it permits the visualization of individual oreshoots, furthering the understanding of the position and controls of the high-grade oreshoots on a deposit-scale. The overprinting and structural relationships between vein sets, alteration and the regional fold structures signify the syn-tectonic, long-lived, multi-phase mineralization during the Lufilian Orogeny, making Frontier Mine a distinct and unique deposit in the Zambian Copperbelt.
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