Ascent and Emplacement of Mafic Magmas in the Mid-to-Lower Crust: 3D Modeling of Nickeliferous Gabbroic Intrusions in Namaqualand, South Africa

Low-viscosity mafic magmas commonly ascend through the brittle upper crust as sheet-like dykes and sills. At mid- to lower-crustal depths, high temperatures and weak wall rocks are expected to inhibit fracture-controlled propagation, although preserved field examples are rare. Four hundred and three exploration boreholes were used to reconstruct the three-dimensional geometry and contact relationships of 1060–1030 Ma Koperberg Suite gabbronorite intrusions at Hondekloof, Namaqualand, South Africa. The intrusions were emplaced into partially molten granulite-facies gneisses and form three principal isolated lensoid intrusions, accompanied by numerous smaller intrusions, across 1.8 km of strike. The principal lenses have a combined modeled volume of approximately 2.3 × 106 m3 and preserve sharp internal contacts between successive magma batches, wall-rock xenoliths, back-veins, reaction margins, and basal sulfide concentrations. Sharp internal intrusive contacts document successive magma inputs and support incremental assembly of the principal lenses. Partial melting, fabric development, and reaction along intrusion margins record viscous wall-rock accommodation during emplacement. The isolated geometries and modeled stringers that widen upward into larger bodies are consistent with arrest and amalgamation into transient magma reservoirs. Repeated use of emplacement pathways is inferred within the preserved corridor; the complete ascent route from depth is not preserved. The specific controls on arrest remain unresolved, but likely involved wall-rock rheology, structural and lithological anisotropy, pathway geometry, and variations in magma supply.