HR: 0800h
AN: GP41B-03    [Abstracts]
TI: Magnetic Layering in the Great Dyke of Zimbabwe: Implications for Emplacement and ore Genesis
AU: * Butak, K C
EM: kbutak@siu.edu
AF: Department of Geology, Southern Illinois University 1259 Lincoln Drive, Carbondale, Il 62901, United States
AU: Ferré, E C
EM: eferre@geo.siu.edu
AF: Department of Geology, Southern Illinois University 1259 Lincoln Drive, Carbondale, Il 62901, United States
AU: Mathez, E A
EM: mathez@amnh.org
AF: Department of Earth and Planetary Sciences, American Museum of Natural History, New York, Ny 10024, United States
AU: Belley, F
EM: fanfan24@siu.edu
AF: Department of Geology, Southern Illinois University 1259 Lincoln Drive, Carbondale, Il 62901, United States
AB: The Great Dyke of Zimbabwe (GDZ) is an elongated mafic-ultramafic layered complex (3-11 km x 550 km) emplaced at 2575 ± 5 Ma in the Archean Zimbabwe Craton. It consists of 5 linked layered subchambers with a funnel shape in cross-section. The igneous layering in the Great Dyke dips gently inward to the center of each subchamber. All subchambers exhibit similar stratigraphy and consist of a lower ultramafic series capped by gabbros. The GDZ was formed by emplacement of successive magma batches that differentiated in-situ. Magmatic fractionation and mixing processes resulted in the development of a prominent compositional layering and ore concentration. Specimens come from a 592 m-long borehole core drilled through the mafic and ultramafic sequences. The magnetic susceptibility (K) was measured in low-field (LF) at a 1.5 m spacing, and in high-field (HF) at a 7.5 m spacing. K_LF ranges from 113 to 100000 x 10^-6 [SI], while K_HF ranges from 109 to 979 x 10^{- 6} [SI]. The variation of K_LF and K_HF allows for the identification of ferromagnetic and paramagnetic contributors to K. The mode of ferromagnetic oxides varies periodically and defines several layers with high magnetite content. The paramagnetic contributors show a slow increase through the lower mafic rocks. One section of the core in the lower mafic sequence displays anomalous magnetic characteristics such as a large increase in both low and high field magnetic susceptibility, and a decrease in mineral grain size. The magnetic results clearly discriminate rocks of the lower mafic sequence, ultramafic sequence, and rocks that are likely related to a later intrusive event into the GDZ. Additionally, the magnetic data allows for the further subdivision of the lower mafic sequence into magnetic layers with distinct characteristics. The disruption of all magnetic properties at the mafic-ultramafic boundary suggests a dramatic change in crystallization conditions. The slow increase of paramagnetic contributors in the lower mafic sequence reveals a slower change in crystallization conditions. This data seems to suggest a disturbance at the mafic-ultramafic boundary, possibly a reintrusion or extrusion event, followed by undisturbed fractional crystallization in the lower mafic sequence. This information has important implications for the magmatic history of this part of the magmatic chamber, and also for formation of major sulfide ore deposits located a few meters below the mafic-ultramafic boundary. Additional information about the magmatic history near this major boundary could help determine if these ore deposits result from cumulus settling of immiscible sulfide liquid, or by later post-cumulus processes involving late-stage magmatic fluids.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly