MAGNETIC FABRIC CONSTRAINTS ON MAGMATIC PROCESSES: INSIZWA LAYERED MAFIC INTRUSION, SOUTH AFRICA

The Insizwa sill of eastern South Africa is a classic open system magma chamber. Its emplacement is related to the Mesozoic Karoo flood basalts. The sill did not undergo any significant syn-emplacement or post emplacement deformation; therefore primary fabrics resulting from emplacement can be studied, constraining models of ore deposition.

Vertical borehole cores were used to characterize the changes in magnetic properties and fabric throughout the SE portion of the intrusion. On three separate cores, spaced 5 km apart, the lateral continuity of both composition and magnetic fabric was evaluated. Bulk susceptibility measurements of closely spaced samples in all three cores show significant variation in magnetic properties with depth. In two of the cores, changes in magnetic susceptibility correlate strongly with changes in petrology. AMS analysis was also conducted on these cores to determine possible correlation with changes in composition. In general, neither the orientation nor the magnitude of the AMS ellipsoid correlated to the bulk susceptibility or mineralogical changes.

AMS does provide information on flow directions, provided that the source of the magnetic fabric is known. These Fe-rich rocks often contain multiple ferrimagnetic phases (pyrrhotite and magnetite), whose contributions to the magnetic anisotropy must be separated. In addition, mafic rocks often contain both multidomain (MD) and single-domain (SD) magnetite grains, which contain normal and inverse magnetic fabrics, respectively. We will utilize new high-field magnetic techniques in order to overcome these problems and obtain flow directions. Oriented samples collected on the margins of the intrusion will provide an interpretive framework for our observations.

Knowledge of flow direction allows us to test models of magmatic sulfide deposition. If a magma containing a separate sulfide phase forms at depth and rises rapidly through conduits to fill a chamber, the decreased flow rate upon entering the chamber will cause the sulfides to be deposited. The position of the site of injection, determined from the flow field, therefore provides a testable drilling target.