『Abstract
We report paleomagnetic data from samples spanning several grades
of enrichment in the Kalahari manganese field, South Africa, in
order to assess mineralogical aspects of the ore-forming stages,
and also to date these stages through comparison to previously
existing, well dated paleomagnetic results from the Kaapvaal-Kalahari
craton. Our paleomagnetic study confirms a multistage evolution
for the orebodies, with three distinct, ancient remanent directions
preserved. An early diagenetic remanence direction (MAM-1), associated
with “dusty” hematite inclusions (1-10μm) that are omnipresent
in the microcrystalline matrix of low-grade, Mamatwan-type ore,
yields a tilt-corrected paleomagnetic pole (-08.2゜N, 111.1゜E,
dp = 5.6゜, dm = 11.1゜; n = 6 specimens) that is similar to previous
results from the immediately underlying Ongeluk lavas. A late
diagenetic or weak metamorphic overprint (MAM-2), carried by recrystallized
hematite (20-250μm), within both Mamatwan- and Wessels-type ore,
generates a paleomagnetic pole (present coordinates 12.1゜N, 321.8゜E,
dp = 3.4゜, dm = 6.0゜; tilt-corrected 16.1゜N, 317.8゜E, dp = 3.4゜,
dm = 6.4゜; n = 14 specimens) that resembles those from the ca.
1900 Ma Hartley lavas and Mashonaland sills. The MAM-2 overprint
may be related to Kheis thrusting at 1750 to 1800 Ma as previously
proposed or to magma-driven fluid migration during rifting as
the Hartley-Mashonaland igneous event perforated the Kalahari
craton. The third magnetic component observed in our sample suite
(WESS) is restricted to high-grade Wessels-type ore, rich in high
Fe hausmannite and coarser hematite (0.1-1.0 mm), in the immediate
vicinity of north-trending normal faults. It yields a pole (54.4゜N,
033.7゜E, dp = 4.7゜, dm = 9.1゜; n = 7 specimens) that is similar
to both the ca. 1250 and 1100 Ma portions of the Kalahari craton's
apparent polar wander path. Either of these ages would be in accordance
with previous multigenetic models for the Wessels event and its
regional crosscutting relationships. Our WESS paleomagnetic pole,
combined with previous paleomagnetic results from the Sishen-Postmasburg
region, temporally links Kalahari manganese field hydrothermal
upgrading with east vergent thrusting in the Griqualand West foreland,
during the early or medial stages of the late Mesoproterozoic
Namaqua orogeny.』
Introduction
Methods
Paleomagnetic results
Discussion
Acknowledgments
References