Fig. 8. Distribution of banded iron formations and shear zones. Black areas; high-amplitude, wide magnetic anomalies interpreted to be due to banded-iron formation material deep in the upper crust. Thin line; trace of the granitoid complex boundary in outcrop
and subcrop. Thick lines; extent of major shear zones crossing the west and north Pilbara Craton, mapped from gravity and magnetic anomalies, except AA the Scholl Shear Zone mapped by geology. Parallel lines; margin of the Pilbara Craton from gravity
anomalies.
Using the above arguments and magnetic, grav- ity, and geological data, I conclude that the upper
crustal structures extend down to about 14 km, the base of the upper crust indicated by seismic
refraction interpretation.
The minimum geographical extent of the green- stone belt synclines that extend well down into
the upper crust is shown by the extent of high-am- plitude long-wavelength magnetic anomalies Fig.
8, and by the axes of the gravity highs. These indicators allow deep synclines to be mapped in
areas of cover, and, importantly, show that deep synclines occur in both the eastern and western
part of the Pilbara Craton. The deep synclines are displaced by major shear zones in the west
and north. The similar amplitude of the gravity anomalies
throughout the
craton is
consis- tent with the granite and greenstone structures
extending to the base of the crust throughout the craton.
7. Cause of the large magnetic anomalies
The high-amplitude long-wavelength magnetic anomalies are inferred to be caused by volumi-
nous bodies with mean apparent susceptibilities of 0.1 – 0.2 SI. If these anomalies are due to suscep-
tibility alone then this means that in the body the volume percent of magnetite is about 3. In the
Pilbara the three rock types with high susceptibil- ities are large peridotite bodies and small ultra-
mafic bodies with modes of about 0.05 SI, and banded iron formation in cherts, with a mode of
about 0.08 and values up to 1.0 SI Wellman 1999. These rock types also have similar values
outside the Pilbara granitegreenstone terrain Clark and Emerson 1991.
A peridotite andor ultramafic body is an un- likely cause for the anomalies because a the
susceptibility is too low given that the remanence is likely to be less, b it is improbable that the
whole of the body is of this rock type with a cross section of 9 × 12 km extending for hundreds of
kilometres, and c in the area of the greatest magnetic anomaly the residual gravity anomaly is
abnormally low.
Clark and Schmidt 1994 and Clark 1997 discuss the magnetic properties of banded iron
formation in the Hamersley Basin and Yilgarn Block and integrate this work with that on similar
rocks outside Australia. For all areas, bedding parallel susceptibility is typically 0.5 – 2.0 SI, and
the ratio of remanent and induced magnetisation Q is typically 1 – 2. Due to geometrical effects
both the susceptibility and remanence magnetisa- tion are much stronger in the plane of the bedding
than across the bedding. If we assume that the banded iron formations of the Pilbara granite
greenstone terrain have the average values of the above ranges susceptibility 1.0 SI and Q of 1.5,
then their apparent susceptibility is 2.5 SI. If the average apparent susceptibility of the greenstone
belts below 2 km is 0.1 SI then the banded iron formation would have to occupy an average of
0.12.5 = 4 of the volume of the greenstone belt. This seems a bit high, but it is at least possible. The
preferred cause of the long-wavelength magnetic anomalies is thin bodies of banded iron formation
within the greenstone belts, with enhanced mag- netisation below 1.5 – 2.0 km depth. This banded
iron formation horizon, or horizons, may be struc- turally repeated by isoclinal folding, or faults.
8. Relative thickness of the granites
