BCGS Technical Talk – March 15, 2018
Speaker: Obone Sepato, Anglo American
Title: Density and magnetic susceptibility data of the Bushveld Complex, South Africa
Date/Time: Wednesday, March 15, 2018 @ 4:30pm PST
Location: 4th Floor Conference Room, Room 451, 409 Granville St. (UK Building at Granville and Hastings), Vancouver
The Bushveld Complex (BC) is the largest known layered intrusion. This suite of rock crop out in northern South Africa to form the Western, Eastern and Northern Limbs. Most research carried out focuses on the mineralized horizons in the Rustenburg Layered Suite (RLS) of the BC. This study presents a large database of wireline geophysical logs across a substantive part of the stratigraphy of the RLS of the West- and Eastern Limbs. These consist of density and magnetic susceptibility datasets sampled at 1 cm. The major lithologies of the RLS intersected in the boreholes presented are gabbro, gabbronorite, pyroxenite, norite and anorthosite whose density histograms reveal that they are predominantly normally distributed, with density averages of 2.86-3.2 g/cm3. The magnetic susceptibility for these lithologies has a large variation from 10-7 to 13.2 SI with distributions that are multi-modal and asymmetric, which is typical of large layered mafic intrusions. Cross-correlation plots between density and magnetic susceptibility for several boreholes show that the above-mentioned lithologies form clusters (circular to elliptical), which typically overlap. This has been further investigated using k-means classification, to automatically detect these to create a semi-automatic lithology logging system, which has been particularly successful in boreholes from the Eastern Limb.
The final analysis carried out was using wavelet analysis across individual locations in the BC. This has revealed multi-scale cyclicity in all of the boreholes studied, which is attributed to subtle layering created by variations in modal proportions between plagioclase and pyroxene. In addition to this, since layering is generally ubiquitous across layered intrusions, this cyclicity can be assumed to be present across the entire BC. This technique may become increasingly important should the cyclicity in physical property data correlate with reversals in fractionation (demonstrated in the Northern Limb thus far) trends since this may suggest zones of magma addition, challenging the current perspective of four major magma additions as opposed to smaller periodic influxes of magma for the creation of this intrusion.