May 14, 2013

Weekly Geology Picture(s) #1: Chromitite in the Bushveld Complex, South Africa

Posted by Evelyn Mervine

So, I have been quite negligent recently and missed both last week’s and this week’s Monday Geology Picture. My apologies for that! Life has been very busy with things such as writing a paper and preparing for upcoming fieldwork. To make up for missing the Monday Geology Picture the last two weeks, my husband Jackie is going to share two sets of pictures (one today, one tomorrow) from his recent mining / exploration geology field trip in South Africa.

Without further ado, here’s the first guest post from my husband Jackie Gauntlett:

Chromitite seams in the Bushveld Igneous Complex, exposed at Dwars Rivier. Picture taken by Jackie Gauntlett.

Informational sign for the Dwars River Heritage Site in the Bushveld Igneous Complex. Picture taken by Jackie Gauntlett.

The Bushveld Igneous Complex in northern South Africa is an amazing, yet somewhat enigmatic, geological phenomenon. At around 8 km thick, this layered igneous intrusion is the largest in the world by an order of magnitude. Its features also allow geologists a look into the inner workings of large magma chambers. In the image above we see layers of chromitite (a rock made up of ~40% chromite) within an anorthosite package. Conventional thinking from other similar igneous bodies would suggest that these layers were laid down inside a magma chamber one on top of the other during fractional crystallisation. However, outcrops like the one at Dwarsrivier show bifurcating seams which often rejoin and anorthosite xenoliths (rip-up clasts maybe?), alluding to a horizontal intrusion mechanism within a hot chamber. Interestingly, even though bifurcating seams are common, the total chromitite thickness remains constant and individual seams can be traced – across 200 km!