Losberg (translating from Afrikaans as “Loose Mountain”) is probably the most conspicuous land-form in the area of the small town of Fochville. It stands isolated and is geologically distinct from the other main features of the area, the rugged mountain land of the Vredefort Dome. It lends its name to Fochville’s main street, a primary school, and numerous businesses among other things.
The Losberg Complex is a flat-laying layered igneous body that intruded into the rocks of the Transvaal Supergroup. The intrusion is approximately 6.5km long, 3km wide and 120m thick and is situated on the fold axis of the so-called Potchefstroom synclinorium, a large geological structure associated with the Vredeford Dome. The Losberg Complex is of particular interest as it is part of a group of similar layered mafic igneous complexes including the economically significant Bushveld Complex and Great Dyke of Zimbabwe, all approximately 2 Billion years old. Losberg itself holds no economic significance.
The bottom 16m of the complex consists of Harzburgite, a silica-poor, iron and magnesium-rich rock consisting of the minerals olivine and orthopyroxene. No quartz is found in harzburgite.
The middle 25m consists of quartz norite which is made up of calcium-rich plagioclase feldspar, orthopyroxene and clinopyroxene.
The top layer and bulk of the complex is quartz gabbro which consists of calcium-rich plagioclase feldspar, clinopyroxene, and quartz.
Each of the layers has their own internal subsidiary layering.
This dramatic layering and the associated changes in mineralogy and geochemistry is the result of fractional crystallisation.
The intrusion is capped by an unrelated quartzite.
As far as I know, Losberg itself cannot be accessed without entering private land. The waypoint at 26° 33.978’S 27° 27.016’E is a convenient parking spot and offers a beautiful view of Losberg with its layering clearly visible. The cache may be logged from here.
In order to log this cache you need to answer the following questions and e-mail them to me:
1. How may layers can you identify?
2. In which direction do the layers slope?
3.What is fractional crystallisation?
4. Next to the wind pump and dam is a very large thorn tree; a beautiful example of the trees that are ubiquitous in this area. This tree is either a kameeldoring (acacia erioloba) or a soetdoring (acacia karoo). Identify the tree from the following descriptions:
Kameeldoring: Bark is thick and rough and deeply fissured. Whitish to reddish thorns are straight and often swollen and fused at the base. Bright yellow, round flowers are borne in winter to spring. Seed pods are large half moon-shaped, flat and thick.
Soetdoring: Bark is rough and blackish grey to black. Thorns are paired, straight and white, and more prominent on young plants. Bright yellow round flowers are borne in spring to summer, usually on young growth. Seed pods are sickle-shaped like like green beans and open when ripe.
Small versions of this tree grow outside the gate, so there is no need to enter private land.
A photo of you with Losberg in the background is optional.
References:
Danchin, R.V. and Ferguson, J. (1970) The Geochemistry of the Losberg Intrusion, Fochville, Transvaal. GSSA Special Publication 1, pp 689 – 714.
Photo guide to Trees of Southern Africa by Braam, Piet, and Ben-Erik van Wyk. Briza Publications, 2014