Adrian Fiege, Leibniz University of Hanover (Germany)
Svenja Germerott, Leibniz University of Hanover (Germany)
Francois Holtz, Leibniz University of Hanover (Germany)
Ulrich Schwarz-Schampera, Federal Institute of Geosciences and Natural Resources (Germany)
Philip Schuette, Leibniz University of Hanover (Germany)
Hennie Terblanche, Vergenoeg Mining Company (PTY) LTD (South Africa)
Plattekop hill is genetically connected to the Vergenoeg fluorspar deposit (South Africa) and hosts significant CaF2 reserves, wherefore it is of economical interest. Plattekop is situated about 800 m south from the funnel-shaped Vergenoeg pipe which intrudes the uppermost unit of the Rooiberg Rhyolite, building discordant contacts. The rhyolites close to the Vergenoeg structure are characteristically strongly altered, showing an enrichment of Fe-rich sericite and Fe-rich epidote up to several tens of meters from the contact (Goff et al. 2004). The pipe is composed of a supergene hematite-fluorite cap rock and units of magnetite-fluorite, magnetite-fayalite and fayalite. Nevertheless, the mechanisms leading to the formation of the ore deposit are still under debate, with two main hypotheses: i) hydrothermal activity related to degassing underlying magmatic systems (Borrok et al., 1998) and ii) magmatic processes involving liquid immiscibility from a granitic melt (e. g. Crocker, 1985). In this study, we present new constraints on the genetic models, based on i) macroscopic and microscopic descriptions, ii) geochemical analyses and iii) experimental data.
According to our studies and field observations, the broad alteration front which is indicated by alteration phases, zones of intense recrystallisation and strong silicification as well as enrichment in fluorite and REE-phosphates strongly supports the formation of the fluorite-rich deposit by hydrothermal processes.
In contrast, evidence for the second model is also present: field observations show that some textures can be interpreted as mingling horizons, flow structures and volcanic bombs. Additionally, microscopic investigations showing welded parts and remelting structures along the rim of some minerals also suggest the influence of magmatic processes.
Finally, experimental results conducted at 1010 °C, in the pressure range of 1 - 2 kbar and in systems saturated with respect to fluorite show that the existence of two immiscible silicate liquids, a Si-rich (∼rhyodacitic) and a Fe-rich (∼ferrobasaltic) melt, is possible. The field with two silicate melts is enhanced at oxidizing conditions. Therefore, an increase in fO2 may have triggered the formation of these liquids and their separation probably led to the formation of a stratified magma chamber with an upper Si-rich and a lower Fe-rich part.
A complex model involving first magmatic and eruptive processes followed by hydrothermal and later supergene processes is favoured for the evolution of the Vergenoeg deposit and the Plattekop extension.