The Vergenoeg magnetite-fluorite deposit, South Africa: Support for a hydrothermal model for massive iron oxide deposits

Vergenoeg is a fluorite-bearing massive iron oxide deposit that is genetically related to granites of the Bushveld Complex. It consists of a pipe-shaped body containing primary fayalite, fluorite, apatite, ilmenite, and magnetite that cuts Rooiberg rhyolites and is surrounded by stratiform bodies of felsite and fragmental hematite-fluorite debris. With the exception of its abundant fluorite, Vergenoeg is similar to massive iron oxide deposits such as Kiruna, Pea Ridge, and Cerro Mercado, all of which are related to felsic magmatism. Models suggested for the origin of these deposits can be divided into those invoking separation of an immiscible iron oxide magma from the parent felsic magma and those invoking a hydrothermal solution. Vergenoeg is an excellent place to try to resolve this controversy because the primary fluorite permits study of inclusions of the fluid or magma that formed the ore. Fluorite at Vergenoeg lacks inclusions that might represent an immiscible iron-rich melt but contains abundant aqueous inclusions. Inclusion petrography, heating-freezing measurements, and gas analyses indicate that the primary mineral assemblage at Vergenoeg formed from a high-temperature (>500 degrees C), high-salinity (>67 wt % NaCl equiv) fluid that coexisted with a vapor phase rich in COB. Stable isotope analyses of primary fayalite and titanian magnetite from deep in the Vergenoeg pipe yield calculated water compositions for these temperatures that are typical of magmatic water (delta(18)O(H2O) = 7-8 parts per thousand at 500 degrees C). Alteration of the primary fayalite-fluorite-ilmenite assemblage is widespread in the Vergenoeg pipe. It consists of an early assemblage containing ferroactinolite, grunerite, and titanian magnetite, and later assemblages containing various combinations of siderite, hematite, magnetite, ferropyosmalite, stilpnomelane, biotite, sphene, quartz, and apatite, which are most common in the upper part of the pipe. Fluid inclusions related to these alteration assemblages homogenize at 150 degrees to 500 degrees C and have salinities of 1 to 35 wt percent NaCl equiv. Stable isotope analyses of hematite and inclusion waters in fluorite suggest that these fluids consist of a mixture of magmatic and meteoric water. These observations suggest that Vergenoeg mineralization formed from hydrothermal fluids of magmatic origin. Comparison of the fluid inclusion record at Vergenoeg to the more fragmentary record at other massive iron oxide deposits supports a magmatic hydrothermal model for this class of deposit.