Dm. Borrok et al., THE VERGENOEG MAGNETITE-FLUORITE DEPOSIT, SOUTH-AFRICA - SUPPORT FOR A HYDROTHERMAL MODEL FOR MASSIVE IRON-OXIDE DEPOSITS, Economic geology and the bulletin of the Society of Economic Geologists, 93(5), 1998, pp. 564-586
Vergenoeg is a fluorite-bearing massive iron oxide deposit that is gen
etically related to granites of the Bushveld Complex. It consists of a
pipe-shaped body containing primary fayalite, fluorite, apatite, ilme
nite, 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 m
assive iron oxide deposits such as Kiruna, Pea Ridge, and Cerro Mercad
o, all of which are related to felsic magmatism. Models suggested for
the origin of these deposits can be divided into those invoking separa
tion of an immiscible iron oxide magma from the parent felsic magma an
d those invoking a hydrothermal solution. Vergenoeg is an excellent pl
ace to try to resolve this controversy because the primary fluorite pe
rmits study of inclusions of the fluid or magma that formed the ore. F
luorite at Vergenoeg lacks inclusions that might represent an immiscib
le 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-te
mperature (>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 thousan
d at 500 degrees C). Alteration of the primary fayalite-fluorite-ilmen
ite assemblage is widespread in the Vergenoeg pipe. It consists of an
early assemblage containing ferroactinolite, grunerite, and titanian m
agnetite, and later assemblages containing various combinations of sid
erite, hematite, magnetite, ferropyosmalite, stilpnomelane, biotite, s
phene, quartz, and apatite, which are most common in the upper part of
the pipe. Fluid inclusions related to these alteration assemblages ho
mogenize at 150 degrees to 500 degrees C and have salinities of 1 to 3
5 wt percent NaCl equiv. Stable isotope analyses of hematite and inclu
sion waters in fluorite suggest that these fluids consist of a mixture
of magmatic and meteoric water. These observations suggest that Verge
noeg mineralization formed from hydrothermal fluids of magmatic origin
. Comparison of the fluid inclusion record at Vergenoeg to the more fr
agmentary record at other massive iron oxide deposits supports a magma
tic hydrothermal model for this class of deposit.