PACROFI VI - Electronic Program

Fluid Inclusion Chemistry of the Vergenoeg Massive Iron Oxide/Fluorite Deposit, Bushveld Layered Igneous Complex, South Africa

David. M.Borrok, Stephen. E. Kesler

University of Michigan, Department of Geological Sciences, Ann Arbor, Michigan 48109-1063

R. H. Boer

Economic Geology Research Unit, University of the Witwatersrand, Johannesburg 2050, South Africa

I. T. Crocker

684 Pretoria Street, Lilaron-Flat, Arcadia, Pretoria 0018

The Vergenoeg massive iron oxide/fluorite deposit is a 600m deep cylindrical pipe with many similarities to other iron-rich deposits such as Kiruna in Sweden, El Laco in Chile, and Iron Mountain in Missouri. Vergenoeg is hosted by the Rooiberg Felsite sequence, the oldest of three igneous phases in the Bushveld layered igneous complex, but is genetically related to younger granites that intruded the intermediate layered mafic phase. The main Vergenoeg pipe consists of primary fluorite, magnetite, ilmenite, fayalite, and quartz and is surrounded by hematite-fluorite tuff-like bodies, that appear to have been erupted from the pipe. Two models have been proposed for deposits of this type: 1) separation of an immiscible iron-rich melt from a felsic silicate magma and 2) deposition from an iron-rich hydrothermal fluid. Although fluid inclusion studies should permit a choice between these two models, they have not been undertaken in other deposits because transparent minerals like calcite, fluorite, and quartz are either absent or not part of the primary magnetite-bearing assemblage. Vergenoeg provides a good opportunity for fluid inclusion study because coarse-grained fluorite is coeval with all stages of magnetite.

A search of primary minerals at Vergenoeg, found a few iron-rich mineral inclusions in fluorite. These consisted of fayalite and magnetite in a ratio of about 5 to 1, but did not appear to be melt inclusions because they were not circular and were extensively altered to secondary ferroactinolite and ferropyrosmalite. Instead large populations of highly saline, aqueous inclusions were found in fluorite throughout the deposit. Homogenization/final melting temperatures for 402 fluid inclusions in fluorite from various locations and depths up to 592m in the Vergenoeg pipe and surrounding tuff were measured using a U.S.G.S. heating and freezing stage. About 10 percent of the inclusions contained halite, hematite, and/or several other salt and opaque daughter minerals. Homogenization of inclusions took place by vapor disappearance, and daughter crystals persisted after homogenization through decrepitation. Homogenization temperatures ranged from 80 to 463oC with salinities ranging from 1 to 38% NaCl equivalent, forming two populations (one averaging 7% NaCl equivalent and another averaging 21% NaCl equivalent). The two salinity populations are not distinguishable optically, but some vapor-rich inclusions have been observed. Analysis of gases in 38 samples of primary/secondary magnetite, fayalite, siderite, quartz, and fluorite by quadrupole mass spectrometry yielded high values of CO2, Ar, and He above 300m depth and high values of CH4, and N2 below 300m depth. H2S and SO2 are negligible below 300m and have slightly higher concentrations above this mark.

The results of this study show that Vergenoeg ore was probably deposited from an iron-rich hydrothermal fluid rather than an immiscible iron melt. Based on salinity differences, two fluids seem to have been present during ore formation. Fractionation of the underlying granites could have been responsible for the higher salinity brine and the low salinity fluid was likely meteoric or formation water, although both of these assignments must be tested by stable isotope work in progress. The wide range of homogenization temperatures and salinities suggests these two fluids mixed. Early deposition of ilmenite, magnetite, fayalite, apatite and fluorite took place in a highly reducing environment, as indicated by high values of CH4 in ore-related inclusions. Deposition of secondary magnetite, siderite, stilpnomelane, and fluorite took place during more oxidizing conditions, probably accompanied by CO2.