GEOLOGY AND MINERALOGY OF THE BONG ORE DEPOSIT

Before describing the initial Bong facilities, a description of the deposit and its geology and mineralogy would be helpful. The Bong Range is an isolated chain of mountains whose highest elevation is 1,350 feet about sea level and about 950 ft. above the surrounding countryside. It has an east-west strike of about 22 miles.

In many ways the Bong Range is similar to "iron formations" in other parts of the world such as Brazil, Australia, India and the northern iron ranges of the US and southern Canada. Nonetheless, it has its own unique features. The basement rocks in the vincinity are Precambrian gneisses intruded by pegmatites that have been weathered to laterites that are much in evidence in the area. Exceptions are noted in areas where erosion has been much more rapid that weathering and the original unweathered formation is exposed. The weathering process has completely altered all of the feldspar to clays. The quartz, where exposed in pegmatite segregations, has not been altered.

The Bong Range Ore Body

The structure of the Bong Range is that of an isolated remnant of a W-shaped series of anticlines and synclines when viewed in a north-south section through the formation. The principal rock types in the formation are banded itabirites, hornblende schists and mica schists. The boundaries between rock types are not sharp. The formation is of sedimentary origin, metamorphosed and intruded by pegmatite dikes, which are younger that the host rocks of Precambrian age. The formation is believed to be +3 billion years old. The pegmatites vary from from afew inches to many feet width. They are of the typical quartz feldspar type and have produced no wall rock alteration. They are, of course, waste when encountered in mining of ore. There is no evidence of major faulting, but there is some internal post-folding stress that produced slickensides.

The principal ore minerals are magnetite and hematite, with quartz being the principal gangue mineral. There is much hornblende and mica in the horizons that contain much hornblende and mica schists. Hornblende and mica are ferro-magnesian minerals and the overall deposit contains about 9% of these minerals. The best ore zones contain about 0.6% combined lime and magnesia and the poorer zones about 3%. The mean grain size of iron minerals is about 0.15 mm and for quartz 0.12 mm. Three general types of ore are recognized and classified accordning to the percentage of iron that is divalent. Divalent iron produces FeO taht is a constituent of magnetite, FeO-Fe2O3 or Fe3O4. The divalent iron is determined by chemical analysis, but does not necessarily indicate an assay for magnetite because of the presence of martite, which contains divalent iron but is not magnetic. The three ore types are weathered containing 20% divalent iron, semi-weathered containing 50% divalent iron and unweathered containing 80% divalent iron. The overall deposit assays 38% to 42% total iron. The divalent iron is, however, a good indicator of the approximate amount of magnetite to expect. In weathered ore, the magnetite will be relatively low compared to hematite and in unweathered ore magnetite will predominate and the ore will be harder.

Bong Range is a classical example of weathered iron formation under tropical conditions. The weathering agent is probably descending meteoric water containing dissolved carbon dioxide, some organic acids from decaying vegetation and some oxygen. The advance of weathering within the formation is related. with almost mathematical precision, with the position of the water table. The water table is not horizontal, but in this case, curved, conforming more or less to the shape of the mountain. The water table, and therefore weathering, is deeper on the top of the mountains than on the flanks. This is an oversimplification in asmuch as the different rock types have different weathering rates. The various schist zones weather more deeply than the itaberite iron horizons.

For exploration prposes, an adit was driven into the mountain for a distance of about 300ft. The portal of the adit was above the talus slope and encountered firm formation near the portal. Advancing along the adit, it was noted that the ore was weathered for about 150ft. and there was very little evidence of groundwater. At the 150-ft. point, a water curtain was descending over a distance of about one and a half feet. The downpour was continous and in considerable volume.Beyond the water curtain, there was very little water and the ore was essnetially unweathered. Keep in mind that the water table is curved and at this point on the flank of the mountain, it was almost vertical. The water curtain is a remarkable feature of the Bong Range and it represents an approximate boundary between between weathered and unweathered ore. Later work indicated that the boundary was not quite that sharp and a category of "transition ore" was developed. The weathering chemistry may be caused by groundwater containing dissolved oxygen that alters magnetite to hematite. Considering only the FeO component of magnetite, the chemistry may be:

4Fe0 + O2 = 2Fe2O3

Such chemical change results in a weakened physical structure of weathered ore. As previously mentioned, groundwater alters feldspars to kaolinite (clays) and may remove small amounts of silica. Indee in the weathered zone in the adit, the pegmatites were altered to clays and could be dug with a handpick, but beyond the water curtain the pegmatites were hard and unaltered, and the ore had a high magnetite content.

Weathering produces some enrichment and weathered ore will average about 40% Fe and unweathered ore about 38%. The enrichment is due to silica being taken into solution and ferromagnesian silicate minerals being altered. Weathering also produces some limonite (hydrated iron oxide). The weathering also takes some iron into solution and can be tasted in the groundwater and may be ferrous carbonate. On exposure to the atmosphere it reverts to hydrated iron oxides and is a cementing agent in talus material. This iron probably comes from the alteration of ferromagnesian silicates rather than the ore minerals.

Weathering affects concentrator performance. Weathered ore grinds easily in an autogenous mill but highly unweathered ores from deeper horizons are difficult to grind and requires the conversion to SAG milling. Completely weathered ore may contain less than 10% magnetite and will not produce as high an overall iron unit recovery in the concentrator as unweathered ore.


© James V. Thomson and Wolfgang Jacobs