Brown stalactites

Stalactites etc are frequently coloured shades of brown. This is often incorrectly attributed to staining from iron oxides, whereas a more generally correct explanation (at least, in eastern Australia) is the colour comes from a variety of soil-derived humic and fulvic acids.

In caves of northern Tasmania, some of the calcite speleothems are spectacularly coloured shades of light brown through to darkest red and orange. The forest above the caves is a myrtle rainforest with a thick humus layer.
Contrasting with that are some of the caves at Wombeyan, NSW, where the vegetation is more open and sparse in places. The calcite speleothems deposited in caves underneath areas with little vegetation or soil layer are generally white or lightly coloured.
Iron oxides do occur in speleothems, however the colour is usually not an indicator. For example, a ribbon helictite was tested² and found to contain 0,5 % Fe however the speleothem was transparent.

Bacterial iron

Iron oxides and oxyhydroxides can also form their own types of stalactites and flowstone, frequently associated with iron bacteria. These are commonly encountered in the sandstone canyons and caves of the Sydney region, forming orange flowstone deposits and a variety of joint fillings under low pH conditions. See also pseudokarst discussion topic.

One interesting feature at Wombeyan Caves is an ephemeral cloud-like bacterial iron colony in Mares Forest Creek, near the efflux of Mares Forest Creek Cave. Possibly there are some iron sulfide deposits under the stream bed being acted upon by the bacteria, which would gain energy from the sulfur and deposit iron oxyhydroxides. There are also a number of man-made structures nearby which may be the cause of the "bloom". (Since this was written in 2001, a new sewerage treatment plant has been installed at Wombeyan Caves, and it would be interesting to see if the "bloom" ever appears again.)

Dark reddish vugs and veins

At Wombeyan Caves, NSW, a number of dark reddish vugs and veins are associated with iron minerals and massive spar calcite. They occur in Creek Cave (part of the Victoria Arch show cave), Mulwaree Cave (part of the Wollondilly show cave) and others. Dark vugs also occur at Jenolan Caves (e.g. in the Imperial Show Cave), and appear to be mainly calcite with some manganese minerals (possibly burnessite or pyrolusite) but are otherwise similar to the Wombeyan vugs. The origin of these deposits is still the subject of investigation and has been discussed by Osborne (1993)¹.

Reddish clay

Cave mud is usually made of kaolinite, and coloured by iron oxyhydroxides. Its origin is most likely from the weathering of ferromagnesian minerals in the local region, and can also be aolian, or can be from local sources such as weathered igneous dykes and weathered detrital sediment. Samples of particularly cloying clays from caves at Wombeyan and Jenolan were examined using XRD by the Australian Museum, and were found to be kaolinite.

Dykes

Mafic dykes exposed in cave walls are often a source of iron and clay from the weathering of ferromagnesian minerals. Some filled joints resemble dykes, for example, at Wombeyan Caves, a considerable quantity of volcaniclastic material has filled joints in the marble bedrock and this resembles a dyke when weathered. Often, the surface of these dyke-like bodies are reddish brown due to iron oxyhydroxides. Thin section reveals the weathered pyroclastic nature of the material. At Jenolan Caves, dykes are better known as a source of manganese. Iron oxyhydroxides are present to a lesser extent.

Oxidised pyrite

When a crystal of pyrite oxidises in fresh oxygenated water (such as in a cave), it liberates sulfuric acid and alters the pyrite crystal to a cube of iron oxyhydroxides. Under the microscope, many samples of Jenolan sediment, ferroan dolomite and dykes* have these tiny oxidised cubes. In reflected light, they are often reddish or dark brown, and some are a deep blood red. In transmitted light, in thin sections, they are opaque and look like black squares or octahedrons.
* Fresh samples of some dykes at Jenolan have unoxidised pyrite.

Massive goethite and limonite

Jenolan and Wombeyan Caves have deposits of goethite / limonite which occur in several distinct situations:

• On the surface at Wombeyan Caves, massive, earthy or renniform goethite occurs as crystals, concretions and masses, associated with sediments and weathered pyroclastics. The iron oxyhydroxides appear to be chemical sedimentary, in some cases forming a cement around quartz grains. Some internal vugs have tiny goethite stalactites. The material is sufficiently rich in iron to have been used as a small test site for iron ore. The original source of the iron in these deposits is not exactly known, but may have migrated from one or more possible sources: nearby dacites, Columba Granite, a gabbro source, or, more intriguingly, a tertiary basaltic source. The nearest basalt is an eroded outcrop about 4.5 km away.
• On the surface at Jenolan Caves, similar concretions occur near the edges of the limestone. Thin sections under transmitted light show opaque and orange crystalline goethite surrounding grains of pyroclastic minerals and sediments. Again the original source of iron is unknown, but there are several potential sources: nearby mafic to intermediate dykes contain pyrite; the volcanics to the east; the pyritic radiolarian cherts to the west. Another intriguing possibility is the migration of warm water along a major fault which lies along the western side of the Jenolan Caves Limestone.
• At Jenolan caves, there are some wonderful examples of ironstone concretions in the Imperial Cave, in the Diamond Mines area.
• Massive reddish ochre occurs with aragonite and various black manganese minerals in several caves at Jenolan. Much of this material appears to be highly weathered ferroan dolomite, where the original ferroan dolomite was pyritic. If you go on the Temple of Baal cave tour or the Ribbon Cave tour you may see some of this material.
• Also at Jenolan Caves, both on the surface and underground, some iron- or pyrite-rich palaeokarst sediments are surrounded by a ring or concretion of ironstones.

References

1. R.A.L. Osborne, 1993. "The History of Karstification at Wombeyan Caves, New South Wales, Australia, as revealed by Palaeokarst Deposits." Cave Science Vol. 20, No. 1, July 1993. Transactions of the British Cave Research Association.

2. Jill Rowling, 1997. "Ribbon Helictites at Jenolan Caves, NSW Australia." Proceedings of the 12th International Congress of Speleology, Vol 1, Symposium 7: Physical Speleology, p263-266" See also helictites article.