On the west bank of Paddy’s River about 0.5 km SSE of the Cotter River is the Cotter Caves & Paddy’s River Mine. The area around the Cotter Caves & Paddy’s River Mine contains a variety of geological features that developed during the Middle Silurian age.
The area around Cotter Caves & Paddy’s River Mine you can find Xenoliths up to 15 cm across. A xenolith is a rock fragment that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption.
There are several sedimentary lenses in the area also. A lens or lentil is a body of ore or rock that is thick in the middle and thin at the edges, resembling a convex lens in cross-section. One of the limestone lens within the Cotter Caves & Paddy’s River Mine forms a prominent ridgeline. This limestone lens is about 100 metres long and up to 40 metres thick. Macro-fossils of marine animals called Crinoids are present around Cotter Caves.
There are three caves within the limestone lens and they are known as Cotter Cave, Powder Store Cave and Blasted Cave. Cotter Cave is about 65 metres long and is the largest cave of the three. Cave formations, are present in two of the caves but they are mainly broken and dirty due to vandalism. The limestone lens is in contact with a magnetite skarn deposit on its south western side. Skarn are hard, coarse-grained metamorphic rocks that form by a process called Metasomatism. Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place.
Opposite Cotter Cave on the side of a steep gully is Paddy’s River Mine. A large ore deposit can be seen protruding from the surrounding vegetation. Two entrances to an underground mine were made in the late 19th century but both entrances have been completely sealed in 2012 due to unsafe conditions. Mining was primarily for magnetite but copper and silver was also mined. More than 60 tonnes of magnetite ore was mined from the site in 1895. Mining was stopped at the site due to it not being economically viable. The Bureau of Mineral Resources estimates that there are over 1 million tonnes of Magnetite Ore at site was well as 83 other mineral types including gold, silver, copper, lead and zinc in small amounts.
More information about the Cotter Caves & Paddy's River Mine is available on the many signs around the GZ. My source of information for this Earthcache is Wikipedia, Bureau of Mineral Resources, Geoscience Australia and McQueen, Ken G.; J. R. Caldwell; P. W. Millsteed (July 1988). "Primary and Secondary Minerals at the Paddy's River Mine, Australian Capital Territory". Australian Mineralogist.
To log this Earthcache you MUST visit the listed coordinates and obtain and email the answers to the following questions. You do not need to wait for permission to log your EarthCache. If there is a problem with your answers, I will contact you for Clarification. Please do not place the answers to the questions in your logs.
Question One: How many tonnes of Magnetite were mined from the site in 1895?
Question Two: How many different types of minerals have been identified in the area around Cotter Caves & Paddy’s River Mine?
Question Three: How many tonnes of Magnetite ore did the Bureau of Mineral Resources estimate was in the Paddy’s River Mine rock deposits?
Question Four: Find a piece of Magnetite near the Paddy’s River Mine. Describe the colour, texture and any unique qualities of the piece.
Optional: Post a photo of yourself exploring the Cotter Cave & Paddy’s River Mine site area.