Blue Lake

Erosion of the basement gold-bearing Permian-aged schist rocks began around 20 million years ago, forming a concentrated deposit of quartz river gravels interspersed with lignite coal beds. 10 million years ago, Lake Manuherika (which covered most of Central Otago at the time) formed on top of the quartz gravels, depositing layers of mudstone with well-preserved leaves, and lizard and mammal bones. Uplift of the region began around 5 million years ago, adding layers of gravels on top of the lake sediments and tilting the units to the southwest by pushing Rakaia terrane (schist bedrock) up into a ridge along the Blue Lake Fault.

The Blue Lake was formed by hydraulic elevating and sluicing for gold in a small gully east of the town during the late 19th and early 20th centuries. The lake itself formed after mining ceased, leaving a large pit, some 40 m below the town's main street, that filled with mineral rich water.

For a full, well illustrated and easy-to-read summary of the formation of this lake and it's gold-bearing deposits, check out the link for the University of Otago gold research page.

The gold is concentrated at the bottom of the quartz gravels where it rests unconformably against the ancient Rakaia terrane rocks on the east side of the lake. It is unknown how much gold still lies within the gravel deposits beneath St Bathans town and to the southwest.

The lignite coal layers can be seen in the quartz gravel most easily down the north west end of the lake walk. When the lignite was exposed during uplift, and subsequent mining, many lignite seams ignited and some burnt for many years, baking the surrounding quartz sediments red and brown. If you look carefully you can even see remnants of what used to be large tree trunks in the layers.