St Bathans
Loop Road, St Bathans, Central Otago
Phone 027 226 3875

The Geology of St Bathans Blue Lake – how did the gold get there? - 2nd Oct 2018

St Bathans Blue Lake has a unique geological history

The St Bathans Jail and Constables Cottage are right next to the Blue Lake in St Bathans, Central Otago. St Bathans was once a bustling gold mining town (one of several historical gold mining towns in the area).

Unlike other lakes in the area, the Blue Lake is an entirely manmade lake; the result of extensive sluicing operations that began in the area in 1873. The Kildare Hill Gold Claim in St Bathans became the deepest hydraulic elevating operation int the world, turning a 120-metre hill into a 68-metre hole. Mining in the area finally stopped in 1932, for fear of undermining the town of St Bathans. Natural drainage from the surrounding hills filled the hole and made it the lake that it is today.

So how did the gold get to St Bathans and the Blue Lake?

Gold is traditionally found in areas where there are schist and quartz veins, as quartz turns into gold. That’s what makes St Bathans so interesting from a geographic perspective.

The St Bathans area is made up of 200-million-year-old greywacke, and its long complicated geological history ended with large concentrations of gold where St Bathans town is situated. Erosion of schist hills at some distance from St Bathans resulted in quarts pebbles being transported and deposited on the greywacke bedrock. 10 million years ago the area was part of a large lake that covered most of Central Otago. Quartz pebbles ended up under this historic lake, resulting in a layer of mudstone on top of the gold, and then about 5 million years ago, mountains began to rise, resulting in tilting of the greywacke gravels. Read more on the geology of the area thanks to Otago University. You can even download a PDF.

Miners sluiced the area away unit they hit what they call a ‘bottom’ or what geologists call ‘unconformity’. The miners followed the gold-bearing surface down the slope until they couldn’t raise the gravel any more, and couldn’t drain the area, forming the Blue Lake. Of interest, an unknown amount of gold still exists beneath St Bathans town and hills to the Southwest.

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