Friends of the Box-Ironbark Forests

Mt Franklin, or Lalgambook, is a volcanic scoria cone with a wonderfully preserved crater. Lalgambook was once thought to be 470,000 years old but a more accurate and surprising date was published in 2013; it turns out to be amongst the youngest volcanoes in central Victoria – a mere 110,000 years old. This is probably way too old to have been witnessed by first nation people, but they certainly witnessed eruptions in western Victoria as evidenced by the Bushfield stone axe, found buried by volcanic ash dated at about 34,000 years old.

Lalgambook would have started with a powerful and impressive display. Deposits along the north flank shows that the initial eruption was highly explosive with fragments of sandstone bedrock ripped up and mixed with scoria. This first unpredictable stage soon transitioned to a steady eruption of gas, ash and scoria fragments. The ash and scoria were thrown high into the air and quickly built the cone – scoria fragments called bombs, now seen along the entrance road, were welded together as they hit the ground. Some of the finer material would have formed an ash cloud that spread eastwards carried by the prevailing winds.

Welded scoria fragments (bombs) of all sizes can be seen along the entrance road to the Mt Franklin picnic area.

Mt Franklin was the largest volcano but several smaller eruption points are closely scattered around the mount. Lady Franklin is on the western flank of Lalgambook and an even smaller cone can be seen on the northeast side. Lalgambook would have been a mighty New Year fireworks display.

Mt Franklin is the tree covered scoria cone and Lady Franklin is the bare cone to the right.

This is the fourth post in our geology series written by Clive Willman. 

Quartz Hill is the site of some of the earliest reef mining in the Castlemaine area. Just
2km north of Chewton, it was one of a handful of huge quartz outcrops that demanded miners’ attention. The problem is not all quartz contains gold.

A few canny miners soon realised that of the three main types of quartz, only one was reliably rich in gold. The big outcrops of ‘massive’ or ‘buck’ quartz (photo 1) are almost completely barren and the narrow ‘spur’ veins that criss-cross the adjacent sandstone and shale are not much better (photo 2).

Photo 1: Massive but useless quartz outcrop at the Quartz Hill mine. The heritage site can be found by taking the Quartz Hill Track off Colles Road but be wary: the track has been damaged by recent rains. Alternatively from Chewton take North Street which meets Quartz Hill Track. 

Photo 2: Spur veins of quartz in ancient sandstone.

The miners left those behind for us to see but completely removed the best quartz – this had formed along a narrow fault than runs north-south along the east wall of the open cut. They worked a distinctive gold-rich laminated quartz, sometimes with thin sheets of pure gold. It had formed millions of years ago when a fluid, carrying dissolved quartz and gold, repeatedly seeped along the fault during dozens of separate earthquakes.

Amazingly, remnants of the ancient fluid are wonderfully preserved in tiny microfractures called ‘fluid inclusions’. The fluid can be probed, analysed and categorised by geochemists who find it consists mainly of water with a little CO₂ and CH₄  (methane) and often a tiny vapour bubble. Clever laboratory manipulations estimate the temperature and depth of the fluid at the time of deposition. So, 440 million years ago this hot watery fluid deposited its precious cargo at a depth of 10–15 km at a temperature of 300°C.

This is the third post in our geology series written by Clive Willman.