John's New England Minerals and General Geology Blog: Garnet Corner

Saturday, 12 April 2014

THE CASCADES, YARROW CREEK NEAR GLEN INNES NSW AUSTRALIA

THE CASCADES, YARROW CREEK

NEAR GLEN INNES NSW AUSTRALIA

The Crossing is in the centre, the long pool is on the left

This section of Yarrow Creek lies between Garnet Corner and The Crossing (click to go to the Blog List page for information on these locations). In dry weather, it is possible to park in the centre of the area, otherwise walking upstream from the crossing is the best option.

It can be divided into three parts. The uppermost is the cascade area itself. Immediately below Garnet Corner, the stream descends steeply through a drop of about 10 metres to a blockup of large granite boulders. There are numerous small pools and cracks on the way down. Some good gemstones have been extracted here using yabby

The beginning of the pool

pumps. However, a lot of sand has washed down from the Garnet Corner diggings as fossickers have dug and sieved over the years. This may have been washed further downstream by floods since the beginning of 2012.

Boulder blockup and the pool

No doubt there will be good finds made amongst the boulders one day, but it will take a lot of work to shift all but the smallest of them.

The middle section is a long

pool, commencing immediately below the boulders. Within 2 or 3 metres of the rocks, the water will be over your head. Good for swimming (brr) and perhaps ideal for a wetsuit and snorkel equipped fossicker. The 13 carat sapphire in the picture came from the edge of the deeper water. There are some large boulders in the pool immediately after the deepest part and then sand takes over. Some nice cutters have been found among the boulders.

Finds including the 13 carat stone

The final section leads to the rock barrier which crosses the creek another 100 metres or so further downstream. Wash is scarce along here but some good stones have come from the pool just above the barrier and by digging under the left bank (facing downstream).

There is an extensive alluvial flat on this bank which probably conceals former channels of the creek. Naturally, no-one knows where they are and they are probably loaded with gems.
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Shoalhaven District Geology.

My finds at Minerama in 2007

Tuesday, 3 September 2013

GARNET CORNER, YARROW CREEK NSW AUSTRALIA

GARNET CORNER, YARROW CREEK NSW AUSTRALIA

This has always been one of my favourite gem hunting spots along Yarrow Creek, made more so by the fact that the track to it is often impassable, meaning months could go by before we could drive down to it. The name was given by us and now the landowner uses it as well. It’s not that the other Yarrow Creek gemstones are lacking at Garnet Corner but rather that pyrope garnets are more abundant.

This naturally makes one suspect that the source of the garnets is nearby, especially as the garnets are all quite rough and irregular here, and this is where we have found our largest specimens (several of around 30 carats). This has not been confirmed, but the geology of this place is different from anywhere else along the creek.

Firstly, the granite outcrops cease abruptly and, after a short space with no rock outcrops, a long but fairly narrow outcrop of a different rock takes its place. This is a gabbro dyke and from its appearance on satellite photos, it is almost certainly a ring dyke. Once past the dyke there is an extensive area, almost circular, where again no outcrops are seen.

Garnet Corner is in the centre. The gabbro outcrop is to the left of it

Secondly, geologists have decided that this is most likely a diatreme, which would normally include fragments of rock brought up from deep down in the Earth’s crust or even the mantle. Perhaps amongst these are fragments which weather to release the garnets. Some prospecting for diamonds has taken place in the nearby Kelly’s Hut Creek, but not only did this draw a blank, but even the normal Yarrow Creek gemstones were absent.

There are outcrops of a pale, crystalline rock in the creek bed at Garnet Corner, but nearly always what is found beneath the sand and gravel is a distinctive green clay, which rapidly turns

Gabbro pieces in the alluvium

brown when exposed to the air. Digging into this has produced nothing of interest. However, it is the chunks of gabbro which have washed down from the neighbouring hill that fossickers are looking for. Their rough surface holds onto the abundant black spinel and the gem minerals.

Downstream of the first granite outcrops the gabbro chunks (I can’t call them pebbles because they are never rounded) rapidly disappear, so easily does this ferromagnesian-rich rock weather away. We don’t know what happens above the last gabbro outcrop as this is on an adjoining property to which we have no access. If there are garnets in the alluvium there, then maybe there is some other origin for the garnets than the one suggested here.

The fact remains, however, that Yarrow Creek and a few of its tributaries are the only local source of pyrope garnet gems. Even the tributary streams could well have prior courses of Yarrow Creek in their drainage areas, something I have long suspected. It’s up to someone else to solve this mystery. Meanwhile, I hope you get a chance to find some of these beautiful gems at Garnet Corner.