Return to Andamooka
22 April 2023 by Johno
Categories: Opal Fields | Opal Fields Characters | Opal Fields History
A short time ago I was contacted by an opal miner from Andamooka who related to me a story of his time at the opal field.
The miner was Geoffrey Cotton and when asked if it was ok to post his story he provided me with the following. I am having trouble with the photographs and hopefully will be able to add these later.
Return to Andamooka
In July 2022 I turned off the Stuart Highway at the iconic Spud’s Roadhouse at Pimba (now with a fancy caravan stopover) and proceeded past Woomera on the comparatively straight sealed highway that leads to, and past, Roxby Downs township and the giant Olympic Dam mine. I was headed to Andamooka to continue my connection with a place I’d once considered my spiritual home.
Since I’d last been there in 1998, when a mate and I had camped in the small caravan park above the old post office and had put down a dozer cut on the southern part of Gunns Gully diggings, the town had changed considerably.
The most noticeable changes were the sealed road into and through town, and the street signs. What the heck? Since when did Andamooka have streets, anyway? Part of the romance of the place was trying to make sense of directions like “Drive up the hill past the bottle motel to where Ron’s place is, that’s the one with the dead D7 in the carport, head past where the drive-in used to be, then left until you come to a red corro shed where old Bert lived back in the 70s. If you get to Polish Jenny’s you’ve gone too far. You can’t miss it.” (Yes you could!)
The other really noticeable change was to the town’s retail and hospitality infrastructure. During Covid the last remaining outback store shut its doors, as did the famous Tuckabox bar/restaurant. Fortunately Bob at the remaining grog outlet (Cellarbrations liquor store) also stocks essential groceries including a lot of frozen food, so if you don’t feel like doing the 30km run over to Woolies at Roxby you can get by quite adequately. You can even get a Weekend Australian newspaper by lunchtime on Saturday, if you don’t mind supporting its Murdochian alternative reality. And every day except Mondays and Tuesdays you can get a burger and coffee and a smile from Julie and her opal-mining mum Bev at the Dine-A-Mite Café inside the community hall. The café normally closes by 2pm, but re-opens Friday afternoon/evening for a meal out and takeaways if you don’t want to dine in with some of the locals.
Also not to be missed on Fridays is the CWA Boo-Teek op-shop. This is surprisingly well-stocked with inexpensive, good quality cast-offs. Someone told me that the stocking is a result of the exodus of people from Roxby township as the Olympic Dam mine swings more towards FIFO. The number of houses on the market in Roxby, and the empty shops in the ‘mall’ there supports this explanation.
A first-time visitor to Andamooka might see one of two towns – a romantic mining town with quirky houses and the remains of mining equipment everywhere and the chance to find an opal chip on the many dumps, or an ugly shopless shithole full of dead cars and machinery in the middle of a dusty moonscape. Those who see the latter spend one night at the Andamooka Progress And Miners Association (APOMA) caravan park on the way into town and turn their rig (fancy tandem-axle van complete with satellite dish, aircon and the works, towed by a shiny Cruiser wagon or Ranger dual-cab) back out southwest early next morning. Often they won’t even noodle on the official opal dirt fossicking dump at the south end of the caravan park, which is kept supplied with potch chips by generous benefactors (I left mine there when I returned home). I think these people miss something – they haven’t met Andamooka on its own terms.
PIC 04 Looking across to town
Most of those who stay for more than a day will end up fossicking on one of the many abandoned dumps out in the various diggings, sieving the white dirt and cracking rocks in the hope of a colourful reward. Those who stay a week or more might also take their blisters and their find – a bit of potch with a hint of colour, a piece of matrix or ‘coloured concrete’ - down to the lapidary club on Saturday or Sunday and pay co-ordinator Greg a whole $5 to use the (very good) diamond saws and grinders there, and get his expert instruction along the way.
I was in Andamooka to mine though. First stop was at the post office attached to Duke’s Bottle House motel to apply for my Precious Stones Prospecting Permit (PSPP). This entitled me to peg a claim. The PO, which is open every day including weekends, includes an underground museum with excellent examples of local opal and associated rocks. There’s also a wide range of cut opals for sale, with the very best gem-quality stones available for viewing on request. And it stocks more normal souvenirs too, of course.
It took the best part of a week for my PSPP and four claim identification plates to arrive, as the applications are processed in Coober Pedy and returned by post via Adelaide. During this time I explored old haunts, focussing on the northern diggings – the top end of Teatree Flat, Gunns Gully, Lunatic and Christmas diggings. Apart from a few fragments with colour that were obviously someone’s cast-off chippings from long ago – possibly done while they were waiting for the oil to finish draining from their ute’s engine, if the nearby rusty remains of an oil filter were anything to go by – I found nothing.
In the 24 years since my last visit little seemed to have changed on the diggings. No new diggings have been discovered, and on the existing ones there appeared only to have been some extension of the margins and mostly, dozing of whatever blocks of ground remained between dozer cuts put down anything up to 50 years before – it’s what I call ‘infill mining’. I think I found the remains of the spoil heap from a shallow shaft we’d hand-dug back in ’69 on the eastern margin of the northern part of Teatree, and in all that time the dozer cuts had come only the maybe 150 metres to be near it. Even the dozer cut we put down in ’98 on the southern side of Gunns Gully backing onto the sandhills was intact, with just a handful of more recent cuts nearby.
This time, the main challenge was to find some prospective ground. The ‘infill mining’ has left virtually no un-mined ground on the major diggings, and I spent hours each day walking up over dumps and down dozer cuts trying to find an untouched block big enough to warrant dozing. As a consequence, miners are forced to dig in deeper ground, increasing operating costs significantly. Some are tackling shallow ground on the margins of diggings like Lunatic, previously thought not to be productive.
I logged the positions of possible sites using the ExplorOz Traveller app on my iphone, for consideration and reinspection later. ExplorOz has recent satellite photos that show clearly each dozer cut and shaft dirt heap on which to overlay position co-ordinates, as well as providing the usual ‘find my way back’ facility. For each likely-looking location I created a ‘notes’ page combining a screenshot from ExplorOz, notes about the geology and photos taken with the phone camera. These notes were invaluable later when deciding where to peg.
The mining methods had changed since my ’98 visit. Back then, it was all dozers with the occasional blower or self-tipper hoist marking a shaft. Now, there are as many large excavators as dozers, and the occasional noodling plant, but I didn’t see a single operational shaft mine. Not that there were many dozers or excavators either, maybe half a dozen of each actually operating occasionally, and a few more out at White Dam. And where once you’d always hear a dozer or blower working in the distance, this time the field was largely silent much of the time.
Once I’d found a ‘likely’ spot (or a ‘least unlikely’ spot?) I pegged out a standard 50x50m claim centred on an old dozer cut in whose wall I’d noticed a small slip (local name for a minor geological fault), where one block of ground had slid down relative to its neighbour, a favourable indication for opal. I was hoping that the accuracy of the various GPS apps in my phone would make pegging the claim and defining its position (required for the Notice of Pegging form) easier than in the past, by providing reliable compass bearings and distances. But no, every time I re-took a bearing it was different, same with distances between pegs. Clearly the GPS accuracy was way off. (Subsequently I’ve discovered that GPS accuracy varies between different phone brands and models – a test in 2020 showed that the iphone XR’s average point precision was actually 21.5m, or 4.5 times its claimed precision – see the full report at https://medium.com/@importanttech/we-tested-mobile-gps-gnss-accuracy-and-found-some-surprising-results-b9ec35873e2exxx ).
So, after a frustrating day wasted with the iphone GPS, I resorted to the tried-and-true magnetic compass and tape measure. The distances between pegs ended up ‘best guess’ because of the up-and-down nature of the mullock heaps. It’s amazing how much error is introduced by running the tape up and over a dump that’s only a couple of metres tall in between the pegs, over just a fifty meter traverse.
The slip I found in the wall of the dozer cut soon petered out. I couldn’t find a decent level either, despite digging down further, so I pulled my pegs and cancelled the claim (easy – just a simple form to fill in and lodge at the PO).
I went back to the notes I’d made of my explorations, and shifted my pegs a couple of hundred metres away. Again pegging took a full day – place the peg, measure, shift the peg, repeat several times - working with an offsider would have made it much simpler. Registration was a next-day affair after lodging the form at the PO, from where it was faxed to the mines department office in Coober Pedy.
This time the claim included an old dozer cut that had intersected some even older shafts and drives. One of the drives was large enough to crawl into, and by shovelling out some of the dirt that had washed into it over the years (I estimated that it had been dug maybe 30 or 40 years previously) I was able to get about 1.2-1.4 metres height from floor to ceiling. The drives were quite extensive, and carried over into the opposite side of the cut, indicating that the claim had probably produced opal – or that the previous miner had been determined to give the claim his best shot.
About 5 metres inside the drive I noticed the tell-tale diagonal indicating a slip. The pebble band came and went but the slip itself kept going, with the displacement of the adjoining blocks roughly 30-45cm.
PIC 11 Slip trends from lower left to upper right. Kopi above the level was very dry, crumbly and greyish here
Instead of getting the claim dozed straight away – and I’d been unable to find an operational dozer for hire at that stage anyway - I decided to follow the slip, digging with a Bosch 36v battery jackhammer. Although I’d also brought along three 240v hammers in anticipation of working a dozer cut – a small Makita hammer, a medium-sized Kango hammer and a large 2000 watt Makita breaker – the Bosch proved entirely adequate. With two 4kwh and one 2 kwh batteries I could dig for about 2.5-3 hours before needing to recharge. This included time spent shovelling the mullock into a plastic tub and dragging it out into the old cut to dispose of it. I was able to recharge the batteries back in the caravan park, giving me (very welcome) time to recharge my own batteries for the afternoon session.
On the first day of digging I found some thin traces of clear and blue potch on the upthrown side of the slip, just in from the wall of the old drive. One of those ‘missed by a whisker’ stories. These traces continued into Day 2 but eventually petered out, without making any colour (fire). So, nothing to do but keep digging… and digging.
Over the next fortnight I followed the slip back out to the wall of the dozer cut, and despite more hopeful signs (strong pebble band, moist clay) there was only one more area of potch, and no colour. This deposit was lenticular (lens-shaped). In this area the slip showed the typical ‘slickensides’ appearance associated with faulting, where the action of one block of ground sliding against the other polishes (slickens) and stripes the fault plane.
Late one afternoon when climbing out of the cut I found a small stone with a thin seam of blue-green precious opal – additional evidence that the area had produced opal. I think the Opal Gods scatter these around to give us heart and keep us digging.
Once I’d broken through into the old dozer cut I reversed direction and started following the slip inwards from the drive, hoping to intersect another old drive that was 5-10m further in and roughly parallel to the access drive. I’d taken my blacklight (UV light) into this drive and had found some more thin potch there, similar to what I’d found already. After a couple of days’ digging more thin potch appeared, and I hoped it might ‘make’ colour, but all it made was disappointment. Such is the nature of opal mining, where maybe one claim in a hundred will provide reasonable pickings. Again, this potch was in the upthrown side of the slip. And again, it didn’t make colour.
Lots has been written about opal field geology, and how opal might have been laid down. These explanations maintain that the structure of the ground is relatively similar throughout a field – for Andamooka it’s sand and/or soil at the surface, usually silcrete boulders beneath that, then a layer of kopi (bleached claystone) and below that a roughly horizontal layer of pebbles, possibly cemented by silica and sometimes with fibrous gypsum present just above, marking the level. Below this is the ‘mud’, the impervious moist clay barrier that’s supposed to have stopped the downward-percolating silica solution and given it time to solidify into opal. It’s also called the ‘toe dirt’ because that’s where the miner’s boots are when digging through the level.
These simplified explanations suggests that the material above and below the level is homogeneous within each of the two layers. Nothing could be further from the truth. Every 30cm forward I progressed, I squared off the face of the drive and photographed it and the floor of the drive. This helped me to build up a mental picture of the structure and composition. Within the space of a metre vertically and horizontally the ground varied between moist fine-grained clay, to dry, crumbly clay, to hard almost ‘fired’ claystone which required me to swap the jackhammer chisel for a point. The level pebbles came and went and erratic flattish quartzite boulders up to 60x30x20cm sat above, on and under the level.
Surprisingly, the potch was within the dry, crumbly clay – theoretically, it should have been at the interface with the impervious moist clay of the toe dirt. I’m not sure that even by the time Andamooka is completely worked out that we’ll fully understand how opal is deposited here.
PIC 20 Squared-off face of drive shows complex geological structure. Brown colour is iron oxide staining. Grey claystone near the top was quite hard
By late August I’d run out of time to arrange for a dozer to excavate the block of ground behind where I’d been digging. I’d figured it would take me a month to work through the base of a decent-sized cut by myself, and I only had a week left. There was no guarantee that the claim would produce anything better than more thin seams of potch anyway – that’s the nature of opal mining. As I contemplated the heap of dirt that I’d pulled out, I couldn’t decide whether that was ‘it’ for me, or whether there’d be a ‘next time’.
P.S. If you’re in Andamooka long enough you’ll see some amazing sights. And not all the colour is locked up underground.
PIC 22 ‘Sky opal’ is a pretty substitute for the real thing
Geoffrey Cotton
April 2023