SCREW TIN – EMMAVILLE’S UNIQUE CASSITERITE FOSSILS

SCREW TIN – EMMAVILLE’S UNIQUE 

CASSITERITE FOSSILS

 

From the Mindat Entry on Doctor's Gully Emmaville

Marine fossils have been found in many places in NSW, of course – even in some of the ancient sedimentary rocks near Emmaville – but what makes the fossils described here unique is the fact that they are preserved in cassiterite (SnO₂). Generally speaking, most marine fossils are preserved as calcite or quartz or simply as impressions in what was once soft mud or sand. To the best of my knowledge, Emmaville is the only location where the replacing mineral is cassiterite.

This is a piece of information I’ve known about for many years, but I have no specimens of my own; however I have  found a photograph in the Mindat entry on Doctor’s Gully at Emmaville (here). There may be specimens in the Emmaville Mining Museum (there ought to be) and there probably are in museum and university collections elsewhere, but I don’t have access to these. From the photograph and the reports listed below you will see that the fossils are of two main types.

The first (the so-called screws) are segments of the stems of marine animals called crinoids. These are plentiful in sedimentary rocks ranging from the Silurian to the Permian period and are usually preserved in calcite . Crinoids resemble sea urchins and starfish; however they are attached to the sea floor by long stems made up of individual plates or ossicles. When seen from the edge, sections of these stems do resemble the thread of a screw or bolt, except that a little observation shows that that are not spirals, merely stacked discs. The miners who gave them their name probably weren’t fooled, but similar fossils in rocks have given rise to silly stories about ancient machines being found embedded in solid rock. 

The second type of fossil is of a small gastropod (snail) of a species commonly found in Permian sediments. Similar fossils (preserved in calcite) are common in Shoalhaven Group sediments at Gerroa on the south coast of NSW. Thus we have references to “screw tin” and “snail tin”. Here is a quote from the Wingham Chronicle and Manning Observer (Friday 9^th April 1937, page 1):

“Queerly shaped specimens which Emmaville miners have been finding in their claims, and which they have been calling "screw tin," and "snail tin" from the shape of the crystals, have been identified by Mr. T. Hodge-Smith, mineralogist at the Australian Museum as sea lilies and gastropods (star fish and sea snails) which countless thousands of years have turned into tin oxide. Such specimens have not been found anywhere else in the world, and prove that many thousands of years ago the sea swept over what is now the Tablelands.” Why this particular newspaper chose to report this information I can’t imagine. Did they issue a press release in 1937 after a field trip to investigate reports of cassiterite fossils being found near Emmaville?

The locality I’ve known about is Doctor’s Gully, an alluvial tin area near the Emmaville Golf Course (Gulf Road). However, it turns out that several other places nearby have also produced these fossils and there might have been others. It was just plain tin to the miners and worth money as such, so into the bags went beautiful specimens any of us would be proud to own. From the maps you can see that Doctor’s Gully has at least two tributaries gullies, called “Steele’s Gully” and “Charcoal Gully”. They are listed in the various reports as being 2km north of Emmaville. The Grafton-Maclean Mine Data sheets describe them as “fossil placers (fluvial)”. The Mine Data reference for all three is 1166. There is an additional reference (1266) to “Screw Tin Gully”, 3km north of Emmaville, described as “modern placer (fluvial)”. I can find no further reference to Screw Tin Gully but it is apparently in the same general area as the others.

Don’t imagine that you can just turn up at Emmaville and expect to find examples of these very special fossils. Read Jim Sharpe’s account of his experience in the Newsletter of the Mineralogical Society of NSW for November 2013 (here). Finally, a scholarly report on these fossils appeared in the Journal and Proceedings of The Royal Society of NSW for 1952. The article, by LJ Lawrence, may be downloaded here. It’s nearly at the end of the document and no plates have been reproduced. Note to the Royal Society: it’s time to make your records freely available in modern PDF format!

From Rasmus 1972. Check DIGS for the original

From the Grafton-Maclean Mine Data records

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Shoalhaven District Geology.

SURFACE HILL GEM DEPOSIT GULF ROAD EMMAVILLE

SURFACE HILL GEM DEPOSIT GULF ROAD EMMAVILLE

Location of Surface Hill with the basalt outliers marked

 How often have you wished you were around in the “good old days”? It must have been easy to pick up gemstones then. Maybe.

“Australian and New Zealand Gemstones” (1972) by Bill Myatt has this statement on page 298 in a section about Emmaville. “The Gulf Road is not good in rainy weather, as it crosses four creeks; the crossings, however, are all cemented. Sixteen miles out, Bill Frappell’s Topaz Farm is on the left, where camping and fossicking is allowed on payment of a fee, the money going to charity.” I don’t know when this arrangement ceased, but I would guess more than 30 years ago.

The place we are talking about is correctly called Surface Hill. If you’re searching for information, don’t get confused with the many other Surface Hills around, especially the one on the Timbarra goldfield near Tenterfield. The name refers to the fact that the alluvial wash there was on the surface of the ground rather than in a creek bed or its banks. The place is about 2 km south of the Gulf Road, between Flagstone and Little Flagstone Creeks. Both Minerama and Emmaville Gemfest have conducted field trips there. The last I knew, it was part of the property called “Willow Creek”, where James and Kerry West conducted their business “The Fruit Salad Tree Company” (multi-grafted fruit trees). I believe the property has recently been sold.

Naturally, the old time miners were not interested in topaz. They were after tin (cassiterite) which was eagerly sought all over the Emmaville and Torrington districts from 1872. The cassiterite originated in fluids associated with the nearby crystallising granite. There are literally thousands of veins, dykes and impregnations which formed from the early Triassic Mole Granite. Some are within the granite itself, others are in the adjacent intruded rocks, such as those at Webb’s Silver Mine. At the Emmaville Emerald Mine, the pegmatite dykes can be traced from the intruded rocks into the granite itself.

From TWE David. Basalt outliers marked

The story at Surface Hill is entirely different. Here the cassiterite is found as waterworn grains in an old stream deposit. Let me quote TWE David (from his 1887 report on the Vegetable Creek Tin-Mining Field, DIGS reference R00031676, page 45): “The Tertiary gravels at Surface Hill form three outliers. At Surface Hill a flat-topped ridge of claystone is capped by a bare oval patch of tertiary gravel. The deposit has an extent of 1 acre and a thickness of 1 to 2 feet. The pebbles of which it is composed are principally quartz, from 1 to 6 inches in diameter, and as smooth as eggs. A great deal of tourmaline, as well as topazes, beryls, emeralds, and stream tin (alluvial cassiterite ed), are mixed through these gravels, which from their higher position appear to be older than the similar gravels underlying  two neighbouring outliers of basalt. In its lithological character, and in its stratigraphical relation to the “deep leads” this small outlier of gravel closely resembles the tertiary pebble beds at Cope Hardinge, near Tingha. The importance of these outliers rests not so much on the minerals contained in them, though that has been considerable, as on what they teach of the former wide distribution of stream tin deposits in early tertiary time. The gravels at Scrubby Gully and Surface Hill are over 6 miles distant from one another, the former being 3,360 feet above sea level, and the latter 2,460 feet; while those in the Ruby Hill outlier have an altitude of 1,885 feet, and are over 25 miles distant from Scrubby Gully.”

The mention of topaz, beryl and emerald should get any self-respecting fossicker’s heart racing. I’ve been out to Surface Hill a few times and never failed to find topaz, though not the other minerals. I have no doubt that this is because of the similar specific gravity of beryl to quartz: We look for topaz in the sieve centre because it is dense but forget the less dense beryl among the quartz fragments. It is the same story at Blatherarm and Scrubby Gully. You won’t find alluvial beryl unless you are looking for it.

David has more to say on page 164. “Beryl occurs most frequently in small waterworn round or oval prisms from ½ to 1 ½ inch long and from 1/8 to ¼ inch thick. The localities where it is most abundant are Surface Hill and Kangaroo Flat. The majority of specimens are pale green, and many are colourless. The gems generally contain a number of minute cavities, which of course detract immensely from their value. Some specimens from Kangaroo Flat, which have been cut and polished, are valued at about £4, the cutting and polishing having cost about £3.”  

The only other major source of information on Surface Hill I’ve been able to locate is in the Grafton-Maclean Mine Data records (DIGS Reference R00056102). There are two listings: Surface Hill Northwest (GR1012) and Surface Hill (GR1039). The descriptor for both is “Sn, topaz – industrial fossil placer (fluvial)”.

The 1906 SMH report

Surface Hill Northwest pretty well matches TWE David’s description; there is both a surface deposit and a deep lead under a small basalt cap. A large race carried water to work the gravel which is dated to A Bouveret in 1883. NOTE. The two dams on the creek running south and the existing water race from the creek could date from E Sturtridge’s work in 1906, which is not mentioned. See the Sydney Morning Herald note for 30^th June 1906 (here).

Surface Hill proper is a deep lead deposit with workings dating from 1878. To quote: “Numerous shafts and pits on side of hill. Wash around shafts has rounded white pebbles of quartz. Has known association with blue topaz and alluvial cassiterite.”

What would you see if you were able to get permission to go out there? Firstly, I suggest that you follow the water race from the creek and see where it goes. This is Surface Hill Northwest. All the topaz and beryl would have been discarded where the gravel was washed. The area where the surface gravel was removed is where I picked up numerous piece of topaz in 2001 solely because of the glint of the sun from cleavage faces. Find where the deep lead was mined.

The track continuing around the hill without crossing the creek should go to the main Surface Hill deep lead workings. Once again, beryl and topaz should be in any discarded gravel piles or in the gully where water carried away the waste.

This link here will take you to the ALF thread on Surface Hill, from which I “lifted” Wwoofa’s photograph below.

Field Trip to Surface Hill. Photo "Wwoofa", Australian Lapidary Forum

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Shoalhaven District Geology.