John's New England Minerals and General Geology Blog: 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!