WARRNAMBOOL GEM CLUB

FOSSICKING IN SOUTH WEST VICTORIA

South west Victoria is predominately a farming area. Warrnambool, the regional centre, is located on the coast and has a population of some 30,000 people.

Fossicking sites are limited in the region as most land is privately owned and generally inaccessible to fossickers. Currently (2010) club members fossick at three main locations in the vicinity of Warrnambool.

These locations are:

  1. The Mount Shadwell quarry (operated by the Moyne shire) which is located at Mortlake, which is some 45km inland (north) of Warrnambool. The quarry produces road making materials (different grades of scoria). In the quarry volcanic "bombs" can be found, some of which contain gem peridot.

   2. Lake Keilambete (Fentons quarry, privately owned) which produces limestone for soil conditioning purposes and for road material. A variety of fossils including sharks teeth can be found at this quarry. PLEASE NOTE THAT THE QUARRY HAS BEEN SOLD TO 'PETER AITKEN TRANSPORT'. THE NEW OWNERS HAVE GENEROUSLY AGREED TO ALLOW CONTINUED ACCESS. HOWEVER FOSSICKER MUST CONTACT THE NEW OWNERS (PETER & GEORGE) AHEAD OF THE VISIT (PH. (03) 55676378) AND MUST HAVE SIGNED A CLUB INDEMNITY FORM PRIOR TO ENTERING THE QUARRY. PLEASE ENSURE THAT YOU ACT APPROPRIATELY TO ENSURE ACCESS TO THIS FOSSIL SITE IS MAINTAINED FOR ALL OF US!!!!!!!!!!!!!!!

  3. Tarrone quarry (a commercially operated quarry with restricted access) which produces basalt aggregates, within which are found zeolites including calcites and aragonites. It is essential to obtain permission before entering this operating quarry!

Mount Shadwell  Back to top

Mount Shadwell is mainly of interest to fossickers for the peridot which can be found at the quarry.

Over many years, members of the Warrnambool Gem Club have been collecting peridot from the Mount Shadwell quarry at Mortlake in S.W. Victoria. The peridot is found in volcanic bombs which are spread throughout the volcanic ash/scoria which forms the volcanic cone which is Mt. Shadwell.  Back to top

Over recent years club members have been finding increasing amounts of orange/golden/yellow peridot as well as the "normal" green peridot. Many variations in between these two extreme colours have also been found. Typically the rich green peridot has been found in the black scoria in the quarry whereas the "golden peridot" has been located in the red scoria. However this is not always the case, as single bombs can be found containing both yellow and green peridot as well as many other colours in between these two extremes.

   Peridot is the name generally given by lapidary enthusiasts to gem quality olivine. Olivine is a "magnesium iron silicate" which follows the formulae (Mg,Fe)2SiO4. This mineral is one of an isomorphous series which varies from the magnesium silicate "forsterite" (Mg2SiO4) to the iron silicate "faylite" (Fe2SiO4). this variation in the amounts of iron and magnesium influences the different colours in the peridot found at Mortlake.    Several of our club members have good collections of faceted peridot which show the variation in colours present in the material that can be fossicked from Mt Shadwell. The following photos show some faceted gems cut by Brian Alexander and Alan Wood,  Warrnambool Gem Club members. These gems range up to 4.6ct in weight and provide a good indication of the wide range of colours available. The larger gems often contain small inclusions but still cut very attractive gemstones.

Fossicking for Peridot at Mt Shadwell

   Mt Shadwell is well known locally for its volcanic bombs, which range in size from a few centimetres across to huge specimens over a metre long; although the majority of bombs would measure around 10 to 15cm in diameter. Most of the bombs have spectacular olivine cores with a thin coating of scoria on the outside. Unfortunately, the olivine core usually consists of small crystals the size of grains of sugar. The bombs which we gem hunters seek are those which contain peridot of a size suitable for faceting. You generally have to break open many of the "sugary bombs" before you strike a "gemmy" bomb with suitable material within it. If you are lucky you may also find an occasional bomb yielding anorthoclase of a size suitable for faceting a nice gem.

    Mt Shadwell is the type of place you can visit and find "plenty" or "nothing" if you are looking for facetable peridot. I have visited the quarry on several occasions when I found no facetable material, while at the same time some of my companions were finding nice bombs with gem material within. However, my last 7 visits have yielded several bombs containing good gem material each time. Each "good bomb" can yield several "cutters". So like many fossicking locations, you have to be digging in the "right spot" at the "right time"!

   During the last 12 months our club has hosted visits by several  clubs, both Victorian and interstate.

   If you plan to visit Mt Shadwell quarry, you should first visit the Moyne Shire offices at 1 Jamieson Ave., in Mortlake to sign a consent form. The staff at the shire offices are very helpful and can put you in contact with Alan Wood, a Warrnambool Gem club member who lives in Mortlake (except when he migrates north during winter, on fossicking trips) and who is usually only to happy to help visiting fossickers.   

   Alan has become the local "expert" on the minerals to be found at Mt Shadwell. He has an excellent collection of "gemmy bombs" from the Mount and a good display of peridot gems which he has faceted from such bombs. While locals take these "bombs" for granted, there are very few places in the world where such volcanic bombs can be found. Calcite "micromount" specimens can also be collected at Mt Shadwell with little difficulty.

   While you are at the quarry, a good place to look for the peridot is in the large stockpile of "volcanic bombs" that the quarry operators (the Moyne shire) stockpile for fossickers to work through. Many good pieces of peridot have been obtained from that stockpile!

   When visiting the quarry you should wear a high visibility vest and follow the directions of the quarry foreman. The main equipment you need is a good geo pick/hammer and something to carry your finds in. Good footwear and long trousers are also essential.

Notes:

1. Isomorphism: is the phenomenon where the atoms of one element (such as Fe, iron) replace the atoms of another element (such as Mg, magnesium) in the crystal lattice to produce a continuum of minerals between the two extremes.

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LAKE KEILAMBETE   Back to top

    Lake Keilambete near Terang is a half hour drive east of Warrnambool. Here fossickers can search for marine fossils in Ian Fenton’s limestone quarry on the edge of the lake. Lake Keilambete is a maar volcano, approximately 50,000 years old, located some 4km northwest of Terang. There are many such maar volcanos in the western district. They result from explosions occurring when hot magma rises into rocks which are not far from the surface and which contain lots of ground water. High pressure steam is produced, which blasts the magma and surrounding rocks into clouds of small fragments. Lake Keilambete’s circular crater, which resulted from such an eruption/explosion, is quite beautiful. It is some 2.4km in diameter and 40m deep, with the lake up to 9m deep. (Ref. 1)  The crater is surrounded by a tuff ring with limestone below the tuff. It is this limestone and tuff that is mined in the quarry and it is in the limestone that the fossils can be found.

    Ian Fenton has been operating the quarry since the late 1960’s for material to make farm tracks with the limestone also being used for ‘soil conditioning’. Over that time Ian has collected a good set of fossils. Of particular note is his very impressive collection of fossil sharks teeth, one of which is shown below!   Back to top

During  field trips a number of impressive fossils have been collected.  At the quarry a variety of 40 - 50 million year old fossils can be found, ranging from whale vertebrate and the sharks teeth to many different types of shells. Fossickers wishing to visit the quarry should contact the owners prior to visiting the area.

TARRONE QUARRY:     Back to top

    ‘Tarrone’ quarry is located a half hours drive north-west of Warrnambool. This quarry, which is not open to the general public, is a working quarry and is inaccessible during the week when excavation and blasting work is undertaken. It is necessary to obtain special permission if you wish to fossick there, and members of the Warrnambool are only able to obtain access the quarry because a club member works at the site. The field trips to Tarrone enable the collection of zeolite specimens; club members having found a number of beautiful aragonite and calcite specimens in recent visits. The fossicking involves the use of hammers and rock chisels in the breaking open of basalt boulders in the search for minerals/crystals. Typical finds include calcite crystals plus aragonite and augite specimens, some of which are shown in the  photos below.  Back to top

References:

1.    Volcanoes in Victoria, Royal Society of Victoria, W.Birch, 2000

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FOSSICKING FOR FOSSILS AT LAKE KEILAMBETE

FOSSIL SHARKS TEETH FROM FENTONS QUARRY