• Document: s: Wilkinson, H. L. Title: The deep leads of Victoria (Paper & Discussion) Date: 1905 Persistent Link: File Description: The deep
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Library Digitised Collections Author/s: Wilkinson, H. L. Title: The deep leads of Victoria (Paper & Discussion) Date: 1905 Persistent Link: http://hdl.handle.net/11343/24308 File Description: The deep leads of Victoria (Paper & Discussion) 246 VICTORIAN INSTITUTE OF ENGINEERS. THE DEEP LEADS OF VICTORIA. BY MR. H. L. WILKINSON. 7 The importance of alluvial mining to Victoria can be gauged by the total amount of gold produced from this source since the discovery in 1851, for out of the 66,000,000 oz. that Victoria has produced, some 40,000,000 oz.were alluvial gold, and at the present time of the 8oa,000 oz. raised annually, 250,000 oz. are alluvial, of which a good deal more than half was from mines operating on the deep leads, properly so called. As the shallower alluviums get worked out, and quartz mining has to deal with more complicated and .difficult problems as depth increases, the importance of the 000 miles of proved deep leads and others which Victoria possesses will become enhanced. Most of the gold found during the first ten years was from shallow placers, the runs of gold were easily found and cheaply worked. These shallow deposits occupy the beds of streamlets that fed the river systems of the Upper Tertiary periods, the deposits of which have been preserved, and form what is now known as " Deep Leads." It is to a short description of these auriferous deposits, their location, draining, and working that this paper is devoted. GEOLOGICAL HISTORY AND DISTRIBUTION OF DEEP LEADS OF VICTORIA. The Ordovician, or Lower Silurian deposits were during the Devonian Period thrown into immense folds, having a general strike of N. and S. The strata were very much uptilted, and the higher portions were thrown to such a level that the silurian rocks, which form the present " divide " of Victoria, are only the re- mains of the m'oun'tainous plateau which occupied the State. Underlying these folded sedimentary rocks, and forming im- mense intrusions and dykes, are igneous rocks of varying compo- sition, which must be looked upon as the ultimate source of the Deep .Lead gold. During the uptilting of the Ordovician strata and the intrusion of the Granites, Diorites, and other igneous rocks, quartz veins were formed in the various cracks and fissures. which were in a N. and S. strikeowing to the E. and W. thrust which produced them, by the meteoric water whidh permeated the Ordovician rocks above the critical temperature. These waters were (during their circulation) charged with gaseous emanations from the underlying and adjacent highly-heated plutonic rocks, THE DEEP LEADS OF VICTORIA. 2 47 which are now showing themselves as granite hills in various parts of Victoria. These gaseous emanations being charged with the various minerals, including gold, were taken into solution by the highly-heated waters which collected in the cracks and fissures above mentioned. Whilst rising to the surface these solutions deposited and formed as their power of holding the silica and other minerals in solution lessened (owing to the decrease in its temperature, and other causes) the quartz reefs which were the immediate source of the Deep Lead gold. This is only mentioned because of the important bearing these plutonic rocks and their chemical composition have on the rich- ness of the Deep Lead systems; mudh of the gold which they con- tain was derived probably from reefs whidh have long since dis- appeared, leaving only the harder plutonic outcrops, the exact chemical composition of which might give some clue to the probable richness of the neighbouring Leads, which, being covered with some hundreds of feet of Basalt and •heavily charged with water, it is impossible to test with any degree of certainty. The so-called Auriferous zones, which stretch in parallel N. and S. bands across Victoria being, in the opinion of the writer, merely a reflection of the chemical composition of the underlying and outcropping plutonic rocks. This immense Ordovician plateau, with its quartz veins, at an elevation much greater than the present dividle, was intersected by valleys, and was subjected' to a rainfall, to Glacial, and to other denuding forces far more powerful than those ar present reduc- ing its elevation. Much of the earlier erosion of this Ordovician plateau was done by Glacial action ; there exists evidence to show in the glacial conglomerates of Bacchus Marsh and those which form the beds of the later fluvatile valleys at Charlotte Plains, Moolort, Pitfieldi, Heathcote, etc., that glacial valleys were formed at much lower level than the later deep lead deposits, and with much steeper sides. These Glacial deposits are of bluish clay, with numerous granitic boulders (upi to 3 or 4 feet diameter) and iron- stone, quartz, quartzite, and slate striated

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