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Researches in British Mineralogy. III. Native Gold. N the first part of these researches, which appeared in 1867 in the November Number of this Magazine, I communicated the results of my investigations into the chemical composition and geological occurrence of the native gold which is met with in the quartz lodes and alluvial river-deposits of North Wales. Since that time the recent discoveries of gold in the north of Scotland have attracted much attention, and rendered the subject of the occurrence and distribution of the noble metal in the British Isles one of somewhat more than ordinary interest-a circumstance which has induced me to extend this inquiry into an examination of the native gold from all the British localities whence I am able to procure authentic specimens. Although it will be found that, even from the most ancient periods in English history, statements of discoveries of gold have been from time to time recorded as having been made in numerous places throughout England from north to south, none of them seem to have at any time led to a practical or successful result; and certainly the majority of such accounts do not upon scrutiny appear to be entitled to any confidence being placed either in their correctness or veracity; in fact the only districts in England where unquestionable evidence of the presence of native gold can be obtained are Devonshire and Cornwall. * Communicated by the Author. Phil. Mag. S. 4. Vol. 37. No. 250. May 1869. Y It must be remembered, however, that gold in minute quantity is undoubtedly contained in some of the English lead and copper ores, especially when these ores are strongly argentiferous, as will be hereafter noticed. Still the total amount of gold present in such cases is so extremely insignificant as neither to entitle them to the appellation of gold ores proper, nor even, in most if not all cases, sensibly to augment the value of the ore itself, since, as a rule, its value would scarcely be found to cover the expense of its extraction or separation from the other metals with which it is associated in the ore as it comes from the mine. In Devonshire the Poltimore mines at North Molton are traditionally reported to have been worked for gold by the Romans during their occupation of Britain; and in 1853 the brown and red "gossans" forming the substance of this lode at its surfaceoutcrop or backs," as it is called by the miners, were treated as gold ores, and stated to yield between one and two ounces of fine gold per ton of "gossan." Still, although a not inconsiderable amount of gold was extracted from them, the final result of the adventure did not prove remunerative. The chemical examination of these "gossans," made by myself in 1854, indicated that they were a product of the decomposition of slightly auriferous metallic sulphides (principally iron), due to oxidation and other atmospheric influences; and whilst it proved that these gossans did contain a small amount of gold, it at the same time showed that the actual amount present in them was much less, and much more irregularly disseminated throughout the mass, than had generally been reported. 66 Native gold along with stream-tin ore (cassiterite) has also been washed out of alluvial deposits at Shepstor in Dartmoor, which have no doubt been formed from the débris arising from the disintegration of the granitic rocks of that district. Native Gold from Cornwall. In this district the occurrence of gold in situ in its original rock-matrix is extremely rare; but it is reported to have been found in a cross course in Huel Sparnon, and the gossan of the Nangiles mine is said to be auriferous. In the British Museum a specimen of gold in quartz along with argillaceous or possibly chloritic slate is exhibited; but as no locality is given besides Cornwall, I am uncertain as to whether it is to be regarded as thoroughly authentic or not. In a recent visit to the north of Cornwall I found that the argentiferous tetrahedrite (Freibergite), chalcopyrite, and galena from a lode at Bound's Cliff near St. Teath, contained a minute but distinct amount of gold; but I have as yet not been able to determine its amount quantitatively. It has long been known, however, that the alluvial deposits in the beds of many of the Cornish rivers contain native gold in the form of nuggets and small rounded or flattened grains or scales; and it appears not improbable that the gold forming the ornaments of prehistoric origin frequently found in Cornwall and the neighbouring counties may have been derived from this source. A nugget of gold from these alluvial deposits in the collection of the late Sir Charles Lemon weighs more than one ounce, and the largest nugget recorded is stated to have weighed 2 ozs. 3 dwts.; but these are quite exceptional instances, as in general the weight of the particles does not exceed a few grains. Most frequently the gold is found associated with the so-called stream-tin (cassiterite or oxide of tin); and when the streamworks were, as anciently, an important source of the tin-production of Cornwall, a not inconsiderable quantity of native gold seems to have been obtained during the washing of the alluvial deposits for tin ore. As these deposits have now been nearly, if not altogether, exhausted, it has become extremely difficult to meet with authentic specimens of Cornish gold at the present time. In the British Museum may be seen a specimen of stream-tin ore with gold from the Carnon stream-works; and Messrs. Greg and Lettsom mention gold being found at the Crow Hill streamworks at Trewarda, Kenwyn, at Llanlivery near Lostwithiel, and at Laddock near Grampound. As no analysis of native gold from Cornwall is to be met with in any of the works of mineralogy which I have examined, I have for several years been endeavouring to secure an authentic specimen for the purpose of determining its chemical composition, but only succeeded last month, when Mr. W. J. Henwood, F.R.S., kindly sent me a specimen, obtained through Mr. Petherwick of St. Austell, from St. Austell Moor. The gold thus obtained was of a rather deep gold-colour, but of a dingy lustre, the surface of the particles being worn and rounded by attrition; it consisted of nine minute nuggets of very irregular forms, the largest of which weighed 2-1 grains, whilst their aggregate weight was only 5.46 grains. The specific gravity, determined upon the entire quantity at disposal, was found to be 16-52 at 60° Fahr.; and the chemical analysis gave the following numbers as the percentage composition of the gold itself:— results which closely approximate to the composition of the gold from the Clogau mine in North Wales previously examined by me, and which correspond to the formula Au Ag, which by calculation requires 90.88 per cent. gold to 9-12 per cent. silver. The geological age of the appearance of this gold is, I believe, identical with that of the intrusion of the stanniferous and auriferous granites of Cornwall, which appears to be some time between the Silurian and Carboniferous periods; and it seems not improbable that the greater portion of both, the stream-tin ore as well as the native gold, was originally disseminated throughout the mass of the granite itself, and subsequently, through its disintegration, became set free and washed down into the alluvial deposits formed by the rivers. Native Gold, Wicklow, Ireland. To the good offices of Mr. F. Jennings I am indebted for two small nuggets of gold from the Wicklow alluvial deposits, which have enabled me to determine their composition. The specimens were rounded irregular masses, and evidently contained internal cavities, so that the determination of their specific gravity is not to be relied upon. Their colour was a rather brassy golden hue, whilst the external surface of the gold was abraded and possessed a very dingy lustre. The specific gravity of the one nugget was found to be 15.07, whilst that of the other was only 14.34-probably owing to the existence of internal cavities or quartz particles. The analysis was conducted upon the first-mentioned of these nuggets (spec. grav. 15:07), and in precisely the same manner as described in the case of the gold from the Clogau mine in the first part of these researches. The results obtained gave the following percentage compo sition :— These results do not coincide with the analysis of Mr. W. Mallet, published in the Journal of the Geological Society of Dublin, vol. iv. p. 271, but are almost identical with those obtained by Mr. R. Scott, published in Sir W. Wilde's 'Catalogue of the Gold Antiquities in the Collection of the Royal Irish Academy,' 1862, when the amount of iron obtained by him is subtracted*. *Mr. Scott informs me that this was probably due to entangled pyrites, and that he also found a trace of copper present. |
