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Art. XLIX.—Notes on the Occurrence of Native Lead at Parapara, Collingwood.
[Read before the Otago Institute, 8th July, 1902.]
During a geological survey of Collingwood County in 1887 I was informed by some miners of the occurrence of lead globules in the gold drifts in their claims, the quantity in some cases being said to be so great as to seriously interfere with the recovery of the gold on account of the ripples in the sluice-boxes becoming choked with the lead. The prevailing belief among the miners then, and now, was that the lead was ordinary shot, which had been used by sportsmen in the chase after native game in bygone days. This explanation seemed to me insufficient to account for the large quantities of lead obtained at the periodical “wash-up”; and in that year, at my request, Mr. H. P. Wash-bourn, of Parapara, forwarded a sample to the Geological Survey Department at Wellington for examination. The sample was submitted to the late Mr. William Skey, Government Analyst, who reported that it was native lead.
In a paper read before the Wellington Philosophical Society, Mr. Skey stated that the lead globules were coated with a thin incrustation of lead-carbonate, while some exhibited spangles of gold upon some surfaces. He added, “They have a great scientific interest, owing to the fact that the gold is in actual contact with the lead, often, indeed, actually surrounding it.” The sample submitted to Mr. Skey was unfortunately very small, but, so far as it could be tested, he stated that the lead appeared to be unalloyed with any metal whatsoever.*
For some years past the Parapara Hydraulic and Sluicing Company (Limited), of London, has been carrying on operations at Parapara, principally in the old Glengyle and Hit or Miss alluvial claims. Mr. William Beetham, the local managing director, in the course of a conversation with me last February in Nelson, mentioned the trouble and extra labour often occasioned at the periodical “clean-up” by the presence of round shot-like globules of lead which collected in the ripples with the gold. The lead, he said, occurred in considerable quantities, and as an approximate estimate he thought about 25 lb. was obtained for every 100 oz. of
[Footnote] * Trans, and Proc. N.Z. Inst., 1888, vol. xxi., p. 368.
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gold won by sluicing. Subsequently Mr. Beetham kindly handed me a sample for examination, weighing about 2 lb. The lead in this sample was very soft and malleable, and coated with a thin incrustation of lead-carbonate. It consisted principally of small irregular globules, varying from 0·1 cm. to 0·4 cm. in diameter, and flat thick plates varying from 0·25 cm. to 1·00 cm. long. Some of the latter weighed 10 gr., and of the former 5gr. None of the globules of lead exhibited spangles of gold as reported by Mr. Skey, but several portions, weighing 200 gr. when cupelled, in every case showed the presence of both gold and silver, as under:—
| Gold. Grains. |
Silver. Grains. |
|
|---|---|---|
| Round globules | 0·005 | 0·01 |
| Flat pieces | 0·007 | 0·02 |
Careful chemical tests showed the lead was perfectly pure and unalloyed with metal of any kind. Arsenic and antimony were specially looked for without success.
The rocks in the neighbourhood of the Glengyle Claim are crystalline limestones, quartzites, mica-schist, and talc-schist of probably Lower Silurian age, associated with masses of gabbro, generally much altered. Glengyle Claim itself occupies a deep narrow gutter, which is believed by Sir James Hector, who made a detailed examination of the locality in 1890, to mark the course of a large fault or slide.* With such a variety of rocks it is impossible to determine which formed the original matrix of the lead.
Metallic lead is very subject to oxidation, and for this reason is rarely found in its native state. The authentic recorded instances of its occurrence are very few, and in no case is it abundant.
[Footnote] * Geol. Rep. and Explo., p. xii.