Other formats

    Adobe Portable Document Format file (facsimile images)   TEI XML file   ePub eBook file  

Connect

    mail icontwitter iconBlogspot iconrss icon

The Pamphlet Collection of Sir Robert Stout: Volume 75

Thames, North Island of New Zealand

Thames, North Island of New Zealand.

This gold-field, in the Coromandel Peninsula of the North Island, presents a new type of country-rock, as compared with those considered above, all of which (except that of Charters

page 22
Fig. 4.

Fig. 4.

Vertical Section to Illustrate Bunches of Quartz of Reefton Gold Field.

A - - Soft broken country

Towers) were sedimentary strata. The country-rock of the Thames district (probably, for its area, the richest ever yet page 23 worked) consists of alternating hard and soft bands of hornblende-and augite-andesites, of Lower Tertiary age. Auriferous lodes occur in both hard and soft (decomposed) andesites, but they are not profitably rich, except where they traverse the latter. The undecomposed rock is extremely hard, and usually greenish or dark blue. It is locally called "blue-stone." The decomposed rock varies from almost pure white to gray and brown, and is often highly impregnated with pyrite. Its local name, "kindly sandstone," roughly indicates its appearance. The name propylite, proposed by Rich-thofen for an altered andesite like this, and applied to the "kindly country" of the Thames by Mr. James Park,* lately Director of the Thames School of Mines, is similarly used in the present paper. This propylite contains a large number of auriferous lodes, practically parallel, and generally striking N.E. The gold they contain is usually alloyed with 30 to 40 per cent, of silver. In some parts of the field, the silver constitutes 50 or 60 per cent, of the bullion obtained.

Prof. Hutton has shown that the propylite consists entirely of the products of decomposition of the unaltered andesite with which it is associated. (Becker, in his work on the Corastock Lode, has shown a somewhat similar decomposition of the andesite and diabase which bound it). Prof. Hutton gives it as his opinion that the origin of the gold in the auriferous lodes will probably be found in the pyroxenes of the andesites, and strongly advises chemical examination of the andesites to clear up this question. Mr. Park, in his report already cited, agrees with Prof. Hutton, and promises further investigation. Both these authors ascribe the alteration of the hard andesite into propylite to heated waters, carrying hydrogen sulphide and other acid vapors. These waters, having a high temperature, must have been, of course, ascending waters. This narrows the inquiry to two questions:

1. Do the gold and silver of these lodes form a part of the crystalline constituents (pyroxene or other silicates) of the original andesite, and did the heated waters, decomposing these

* See Mr. Park's admirable report on the Thames gold-fields, printed as an appendix to the New Zealand Mining Report of 1893.

"On the Bocks of the Hauraki Gold-field," by F. W. Hutton, F.G.S., Austral. Adv. Sci., vol i., 1887, pp. 245-274.

page 24 crystalline constituents, dissolve the gold and silver and carry them into the lode-fissures where they are now found? or,

2. Did the heated waters obtain their gold and silver from rocks at greater depths, whence they were carried into their present position?

If the first question is to be answered in the affirmative, then analysis of the unaltered rock, and especially separate analysis of its isolated crystalline constituents, should detect gold and silver. Otherwise, it seems reasonable to conclude that some such hypothesis as is stated in No. 2 is the true one. So far as I am aware, no analysis of the crystalline constituents of the Thames andesites has been made hitherto, though Prof, Hutton mentions that in one instance gold has been found in pyrite not actually taken from the lode itself.

The, Moanataiari Tunnel.—A very good section of both the unaltered andesite and the propylite, as exposed in a deep level, away from the oxidizing influence of the ground-water, is afforded by the Moanataiari tunnel, which has been driven 3125 feet into the Kuranui hill, and is 1530 feet beneath the surface at its inner end. This adit crosses several belts, both of hard and of kindly rock. The gold-bearing veins intersected by it are in many cases associated with the latter. At 1500 feet from the mouth, the great Moanataiari fault was struck, bringing hard unaltered country opposite the kindly propylite, Tables XVI. and XVII. contain the results of analyses, taken as indicated in the plan of the tunnel, Fig. 5. In every case in which the sample was concentrated, the silver as well as the gold in the concentrate was determined; and the usual examination for insoluble sulphides was made in this field as in all others.

Tables XVI. and XVII. exhibit a striking difference between: the hard andesite and the kindly propylite, both as to their gold and silver, and as to their sulphide contents. Taken together with Table XVIII., they point to the conclusion that the gold and silver of the propylite are not derived from the pyroxene or from any other constituents of the unaltered rocks, but must have their origin in some rock not reached by any of the present mine-workings.

Prof. Hutton* thinks there "can be no reasonable doubt that

* Op. cit., p. 265.

page 25
Fig. 5.

Fig. 5.

page 26

the gold came out of the volcanic rocks, and was not brought into them from below," and gives five reasons for this opinion. But he admits that no chemical examination of the country-rock has been made, and also (on p. 269) that Sir James Hector, who has had large experience of this field, believes that the gold and silver came from a source lower than the andesites which bound the reefs. The evidence supplied by Tables XVI, XVII. and XVIII. will throw additional light on the subject.

It will be noticed in Table XVI. that there is a great difference in fineness between the gold in the favorable country-rock and that found in the quartz lodes running through that rock. Seven samples taken from various reefs crossed in the tunnel gave an average fineness of 6-3.18 gold and 36.82 silver, while the average fineness of the bullion obtained from the pyrite of the country-rock is 22.93 gold and 77.07 silver. Considering the fairly large number of samples analyzed, this difference can scarcely be regarded as accidental. It will be interesting to note whether future analyses of the pyrite from the propylite confirm it.