Art. XLVI.—On the Occurrence of Native Mercury near Pakaraka, Bay of Islands. (With Illustration.)
[Read before the Auckland Institute, October 10, 1870.]
A Few miles south-east of the Omapere Lake, near the Bay of Islands, lies a group of hot and cold springs, of great interest on account both of the peculiar mineral character exhibited by them, and the deposits of mercury and cinnabar that some of them form.
As long ago as 1866, it was known that mercury had been found at these springs, but, as only very small quantities had been obtained from one spot alone, it was the general opinion that some person had broken a thermometer when taking the temperature of the water. Having, however, been informed that small grains of cinnabar had also been found in the sand, I was induced, in June, 1869, to make a visit to the place. This visit, owing to circumstances over which I had no control, was unfortunately a very hurried one, and limited to a few hours only, but, thanks to the guidance of my friend, Mr. Henry Ireland, of Waimate, it was very successful, for not only did we find mercury in the sand in the place already alluded to, but also in two other localities, in veins in the rock, accompanied by cinnabar and sulphur.
The series of springs which I have to describe are, I believe, the most important in the district, although many others exist. They commence with a group of large cold springs (a), situated in a crateriform hollow in low undulating fern-covered hills composed of brown, soft, argillaceous, thin-bedded sandstone, with occasional beds of lignite. These rocks are probably not of an older date than Upper Miocene, and may be even of Pliocene age. After leaving this hollow the stream flows through a narrow gorge, about a quarter of a mile in length, which opens out into another small hollow, in which, among others, some warm springs are found (b and c), containing mercury and sulphur.
The stream then flows through another gorge, and again emerges into a level irregularly-shaped hollow, of much larger dimensions than either of the others, in which numerous springs, both hot and cold, are situated. On leaving this hollow the stream flows through a narrow passage, and empties itself into a small lake.
The water of this stream both smells and tastes strongly of sulphuretted hydrogen, and is decidedly acid. Most of the springs are cold, some are tepid, and a few, situated in the centre of the last and largest hollow (e and f), are hot, but the temperature of the hottest of them is probably not much more than 120°F. In one of these hot springs (e), deposits of alum and sulphur are found; many of them show traces of sulphur, and almost all have deposited silica, although not in large quantities.
We commenced our search at the place where mercury had previously been found (marked g on the sketch), but for some time without success, owing to the soil having been a great deal disturbed by former explorers; at last, after turning aside the small stream that here joins the larger one, and carefully washing in a tin plate the grit in its bed, we succeeded in obtaining a few specks of cinnabar and a few small globules of mercury. Thus encouraged, we continued our search, but with only moderate success, and as it was getting late, we thought it advisable to examine other localities, in order more effectually to test the thermometer theory. Accordingly, we crossed over the hills to the sources of the stream (a), but in our necessarily hurried examination, could find nothing. We then followed down to the second hollow, and began, a little before dusk, trying the spot marked b, where we at once found grains of mercury in the sand, and on continuing our search, we discovered a small vein in the soft decomposing sandstone. This vein was from a quarter to half an inch in width, open in the centre, and lined on each side by a black ore of mercury, and contained, both in the lining and centre of the vein, globules of mercury, often of considerable size, and accompanied by deposits of sulphur. Soon afterwards we found a precisely similar vein a few yards distant (at c). Mercury was also distributed in fine globules through the rock on each side of the veins, and on breaking this down and washing it in a small tin plate, I obtained about two-thirds of an ounce in half an hour, besides many small angular grains of red cinnabar. Much of the mercury, however, escaped me (perhaps an equal quantity to that which I obtained), for the particles were so minute that much of it floated on the surface of the water, and refused to run together. Even the larger particles amalgamated with difficulty, owing either to the sulphur, or to a black greasy hydro-carbon, which is found at all the springs where mercury occurs, and which may be derived from the lignite beds at or near the surface, or may have a deeper origin.
The black mineral found in the veins is a sulphide of mercury with some iron. It is insoluble in boiling nitric or hydrochloric acids, but dissolves readily in aqua-regia. It is generally black and dull, but sometimes the colour is lead grey, and the lustre sub-metallic. Streak metallic, lead grey, powder black, sometimes mixed with particles of red, or reddish black. Hardness about 5. S.G. = 9.224. Before the blow-pipe it gives a greenish yellow bead, with borax.
I also took home with me a small bottle of water from the spring at g, and found it to contain zinc, manganese, silica, and free sulphuric and hydrochloric acids. I could obtain no trace of mercury. The specific gravity of the water was 1.04. On passing sulphuretted hydrogen through it no precipitate was formed, but it turned a beautiful pale blue colour, very like that of Te Tarata, and other hot springs in the interior of the island.