Ohaeawai Cinnabar Deposits.
The Ohaeawai Hot Springs quicksilver deposits, on the mainland some distance north of the Hauraki Peninsula, are
of great importance on account of the evidence which they furnish in connection with the genesis of solfataric oredeposits
The basement rocks consist of marly clays and greensands of Lower Tertiary of Upper Cretaceous age, which are covered with flows of basalt and beds of scoriæ. It is agreed by all geologists that the basalt constitutes the youngest rock-formation in the district. The surrounding country is studded with old craters, and there is everywhere evidence of former intense volcanic activity.
The hot springs around which the quicksilver-deposits are clustered are situated about two miles south-east of Lake Omapere, which itself occupies the site of an old crater. They occur along the edge of a flow of basalt, which is overlain at this point by deposits of calcareous and siliceous sinter and solidified siliceous and carbonaceous muds, through which sulphur and cinnabar are finely disseminated. There are also deposits of pyrites with or without cinnabar, in some cases containing traces of both gold and silver.* The sinters also contain gold and silver.
The ground around the springs is generally very hot, and all attempts to develop the quicksilver-deposits have been frustrated by the large volumes of hot water encountered at shallow depths below the surface.
The district has been examined at different times by Captain Hutton, Sir James Hector, A. McKay, and the author; but the best description is that of André P. Griffiths, who conducted extensive prospecting and mining operations there in 1895 and 1896. The mining operations and borings disclosed many important details which could not be gathered from a surface-examination.
The iron-pyrites occurs in masses near the basalt, and also filling cracks and fissures in that rock. The thickness of the pyritic masses varies from 3in. to 3ft., but their other dimensions are extremely irregular. Close to the pyritic masses there is a hard white siliceous sinter from 8 in. to 10 in. thick, which Griffiths found to contain gold and silver in places. One assay of the sinter gave a value of £3 per ton, but unfortunately the proportion of gold and silver is not given† The cinnabar generally occurs lining small cavities and cracks in the solidified muds and sinters surrounding the original fissures in the basalt. It also occurs impregnating the sinter in an extremely finely divided form. Sulphur occurs the sinter in larger proportion than either the cinnabar or pyrites.
[Footnote] * André P. Griffiths, “The Ohaeawai Quicksilver-deposits,” Trans. N.Z. Inst. Min. Eng., vol. ii, p. 48.
[Footnote] † André P. Griffiths, loc. cit., p. 50.
The hot springs give off large quantities of H2S, and occasionally a little steam. The gas escaping through the water of the pools and small streams is partially oxidized, liberating sulphur, which imparts a milky-white colour to the pools, locally known as “white lakes.” The beaches of the so called “white lakes” consist of sulphur mixed with magnetic ironsand and a small proportion of alum. Sulphur is also being sublimed at the vents of openings in the rocks from which H2S and SO2 gases escape.
The prospecting-work conducted by Griffiths disclosed some interesting features. A deposit of cinnabar and pyrites crops out at the foot of the hills to the south-west of the main deposits. A shaft was sunk near it, and cut the lode at a depth of 35 ft. The ore was 2 ft. thick, and consisted of small crystals of pyrites cemented by cinnabar. At this depth there was a strong evolution of H2S, and the heat of the rocks increased so rapidly with the depth that mining was extremely difficult.
It is noteworthy that the outcrop of this lode was found close to the charred trunk of a tree partially imbedded in hard siliceous mud. The trunk and roots of the tree were coated with a thin film of cinnabar, as also were some pieces of fossil kauri-gum found near the roots.
A small trench was sunk over a small fumarole; and at a depth of 10 ft. the temperature of the rock was found to be 185° Fahr.
No. 1 borehole, cased with 3in. piping, was put down to a depth of 10 ft. where it encountered the edge of the basalt. At the same time it struck a fissure from which hot mud was projected a height of 60 ft. for about forty-eight hours. The mud was succeeded by boiling water charged with H2S gas, which was found to issue at a pressure of 30lb. per square inch.
Griffiths further mentions that the richest deposits of cinnabar were found in close proximity to the hottest fumaroles, and that at very shallow depths a temperature was soon reached which precluded mining operation being carried on.
The Ohaeawai hot springs cinnabar-deposits, although never likely to be turned to economic account, are of great scientific importance from the light which they throw upon the formation of sulphide ores by solfataric actions. The deposits are still in process of formation, and metallic sulphides have been, and are still being, deposited in underground fissures and at the surface, together with the sinters which form the matrix.
The hot springs and fumaroles owe their existence to the eruption of the basalt, but the basalt is manifestly not the
source of the metals. The source may not be deep-seated, but that it exists at some distance below the flow of basalt is almost certain.
The waters of the Ohaeawai springs were found by Captain Hutton in 1870 to contain zinc, manganese, silica, free sulphuric and hydrochloric acids, but not traces of mercury.* A sample of the water analysed by W. Skey in 1896 gave the following results:—
|Grains per Gallon.|
|Protoxide of iron||2·23|
[Footnote] * F. W. Hutton, “On the Occurrence of Native Mercury near Pakaraka, Bay of Islands. “Trans. N.Z. Inst., vol. iii, 1871, p. 251.