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Volume 23, 1890
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Art. XXXIII.— On the Microscopical Structure of the Ohine-muri Gold.

[Read before the Auckland Institute, 4th August, 1890.]

Plates XXXIII., XXXIV.

During my long residence in Ohinemuri I visited most of the mines, and many localities where gold and other minerals occur in more or less considerable quantity, with the view of collecting a complete series of microscopical specimens for a monograph to be published in co-operation with several continental mineralogists and chemists. The material accumulated during fourteen months affords good examples of almost every known variety of gold and silver, and a number of specimens which may help to throw light on some yet obscure points in connection with the origin and distribution of these metals. The subject bearing special relation to New Zealand mineralogy, these short remarks may be of interest to the members of the Institute.

To show the structure of the gold, and the various modes of occurrence in a pure or alloyed state, coloured plates are absolutely necessary, and the specimens have to be drawn on a very large scale; which methods could not be adopted in this paper, through want of space, and difficulties in the execution of the coloured illustrations. The granular or finely crystalline nature of the surface could consequently not be represented in all its minute details.

Nearly all the specimens were obtained by myself on the spot, to prevent any mistakes about the exact localities and nature of the rock and surrounding country. A few were picked out from prospects brought by miners and prospectors for microscopical examination, but only in those cases where full particulars were given, and where the composition of the material left in the dish showed the mineralogical character of the rocks or the loose debris in which the gold was deposited mechanically after the disintegration of its original matrix.

The ordinary process of washing out a prospect in the dish causes, of course, distortions, striation, or compression of the soft, delicate specimens, which, especially in the fine arborescent and filiform varieties, destroy most of the primitive shape and surface. Even the most careful manipulation produces often a slight flattening or scratching, which may easily be mistaken for imperfect crystallization, though under higher power the cause is generally easily detected. The gold found in loose debris has undergone many changes through mechanical

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action, and particles transported by water-currents are of course much worn—generally to such an extent as to make the original form unrecognizable. Whenever possible the particles of gold were left in situ, and figured in this position with a portion of the adhering quartz or other minerals.

It is not the purpose of this brief outline to treat in detail the many theoretical questions relating to the formation of gold and the causes of its patchy distribution throughout the gold-bearing region of this province. We are not yet in possession of a sufficient number of facts to answer any of those questions. “Where the gold is, there it is not; and where it is not, there it is,” has been said with some cause of the Thames Goldfield, and we must be satisfied with this paradoxical explanation until further researches on the spot and in the laboratory allow us to come forward with substantial evidence.

Gold is very widely distributed within the boundaries of Ohinemuri. Probably all the minerals which are auriferous in other districts also bear gold in the Ohinemuri region. No careful examination of several species, which are generally very small, even microscopical, has yet been made. The mineral most productive of gold is iron-pyrites, and the microscopical investigations support the theory that most of the gold of the Thames and Ohinemuri districts was originally deposited as a constituent of the pyrites. It is invisible in the pyrites, but it becomes visible when decomposition takes place, though evidently not in the original (molecular?) form. The minute particles unite into granular or vermicular masses, which may be found in the crust of oxide of iron enveloping a nucleus of but incompletely-decomposed sulphide. Through some agencies not yet known, the gold set free is generally removed in a soluble state, and deposited, simultaneously with quartz, in veins, or after the formation of quartz veins, in fissures, &c. Thin coatings on ferruginous minerals may have been produced through some electrolytical process. These delicate coatings were undoubtedly precipitates from solutions containing a considerable amount of gold and other minerals. A portion of the gold in quartz veins has apparently been deposited by mechanical means, though it is difficult to see how it could find its way through the compact rocks.

Most of the Ohinemuri gold is more or less crystalline. As crystals are always distorted or elongated, when free, or the faces slightly curved. In some cases this may have been caused through contact with hard fragments of quartz during compression of the rocks, or after its isolation. Mostly, how ever, the intact finely granular surface indicates that the crystals were formed in this imperfect condition. No perfect octahedron was discovered—generally but a small portion of a

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crystal projects from an irregular crystalline mass. A great number of crystal-faces always cover the more delicate arborescent forms of gold and silver. The incrustations of gold on quartz, due to infiltration in wide fissures, are distinctly crystalline.

An exceptionally fine specimen of dendritic gold, found by Mr. Reid, of Owharoa, near the Radical Mine, was covered with large projecting crystals, arranged with a certain regularity. In most branched specimens the crystals are spread over the surface without distinct order, though under low power the planes seem to be overlapping each other, especially when they are of nearly uniform size. Even a granular surface suggests crystalline aggregates under low magnifying-power, owing to the brightness of the protruding grains. Also, the apparently-corrugated surface of their laminÆ, especially those filling narrow fissures in quartz, bear often a strange resemblance to crystalline structure

It goes without saying that by breaking up auriferous quartz in the mortar the gold is very considerably changed, and but for the different angles and striation of the faces, caused by friction, and irregular protuberances, many pounded specimens might easily be taken for irregularly-developed crystals.

In a few instances the octahedral edges are salient, so as to form a distinct ridge. Twins are very rare, and the twinning obscure. Surface striation is occasionally seen on the larger faces. When the striation is not in accordance with the laws of isometrical crystallization it must be accounted for by contact with striated surfaces, quartz, hÆmatite, or other crystals.

Traces of crystallization were also observed in the amalgam from a tunnel near Owharoa.

With the true crystals of gold and silver we meet occasionally specimens which resemble abnormally-grown crystals very closely, but which are, as measurements of the angles show, not isometric. The irregular growth and ill-defined outline make careful measurements very difficult. But few other mineral species were observed in their proximity, among which ironsand is the most common. How these irregular forms originated is difficult to explain. Most likely they are pseudomorphs. Further finds must be awaited before conclusions can be drawn as to their formation and the composition of the species, the forms of which they take through infiltration in cavities left after the dissolution of the contents.

Some of the arborescent forms are crystallized, the branches being set at an angle of 60° 120°, corresponding with the dodecahedral angle. Others show no sign of crystallization,

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but simply a lichen-like arrangement of the branches. These are always argentiferous in a high degree, and their colour varies from light-yellow to pure silver-white. Figs. 48–50 show the three principal arborescent varieties.

Coarse, angular, often partly-crystallized specimens of gold occur principally in small pockets along the road leading from Owharoa to Waihi and Waitekauri. This gold is remarkably free of silver, compared with that obtained in the mines at Owharoa. The sharp edges prove clearly that it was not carried a long distance; but tunnels driven in the hill-side led to no satisfactory result. It is to be hoped that further prospecting will be done in this very promising locality.

Granular gold is the most common in Ohinemuri. The best prospects were washed out from soft debris near the Hauraki camp, Karangahake, which showed all the different modifications. Figs. 1 and 2 illustrate typical specimens. They are probably derived from a highly ferruginous rock, not unlike that successfully worked in the Woodstock Mine. The particles are often very minute, globular or elongated, but with rounded ends, often filamentous in the form of a rosary, or botryoidal, &c. These forms appear to be limited in their distribution to veins rich in iron-ore, and the so-called “blue veins,” which, when oxidized, disclose the gold often to the naked eye. Extremely small almost spherical particles are present in almost all the blue veins, especially those containing a fair percentage of silver. Treatment with acids shows that the gold is present in a very fine state, though when exposed to oxidizing agencies comparatively large agglomerations of grains are seen. It is likely that the gold exists in most pyrites in molecular form, and that during the process of decomposition the molecules join, forming these tubercular aggregates. Figs. 32–35 show the grains of gold in decomposing pyrites. When the oxidation is not sufficiently far advanced to render the mineral friable, the gold necessarily does not come in contact with the mercury in the battery. In the tailings fragments of an ochreous colour containing gold are often found, which accounts for the great discrepancy between the assayed and crushing value of the ore. Roasting and the chemical processes lately tried on the Thames and Ohinemuri Goldfields will eventually save the greatest part of this gold.

Enormous quantities of iron-pyrites, which we may suppose were auriferous, have entirely disappeared, leaving but the walls of quartz. It is clear that the gold they contained was deposited in other places. Borings at the lower levels will no doubt lead to its discovery in very rich veins.

Larger masses of gold, irregular in shape but with a smooth surface, similar to alluvial nuggets, are found occasionally. A specimen about 4 millimetres in length was washed out of a

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creek near Mackaytown. Another from Owharoa shows indication of crystallization (fig. 51).

Filiform gold is found in all auriferous deposits, though only in small isolated individuals. The threads are bent or twisted in various manner. They contain a high percentage of silver, and some pass into pure silver. Parts of the surface are finely crystalline, but generally of a somewhat dull appearance. Thin, long filaments often radiate from compact masses.

LaminÆ are rarely found in Ohinemuri. The best locality I know is the Waterfall Creek, near Owharoa, which also yielded a few grains of platinum. The scales of gold are much worn, and were probably washed out from the boulders on the bank of the creek. The fact that those boulders contain coarse gold, platinum, silver, mercury, and cinnabar ought to tempt miners to prospect the back country.

Vermicular masses of gold, with smooth or finely-granulated surface, are found in ochreous veins, and are evidently the product of disintegration of argentiferous “blue veins:” often they occur with solid masses, or form parts of them. They appear to have been formed under peculiar circumstances, different from those to which the other varieties are ascribed. They either traverse the ironstone or form a partial envelope. In very few cases a kind of regular arrangement was noticed (fig. 30), which may be the result of imperfect crystallization. These vermicular specimens were always more or less argentiferous. Figs. 30, 31, 37, 38, 39, show the different varieties.

The crystal of quartz, fig. 29, contains cavities of irregular contour, thus differing from most cavities from the same district, which are very fine, and run parallel with the main axis of the hexagonal prism, always near the centre of the crystal, with what I take to be exceedingly fine particles of gold. The specimen is so far unique, so that the nature of the enclosed matter could not be ascertained without risk of loss or destruction.

A natural amalgam occurs near Owharoa, in a short tunnel driven by Mr. Louis Dihars in the side of a hill. A loose boulder which we broke up in the drive showed a brilliant surface, due to millions of very minute globules of mercury. Some of them were saturated with gold and silver, and crystal-faces were visible. Streaks of cinnabar traversed the quartz. Unfortunately, but one rich boulder was discovered, but it was sufficiently large to indicate the presence of remarkably rich mineral deposits in localities which yet await thorough prospecting.

In conclusion, I may say that the microscope indicates all through Ohinemuri the existence of a number of most valuable minerals, many of which are left unnoticed by the prospectors,

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who, as a rule, concentrate their attention on gold. The establishment of the School of Mines at the Thames affords every facility for acquiring a thorough acquaintance with the mineralogy of the goldfields. It is to be regretted that recent failures, and still more dishonest transactions, have brought discredit on such a rich and extensive goldfield as Ohinemuri, which must necessarily become some day one of the first gold-producing districts of the world.

Explanation of Plates XXXIII., XXXIV.
Plate XXXIII.
  • Figs. 1, 2. Granular gold.

  • Figs. 3–15. Gold in quartz-veins.

  • Figs. 16, 17. Incrustation on hÆmatite.

Plate XXXIV.
  • Figs. 18–28. Crystals of gold, and pseudomorphs.

  • Fig. 29. Crystal of quartz enclosing gold (Owharoa).

  • Figs. 30, 31. Vermicular gold in ironstone (iron-oxides).

  • Figs. 32–35. Granular gold in decomposed iron-pyrites.

  • Fig. 36. Surface-coating of gold on iron-nodule.

  • Fig. 37. Vermicular gold.

  • Fig. 38. Compact gold in ironstone.

  • Fig. 39. Thick coating of gold on ironstone.

  • Figs. 40, 41. Gold in decomposing iron-pyrites nodules.

  • Figs. 42, 43. Filamentous gold.

  • Figs. 44, 45. Granular gold.

  • Figs. 46, 47. Filiform gold.

  • Figs. 48–50. Arborescent gold.

  • Fig. 51. Compact gold with octahedral planes.

  • Fig. 52. Stellar gold on quartz and iron-oxide.