OUR GOLD MINES. [COMMUNICATED.]

Daily Southern Cross, Volume XXIV, Issue 3399, 8 June 1868, Page 3

OUR GOLD MINES. [COMMUNICATED.]

In a former article, published in the CROSS of tbe 22nd ultimo, upon this subject, the uature and character of mineral veins were briefly explained. With reference to that article the writer has bten informed that his remarks would not be understood by unread persons employed upon the diggings ou account of the number of technical and scientific terms which it contained. In continuing his remarks the writer will avoid, as far as the subject will admit of, all terms that are beyond the comprehension of the unread miner, and as far as he is able he will convey such information as may enable him to obtain the object of his »eaich— gold. Many of the persons who visit our goldfields with the view of mining have not the slightest knowledge of geology, or the different modes of identifying the most likely places in which the precious metal may be fouud ; they waste the few pounds they may have brought with them in fruitless attempts ia making holes (for they can be called by ■o other name) in places where there it not the slightest chance of obtaining the object of their search. But it may be said that the inexperienced miner has discovered the localities of gold Without any previous knowledge of the places where it is to be found. Even then, a few hints may be useful to enable the discoverer to follow up his discoveries successfully and obtain the full yield bis claim is oapable of giving. . The hills and mountaius ab the gold-diggings in the Thames district have been formed by nature without any regard to uniformity ; they are one mass of unevenuesa. The islands of New Zealand having been in former times Bubject to severe volcanic action, is it not possible that the mountains at the Thames have been thrown up by a severe explosion, and the mineral matter, when in a molten form, deposited in the veins or rents caused thereby ? The stratiform deposit", or different compositionsjof the rocks, aocording to geologist*, are subdivided into two great olaßSes — primary and secondary. The former seem to have been called into existence before the creation of organic matter, because they contain no remains of [vegetable or animal beings ; while the latter are more or less interspersed, and sometimes replete, with organic remains. The primary strata are characterised by the nearly vertical or highly-inclined position of their planes. The second are, for the most part, in a nearly horizontal position. In every mineral plane the inclination and direction are to be noted: the former being the angle which it forms with the horizon, the latter the point of the azimuth or horizon towards which it dips, as west, north, east, or south. The direction of the metallic vein or bed is that of a horizontal line drawn in its plane, and which is also denoted by the point of the compass, since the lines of direction and inclination are at right angles to each other. The first may always be inferred from the second, for when a vein is eaid to dip to the ea-t or west this implies that its direction is north aud south. The smaller sinuosities of the mass are not taken into account, just as the windings of a river are neglected in staling the line of its course. If masses of the metal are discovered the miner may be sure that he has hit upon a different kind of rock, which does not extensively spreat in parallel planes but ii regular heaps : this is exemplified in some of the best claims at the Thames. The principal rocks in which native gold is found are quaitz and talcose, aud it is often contained in pyrites, which are a composition of iron, sulphur, and a very small quantity of gold. Mundic is the term given for pyrites by the miners. It obtained its name from beiug capable of striking fire with steel, and from the Greek word pur, fire. The gold rocks of Now Zealand are to a great extent micaceous or talcose schiht— that is, the leaves or layers of the crystal may be either thick or very thin: they sometimes separate easily, and sometimes separate with great difficulty. When the leaves or layers are thin and separate easily, the structure is said to be foliaceous. Mica is a striking example, and the term micaceous is often used to describe this kind of rock. When the leaves or layers are thick, the term tabular is applied; quartz and heavy spar afford examples. The mica is sometimes arranged in stellar or star shapes, with veins or beds of quartz or silica. The gold is mostly confined to these veins, though also found to some extent in the rock or casing either side. The schist or slaty structure |is often half decomposed or rusted. Tue quartz is usually more or less cellular, having caves or smaller caverns in it, or wanting in perfect compactness, and sometimes tabular j yet it is at times quite solid. Iron pyrites is frequently present, and by decomp sition it stains the rock with iron rust. There are other minerals often associated with the gold, such as copper pyrites, blende, galena, anglesite, sulphur (in minute yellow crystals, proceeding from the decomposition of pyrites): this is very prevalent amongst the gold taken from the Waiotahi creek. Heavy spar is sometimes a large constituent of the vein, and fluor spar is now and then present. The peculiar appearance of the quartz, somewhat cellular, or, as the miners term it, having the form of "rats' teeth"— more or less rusted, and its position in veins through an imperfect shale or brittle structure, and generally not firmly attached to the enclosing walls -affords the best indication of the presence of gold, though the absence of all these conditions is not evidence that no gold is to be found. The grains of gold may sometimes be seen in the cavities of the quartz, or it sparkles on a surface of fracture. But very commonly a mass of quartz that shows Dothing to the eye, yielda gold on trial. The examination of a gold-producing rock is an extremely simple process. The quartz composing the veins or leaders is first pounded or crushed up fine, and sifted ; a certain quantity of the sand thus obtained is washed in a shallow iron pan or tin dish, and, as the gold siuks, the material above is allowed to pass off into some other receptacle, but not as is frequently done upon our diggings at the Thames and Tapu Creek, where they allow the dirt or material to flow away with the current of the stream, being satisfied with the test of a first washing. True, they get by the first washing the largest and heaviest part of the gold which is left in the angle of tbe p»D, but by saving the refuse of the first operation and repeating the process a further portion is obtained ; and when the bulk of sand is thus reduced to a manageable quautifcy the sold is amalgamated with clean mercury or quicksilver or compounded with quicksilver ; the compound or amalgam is then strained through a chamois leather bag to separate any excess of mercury, and finally is heated, and the mercury expelled «>r dissolved, leaving the gold . In this way, by successive trials with the rock, the proportion of gold is quite accurately ascertained. It i« the same process used with the larger crushing", though on a small scale. Mercury or quicksilver unites reauly with gold, and thus separates it from any associate! or connected rr.ck or sand ; and it is employed in all the American and Australian gold minings, though mach gold may be oftenobtained by oimple washing without amalgamation . Masses of quartz with no indications of gold, examin d in the above way at one of the Michell Creek workings, afforded an average of eight pennyweights to the hundredweight of quartz. When the gold is mixed with copper or silver, the mode of separating the copper depends on the process of cupellation ; and that of separating the silver on the power of nitric acid to dissolve silver without acting on the gold. For the benefit of the inexperienced miner I will give the process of cupellation : it consists in heating the assay or material under chemical examination ia a small cup which is called a cupel, made of bone ashes, or in a cavity or pit containing bone ashes, while the atmosphere haafree access The heated metal is oxydated (so called because, when thus heated, oxygen gM, one of the constituents or materials of the atmosphere, combines in many cases with some parts of the assay), by the air passing over it, and the oxyd formed siuks into the porous cup, leaving the precious metal behind, ahe shape of the cup or cavity should be similar to a barrel or tub. In order to fuse, the *lloy, or impure metal, and still have the atmosphere circulating over it, the cupel is placed in a small oven shaped vessel, called a muffle. It is of infusable stone ware, and has a number of oblong holes through which to admit the flame from the fire, and give exit to tbe atmosphere which passes into it. The oven, or muffle, is inserted, in a hole fitting it, in the side of a vertical furnace with the open mouth outward, and nearly even with the exterior surface of the furnace. The fire is made within thefurnaoe, below, around, and above ; and after beating up the cupel ia put in the oven with the away in its shallow cup-shaped cavity. It thus has the heat of the furnace to fuse the assay, and the air at the same time is drawn in over it through the large opening of the oven or muffle. The oxygen of the atmosphere mixes with the lead of the assay and produces an oxyd, wbich oxyd sinks into the cupel or cup, leaving the silver or gold behind. The completion of the process is at once known by the chaDge of the assay suddenly to a bright shining globule. In j the cupellation of the assay containing copper, lead is melted with the assay. The lead, being fused in a draft of air, oxydises, and aUo promotes the oxydation of the copper, and both oxyds disappear in the pores of the cupel, leaving the gold behind, and the silver alloyed or cpnneored with it. In this process, the gold is melted with three tawits weight of silver, a quartatiqn as i v ifi b#

profeBiion»l avsayers, the gold being one part out of four of the alloy, in order Dy its diffuiion to effect ft more compete removal of the silver ma well as the oontaiusd copper. The cupel is placed in the heated furnace, and the gold, silver, and lead on the cupel ; the heat is coutinued until the surface of the metal is quiet and bright, when the cupellation ia finished ; the metal then is slowly cooled and removed. The button obtained, after annealing it by bringing it to ft red heat, is rolled out into a thin plate, and boiled in atroDg nitric acid. The process ia repeated two or three times, with a ohange of the acid eaoh time, andthesiWer is thus finally removed. Atthemint, hal » gramme of the <*old is submitted to assay. Th assay-gold and quart«tion silver are wrapped in sheet of lead weighing about ten times as much as the gold under assay. After oupellation, the plate of gold and silver, loosely rolled into a coil, m boiled ten minutes in four and a half ounces ef nitric acid of 20' to 22' Reaumur ; the acid is then poured off and another portion of stronger acid is added, about onehalf tbe former quantity, and boiled ten minutes j then the same again. The gold thus purified is washed and exposed to a red heat, for the purpose of dryingand annealingit, and then weighed. I bare but imperfectly explained tbe process of eucellttion for the benefit of the miner, in order that he may know what is done with the gold when it is placed ia the hands of tbe assayer, and to enable him to perform the process without professional aid.— l am, &c., Stbata. (To be continued.)