THE GEOLOGY OF MINES AND MINERALS.

Otago Witness, Issue 2490, 4 December 1901, Page 75

THE GEOLOGY OF MINES AND MINERALS.

By Professor Pabk.

Chapter, 11. CLASSIFICATION OF MINERAL DEPOSITS.

Attempts have been made by Yon Cotta, Phillips, and others to classify mineral deposits according to their form, supposed origin, and composition ; but a classifica■ion that will fulfil all conditions has not yet been formulated. Outward form seems to offer the most convenient basis by which deposits may be classified. Adopting this sjstem, they may be arranged as follows : —

I. — Stterficiai, Deposits. II. — Stratified Deposits, (a) Constituting a bed or stratified deposit. (ij) Disseminated through sedimentary beds. (c) Deposited from aqueous solutions. UL — Unstratified Deposits. (1) IEKEGXLAK. (a.) Deposits of volcanic origin. (v) Stockwork deposits, (c) Contact deposits. (l>) (Fahibands. (c) Impregnations. (2) :regt;lab. (a) Segregated veins. <B) Gash yeins. (c) True or fissure veins. SrrERFiciAL Deposits. These embrace superficial alluvial de-

posits of all kinds, whether beach sands, river gravels, lake deposits, or glacier drifts, containing" loose particles of gold, tin, platinum, iron, or precious stones ; ut exclude the alluvial deep leads of Victoria and New South Wales, which are often covered with a thick sheet of lava, and are, in consequence, worked from iiafts or adits. Examples. — Familiar examples of goldfaring gravels are the alluvial drifts of Victoria, West Australia, Klondyke, California, and New Zealand. The principal alluvial fields of this colony lie in Otago, Westland, and Coilingwood. No alluvial gold worth mention has been found in the North Island. Gold-bearing -beach sands containing ironsand, mostly magnetite, occur along the shores of Westland and Southland, and the Gold Coast of West Africa. Among later alluvial gold discoveries, that at Cape Nome, in Alaska, is the most important. A sloping plane or tundra extends from the sea back to the foot of the mountains. It consists of a succession of beds of sand, gravel, and clay resting on the upturned edges of the palaeozoic basement rock. The surface is covered with a layer of tundra moss and decaying vegetable peaty matter to a depth of 18in or 24in. The gold-bearing matter is found along the beach, being appai'ently a rewash of the tundra terrace gravels and eands.

Position of Pay-wash. — Experience has shown that alluvial gold is deposited along the inner s.de of the curves in the course of the river. Any stratum of good gravel in the opposite bank was evidently laid down at some previous time, under similar conditions The accompanying diagram will show the occurrence of the gold more clearly. The gold is coarsest at the head of the bar, gradually becoming finer and scarcer, and spreading out below, until the dirt is too poor to pay. »?o k-^g as the river retains its course, the paystreak will continue to form as the bar is extended from year to year.

The average uniform size of the gravel seems to show that the gold is only deposited where the current has a certain medium velocity. At the points where the bends in the river are made more permanent by rocky banks, the pay-gravel \<s generally deeper, and of greater extent than elsewhere. The richest gold does not collect in the deep pot-holes in the river, as might naturally be expected, but on the bars or rocky ledges below. The reason

Many of the gold-bearing terraces on West Coast of New Zealand, and in the higher valleys of the Rocky Mountains, 'bear eA'idence of glacier-river origin, while some are composed of a rewash of glacier gravels and morainic mailer.

the difficulties of working the pay-wash with success. At Wetherstones and Blue Spur, Pleistocene river gravels have become consolidated into 'ements. On the West Coast of the South Island

larger nuggets, which have been found of ali shapes and sizes, up to thousands of ounces, the coarse gold found in claims generally assumes f> bean-shaped form. The finer gold occurs as small heavy shots, but more often as thin flaiies, langing from the smallest particle, which almost floats in water, to pieces like bran. In r:vcr claims, where the gold ha*, been deiived from the erosion and denudation of goldbearing lodes in the vicinity, fragments of quartz with adhering nieces of the precious metal are often met with.

Associates of Alluvial Gold.— -The ronstaiU associate of allavial gold in all countries i's magnetic ironsanci, the agencies vhii.h led to the concentration of the gold faring also acted on the even more abundant magnetite. It is found that -vhatever hea\j oi es or minerals are associated with gold in the original country rock are also found with it in the resulting alluvial drifts.

In CoJlingwood, near Nelson, in New Zealand, considerable quantities of naiive lead, in the form of round shot, were found for many years in the sluice boxes, with the gold In places it was so abundant as to choke up the ripples, and was thrown away as useless. Samples collected by the author, and submitted to chemical examination at the colonial laboratory; disclosed the fact that each shot contained a skeleton of pure gold.

- The tundra gravels bear no evidence of glacier origin, but appear from their bedded and water-worn character to have been deposited in a lake-basin or in a shallow sea near the estuary of a large river. | Origin of Alluvial Gold.— Alluvial gold probably originated from the erosion and denudation of gold-bearing veins or country during countless ages, resulting in the concentration of the gold in leads, or channels, by a process of natural sluicing, i3ome writers have thought that the occurrence of occasional large nuggets in gravel drifts was an evidence that alluvial gold had been deposited in situ from goldbearing solutions circulating through the gravels, but there is very little data to support this contention. Country that was once overrun by glaaers is found to contain two classes of transported matter— one transported jr ice, the other by water. The first forms , tumbled morainic masses, the latter water- | -worn gravels and sands deposited by the liver draining the .ottom of the glacier. Both classes are often mixed at certain points. When the country is gold-bear-ing the glacier gravels and morainic matter contain gold. The moraines are formed ty ice which cannot separate the gold from the rock debris; hence the gold is scattered through it just as the glacier hap- ( pened to diop it. The glacier gravels, on the other hand, being largely a rewash of the moraines, contain the gold in a more . $<«tceuti'*fced iot&k

for this may be that pot-holes generally occur in narrow, rock-bound pails where the velocity and scour is too great to allow the permanent settlement of gold particles, Occurrence of Gold in Drifts.— ln ,lluvial deposits the gold is found in a layer of pay-wash resting either on the slate or schist bottom, or on a false bottom In river and creek workings, the pay-wash lies invariably on the slate bottom ; but in the alluvia of old lake basins it frequently rests or. a false bottom of stiff clay or cement, In cases where the alluvia is of considerable thickness, there often occur two or more streaks of pay-wash resting on successive false bottoms. A typical section of the river claims of the Buller River, in Nelson, is fchown m the following diagram: — ( .

A section of the Mamihenkia old lake basin at Ida Valley show;: a typical eximple of the jeeurrence of pay wash 1 eating on a false bottom, -Throughout this basin the lacustrine beds have been tilted i4mofct ou edjje^ thereby grea>t^ iocrea&iojn

(a) Recent gravels. (cD (b) Grey quartz sands. . (c) (c) Ferruginous quartz sanis. (i) of Xew Zealand the alluvial workings may I be divided as follows: — I I —Cement Claims. J _, . II. — AixrviAL Claims. (a) Woikings on slate bottom. I («) Workings in consolidated gravels resting on sandstone bottom, and at Ross on false bottoms. •

Carbonaceous sand 9. Aunterous wash. Stiff clay. (g) Scliist. ; In Takaka, near Collingxrood, the alluvial gold, besides magnetite, is associated with grey-coloured grains of osmium, iridram, and nlatinum ; in Baton, with platif TTT .., . . , Hum; on the W e?t Coast and in the nords h iß> locall kn(r|rn as rubies . in Centra] OtagQ masseg of gcheelite and magnetite Throughout A usthe a]lavial go]d ig often ttßSociated "with gem sands containing topaz, zircon, spinel, rubies and garnets. (To be continued.)

(c) Workings in glacial drift. (d) Workings on black sand beaches. Cements. — These onsist of consolidated quartz grits and gravel's, associated with the New Zealand coal measures of lower tertiary age. They vary from lft to 6ft in thickness, and have their greatest de-

velopment at Cement Town. Wash on Slate Bottom. — This occurs on the banks of all the streams and gullies intersecting the gold-bearing series- The

gold is generally coarse, and frequently associated with quartz. In most cases it does not appear to have travelled far. Wash on Sandstone Bottom. — The ongiomerates or consolidated gravels rest comfortably on a Drown sandstone, on which the layer of pay-wash rests. The best results have been obtained from a rewash of these gravels. At Ross the consolidated gravels form high terraces, the pay-wash resting on false bottoms, of which at least six occur in the district. Glacier Drifts —At Hokitika and Greenstone many large and rich claims have been worked in these deposits for a number of years, with highly payable results. Black Sand Beaches. — The black sand is magnetite, mostly derived from the chloritic schists in the neighbourhood. The gold

occurs in excessively fine particles, and is oncent^ated with the black sand in layers alonp the beach, the laving action of the ?'-Miealing waves having removed the lighter particles of quartz &and. form pi Alluvial Gplti^— Eice^ting the