ZINC MINERALS.

SMITHSONITE.

The smithsonite (zinc carbonate) is principally the ferruginous variety, deposited by replacement of limestone. Some cavities in it are lined with druses of purer smithsonite.

FERRUGINOUS VARIETY.

Main fracture

The ferruginous smithsonite is medium to dark gray where free from oxidation but is mostly stained from light to dark brown by hydrous oxides of iron and manganese. It is very fine to microgranular, and nearly all of it has a marked lamellar structure. The lamellae are from 1 to 7 or 8 millimeters thick and are separated by thinner spaces, lined with films or welldeveloped druses of copper, zinc, iron and manganese minerals, and, in a few places, calcite. The lamellae range in shape from nearly perfect planes to intricately contorted forms that are closely related in position to fractures preserved from the original limestone.

This relation indicates that the zinc solution,

FIGURE 1.-Sketch showing relation of lamellae of smithsonite to open part of a fracture. Solution spreading in fan shape from the opening produced concentric lamellae concave to the opening. Part inclosed in solid lines sketched from a specimen.

after invading the limestone along the fractures, was diffused laterally and produced a rhythmic replacement of the limestone. Where a single fracture was of uniform width and the limestone of uniform permeability replacement formed lamellae with little or no curvature. Where a fracture contained a relatively open part between tight or sealed portions the solution spread like a fan into the rock and the replacement lamellae assumed concentric positions concave toward the open part of the fissure, as shown in figure 1. Where the solution spread from two intersecting fractures and the adjacent rock was of uniform permeability the lamellae became locally convex to the junction line of the fractures and curved in either direction to positions parallel to each fracture, as shown in figure 2. Where a network of fractures was present the shapes of the lamellae were correspondingly complicated (fig. 3), and any variations in permeability produced still further complications (fig. 4).

This prominent lamination is a feature that distinguishes the smithsonite at Ophir from the bulk of that in the Tintic district, Utah, and at Leadville, Colo., which is massive or contains only cavities due to leaching. The difference is believed to be explainable by the influence of geologic conditions on the chemistry of deposition.

In the Tintic and Leadville districts the waters that deposited the zinc as carbonate in limestone first descended through a considerable thickness of rhyolitic rocks and became charged with material leached from them, notably alkali carbonates or bicarbonates and

Fractures

silica, together with other compounds of no importance in the present discussion. Carbon dioxide forms a small proportion of the rain water entering the earth, but it is held there in the form of carbonates after the water has become exhausted either by evaporation or by the formation of hydrated minerals. These carbonates are readily soluble in the next rain. water that reaches them, and by many repetitions of this process in a thick formation of porphyry a considerable quantity of carbonate accumulated in the water that reached the ore bodies

in the underlying limestone. The zinc on oxidation from sulphide was carried in solution largely as sulphate and also as bicarbonate, the latter due to reaction with alkaline bicarbonates or with any excess of carbon dioxide present in the solution. The extent to which such reaction took place depended upon the relative concentrations of the different constituents, but the effect was to form the least soluble compound. Were such a solution to become supersaturated or to lose excess carbon dioxide, zinc carbonate, the least soluble mineral that could be formed from the solution, would crystallize.

When a zinc sulphate solution attacks limestone, the limestone goes into solution as sulphate and is replaced by a molecularly equivalent quantity of zinc as carbonate. The formation of a deposit by this simple molecular interchange would involve a shrinkage of 37 per cent; but with a sulphate-bicarbonate solution of zine this loss of volume could be in part or wholly compensated by deposition of the zinc that was already available as carbonate, and the zinc carbonate actually formed could thus replace the limestone volume for volume instead of molecule for molecule.

In the Ophir district there is no evidence to suggest that volcanic rocks or sedimentary rocks of similar chemical composition ever over

lay the limestones which now form the crest of the ridge. Furthermore the ore bodies lie close to the crest of the ridge, where there has been a minimum amount of erosion, and water from the surface would therefore not take noteworthy quantities of alkalies and silica into solution. Any carbon dioxide present would aid in the conversion of limestone into soluble bicarbonate. This solution, after converting the zinc blende in the sulphide ore body to sulphate and passing downward into limestone, would cause reaction between zinc sulphate and calcium carbonate, with the resultant replacement of limestone by smithsonite-a replacement molecule for molecule. Additional zinc carbonate could be deposited from a combination of zinc with carbon diox

The shrinkage due to the molecular replacement is expressed, approximately at least, by the quantity of voids in the lamellar smithsonite. The occurrence of a lamellar rather than

Fractures

Fractures

any other arrangement may be attributed to a rhythmic process of deposition, analogous to that forming the Liesegang rings. After deposition of the first lamella the solution permeated onward and attacked the area represented by the second lamella and the intervening space. Replacement followed, the intervening space representing the amount of resulting shrinkage. Continued permeation of the rock alternated with successive stages of replacement, until the supply of available zinc was exhausted.

It may also be suggested that the position of the ground-water level, general or perched, is a factor in determining whether replacement goes on volume for volume or molecule for molecule. If limestone is replaced by smithsonite in the relatively stagnant water at or a little below ground-water level, as it evidently was at Leadville, the entire quantity of rock to be replaced may become saturated with solution before the slow process of replacement has progressed ap

1

Llesegang, R. E., Geologische Diffusionen, p. 180, 1913. Review by A. Knopf in Econ. Geology, vol. 8, p. 803, 1913.

preciably, and the rock may be replaced volume for volume if enough zinc is available and enough carbon dioxide in solution is present to supplement that derived from the replaced limestone. If a locally impervious stratum impounds water above the general ground-water level, as occurred in the Tintic district, the opportunity for replacement volume for volume is likewise favorable. Away from either of

fill the void. Thus the lamellar variety of zinc carbonate may be regarded as characteristic of replacement along watercourses above local or general ground-water level.

DRUSY VARIETY.

Druses of smithsonite are for the most part very inconspicuous in this district. The most prominent noted were not over 1 millimeter thick and ranged from pale brown to light gray and pale green. All the druses noted are separated from the ferruginous smithsonite by films of limonite, a relation suggesting derivation through decomposition from the ferruginous variety. The drusy smithsonite was deposited before any of the copper carbonates. These relations are the same as those noted in the Tintic and Leadville districts.

HYDROZINCITE.

The only material resembling hydrozincite noted was a few white streaks along or across lamellae of smithsonite. None could be separated with sufficient purity for identification. The relations of this material to the smithsonite are apparently the same as those of the malachite described on page 9. Although it has been found close by malachite, its relation to that and the other copper carbonates is uncertain.

MINOR ZINC MINERALS.

The scarcity of calamine is a characteristic feature of the Ophir deposits; it was noted only in a few small white patches along cavities in smithsonite. These patches consisted of microscopic crystals

that could not be separated cleanly from smithsonite. No typical flat prismatic crystals of calamine were found. Small quantities of gelatinous silica obtained by dissolving material composed principally of earthy hydrous oxides of iron and manganese suggest the presence of a little calamine in these decomposition products of smithsonite. The meager evidence at hand indicates that some calamine formed on smithsonite before oxidation of the smithsonite, and that some formed after or as a result of oxidation. No calamine of later origin than any of the copper carbonates was noted.

A few films of gray to pale-brown clay were found coating cavities in smithsonite. This clay has an index of refraction of about 1.58 and is decomposed by hydrochloric acid, with the separation of silica-properties analogous to those of the zinc-bearing clays studied by the writer at Leadville. It was the last of the zinc minerals to form but was not identified in specimens containing copper minerals. The scarcity of calamine and zinc-bearing clay are attributed to the fact that the descending waters that formed the oxidized ores passed only through limestone, as shown on page 4, and had very ittle opportunity to take silica and alumina into solution. Conditions controlling the formation of hydrozincite, the basic zinc carbonate, by the alteration of smithsonite, the normal carbonate, are only vaguely understood. It is known that the basic carbonate instead of the normal carbonate will be deposited on supersaturation from solutions containing no excess of carbon dioxide, and it may be assumed that such solutions can take carbonic acid from smithsonite, replacing it with an equivalent quantity of hydroxyl, and form the basic carbonate-hydrozincite. The question why hydrozincite and malachite form close by each other and also close by aurichalcite, whose chemical formula is equivalent to the sum of the formulas of these two minerals, will be considered after the copper carbonate minerals have been described.

COPPER MINERALS.

MALACHITE.

Malachite, the green basic carbonate of copper, has two modes of Cecurrence in association with the zinc carbonate ores at Ophiras a deposit formed by the replacement of lamellar smithsonite and as druses lining cavities in smithsonite.

The degree of replacement varies, there being all gradations from Smithsonite containing a few scattered green spots or streaks to practically pure malachite in which only the lamellar structure of the former smithsonite is 'preserved. Thin sections of the spotted or streaked facies show the malachite to consist of typical radiating