Mechanical Deposition of Sedimentary Mineral Deposits

In some cases, the ore mineral, on account of its resistance to weathering and its hardness, survived the destruction of many of the rock minerals; and, during sedimentation, on account of its weight, it was sorted out from the fragments of quartz and other resistant minerals. Thus, the ore-mineral was deposited, without being chemically changed, in a layer by itself or approximately so. Black magnetic sand illustrates this process of concentration, and if consolidated, it would form a bed of iron ore. Placer deposits of gold, platinum, tinstone, monazite, etc., are natural concentrations of heavy minerals, separated from the lighter substances by the sorting action of running water. Clay beds owe their concentration to the lightness and fineness of grain of the clay, allowing it to be carried farther by water than the heavier and coarser materials.

Chemical Deposition of Sedimentary Mineral Deposits

In the weathering of rocks, iron and manganese are dissolved as carbonates by water containing carbonic acid, and are carried down to swamps, lakes, and seas, where they have been deposited as beds of ore; under some circumstances they have been deposited as carbonates (siderite and rhodochrosite), and under others, as oxides (limonite, hematite, pyrolusite, etc.) When the sedimentary rocks have undergone metamorphism, the character of the ore may have been altered; for example, limonite changed to hematite or magnetite.

The vast hematite deposits at Bell Island, Newfoundland, are sedimentary. The siliceous magnetite of Moose Mountain, Ontario, and the siderite of the Michipicoten District, Ontario, may be of sedimentary origin. Gypsum and salt are examples of minerals deposited by the evaporation of seawater.

 

Ore Deposition by Water from Surface

In this class are ore-deposits that have been formed by the action of meteoric waters, that is, water derived from the air (rain, etc.). They are found mostly in sedimentary rocks, particularly limestone, shale, and sandstone. The ore is rarely in fissure veins, but mostly in deposits of an irregular kind.

Gash veins are fissures of no great extent, not much more than enlarged joints. Runs are narrow ore-bodies following the stratification. Flats are more extensive deposits between the layers. Pitches are similar to flats, but following inclined joints; they are characteristic of the lead and zinc ore deposits of Wisconsin. Breccia veins are formed by ore filling in the spaces between broken rock in a fissure. Some ore deposits of this class have been formed in old underground water courses and in caves.

What minerals are found in Sedimentary Mineral Deposits

The principal metal ores found in these deposits are sulphides of zinc, lead, copper, and iron, and oxides of iron and manganese; they yield a large proportion of the world’s production of lead and zinc. The large lead-zinc deposits recently developed at Pine Point on Lake Athabasca, are of this type. The carnotite deposits of Colorado, yielding uranium, radium, and vanadium, are found in sandstone. In Westmoreland Co., New Brunswick, and in Cumberland Co., Nova Scotia, chalcocite has been found in nodules and seams filling the crevices of sandstone.

Among non-metallic minerals, barite veins are sometimes of this class. It is possible that many of the ore bodies not obviously connected with igneous intrusive rocks have been formed in association with intrusions that are still hidden; for example, the fluorspar veins of the Madoc district, Eastern Ontario, and the barite deposits near Windsor, Nova Scotia, may have been formed in this way.

Concentration of Ore Bodies by Weathering

By weathering or by the action of magmatic water, an ore body may be concentrated in two ways;

1. by the removal of valueless material so as to leave a richer ore;
2. by solution of the valuable material and re-deposition at a lower level.

By Removal of Valueless Material

Ore bodies formed in this manner are called residual ore bodies; some examples are the following:

1. The siliceous iron ores of the Lake Superior region have been concentrated in places where the conditions have been favorable. The underground waters have removed the silica and left the iron mostly as hematite. Nearly all the ores mined in the Lake Superior district are residual ore bodies of this kind.
2. Some limestones have nodules of phosphate of lime scattered through them. The limestone has been dissolved, leaving the phosphate as a valuable residue, as in Florida deposits.
3. Gold veins may be richer at the surface, where the gold has been left behind in the process of weathering and washing away of the vein matter, particularly when the gangue has been pyrite.

By Solution of the Valuable Material

At the surface, where air and water are acting together, sulphides are oxidized to sulfates, and carried down in solution. When they reach the ground water, which keeps out the air, sulphides are formed again and deposited. The result of this in the case of copper pyrite, is to leave a leached zone of gossan at the surface, reaching down to the level of the groundwater. At or below the ground-water level, the zone of enrichment may begin and continue as far down as the enrichment solutions have reached. Below this, again, the original ore will be found. Since most sulphides contain iron or are mixed with pyrite, the oxidation and leaching leave a residue of limonite, called gossan or iron capping. Whenever gossan is found, it is always a good indication to the prospector: it may cover something of value.

Mineral Associations

There are a number of mineral groups occurring so commonly that it is useful to the prospector to carry their association in his memory. Here are some of the important ones:

• Gold with quartz, with pyrite, with copper pyrites, with pyrrhotite, with zinc blende, with galena, with tellurides, with stibnite.
• Silver with galena, with zinc blende, with cobalt and nickel minerals.
• Galena and zinc blende.
• Copper pyrites and pyrite.
• Galena, barite, and fluorspar.
• Fluorspar, barite, and celestite.
• Native copper with a group of silicates called zeolites.
• Tinstone, wolframite, tourmaline, topaz.

Relative Ages of Rock

Doubtless mineral deposits began to form as soon as there was a solid crust and the process has gone on up to the present; but the records of the rocks show that the concentration of ores into ore-bodies has been faster at some periods than at others. Valuable mineral deposits are found in rocks of every age, but there have been special periods of disturbance, such as mountain-building, folding, volcanic activities, and special conditions of sedimentation, during which ore deposition has been more active than in the intervals between these periods.

The relative ages of stratified rocks are determined by the order of the strata, the younger being above the older. Even where folding and erosion have made the succession less plain than it is when the strata are horizontal, careful study in the field, aided by chemical and microscopical analysis and the evidence of fossils, solves the problem. The relative age of intrusive rocks is determined by evidence of various kinds. For example, an igneous rock must be younger than the youngest rock which it intrudes.

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Times of Forming Mineral Deposits

In somewhat the same way, the time can be judged at which a mineral deposit has been formed. If it is a sedimentary deposit, its age is the same as that of the sedimentary rocks with which it lies. The time of formation of veins and other such ore-deposits, and of magmatic segregations, must be after the consolidation of the youngest rocks in which the deposits are found. For example, many of the fluorspar veins of the Madoc area occur in the youngest rocks of that district, sandstone and limestone of the Ordovician period; they were therefore formed after those rocks. The gold veins of Nova Scotia are in rocks of Cambrian (or possibly pre-Cambrian) age, and are therefore younger than those rocks. The veins of the Kirkland Lake gold area in Northern Ontario, are associated with igneous rocks that intrude conglomerate and other sedimentary rocks, preCambrian in age; this fixes a limit for the age of the veins.

Great Periods of Ore Deposition

When such evidence is studied throughout the rock record, the following periods of ore deposition in North America are made out (beginning with the oldest):

• Precambrian—Iron, gold, silver, copper, nickel, cobalt.
• Devonian—Gold, copper.
• Cretaceous—Gold, copper, silver, lead, zinc.
• Tertiary—Gold, silver, copper.

In Canada are valuable ore deposits that have been formed in each of these periods.

The gold, copper, and some of the iron ores of Nova Scotia, New Brunswick, and Eastern Quebec, were deposited in the Devonian, at a time of intense folding, in connection with granite intrusions of that period; the intrusion to which we owe the asbestos field of Southern Quebec may belong to the same time. The zinc-lead veins of the Gaspé Península were deposited in Devonian shale and limestone, intruded by porphyry and syenite.

To the pre-Cambrian times must be referred the deposition of the silver, gold, copper, and iron ores of Western Quebec, Ontario, and Manitoba. The silver-cobalt-nickel ores of the Cobalt and Thunder Bay districts, were deposited in connection with intrusions of diabase. The copper-nickel ore of the Sudbury area is found at the contact of a great norite laccolith with pre-Cambrian granite, gneiss, etc. The gold veins of Northern Ontario are found closely associated with intrusions of granite rock. The great sedimentary deposits of low-grade siliceous iron ores found in the Lake Superior region and in other parts of Ontario, were laid down in preCambrian time.

In British Columbia, there was a long period of igneous activity in late Jurassic and early Cretaceous times, the result of which was the great granitic intrusion of the Coast Range, around the edges of which, mostly in the Cretaceous sedimentary rocks, have been found valuable deposits of gold, copper, and silver ores. The silver-lead-zinc ores of the southern border districts are referred to the same period of activity.

Activity of a very different kind, vigorous plant growth, is responsible for the extensive Carboniferous coal beds of Nova Scotia and New Brunswick, and the equally important British Columbia and Alberta coal beds formed mostly in Cretaceous times. The lignite beds of British Columbia and Saskatchewan are partly of Cretaceous and partly of Tertiary age.