Mineral Surface Outcrops and Showings

Mineral Surface Outcrops and Showings

The search for mineral deposits can be made to produce better results if special attention is given to certain surface appearances. Experience has shown that minerals sought for in a known area are distributed in definite relations to the rocks of that area. The valuable deposits should be searched for in the zones thus determined by the geologist. The geological map should be carefully studied by every member of the party who is competent to do this. The mineral belts, or zones, are exceptional tracts of country; outside of these tracts, the country may be barren. For example, the gold belts of Northeastern Ontario and Northwestern Quebec are cut off on the east by a wide stretch of barren granite. In British Columbia, there is a great granite batholith, around the edges of which are found many valuable mineral deposits, but within which the country is mostly barren; although mineral deposits have been found around “islands” of sedimentary rocks, which are inside the general contact line of the granite and sediments.

Veins

Many of the mineral deposits found in Canada occur in veins or similar structures. These often show at the surface, with the characteristic minerals visible; or, the minerals may be seen on breaking the surface material. Examples are; gold-bearing quartz veins; the calcite veins of the Cobalt silver area; the fluorspar veins of the Madoc area; and some lead and zinc ores in British Columbia.

The first discoveries are usually made by reason of such easily seen outcrops; after these have been staked, comes the harder task of finding veins not so easily visible. This is accomplished by uncovering rock in the most likely places, by removing moss, and by trenching. Since most of the veins in any one area usually run in the same general direction, trenching should be done systematically across this general direction. If the vein material were such as to be easily removed by weathering, the position of the vein may be marked by a longish hollow, which may be somewhat ditch-like. On the other hand, veins with quartz as filling may stand up as straight ridges or dikes. The covering of hidden veins, as well as of ore bodies of other descriptions, often gives indications of what is beneath.mineral_surface_outcrops_and_showings

Capping and Stains

The materials constituting capping, or gossan, and stains are secondary minerals that are formed by the weathering of the valuable minerals sought for, or of minerals that accompany them. The commonest of these Products of weathering is iron rust or limonite, which results from the weathering of pyrite, pyrrhotite, copper pyrites, zinc blende, and other sulphides containing iron. It should not be forgotten, however, that a rusty stain or capping may be due to the weathering of carbonate, of iron, or even of basic rocks high in iron. Pyrite, pyrrhotite, and copper pyrites weather most rapidly; hence, iron capping, or gossan, as this material is called, is most likely to cover these minerals or the valuable ore deposits associated with them. The color of gossan is usually brown, but it is sometimes yellowish or reddish. In a glaciated country like Canada, in many places the weathering has not gone very deep since the surface of the rock was cleaned off by the ice sheet. For this reason, the unweathered mineral can often be got a few inches below the rock surface. But in some places, the gossan may be many feet deep. The nature and distribution of the gossan depends to a considerable extent upon the sulphide that has weathered to form it. Pyrite gossan may be transported and spread a considerable distance, but gossan from copper pyrites is more likely to stay where the copper pyrites was. The Sudbury copper-nickel ore bodies, the Anyox, B.C., copper ore body, and the Flin Flon ore body in Northern Manitoba, were discovered by means of gossan. In northwestern Quebec and northeastern Ontario, gossan has been the guide to the discovery of many bodies of copper-gold ores.

Other useful stains are: green carbonate of copper, malachite, and the less common blue carbonate, azurite; erythrite, or cobalt bloom, a pink or red mineral formed by the weathering of cobalt arsenides; and annabergite, or nickel bloom, the corresponding nickel mineral, which has a green color. In the silver areas of northern Ontario, cobalt bloom has been the prospector’s guide to many valuable discoveries, but in a number of cases, the veins thus located have proved to be poor in silver. Cobalt bloom is often found in mineral districts that yield no silver; for example, it has been found in the Madoc area, and has been noticed in the Rainy Lake district. In the Bird River area, northern Manitoba, it is the surface indication of cobalt nickel ores.

Float

Float is mineral or rock that has been moved from its original position:

  • (a) by sliding or rolling down hill;
  • (b) by the action of streams;
  • (c) by glacial action.

In unglaciated country, float is traced back to its source by working uphill or upstream. In glaciated country, the source must often be sought in the direction from which the ice came, as indicated by glacial grooves. This information is often given on geological maps. Two or three of these means of transportation may have been used in carrying a piece of float. In mountainous areas the float has usually come down-hill and: down-stream whether glaciated or not.

The float should be followed back until no more is found. If, at that point, the ore body is not discovered as an outcrop, a trench should be started and run in the same direction until the ore body is reached. The condition of the float gives some indication of the distance it has traveled. If sharp- edged and fresh-looking, it is probably not far from the ore body; if rounded and worn-looking, it may have traveled a long distance. Still, a piece of pyrite or other easily weathered material may owe its shape and appearance to weathering, and may be quite near its source. Float found in the beds of streams may have come from the steep banks or it may have been swept downstream by a swift current; its sources should be sought up-stream, and the banks should be examined at and above any point where the float is noticed.

“In Nova Scotia, the gold-bearing veins and saddle reefs are in a district that has a widespread mantle or drift. The discoveries are made by first finding float, i. e., loose blocks of gold-bearing quartz in the drift; then, tracing back toward the direction of glacial advance, finding more float until a point is reached where this ends (this distance is usually measured in hundreds of feet, not miles) ; then trenching to bedrock, and finding the ore in place.”

In Gaspé, Quebec, lead-zinc ore bodies have been discovered by tracing float up-hill to its source. The higher parts of the Gaspé Peninsula escaped glaciation; so any movement of loose blocks of rock or mineral that has occurred there has been due to other causes. “The country is covered by heavy overburden, and, in consequence, outcrops are few. The presence of ore is usually detected by finding pieces of galena in the float. These sometimes form large rounded masses, weathered brownish; as a rule, they have not traveled far, and by trenching up-hill from such float, vein outcrops can usually be uncovered. In other cases, actual outcrops of veins are exposed. Most of these outcrops consist of chambered quartz, from which the zinc blende and often, also, the galena, has been leached. The quartz is white, or perhaps stained brownish; the amethystine variety, common underground, is usually bleached white in the surface exposures.”

Some Natural Aids to Discovery

The experienced, observant prospector learns to take advantage of various natural aids to the discovery of mineral deposits. It sometimes happens that a running stream, after cutting its channel down through the overburden, exposes a vein of quartz or a rusty zone when it has reached bed-rock; or, wearing its way down still deeper, it may have left the section of a mineral deposit exposed in the walls of the canyon. It is not uncommon for discoveries to be made by examining the upturned roots of large trees that have been blown down. The burden of the roots may be gossan or some other suggestive material. The Waite-Montgomery mine in the Rouyn district, Quebec, was discovered in this way. In a mineral country, it is good business to examine all large upturned roots. Sometimes these roots have been spread out on rock, a patch of which is left bare for inspection. Such bare spots have sometimes been found very interesting. Discoveries have been made by examining the mounds of earth at the mouths of groundhogs’ burrows. Even well-worn trails have sometimes uncovered suggestive looking float or an outcrop of quartz. When one remembers what a very small part of the rock surface of the earth is open to view, it is not wonderful that any accidental uncovering of a bit of this surface now and then leads to an important discovery.

Panning

Panning is used in tracing back fine grained material to its source. “This is quite analogous to float tracing. There are some special features about this, however, that require special mention, particularly, since two different kinds of information may be sought by the method: first, tracing the transported material back to its source in ledge; and, second, the discovery of placer, that is, ore in the loose material. In the first case, the method is to work back along the direction of glacial advance or up-hill, as the case may be, until the showing ceases; then dig, as in the case of float tracing. By this method, the tungsten deposits of the Yukon are found. In the second case, the most favorable surface indication is rusty- looking gravel in which the pebbles are well weathered, in the beds of old streams in known mineralized districts. The prospector is not so much concerned with the surface gravel, but attempts to do this panning on the bottom, or lower bed, near bed rock. Even if gold or platinum be not found by the prelimínary panning operations, it is considered a favorable indication to find a residue of galena, garnet, or other heavy minerals. In the Cariboo district, B.C., the gold-bearing gravels have a pronounced rusty appearance.” (T. L. Tanton).

Magnetic Attraction

If the prospector’s compass varies to a considerable extent from the magnetic north, this may indicate the presence of bodies of magnetite or pyrrhotite. A special instrument, the magnetometer, has been devised to measure the attraction that is exerted by such bodies. In the Sudbury nickel-copper area the Levack ore deposits and those of the Falconbridge mine, composed mainly of pyrrhotite, were discovered by magnetometer surveys made with the old- fashioned instruments, followed by diamond drilling. More recently modern magnetometers, measuring a fainter magnetic attraction from a greater depth, have been of assistance in discovering other important ore bodies, more deeply buried, in this district. In northwestern Quebec a number of discoveries have been made by means of the dip needle and magnetometer. The copper-gold ores of that district generally have associated with them enough pyrrhotite to make them distinctly magnetic. The Aldermac mine was discovered by this means.

The aeromagnetic method of detecting magnetic bodies, which was developed during World War II to detect enemy submarines, has been extremely useful in locating buried deposits of magnetite and pyrrhotite. The important magnetic iron mine at Marmora, Ontario, was found by this means under a cover of over 100 feet of limestone. A series of aeromagnetic maps in the Bathurst area of northern New Brunswick directed the diamond drilling that disclosed several zinc-copper-lead deposits of major size, the pyrrhotite that accompanied the other sulphides being responsible for the magnetic attraction. In other cases mineralized geological contacts containing enough magnetite and pyrrhotite to attract the magnetometer have been traced, either from the air or on the surface, and drilling along these contacts has disclosed bodies of ore. The extensive series of aeromagnetic maps made and issued by the Geological Survey, Ottawa, is being extended rapidly to cover vast areas of Canada. It is of more use to companies that can pay for expensive campaigns of diamond drilling than it is to individual prospectors.

Electromagnetic Method

Some years ago determined attempts were made to use the electrical conductivity of sulphide minerals to detect them beneath the surface. The various methods tried out were not, however, particularly successful. More recently, with the help of newly invented electronic instruments, the present E-M (Electromagnetic) method of measuring the electric conductivity from a low-flying airplane has been notably successful. The high-grade copper-zinc-lead deposits near Newcastle, New Brunswick, were found in 1954 by this means. As sulphide deposits are so common in Canada, it seems likely that E-M will be of increasing importance in this country.

The Eotvos torsion balance, while not an electrical instrument, may be mentioned here. It measures the attraction of the earth, which is greater in places where there are large bodies of the heavier minerals. All these, and other scientific instruments coming into use, are beyond the scope of the ordinary prospector. They are expensive and require expert knowledge and handling. This kind of prospecting can be used in areas where the overburden is too deep for the pick and shovel man. It is not infallible, but it gives valuable indications that may be followed up by diamond drilling.

Topographic Features

By topographic features is meant the appearance of the surface of the country as made by hollows, hills, trench-like depressions, ridges, etc.

The asbestos veins of the Thetford-Black Lake district usually occur in the small shallow trenches that crisscross through serpentine rock. These trenches should be examined very carefully, because the asbestos may be hidden by the dirt that fills them.

Large natural trenches sometimes show the position of weathered-out veins in the Cobalt district Fault-line scarps, that is, precipices or cliffs formed by faulting, are favorable places to search for veins, which are often found at the base of the scarp or cliff. Examples of this are seen at Thunder Cape and other places in the Silver Islet area, Ontario; the Casey-Cobalt is another example.
The talc deposits of the Ottawa Valley are found in smooth-sided pits, which are characteristic topographic features.

The fluorspar veins of the Madoc area often occur in depressions; and on the surface of the veins, there is a gravel composed of granular fluorspar, barite, and calcite.

In all these cases, the depressions are caused by the easy weathering of the vein matter. In other cases, the vein filling may resist weathering to a greater extent than the country rock does, and thus form a ridge.

Glacial Hollows

It happens that in eastern Canada the lines of folding, with consequent weakening of the rocks, are roughly in the direction of movement of the great glacier. As a consequence, these zones of weakness in the rocks have been hollowed out by the action of the ice. The long hollows thus formed are often shown by long lakes and streams. These zones of crushed or schisted rock are often good prospecting ground. A favorable prospecting area can sometimes be picked out in this way, even when the geology is not very well known. The map in Fig. 138 shows this feature very well. The long lakes strike about NE-SW. There are also long strips of rocks striking in the same direction. These are remnants of the eroded folds. As is often the case, there is another direction in which the rocks have been weakened by the strain. This is marked by the numerous diabase dikes, which strike about NW-SE. The strike of the veins and other mineral deposits may follow either or both of these directions.

Rock Associations of Minerals

Some valuable minerals are found, usually or exclusively, in particular rocks. Thus, serpentine rocks are the home of asbestos, chromite, magnesite, diamonds, and platinum. Asbestos and chromite are found in workable quantities in the serpentine rocks of the Black Lake-Thetford area, Quebec. “The serpentine rock where asbestos is found is rusty on the weathered surface, and is traversed by depressions of lighter-colored material; in these depressions are found the veins of asbestos. Commonly, there is a fluff on the asbestos, which causes dirt to cling; but at certain places, where the covering is very thin, the characteristic silky lustre shows through, when viewed at the proper angle. In certain localities, the serpentine rock is but slightly weathered, and the green or yellowish-green color of the rock is a striking fatigue. On this slightly weathered rock, the asbestos is recognized by its peculiar physical properties. There are no distinctly colored gangue minerals, such as quartz or calcite, in asbestos veins. The ‘slip-fiber’ asbestos occurs along certain joints of the serpentine rock; and should this be the first observation of the prospector — it is a guide in the search for the transverse-fiber asbestos.” Missinaibi SheetLegend

Attention should be given to rocks other than those associated with the valuable minerals that are being specially searched for. For instance, in north-eastern Ontario, serpentine rocks are frequently met with in the gold fields; these rocks may be productive of asbestos, chromite, nickel ore, magnesite, platinum, or diamonds. Copper ores are sometimes associated with serpentine rocks. The prospector in that region should therefore be acquainted with serpentine rock and with the valuable minerals that may be found in it.

Geological Relationships

Geological relationships are a guide to prospecting; and since these are determined by the study of outcrops, they should be included in the study of surface indications. Examples: A number of valuable minerals are found in the Ottawa Valley region, at the contact between crystalline limestone and intrusive rocks, such as pegmatite. “The graphite deposits occur in masses and zones in the Grenville limestone, at the contact with gabbro, anorthosite, pegmatite, and rocks of intermediate type. The mineral associations in the deposits are pyrite, pyrrhotite, pyroxene, scapolite, calcite, and sphene. The deposits are commonly in a rusty-colored depression, in which there is usually loose, granular, rust-coated pyroxene, and calcite. At Calabogie, the fresh unaltered material appeared on the glaciated surface.”

“Magnesite is found in Grenville limestone in large irregular masses, more or less intimately associated with dolomite. The associated minerals in the deposits are serpentine, white diopside, and phlogopite in fine flakes. On the weathered surface, it appears white or buff, and shows a peculiar pitting due to the removal of the associated dolomite,” (T, L. Tanton, Geological Survey, Ottawa).

“Molybdenite occurs as pegmatite segregations in syenite and quartz-syenite. At Quyon, Quebec, it is associated with pyrite and fluorite in syenite. On that part of the surface which appears freshly glaciated, these minerals appear in their unaltered state; elsewhere, the deposit has the appearance of a greenish gray, rusty rock. The surrounding rock is pink. Molybdenite also occurs at the margins of Grenville limestone, in granite, gneiss, or pegmatite. These concentrations of molybdenite are disseminated along with pyrite and pyrrhotite through a deep-green pyroxenite.” (T. L.

Commercial deposits of feldspar are in coarse- grained pegmatite dikes, which appear white on the surface, the quartz glassy, and the feldspar porcelain-white by weathering. These dikes sometimes cut through granite or gneiss, in which case, the feldspar is pink beneath the surface. Other dikes are in crystalline limestone; the feldspar in these is white throughout.

The gold veins of northeastern Ontario and northwestern Quebec are found at or near the contact of Timiskamian sedimentary rocks, with intrusive porphyry, granite, and syenite. “Gold deposits, which are found in numerous localities in the province, from the Quebec boundary on the east to that of Manitoba on the west, are, in many cases at least, genetically connected with granites to which the name Algoman is applied. In a few cases, the granites are represented by more basic rocks. The gold deposits are found chiefly in Keewatin schists, but a few occur in Timiskamian clastics. It is worthy of note that the only gold deposits of the province that have been proved to be of great economic importance are found in, or in the vicinity of, these Timiskamian fragmental rocks.”

“The gold deposits of Boston Creek supply another example of gold being derived from acid intrusions of the Algoman age. The granite, syenite, and feldspar-porphyry exposed in this area by erosion, are probably different facies

[variations] of a plutonic rock that underlies the whole area. The gold generally occurs near these acid rocks. The presence of a number of gold-bearing veins along the contact of the intrusive porphyry and older rocks at Boston Creek, as in many other parts of central Canada, and the frequent occurrence of auriferous quartz veinlets in the porphyry and granite, suggest the relationship between the intrusives and the veins.”

Contacts

Igneous intrusions are masses of rock that have been formed by melted rock material that has pressed or broken its way into rocks already formed. The places where the intrusive rocks are joined to the other rocks are called contacts. Some of these intrusions of igneous rocks are of vast dimensions, and are believed to go to great depths. They are called batholiths. A good example is the granite batholith forming the Coast-Range of British Columbia and Yukon. Good prospecting ground is sometimes found around the edges of the great masses of igneous rocks, but the ground is usually more favorable around the smaller masses that are believed to be offshoots from the main body. When a mass of granite, diabase, gabbro, or other igneous rock, has forced its way through an older rock, it has chosen for its path the weak places in that rock and has further weakened it; sometimes for a long distance from the contact. The liquid supplying the valuable minerals has followed the same path or the new easy roads opened up by the intrusion, and have deposited their valuable burden in these structurally weak places, marked by breaks, faults, shear zones, or a schistose condition of the rocks. It follows that the favorable ground for prospecting it to be sought at and near the contacts of the igneous rocks with the rocks invaded. Often the good ground extends into the older rocks for several miles from the intrusive rock, and the conditions seem to be more favorable for this when there are a number of offshoots from the main body. These offshoots are sometimes different from the rock forming the larger mass. For example, in the neighborhood of a granite batholith, there may be dikes and bosses of granite, porphyry, aplite, syenite, and lamprophyre, all believed to have been derived from the same melted mass deep down in the earth. These favorable areas can often be picked out on the geological map by the variety of colors in small patches.

outcrop