In the examination of an undeveloped prospect a decision must be arrived at from an inspection of the outcrops and the exposures in a few shallow pits. Prospects that are offered for sale rarely expose any important quantity of good ore, work having usually been stopped when the immediate exploration no longer yielded favorable results. It should be borne in mind, furthermore, that a great majority of prospects have been examined many times, and if of conspicuous promise, would have been acquired for development. In most cases the problem for the engineer, therefore, is to discover the traces of a valuable mineralization that has disappeared through solution, or the conditions that indicate the possibility, or probability, of underlying secondary enrichments.
The available data being meager, every feature should be the subject of careful study—the condition of the outcrop, the structural relationships, the associations of minerals, the alterations of wall rocks, the chances for underlying enrichments, and so forth, as well as the actual assay value of the material exposed.
It is frequently advisable to spend the time and money necessary to have trenches dug at various significant points along a promising outcrop; such exposures, even if the depth attained is slight, often disclose conditions that are not apparent at the actual surface, as for example, the existence of soft minerals and the distribution of the various minerals through the mass of the deposit. Trenches also permit samples to be taken from points that were not accessible during former examinations.
Strong, persistent outcrops of uniform width may be taken to indicate the probable size and character of the underlying vein, whose persistency in depth is likely to be proportional to the length of its outcrop. Fissures that may be traced for long distances on the surface are commonly found to be equally persistent in depth, while short, branching and irregular outcrops are usually indicative of similar irregularities in the underlying deposits. Outcrops that comprise a series of large irregular masses, perhaps connected by narrower veins, are in general likely to become smaller in depth.
In a vein or deposit that is harder and more resistant than the enclosing rocks, erosion is likely to be halted at the widest part, that part offering the greatest resistance to erosion and the greatest protection to the adjoining rocks. Such outcrops, therefore, are likely to be larger than the average of the underlying deposit. The converse of this condition is also true. A vein or deposit that is softer and less resistant to erosion than the enclosing rocks is likely to become larger in depth. Erosion tends to remove the surface of such a deposit faster than the adjoining wall rocks, forming a depression, the walls of which tend to close together upon the removal of the intervening soft material; occasionally, a narrow and inconspicuous gouge-filled fissure is the only surface indication of an important vein of soft minerals.
In disseminated deposits the most intense primary mineralization is likely to be connected with a thorough brecciation, as are also disseminated enrichments. The study of post-mineral fracturing, therefore, is of as much importance in the investigation of these deposits as is the study of the minerals of the outcrops themselves.
The outcrop of a vertical vein is a straight line, whatever the slope of the surface, and the outcrop of a horizontal bed follows a contour around all hillsides. A vein of intermediate dip, however, outcrops to the right or to the left of its strike at any horizon in accordance with its dip and the slopes of the surface. An idea may be obtained in the field as to the dip of a vein by paralleling a book or other plane surface with several parts of the outcrop at different elevations, while in mapping, simple trigonometric calculations or graphic solutions will give the heights or horizontal positions of the vein at any desired point: contouring is expensive and is usually unnecessary.
An outcrop situated on a steep hillside is likely to overturn in the direction of the slope of the hill; loose fragments of the partly disintegrated outcrop become mingled with the surface material, and when at some distance from the parent mass are widely separated and constitute “float.” The apparent dip of a vein that outcrops along a steep hillside is, therefore, likely to be flatter than its true dip below the mechanical influence of erosion.
Mineralization occasionally finds local expression in the topography. Silicified areas and resistant outcrops may form prominent ridges or knobs, and soft or altered rocks may result in depressions or saddles in the ridges. Faults are occasionally prominent topographic features where one wall is notably more resistant to erosion than the other, but, in general, unless of great displacement, faults are not likely to be represented in the topography. Different kinds of rocks upon weathering produce characteristic topographic outlines, as, for example, the plateau-like hills and steep talus slopes of horizontal sedimentary beds, or the ragged outlines common in areas of volcanic rocks. While topographic relief is rarely significant in the examination of mining properties, it may be a valuable guide to the prospector, as many mineralized areas are connected with low, rounded foothills of notably different outline from the general relief of the region.
Deposits formed at slight depth below the surface are characteristically irregular in their upper part, and tend to consolidate and to become structurally more regular with greater depth.
The outcrops of ore deposits that have yielded to oxidation are commonly porous and cellular, owing to the removal of certain of the original constituents in solution. Such outcrops are favorable in that they indicate solution and possible secondary enrichments of the dissolved substances, or, at least, the original presence of easily dissolved minerals which in their unaltered state may have been valuable. A massive, tight outcrop, on the other hand, is usually indicative at slight depth of the typical value of the deposit, which has presumably been but little affected by secondary agencies.
Upon the oxidation and leaching of an ore composed of sulphide minerals in a resistant gangue, the outcrop is likely to
retain the casts, or open spaces, left by the removal of the sulphides. In many instances outcrops exhibit abundant casts of minerals that have completely disappeared, and whose presence originally would not be suspected without a close examination for such evidence.
The crystallization of pyrite appears to be interrupted when it contains a notable proportion of copper. Pure, barren, pyrite commonly crystallizes as cubes or other isometric forms, while chalcopyrite is commonly distributed in an irregular manner; the crystallization of cupriferous pyrite, in general, is wavy and irregular, although occasional cubes may be noted. A less strongly emphasized but still marked relation exists between pure, barren galena and highly argentiferous galena. This mineral is less likely to be well crystallized when it carries an important proportion of silver. An examination of the casts of these sulphides in a leached outcrop, therefore, is likely to yield evidence to some extent in regard to the value as well as to the kinds of the sulphides originally present.
The minerals present in outcrops are those primary constituents of the ore most resistant to solution, and the most resistant of the products of alteration. The most resistant of the primary minerals is usually quartz, which commonly forms a large proportion of the outcrops of the deposits in which it is an important constituent. Magnetite and specularite are also relatively resistant minerals. Of the products of oxidation and hydration kaolin and limonite are the most resistant, and are commonly found in or close beneath the outcrop. Sericite likewise is resistant, but is likely to be changed to kaolin where sulphuric-acid solutions are prominent in accomplishing the surface alteration.
The oxides of manganese are resistant, and are frequently found in large quantities in the oxidized parts of ore deposits, even those in which manganese forms a quite subordinate proportion of the primary ore.
Gold is a resistant mineral, and is frequently concentrated at the surface and in the oxidized zone, as is discussed in a preceding paragraph.
The more resistant of the ore minerals are galena and its oxidation products, anglesite and cerussite, chloride of silver, native silver, cuprite, native copper, chrysocolla and to a lesser degree the carbonates of copper, all of which minerals are likely to be left behind as residual ores during solution and migration.
In copper deposits that have been subjected to thorough leaching and alteration, the outcrops are likely to consist of soft white kaolin and quartz, carrying at the surface, perhaps, trifling quantities of limonite, manganese oxide, and chrysocolla as stains. Such outcrops are most favorable for the existence of chalcocite enrichments in depth.
In a series of articles entitled Surficial Indications of Copper, Frank H. Probert has given a cogent discussion of the outcrops of copper deposits. He says:
The nature and genesis of an ore deposit affect the topographic expression of the outcrop. It is, of course, necessary to distinguish between present contours and those of the mineralizing period. Elevated areas are frequently the result of comparatively recent fracturing; hence processes of oxidation are seldom advanced. Precipitous slopes mean rapid runoff. Gravitative stresses may cause a constant widening of fractures.
The thorough alteration of an intensely mineralized mass may remove all traces of the primary ore minerals, while adjacent rocks that received a scanty mineralization only and so remain relatively unaltered, often retain traces of the primary minerals and so furnish a clue to the original nature of the principal deposit.
The oxides of iron, either massive or as stain, are frequently the most prominent constituents of outcrops and of the superficial parts of ore deposits that have suffered oxidation. The condition and occurrence of these minerals are often indicative of the character of the mineralization in depth.
The final product of the oxidation and hydration of iron minerals is limonite, and the other oxides of iron on thorough alteration yield this mineral. Magnetite in an outcrop is commonly present as an unaltered primary mineral, and may not be taken to represent the alteration product of a sulphide mineralization. Specularite, of characteristic crystal form, is likewise a primary mineral, and not the result of the alteration of sulphides: micaceous hematite should be distinguished from specularite, as it is frequently found as the alteration product of sulphides, the micaceous structure having been developed by stress exerted after its formation. Certain gangue minerals, such as lime-iron garnet, yield limonite upon oxidation; in most cases limonite of this origin occurs as soft earthy masses, mixed with unaltered contact minerals as, for example, partly decomposed epidote, and is commonly distinguishable from limonite resulting from the oxidation of sulphides.
In many outcrops in arid regions whose underlying ore deposits have been explored, the limonite resulting from the oxidation of pyrite occurs as a massive brown mineral, while the chalcopyrite of the original ore is represented by seamlets and patches of soft, bright-red hematite.
Features of outcrops that indicate the possibility of enrichments in depth are the residual indications of a good primary mineralization together with a porous or brecciated structure, or the presence of post-mineral fractures.
A majority of enrichments of secondary sulphides are probably due to the migration of sulphate and sulphuric acid solutions; these solutions frequently leave traces through the presence of a kaolinitic alteration of the associated rocks, or of kaolin in the outcrop or oxidized zone. Resistant minerals that remain after a thorough alteration of this kind are kaolin, limonite and quartz. Feldspars are completely altered and sulphides are absent. The presence of unaltered feldspars or of most of the other usual gangue minerals except quartz indicates a partial alteration at best, and the presence of sulphides indicates an incomplete solution. Galena is a resistant mineral as compared with other sulphides, and not infrequently overlies important enrichments of other metals, but the presence of unaltered pyrite, chalcopyrite or zincblende renders it unlikely that important enrichments will be found through deeper exploration.
The outcrops of suspected disseminated chalcocite enrichments should contain little else than kaolin, limonite and quartz, together with, perhaps some unaltered sericite, itself a product of primary altering agencies. The presence of pyrite in such an outcrop or in the oxidized zone is a most unfavorable sign; cases are known where pyrite was found above secondary chalcocite enrichments, but in every case it was partly altered, crumbly, and was contained within thoroughly kaolinized rock, the supposition being that the copper was leached from the pyrite under conditions that did not permit a complete oxidation of the sulphur. Where the containing rock shows the outlines of the feldspars the kaolinization must be considered unsatisfactory.
Efflorescences of soluble salts in the outcrops or at slight depth below them are frequently instructive. An efflorescence of copper sulphate is indicative of the solution of chalcocite, and may or may not be mixed with iron sulphate. An efflorescence of iron sulphate with but little copper is commonly indicative of pyrite at no great depth. Sulphur and a yellowish-green sulphate of iron are probably not formed except close to oxidizing pyritic sulphides. Other efflorescences frequently found are alum and zinc sulphate, the latter characteristic of primary ores below any zone of chalcocite enrichment; complex sulphates of aluminum and other bases are also formed in the leached zone.
Where large quantities of pyritic sulphides have oxidized and have in part been removed in solution it is not uncommon to find the conglomerates of stream beds cemented by limonite, and, in some instances, carrying oxidized copper minerals; the presence of such conglomerates may be taken to indicate conditions under which migration and enrichment may have taken place within the deposits themselves.
The elevation of an outcrop as compared with the drainage level, or the elevations of the known enrichments of the district, are important factors in the consideration of possible secondary enrichments.
Disseminated chalcocite enrichments appear to be confined to regions of slight rainfall: important enrichments of copper and other metals are found in veins that have suffered post-mineral fracturing under all climatic conditions, except where rapid erosion or glaciation continuously exposes fresh surfaces of primary ores and thus does not permit the operation of surface agencies.
The several types of rock alteration are discussed in a preceding chapter where their close relationship with mineralizing processes is emphasized. In the field it is usually possible to distinguish between a primary or hydrothermal alteration of the rocks and the results of ordinary weathering, and also between either of these and kaolinization, but where such distinction is doubtful, slides should be cut and examined under the microscope, when the type of alteration will become apparent. A “highly altered condition” means nothing unless its type and probable relation to mineralization are understood.
In the Tintic district, Utah, as described in correspondence from Tom Lyon, the orebodies on the west side of the district occur as pipelike masses which extend more or less horizontally for several thousand feet. At the Mammoth mine one of these orebodies outcrops. This outcrop has been almost entirely stoped but, judging from the remnants, it was an iron-stained mass of vein material and limestone containing silver and lead. On the east side of the district the orebodies of the Tintic Standard, North Lily, and Eureka Lily occur in the Ophir formation beneath the Packard rhyolite flow. The mineralization and ore occurrence in this territory is manifested by an intense alteration and bleaching of the rhyolite and numerous dikes containing quartzite pebbles, called locally “pebble dikes.”
Upon the kaolinitic alteration of rocks carrying pyritic mineralizations the products of the alteration are likely to be similar whatever the original nature of the individual rocks. On Shannon Mountain, Arizona, granite, porphyry, and shales have all suffered intense kaolinitic alteration, and the resulting mass of kaolin, sericite, and quartz with associated limonite and chalcocite may rarely be differentiated in the field into parts representing the original types of rocks. In granitic rocks that were not sericitized before suffering kaolinization the quartz phenocrysts remain clearly distinguishable, and in hand specimens tend to obscure the degree of alteration; a kaolinitic alteration, however, that has not obliterated the outlines of the feldspars cannot be considered thorough, and outcrops of this nature are rarely underlain by important enrichments. Thorough leaching and kaolinization usually remove the iron as well as the other bases, and the dumps of workings in the leached zones are commonly white in color, in sharp contrast with the brown or red color of the surface. In deposits that originally carried abundant pyrite, much limonite may remain in the leached zone. The line of demarkation between the leached and the enriched zones in such cases is well marked, the former being stained by iron, while the latter is white in color.
The minerals developed by contact metamorphism are, in general, resistant to oxidation and erosion, and are likely, therefore, to form conspicuous outcrops. The tightness of these
minerals and the slowness with which they decompose under the influence of surface agencies protect the sulphides contained within them and so in large measure prevent migration and enrichment. A further hindrance to secondary enrichment in deposits of this nature is the usual presence of active precipitants. As a result of these conditions secondary enrichments are rare in contact deposits except where the sulphides were present in large masses: in all other cases the outcrops of contact deposits are likely to be indicative of the value contained by the deposit at all depths.
Certain deposits of surface origin present close imitations of the outcrops of ore deposits, but are not underlain by valuable minerals. Bog iron ore (limonite) is a familiar example of surficial deposit not connected with any underlying mineralization, and many deposits of the oxides of manganese also occur in like manner. Nodular concretions of manganese oxides that form on the sea bottom are likely to be concentrated into such superficial deposits, as is also, upon erosion, the manganese contained in small quantities by many rocks. Bog iron ores commonly contain casts of vegetable remains, sand, and silt, and may also be recognized by their bedded form. Surficial deposits of
limonite and manganese oxides may generally be distinguished from the outcrops of mineral deposits through their lack of associated minerals characteristic of outcrops. Furthermore, after slight exploration their structure is revealed and their superficial nature becomes apparent. Pyritic deposits are occasionally met with where the sulphides have replaced roots, or other organic matter, and whose superficial formation is evident.
The investigation of an outcrop is largely a search for residual conditions indicating that an important mineralization has been removed from the surface by oxidation and solution. It is often advisable to have slides cut from specimens of an outcrop and its associated rocks, and to have them examined under the microscope. Information is thus gained of the character and degree of the alteration, and in many cases of the mineralogical nature of the original ore and of the distribution of the several minerals. Specimens taken for this purpose should be chosen carefully, and a record of them kept in the same way as is the custom with samples taken for assay; furthermore, it is best always to reserve a specimen, preferably part of the same piece that is sent away, for purposes of comparison upon receipt of the results of the examination. Microscopic examination is of great value, if only for corroboration of the evidence gathered in the field, but may also yield information and indicate possibilities that without it would not be suspected.
Augustus Locke has made some timely observations on exploration for new mines. In part he says:
There have been three periods of ore discovery in the Western States: first, that of great new deposits; second, new mines in old districts; third, new ore in old mines, near known old ore. We are now in the third period, but there is yet the possibility that many new mines and mine areas await discovery by a finer ore-hunting art.
There may be many kinds of hidden ore; it may be hidden by wash or, as in Canada, by moss and water. Where the cropping of a copper deposit is high in pyrite or low in neutralizer (such as calcite or feldspar) and where the ore is low in iron or lies in a forested area where organic acids may dissolve the limonite from the surface evidence, the cropping may be obscure. Imposing deposits may yet be found of this type.
Spotty garnet-limestone deposits repel the engineer because few mines are found in them. Near these, however, may lie rich deposits in the limestone that weathers soft and is covered.
Bulky massive sulphide deposits, especially pyritic and especially low in rock relics, may weather to a sand, pinched down to only a portion of the width of the sulphide itself. There are possibilities of new orebodies here.
He concludes that
the search for new mines will become a more studious, deliberative affair. Not much longer will good practice allow the scout to see prospects, so many in a month, in separated districts. Rather, it will involve examinations, not so much prospect by prospect, as district by district. Under certain conditions it will subject the particular district to long study.
Science has recently come to the aid of the prospector and geologist by furnishing tools, in the shape of geophysical instruments, that will have a profound effect on the art of hunting valuable minerals. Inductive, gravimetric, magnetic and seismic processes have been developed to this end, and as the technique of their use is coming to be more fully understood, they are increasingly valuable means of extending exploration downward. Geophysical methods are far from the old “doodle-bug” or divining-rod systems, which had no scientific basis for their being. There is no doubt that, as the science of geophysics is better understood and can be used to the best advantage, it will be a great aid to the geologist and the examining engineer.
Descriptions of Outcrops
At Nacozari, Sonora, Mex., the outcrops of the Pilares solution breccia are prominent. The country rocks of this district are andesitic and rhyolitic porphyries whose surface over an area of several square miles is stained red, yellow or brown by the oxides of iron resulting from the oxidation of a disseminated cupriferous pyritic mineralization. Shallow leaching and attendant unimportant enrichments of disseminated chalcocite and chalcopyrite are present at many points in the district, but the depth and degree of alteration and enrichment are apparently too slight to yield orebodies of commercial importance. The walls of the gulches frequently show efflorescences of chalcanthite, usually associated with ferric sulphate and sulphur, the earmarks of a near by unaltered pyritic mineralization: a white, probably aluminous, precipitate is present at many points where solutions are oozing out of the rocks, and this, apparently, is also a sign of shallow alteration. In the vicinity of the Pilares orebody, the only important copper deposit in the district, the rock is strongly sericitized, and in this respect differs from the remainder of the district seen by the writer. The outcrop of this ore deposit is composed of a striped breccia, seamlets of ore minerals being separated by long, thin slabs or sharp angular fragments of the country rock that frequently show a curving parallel arrangement of ore and rock areas.
The chief ore mineral is chalcopyrite, with some bornite, associated with pyrite and quartz. In the outcrop, the seamlets residual after the ore are made up principally of micaceous hematite and quartz, with small particles of copper carbonates. Through the brown micaceous hematite, and as individual seams, occurs bright-red hematite, in distribution following that of the chalcopyrite in the ore of the sulphide zone, and evidently residual after this mineral. The outcrop is of small extent as compared with that of the deposit as developed in the deep levels.
At Jerome, Ariz., the conditions exposed in the open pit of the United Verde mine are indicative of intense mineralization. Here the principal country rocks appear to be schist and a rather basic granite porphyry that carries a heavy pyritic mineralization. The residual minerals remaining in the walls of the pit from which the surface ores were mined are white kaolin, quartz, limonite, and oxides of manganese in which occur copper carbonates and silicates. The outcrop is said to have carried gold. Sharp lines of demarkation exist between bands of unaltered schist and the mineralized and altered parts of the deposit. Kaolinization does not appear to have affected the wall rocks. There was an overlying mantle of post-mineral sediments, several hundred feet thick, that effectually masked the presence of copper except where, by faulting and erosion, isolated exposures have resulted. Fully 90 per cent of the surface located as mineral land is covered by rocks having no connection whatever with the ore deposits.
In the copper range country of Michigan the lodes were not characterized by conspicuous outcrops. The whole country has been base leveled by glaciation so that the present topography has no recognizable relation to the ore deposits. The surface is covered by drift or soil, so that an intimate knowledge of the local geology is essential to intelligent prospecting.
At Morenci and Metcalf, Ariz., the lode and the disseminated deposits in porphyry have inconspicuous outcrops. The lodes are represented at the surface by quartz-filled fractures through sericitized and kaolinized porphyry. A light iron stain is common at the surface throughout the intrusive areas, but, in general, the iron as well as the copper has been removed from the mass of the leached zone by solution. Except where the secondary ores actually outcropped, as is said to have been the case with the Metcalf orebodies, there is but little indication of copper at the surface, although a little chrysocolla is occasionally seen, and light, efflorescenses of chalcanthite are common on the walls of shallow workings driven into the porphyry. The most promising areas for disseminated chalcocite enrichments are indicated by abundant casts, residual after an intense primary pyritic mineralization, together with ramifying quartz veinlets associated with a thorough kaolinization of the rock. All of the important enrichments in this district occur high up in the hills; the canyons are in every case in lean primary ore, and extensive exploration of the lower hill slopes has been without result.
At Bisbee, Ariz., according to Ransome, one only of the large pyritic orebodies outcropped at the surface; in this deposit the usual oxidized copper minerals were present in quantities sufficient to constitute ore. The limestone in the vicinity of many of the orebodies is fractured and carries disseminated pyrite. The rusty outcrops of this pyritic mineralization have in some instances indicated the presence of underlying orebodies; slight depressions were noted at the surface above certain orebodies, probably due to subsidence upon the removal in solution of certain constituents of the deposits. Many of the most important of the orebodies of this district, however, are not represented in any way at the surface. The intensely mineralized porphyry of Sacramento Hill taken in connection with the fracturing, silicification and marmorization of the limestone are surface conditions patently favorable to the existence of ore deposits.
At Park City, Utah, the ores occur as lode and replacement deposits in limestone; outcrops are scanty or lacking. Marmorization and silicification of the limestone are present in places, and shattering and discoloration are the chief surface indications of the important deposits.
In the Coeur d’Alene district, Idaho, according to Ransome, the outcrops of the important lead-silver deposits in limestone are inconspicuous. Soil and vegetation cover the hillsides, and the material of the lodes is neither so superior to the enclosing rock in hardness or durability as to form bold outcrops, nor so easily eroded as to produce trenches or saddles in the topography. The courses of the lodes may not be followed at the surface without the aid of test pits or trenches. The ores at the surface carry galena with a little cerussite, limonite and copper carbonates.
At San Pedro, N. M., as Lindgren describes it, copper occurs as chalcopyrite associated with garnet in extensive contact deposits. These deposits are confined to the lower part of a laccolithic roof of limestones. The shaly limestone is in places altered to hornfels and carries tremolite and diopside as coarse crystals: this type of altered rock carries no ore; above these beds occur garnetized beds from 50 to 100 ft. thick and of great horizontal extent, through which the ore is irregularly distributed. The primary ore is composed of chalcopyrite, with some gold, in a yellowish-garnet, chiefly andradite, calcite, and lesser quantities of tremolite and wollastonite. Oxidation has had but slight effect on these deposits, in which apparently there has been no migration or enrichment.
At Virginia City, Nev., the Comstock Lode outcrops for a long distance as siliceous masses and quartz veinlets cementing a fractured zone. This lode, unlike most deposits, occupies a fault of large displacement. The country rocks are propylitized andesites. The lode is continuous over its central part, but branches at either end; while broad and scattered at the surface, it becomes more regular in depth. The principal value at the surface was as chloride of silver. The bonanza ores contained stephanite, polybasite, argentite, and native gold, associated with small quantities of galena and zincblende in a quartz gangue.
At Tonopah, Nev., Spurr says that the important veins showed prominent and continuous outcrops of white quartz; the first samples broken from the veins, although rich, showed no ore minerals, and were of unpromising appearance. The quartz has in places a purplish color, due to minute particles of argentite. The ores near the surface carried chloride, bromide and iodide of silver associated with limonite and oxides of manganese. The country rock is a sericitized andesite.
At Silverton, Colo., the outcrops of the stock deposits of the Red Mountain area form prominent siliceous knobs composed of silicified andesite carrying finely disseminated pyrite, sericite and kaolin. These knobs are thoroughly fractured and contain vugs, cavities, and ramifying caves. The residual ores were found chiefly in the caves, as beds of sandy or clayey material, and on their walls associated with porous, spongy masses of quartz. The oxidized ores carried argentiferous cerussite and anglesite associated with siderite, barite, oxide of iron and kaolin. At slight depth argentiferous galena formed the principal ore; this gave way in depth to argentiferous enargite, chalcocite, bornite and chalcopyrite, which in turn were underlain by lean primary pyritic sulphides.
At Cripple Creek, Colo., Lindgren and Ransome say that the telluride gold ores occur with a very scanty gangue, and the outcrops of the veins are not conspicuous.
As elements of geological structure, the lode fissures at Cripple Creek are exceedingly inconspicuous. They are marked neither by bold outcrops of quartz nor by superficial bands of ferruginous gossan. They seldom fault perceptibly the structures that they traverse, and are not sufficiently different from the enclosing rocks as regards resistance to erosion, to have influenced perceptibly the topographic development of the district. It is this obscurity that retarded the discovery of the ore deposits, and that today renders it impossible to follow the veins over the surface without first stripping off the soil and loose rock, or sinking test pits.
The surface ores here contain dull native gold associated with tellurites in ferruginous clays, kaolin and alunite. Hydrothermal metamorphism was an inconspicuous process in this district.
In the Black Hills, South Dakota, the large Homestake orebodies outcropped as iron-stained chloritic slates, quartz, and porphyry, which in places carried as high as $16 in gold. These altered gold-bearing rocks were not sufficiently resistant to erosion to produce prominent outcrops. In the open cuts it is not possible for the unpractised eye to distinguish the slates that are sufficiently mineralized to constitute ore from those that are practically barren. In general, the rocks within the ore zone appear to be somewhat more completely silicified, and to carry more iron stain than the country rocks, and also to have been subject in greater degree to fracturing and folding. In the surface ores the quartz occurs in three phases: as thin siliceous layers intercalated in the slates, as thin seams that frequently follow the lamination and bedding, but occasionally cut across them, and also as veinlets independent of the structural features of the containing rock, which last are the most important. The gold in these deposits occurs in finely disseminated particles, and appears to be of later origin than the quartz and rock gangue.
In the Mogollones district, New Mexico, the country rocks are a series of flows, fragmental beds and tuffs, composed of soda rhyolite, andesite and basalt. The rocks in the mineralized areas exhibit propylitic alteration; silicification, resulting in a greenish-gray hornstone, is prominent along the veins. Sericitic alteration is absent. The veins are partly filled fissures and partly the result of replacement of the walls along fractured zones. The topography is rugged, and the veins, being harder than the altered wall rocks, form bold outcrops. The veins carry native silver, argentite, chalcocite, pyrite, chalcopyrite, and bornite associated with specularite in a gangue of quartz, calcite and fluorite. In certain of the veins in which copper is present in small quantity the ore minerals are finely disseminated through the gangue. In another type of vein copper forms a large proportion of the total value of the ore, and sulphides are abundant. Oxidation has reached a slight depth only in these veins and secondary enrichment does not appear to have effected any important rearrangement of values. The water level in this district is deep, and the slight depth of secondary alteration and oxidation must be explained by the geologic youth of the region, erosion having proceeded more rapidly than oxidation. The degree to which the rocks have been shattered and the presence of abundant druse-lined cavities in the veins, together with the type of rock alteration, indicate a slight depth at the time of ore deposition, and that no great depth of rock has been removed by erosion since the veins were formed. The oxidized ores found at the surface contained chiefly malachite, cerargyrite, and gold, associated with limonite.