Conclusions Concerning The Localization of
Ore-Shoots:
To the miner the localization of the richer portions of a vein is of more
immediate practical interest than the theory of their origin. A gold-vein is
not a homogeneous mass of auriferous
quartz, of tabular form,
penetrating the rocks like a sheet of paper, but rather as an irregular
occurrence of ore, the composition and shape of which are very variable,
because they are the result of chemical agencies and structural conditions
of great complexity. While the traces of the agencies which precipitated the
ore are obscure, because they have been largely obliterated by subsequent
chemical action, the relation between the vein and its encasing rock can
often be traced by observation. In this direction the miner obtains great
aid from the geologist. The transactions of this Institute and the
publications of the U. S. Geological Survey contain numerous clear
expositions of such structural relations. The monographs on the Leadville
and Eureka mining districts may be especially instanced as affording
striking examples of the direct application of geology to underground work.
Both locations are replacement ore bodies in
limestone and have yielded considerable precious metals that were
present in base metal
sulfides like
galena,
chalcopyrite and
sphalerite and oxides like
cerrusite and
malachite.
Australia. One of the best examples of the localization of rich gold ore came under my notice in 1890 in the Bright mining district. Bright is geographically in the Australian Alps, and geologically in the Upper Silurian slates and sandstones. Though these rocks have undergone metamorphism, and exhibit a well-developed cleavage, yet their bedding has not been obliterated. The veins cross the bedding-planes of the enclosing country both in strike and dip. When investigating the distribution of the ore in the mines of this district, I found that the ore-shoots had a pitch corresponding with the line of intersection between vein and country. This was well illustrated at the Shouldn't Wonder mine, 7 miles from the town of Bright.
The lode was a simple quartz vein from 15 to 24 in. wide, carrying a small percentage of pyrite. It had a strike of N. 28 W. and a dip to the NE. of about 75, while the country dipped SW. 79 and had a strike of S 55 W. The plane of the vein cut across the beds of the country and the intersections thus produced were to be seen along the foot-wall of the lode as lines, pitching 42 to 46 southward. While the foot-wall was more regular than the hanging, and therefore exhibited this feature best, yet the hanging also carried lines corresponding with those observed on the opposite wall. The boundaries of the ore-shoots in the mine followed these lines; and the longitudinal section of the workings, as seen on the mine-maps, proved also that these lines of intersection had an inclination which coincided with the trend of the ore-bodies, as stoped out between the four successive upper levels of the property. At the Myrtle mine, in the same district, there was the same correlation between the pitch of the ore-bodies and the line of intersection of the wall of the lode with the bedding-planes of the enclosing country. The stratification was distinct, the rocks consisting of altered, silicified slates of a gray to gray blue tint. In the stopes above the 700-ft. level the pay-ore was separated from the normal valueless quartz of the lode by a small step, due to the irregular fracture of the vein in crossing two beds of unequal hardness. It marked the line of intersection between lode-plane and country bedding, and also proved to be the boundary of the pay-shoot. In the different portions of the mine the variation in the dip of the country produced variations in the angle of the lines of intersection, and also in the pitch of the ore-shoots.
It is not often that the formation traversed by a vein has such a simple structure as was presented by these Silurian sedimentary rocks; but it is probable that in other districts also the pitch of the ore-bodies may have been determined by structural conditions of a similar kind, which have been obscured, however, by metamorphism. Experience in Colorado has shown that the intersection of fractures favors the occurrence of rich ore-bodies. An interesting example was afforded by the Moon-Anchor mine, at Cripple Creek, in 1899. The ore in the mine occurs in a lode-channel marked by a band of fractured andesite breccia. At the 400-ft. level a small dike of granite, 2 to 6 in. thick, intersects the lode-channel at a place where a counter-fracture also traverses it. A triangle is produced by these intersections, and the ore is proved to surround a block of ground which is also mineralized, but not sufficiently so to be regarded in its entirety as pay-ore. At the crossing of the dike and cross-fractures a very rich body of telluride-ore was encountered.
This reminds me of the Yankee Girl ore-body, mentioned by Emmons. This body of ore was of phenomenal richness, many ten-ton lots being shipped which carried 7 or 8 ounces of gold and 3000 to 4000 ounces of silver per ton. The ore was also rendered remarkable by carrying the rare mineral stromeyerite, a sulphide of silver and copper. Mr. Emmons speaks of the bonanza turning into low-grade pyritic ore as depth was attained. I may add that this change was not gradual, but sudden, and coincident with certain structural relations. At the surface, the vein consisted of comparatively low-grade ore, which led to the finding of a nearly vertical "chimney," averaging only 25 to 30 ft. in diam., of extraordinarily rich ore, consisting of the copper sulphides, bornite and erubescite, with stromeyerite and barite. The gold in the ore was associated with the barite. From the second to the sixth level, at about 500 ft. below the surface, this bonanza proved immensely productive; then, suddenly, a flat floor, dipping and accompanied by clay, crossed the deposit. This flat vein was worked for 90 ft., from the south drift at the No. 6 level, and contained ore similar to that of the Yankee Girl chimney. The latter was found again deeper down, and out of its former line of descent, but it was much diminished in richness, and appeared to merge into the general body of low-grade chalcopyrite and iron pyrite which characterized the lode at the tenth level. This mine and its neighbors, the Robinson and Guston, are idle now. They are in the andesite breccia of the San Juan region. The Yankee Girl chimney was situated, I believe, at the crossing of three lode-fractures, appearing as breaks in the andesite, which was bleached and mineralized where they traversed it. It was a curious feature of this mine, and of the Guston also, that the short, very rich bonanzas of the upper levels gradually lost their definition, that is to say, they became no richer than the intervening portions of the lode. This was interpreted as a "lengthening" of the ore-shoots, which may be true, viewed in one way; but I think that it should be more properly regarded as an impoverishment of the lode, marked by a disappearance of the bonanzas.
In California, especially in that mining region which follows the foothills of the Sierra Nevada and traverses the counties of Amador, Calaveras and Tuolumne, the occurrence of pockets of rich ore, full of native gold, is a notable feature of the superficial parts of the quartz-veins. These pockets appear to be confined to the zone between the surface and the water-level, and to be dependent upon the results produced by the small cross-veins which encounter the main lodes. In 1887 I had the pleasure of extracting, in two hours, a little over 170 ounces of gold, worth about $3000, from one of these pockets. It was at the Rathgeb mine, near San Andreas, in Calaveras County. The main lode consisted of 5 to 8 ft. of massive "hungry-looking" quartz, the foot-wall of which was a beautiful augite-schist and the hanging wall a hard diabase. The water-level was 160 ft. below the surface. Down to this point, the country was oxidized, the hanging-wall exhibiting only slight alteration, while the schist of the foot-wall was softened and decomposed almost to a clay. This was traversed by numerous small veins, which appeared to act as "feeders," forming bunches of rich ore where they encountered the main lode. At the 120-ft. level, south from the shaft, there were some old workings ; and the examination of these led to the discovery of a small seam, about one-sixteenth of an inch thick, filled with red clay which carried a good deal of native gold, as was proved by washing it in a pan. An experienced miner was put to work, with instructions to follow this small streak. It varied in thickness, and occasionally opened out into small lenticular cavities, containing a clay in which the gold was distributed like the raisins in a pudding. Each of these "pockets" yielded several hundred dollars' worth of gold. At length the streak widened to 6 or 8 inches of quartz, lined with clay. The amount of red clay commenced to increase; coarse gold became more frequent; and a big discovery was hourly expected. It was finally made. The vein suddenly became faulted, and at the place of faulting there was a soft, spong, wiry mass of gold and clay with more gold than clay. The first handful I broke, while yet the stope was thick with powder-smoke, contained three ounces of gold. Within the next two hours this pocket gave us $3000, and during the following week it yielded over 20,000, an amount which was obtained at a total cost of less than $200. When it had been worked out, it was easy to observe the conditions which determined its occurrence at this place. The vein had been faulted about its own width, namely, 10 inches, by a small cross-seam, and at this intersection, the pocket lay. The gold was spongy and was intermixed with quartz. The clay which penetrated the whole mass was partly red and ochreous, and partly a gray gelatinous material. In the quartz, and associated with the gold, there were acicular black crystals of pitchblende (uraninite), together with uranium ochre. This association of gold with uranium is uncommon.
New Zealand. Intersections which coincide with enrichments form a notable characteristic of the Hauraki gold-field in the north island of New Zealand. In this district the occurrence of patches of native gold is an important feature of the regular mining operations. When I was there, in 1891, each stamp mill had its "specimen-stamp," a single stamp working in a separate mortar, and employed solely for the treatment of specimen ore. These rich patches occur at the places where the "reefs" or lodes cross bands of flinty quartz. The latter are known among the miners as "flinties." They vary in thickness from a few inches to mere threads of chalcedonic quartz. They are barren in themselves, but have a favorable effect on the gold-veins. The latter are also intersected by cross-veins, producing enrichment similar to that caused by the "flinties." One of these intersections, as seen by me in the Moanataeri mine showed the lode to consist of a series of small seams of quartz, conforming to the structural lines of the enclosing country, which is hornblende-andesite. The cross-vein is a band of soft gray decomposed rock, which also carries a number of small quartz-seams, but only near its crossing with the main lode. The line is parallel to a large fault, to be seen elsewhere in the mine-workings. The "leaders," or quartz-seams are gold bearing, and exhibit marked enrichment at the intersection. The prevailing formation of this mining district is an andesite, which is traversed by soft bands of decomposition, called "sandstone" by the miners. The latter, when penetrated by quartz seams are favorable to the finding of ore. The gold occurrence is essentially sporadic and dependent upon local enrichments, such as have been described. The district is surrounded by thermal springs, and is near the well-known volcanic region of Tarawera, which was active in 1884. The mine waters are heavily mineralized and very acid, so that the metal screens used in the mills are quickly corroded. Tellurides like calaverite and sylvanite have been detected in the ores; but the precious metal is usually found native and in coarse particles, which are frequently coated with native arsenic. The district is one which, I think, if thoroughly examined, would afford many suggestions regarding ore deposition.
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