Ray Lyman Wilbur, Secretary
W.C. Mendenhall, Director
Professional Paper 172
GOLD QUARTZ VEINS OF THE ALLEGHANY DISTRICT CALIFORNIA
HENRY G. FERGUSON
ROGER W. GANNETT
UNITED STATES DEPARTMENT OF THE INTERIOR
OUTLINE OF THE REPORT
Lode mining in the Alleghany district began as early as 1853, and it is estimated that the total lode production from the veins of the district has been in excess of $20,000,000 ( all dollar amounts are calculated at $20.67 per ounce).
The principal veins have northerly strikes with gentle easterly dips and follow minor reverse faults. The paragenesis of the vein minerals indicates three stages of mineralization. The first caused the development of new minerals along the walls of the fissures and at least initiated serpentinization. The major feature of the second stage of mineralization was the deposition of the quartz. Accompanying the quartz were minor amounts of other minerals, chiefly arsenopyrite, pyrite, albite, oligoclase, and barite. The sulphides belonging to this stage crystallized prior to the quartz that surrounds them. The third stage of mineralization was economically the most important, for the gold was introduced at this stage. Quantitatively its major feature was the replacement of the wall rock by carbonate, chiefly ankerite, and the micaceous minerals mariposite and sericite. At the same time the older quartz was fissured, and carbonate and mica, together with various sulphides, graphite, and gold, were deposited within the veins, chiefly by replacement of the quartz.
From a review of the geologic features that appear to control the deposition of the shoots of high-grade ore, areas in which the veins are most fractured will on the whole be most favorable. Fracturing of a vein is largely dependent upon changes in strike and dip of the vein, and such changes are commonly found where the vein is close to serpentine masses, at junctions of veins, and where the vein has been faulted prior to the introduction of gold. Another influential factor is determining the position of the high-grade shoots is the presence of the early arsenopyrite, which has acted as a precipitant for the gold. As the arsenopyrite is found most abundantly in veins near the serpentine conditions are thought to be favorable for reasonable persistence of the high-grade ore in depth.
None of the hypothesis of origin of quartz veins are found to be entirely free from objection. The minerals of the later stage were deposited chiefly as the result of replacement, both of the wall rock and of the quartz.
The report closes with a statement of opinion regarding the future of the district, in which a distinctly favorable view of the outlook for lode mining in the future is taken.
PART 1. GEOLOGY
SCOPE OF THE REPORT
The Alleghany district has been a producer of gold since the earliest days of California gold mining. Here, as in other California gold camps, placers yielded the bulk of the earlier production, but during the last 30 years lode deposits have been the source of nearly all the gold produced. The district differs from other California gold-quartz districts in that nearly all the production is obtained from small shoots of high-grade ore, which may yield several thousand dollars to the ton, and very little is obtained from the ordinary type of lower-grade ore.
LOCATION AND TOPOGRAPHY
The mining camp of Alleghany and Forest is on the west flank of the Sierra Nevada about 20 miles west of the crest. The Middle Fork of the Yuba River forms the southern boundary both of the district and of Sierra County. On the north the ridge north of Oregon Creek overlooks the deep valley of the North Fork of the Yuba River. The district is about 18 miles in a direct line from Nevada City, but owing to the deep valleys that cut the upland plateau the automobile road, which connects the two places, is 31 miles in length.
The has hot, rainless summers, though at this altitude (4,500 feet) the heat is far less intense than in the California Valley or in the towns of the lower foothills, such as Grass Valley. The winters are not cold—a temperature as low as 0o F. is almost unknown—but the snowfall is very heavy.
FIELD WORK AND ACKNOWLEDGEMENTS
The senior author first visited the district in 1913 and wrote a short description of the lode deposits.(1) The increasing productiveness of the district made it desirable that a more detailed study should be undertaken in the spring of 1924, and the senior author spent August and September of that year in the field. The complexity of the problem, as shown by the first season’s field work, made it necessary to devote a second season to the investigation, and in this the Geological Survey was fortunate in procuring the services of Mr. Roger W. Gannett, who had already done detailed geologic work in the district for several of the mining companies. During 1925 Mr. Gannett was in the field from July to October and Mr. Ferguson during parts of July and August. In September 1928, after the completion of the first draft of the manuscript, Mr. Ferguson made another short visit to the district.
Owing to the death of Mr. Gannett in October 1925, just before his field work was to end, the work of preparation of the entire report fell to Mr. Ferguson. He takes this opportunity of acknowledging how deeply he is indebted for Mr. Gannett’s excellent work in the field and his realization of how greatly this report suffers from lack of Mr. Gannett’s assistance in the office.
PART 2. ORE DEPOSITS
HISTORY AND PRODUCTION
Gold is obtained in the Alleghany district from both placer and lode deposits. In the early days of gold mining the placers were most productive. First the gravel of the present streams and low-level benches was washed. Then the Tertiary gravel of the upland ridges was followed by drifting beneath the lava cap and later hydraulicked wherever the situation was favorable.
The stream gravel worked at first was undoubtedly very rich, as it derived gold both from the lodes and from the Tertiary gravel. No record exists of this early work. It was undoubtedly extensive, as the old "dry walls," made by the miners in removing boulders from the streambeds, still exist along the banks of even the smallest streams.
It was not long, however, before the mining of the Tertiary gravel deposits was begun. Those at Minnesota, near the present Irelan Mine, were worked as early as 1852, and by the next year 400 miners were at work there. At about the same time, or slightly later, the deposits of gravel at Chips Flat, Balsam Flat, Alleghany, and Forest were worked. Certainly all were being mined by 1854.
The gravel deposits available for hydraulic mining were less extensive though probably much richer than those of the main Tertiary channel south of the present Middle Fork of the Yuba River; consequently the greatest production was won from drift mines which followed the auriferous channels beneath the lava cap. In 1858 there were 18 tunnel companies in profitable operation at Alleghany, but by 1868 only 25 men were at work, the magnitude of operations is shown by the fact that there were formerly underground connections by means of these drift tunnels between the towns of Alleghany and Forest and between Minnesota and Chips Flat. The Highland and Masonic mine, working a stretch of the channel between Forest and Alleghany, had produced over $300,000 prior to 1867.
In 1874 Forest was said to be "one of the liveliest and most prosperous mineral sections of California." The area of gravel available for drift mining in the vicinity of Forest was much greater than that at Alleghany, and work continued to be active there for a much longer time and has not entirely ceased even to-day. The most famous of the drift mines, the Bald Mountain, began operations in 1872 with an original expenditure for opening the mine of $20,000, and from that time to 1887 it produced $3,100,000, of which $1,300,000 was paid out in dividends.
Lode mining seems to have begun in 1853 at the German Bar mine, on the south bank of the Middle Fork of the Yuba River. A stamp mill was in operation at the Rainbow mine in 1858.(2) From 1872 to 1875 the Plumbago mine produced $100,000, all from hand mortars. The lodes worked in this period, with the exception of the Rainbow were those that were exposed at the present surface. According to local tradition, which seems reasonable, the drift miners, familiar with the tendency of the placer gold to work its way into crevices of the weathered bedrock, regarded the gold of the outcrop of the veins as of the same origin and took out only the upper 2 feet or so.
More favorable opportunities for discoveries beneath the lava were offered by tunnels that crosscut the upper part of the bedrock in order to reach gravel channels. The first recorded discovery of a vein beneath the lave cap was that of the Rainbow mine in 1858. This vein was cut in the gravel tunnel 2,000 feet from the portal, and an incline was sunk on the vein. Some rich ore was obtained, but there was trouble with water and the work was soon abandoned. Apparently some further work was done later, for Burchard in 1883 refers to the mine as having been worked for many years. In the later part of 1881 high-grade ore was discovered not far below the lava, and the shoot yielded $350,000 to 1884. It is said that $60,000 was taken out in a single day and $100,000 in a single month. A single slab of fold-bearing quartz calculated to contain $20,468 in gold was exhibited in San Francisco in 1882. The single outstanding success was the rediscovery of the Tightner vein in 1907 by H. L. Johnson. The discoveries of rich ore in the Tightner and Sixteen to One caused renewed activity throughout the camp, old mines were reopened, and since 1912 production has continued on a greatly increased scale.
In spite of the richness of the ore shoots mined, lode mining has proceeded in rather desultory fashion, and several mines have been worked for short periods and then closed down, to be later reopened when an important "strike" gave new impetus to mining. This seems to have been in large part due to the nature of the deposits. A shoot of enormously rich high-grade ore would occasionally be encountered, and in the period of flush production that followed it proved impossible for the owners, for the most part men of small means, to refrain from "cashing in" a large art of the proceeds. Consequently after the shoot was exhausted the mine was left with a surplus inadequate for the amount of unproductive exploration necessary to discover the next shoot. Therefore, except for a few mines, which have operated fairly consistently, the history of the district has been one of marked ups and downs. The production recorded, beginning in 1891, shows an output to 1930 of more than $13,000,000. For the years prior to 1891 the only data are scattered references in technical publications, principally the early issues of the Mining and Scientific Press, the reports of the Director of the Mint, and official publications oft eh State of California. These publications have been used in compiling notes on the history of the individual mines and, combined with such local information as appears to be reliable, indicate that the total lode production of the district exceeds $20,000,000.
The total for lode production is certainly too low by an unknown amount, possibly of the magnitude of 5 or 10 percent, for a district producing ore of this type is always cursed with the "high grader," and in the early years, before methods of collecting statistics had reached their present degree of accuracy, there were undoubtedly many small mines from which production reports were not received.
The ratio of value of gold produced to tonnage of quartz mined from 1903 is about $16.60 to the ton. It does indicate, however, that on the average, in spite of the smallness of the ore shoots, the returns from lode mining have been higher than in most other California districts.
There are two principal series of veins—those with northwesterly to westerly strike and northeasterly to northerly dips of 25 degrees to 40 degrees, and those with northerly and northwesterly strike and westerly dips, commonly close to vertical and everywhere exceeding 60 degrees.
SIZE AND STRUCTURE
The veins of the district are well defined, and the different vein systems cross the entire area covered by the detailed map. The individual veins are less persistent. The veins vary greatly in thickness. Although in any given level there are parts where little or no quartz is present, corresponding parts of drifts above or below will show quartz, and in most mines the quartz is continuous through the whole developed portion of the vein, though not necessarily so on any single level. On the other hand, there are places, particularly where the vein bows upward or shows a sharp change in strike, where the quartz may attain a considerable thickness. Several veins show such swellings in which the quartz is more than 30 feet thick. Probably 5 or 6 feet is an average thickness for the productive veins, but the variation is so great that any such figure is of little significance. As would be expected in veins in fissures along which there has been reverse movement, there is usually a greater thickness of quartz in portions where the dips are flatter than the average. This relation, however, is by no means universal.
Most of the major veins are somewhat irregular in strike and dip, and in many places the changes are very sharp. The causes of the changes in the course of the veins are as a rule not evident. The steeply dipping veins are distinctly more regular than those with gentle dips. In spite of the fact that the movement along the veins was complex and long continued, it is believed that the total displacement resulting from inter-mineral and post-mineral movement was not large and that most of the movement took place prior to the introduction of the quartz. Mr. Gannett's notes indicate the possibility of a single post-mineral displacement of as much as 40 feet in the Sixteen to One mine, but none of the faults indicate more than a few feet displacement. The best evidence against major movement is found in a study of the intersections of the flat and steep veins. There has also been movement along the steeper veins, though it seems to have been less complex than in and along those with flatter dip. Therefore if the inter-mineral and post-mineral movements were of any magnitude a complex mosaic of fault blocks should be present in the region of the intersection of the veins of the two systems.
ARSENOPYRITE AND PYRITE
The only sulphide minerals belonging to the quartz stage of mineralization are arsenopyrite and pyrite. Both are distinguishable from the same minerals of the succeeding stage by their crystallization and by the fact that their crystallization preceded that of the quartz which surrounds them, so that these early sulphides show crystal faces against the quartz and are also veined by the same quartz which forms the body of veins.
Arsenopyrite is the more abundant and of greater economic importance, as it has in many places formed the nucleus for the deposition of the free fold of the next stage. This early arsenopyrite is commonly found in clusters of radiating prismatic crystals as much as several centimeters in length, either growing out from the wall rock or separated from the wall by a narrow strand of quartz. The course-grained arsenopyrite has been founding all the veins of the district from which any considerable production has been made but seems to be most abundant in veins such as the Oriental, Gold Canyon, and Plumbago, which cut gabbro and serpentine. It is particularly noticeable that wherever a vein is close to serpentine there is likely to be abundant coarsely crystalline arsenopyrite.
Pyrite belonging to this stage of mineralization occurs here and there throughout the veins but is less common than the arsenopyrite. It is not commonly associated with the high-grade ore and seems to be more abundant in the less productive veins.
Quartz forms by far the greater part of the filling of the veins and has replaced an unknown amount of wall rock. Quartz is milky white and of a massive appearance. Individual crystals or grains can not be distinguished except where drusy crystals project into vugs.
CHARACTER OF MINERALIZATION
After the formation of the quartz veins and the accompanying minor alteration of the wall rock came the introduction of additional minerals, chiefly carbonate, sericite, and mariposite, with sulphides, graphite, free gold and small amounts of quartz, chalcedony, and opal. It is not intended to imply that any long interval separated these stages, and they probably overlapped to a slight extent, yet the evidence indicates not only that the crystallization of the early quartz was complete before the introduction of any appreciable amount of these later minerals, for no quartz of the early type cuts or replaces the carbonate, but that movement had taken place along and within the veins of crystalline quartz prior to the introduction of the carbonate.
The minerals belonging to this stage are found both as replacement minerals in the wall rock and within the quartz veins. Most of these minerals occur in both positions, but certain of them, including most of the sulphides and gold, are found only within the earlier vein quartz.
The minerals of the last stage of mineralization are inconspicuous in total amount and of no economic importance.
The products of oxidation are inconspicuous.
The characteristic ore of the Alleghany district is of "high-grade"—that is, it shows coarse gold in considerable amounts. As a rule such ore carries from $2,000 a ton in free gold up to many times this amount.
The high-grade shoots range in size from "bunches" yielding a few hundred dollars to those from which many thousand dollars' worth of gold had been taken. The largest shoot in the Sixteen to One mine yielded a total of nearly $1,000,000. The richest ore in this shoot was confined to an area less than 40 feet square on the vein and came mostly from the 2 feet of quartz next to the hanging wall. A shoot that yielded $736,000 from an area of 14 by 22 feet on the vein was mined in the Oriental mine.
The quartz outside the high-grade shoots may be practically barren, with a tenor of less than $1 a ton, or many contain enough gold to nearly repay the cost of mining. Although barren quartz may adjoin a high-grade shoot, it is usually found that in one or more directions the change isles abrupt than in others, and along such a course for a considerable distance from the high-grade shoot the quartz may contain more gold than the average vein quartz. Nevertheless, the change is in places very sharp, and a single round of shots may bring a drift or raise from almost barren quartz into rich ore.
The high-grade ore rarely extends completely across the vein. It is usual for high-grade ore to be confined to a single strand, with the fold either ceasing abruptly at the edge of the strand or visible for a few inches into the next. High-grade ore is more commonly near the hanging wall than the footwall, especially in the vicinity of lenticular swellings or outward bends of the vein either in strike or dip.
The attempt to determine the factors that control such unusual concentration of gold as that at Alleghany was the prime object of the present studies in both field and office. Naturally a district in which such concentrations exist is a favorable one for the study of the factors that determine the presence of ore shoots, but even the quantities of gold in the high-grade shoots represent relatively small concentrations of the gold considered in terms of percentages. For instance, in high-grade ore with a value of $6,000 a ton the gold represents only about 1 percent of the ore by weight and less than 0.1 percent of its volume.
Although the authors were able to see high-grade ore in place in several of the mines, it is inevitable that a study of this kind must depend largely on information furnished by those who have had more constant opportunities for observation. In this respect the authors were particularly fortunate. For many years all those associated with the mines of the district have observed closely the features of the veins concomitant with the occurrence of high-grade ore, and such observations have greatly aided the studies here reported. To a large extent this section will be devoted to a rationalization of these empirical guides and a consideration of their relative importance.
In the development of mines containing ore bodies of this type small high-grade shoots are, of course, easily missed unless the veins are thoroughly explored' on the other hand, it has been found by experience that mining large amounts of nearly barren quartz in order to avoid missing high-grade bunches is not profitable. If, therefore, intensive development can be confined to the portions of veins in which high-grade ore is likely to occur the chances of profitable operations will be greatly increased. It is probable that future operations will involve exploration of the undeveloped territory beneath the lava. This work will be more likely to be successful if development of the unproductive portions of veins is confined to the minimum of necessary drifts, excessive crosscuts are avoided, and only the parts of the veins that appear to show promise are prospected closely.
The sporadic distribution of the high-grade shoots has at times led operators to attempt to stope and mill whole sections of the vein in the hope of encountering "bunches" that would otherwise have been missed. Even where this procedure has been rewarded by the discovery of high-grade ore it has usually not been profitable, though partial exceptions to this generalization should be noted for the Sixteen to One and Plumbago mines. Moreover, as patches of high-grade ore tend to occur near together, separated by more or less barren quartz, it is, of course, only common prudence to stope the barren quartz surrounding a high-grade shoot.
FAVORABLE STRUCTURAL FEATURES
In all the veins that have been productive there are certain structural features, which appear to be generally associated with the high-grade shoots, and within such favorable areas there are closer guides furnished by the mineralogy of the veins.
The gold reached its present position later than the crystallization of the quartz, and it is believed that the major control of the deposition of gold in the high-grade shoots lay in the presence in the vein of conditions which favored the local shattering of the quartz and consequent ready entrance of the solutions that deposited the gold.
It is well recognized that any irregularity of the vein is a favorable indication. Such irregularities may be caused by changes in strike and dip, sudden swellings in the vein, junctions of veins or splits in the vein (the same thing looked at from opposite points of view), and minor faults. The veins with gentle easterly dips have much more sinuous courses than the steeply dipping veins have proved far more productive. The influence of large masses of serpentine on the courses of the veins has already been noted.
It is estimated that 80 percent of the production of the district has been derived from those portions of the veins where serpentine either formed one wall or was less than 100 feet from the ore. This is believed to be due to at least two causes—the effect of serpentine in causing bends in the vein, both in strike and dip and in branching of the vein, all of which favor later fracturing; and the greater abundance of the early arsenopyrite, which, as shown below, an eff3ctive precipitant of gold, near the serpentine.
A knowledge of the position of the serpentine in relation to the veins may, therefore, be of value in planning development work. To some extent the extension of the serpentine belts beneath the lava can be predicted from the geologic map that forms Plate 1. It is important, therefore, that the position of all serpentine belts encountered in workings, whether drifts or crosscuts, should be accurately recorded and their possible extension relative to the veins carefully studied. The only visible mineralogic guide to the probable presence of serpentine in the vicinity of a vein is the increase in the amount of the mariposite-carbonate mixture ("blue jay") in the altered wall rock. Therefore the presence of abundant "blue jay" should lead the operator to consider whether the vein may not be near a serpentine mass. As the serpentine contains sufficient magnetite to affect the compass needle, it is possible that magnetic surveys, both on the surface and underground, may in places yield information of value as to the position of the serpentine.
The most reliable mineralogic guide is the actual presence of free gold, though not necessarily in amount sufficient to make even milling ore. In a shoot of high-grade ore there is commonly one direction in which the change to barren quartz is not as sharp as in others, so that the ore body has a "tail" consisting of quartz from which a little free gold can be panned. Therefore quartz that shows even small "prospects" may indicate the proximity of high-grade ore.
In the Alleghany district the mill is a valuable adjunct to prospecting. It is the usual custom to mill all quartz, however barren, mined in the course of development work, and whenever there is an increase, no matter how slight, in the gold recovered, the working faces are carefully investigated. It is thought likely that more panning, particularly wherever banded quartz is present, might be of use as a guide to ore. Galena, which is the later sulphide most closely associated with gold, should also be looked for in the concentrates.
Other than the accrual presence of gold, the best mineralogic indication of high-grade ore is the coarsely crystalline arsenopyrite, which has crystallized prior to the quartz. This mineral seems to have been effective in causing the deposition of the fold and is in places partly replaced by gold. It is the impression of the writer that the greater portion of the gold obtained from the high-grade shoots of the Alleghany district was found in close association with arsenopyrite though not necessarily in immediate contact with it.
Experienced miners of the Alleghany district distinguish between "live" and "dead" quartz and consider the "live" quartz a favorable but by no means certain indication of high-grade ore. The distinction between the two varieties is very faint and may often be warped by the optimism of the observer. Nevertheless it appears to be real, though the writer's limited experience did not enable him to make it with any certainty. The "live" quartz has a milkier and less lustrous appearance that the "dead" quartz. Under the microscope it is apparent that this difference is due to the greater degree of microbrecciation in the "live" quartz. It follows, therefore, that, on the assumption that portions of the veins in which later fractures are prevalent are the more favorable; this distinction has some validity as an indication.
PROBABLE PERSISTENCE IN DEPTH
The question most frequently asked us by those interested in the district was, "Will the high-grade ore continue in depth?" To this question it is impossible to give a categorical answer. The Alleghany district stands alone among the districts of the California gold belt in its peculiar type of ore shoots, so that no analogy based on the persistence of ore to great depths in such districts as Grass Valley and the Mother Lode region is pertinent. It may be said, however, that the fear often expressed that the rich shoots are of super-gene origin is without foundation. The gold has been deposited from hypogene solutions and is in no way dependent upon either the present surface r the surface as it existed prior to the outflow of the lava, and there is no reason to anticipate impoverishment in depth on this account. It is possible, however, that there may be a change in the character of the ore with greater depth. The wider distribution of gold in the lower levels of the Sixteen to One mine, as shown by the data given on page 52, is perhaps an indication that with greater depth the gold will end to occur in larger shoots of lower grade. But, on the other hand, ore of the Gold Canyon mine, whose workings are at about the same altitude as the lower productive levels of the Sixteen to One, was of the same high-grade type as has been found elsewhere in the district at higher altitudes, and though "mill rock" was mined at the German Bar mine most of the production was derived from high-grade ore. The mill rock shoots of the Tightner, Plumbago, and Eldorado were at higher levels than either of these. It has been found that barren zones exist even where a combination of favorable features is present, and it is possible, though the depth to which work has extended in any on e mine is not sufficient to say with certainty, that there may be a rough horizontal distribution of barren zones and zones containing high-grade shoots.
In conclusion, the authors are of the opinion that there is no reason indicated by the present study why the mines should not prove equally productive to a much greater depth than had been reached at the time the field study was made. This opinion is, of course, not meant to be taken as an encouragement to deep exploration of a vein in which shallow development has revealed nothing of promise.
The veins of the Alleghany district are considered to have been formed at about the end of Upper Jurassic time, and various lines of evidence lead to the conclusion that the depth of formation was at least 10,000 feet and may have been much greater. The reverse faults of two systems, which are the sites of most of the veins, are considered to be auxiliary to the major reverse fault that borders the district on the east. The later reverse movement along the veins, which continued during the whole period of ore deposition, may have been in part caused by the progressive serpentinization of the basic intrusives.
The minerals of the earliest stage of mineralization were formed by the alteration of wall-rock minerals without the addition of material except water and carbon dioxide. The vein quartz in places shows evidence of re-crystallization after solidification but prior to the introduction of minerals of the carbonated stage.
The minerals of the carbonate stage were deposited after the final crystallization of the quartz and largely by replacement of earlier-formed minerals, both of wall rock and of veins, but also to a minor extent in open fissures. No acceptable explanations are found for the peculiar concentration of the gold in high-grade shoots or for the fact that the gold, though associated with minerals that have replaced the wall rock, is always found in association with the earlier quartz.
FUTURE OF THE DISTRICT
It must be evident to anyone who has studied the body of this report, particularly the section on ore shoots, that the authors consider that the veins of the Alleghany district hold promise of considerable future of production, although owing to the peculiar type of ore prevalent in the district production will continue to be irregular.
There is nothing in the mineralogy of the veins which suggests that the gold of the high-grade shoots is of super-gene origin or in any way related either to the present surface or to that existing at the time of deposition of the Eocene auriferous gravel. In spite of the long history of mining in the district, development of the veins has so far reached only comparatively shallow depth, but if the smaller amount of development work at the lower altitudes is taken into account, there has been no decrease in production with depth. It is of course beyond the ability of the writer to make any predictions as to the ultimate depth to which the veins will prove workable, and as the ore of Alleghany differs so greatly from that of other California district that have been developed successfully to much greater depth, no prediction based on comparison with these districts an be made. It is reasonable, however, to infer a productive depth at least as far below the present average depth of development as the present depth is below the lava overlying veins.
It may be that the deposition of the coarse gold of the high-grade shoots was dependent upon a delicate adjustment of physical conditions such as temperature and pressure and that at much greater depth a wider distribution of gold into lower-grade shoots will be found. The increase in the amount of gold outside the high-grade shoots in the lower levels of the Sixteen to One mine and the "mill ore" of the German Bar mine suggest that this is possible. But high-grade rather than mill ore is by far the principal source of gold in the lowest workings of the district, such as the lower parts of the Sixteen to One and Plumbago mines, and the workings of the Gold Canyon and German Bar mines. It therefore seems reasonable to suppose that no significant change in the general type of the ore need be expected within moderately greater depth on the veins.
The most cursory inspection of Plates 8 and 9 shows that by far the greater part of the development work has been done on veins that crop out in areas from which the lava has been eroded. There is no reason to suppose that veins which lie beneath the lava not prove equally productive. Naturally the exploration and development of such veins, even those which have been definitely located by the placer tunnels, will be a far more expensive process than the further exploration of the veins already in part developed and should hardly be undertaken without capital ample to carry on a long campaign of un-productive exploration. To give quantitative expression to this opinion of the future possibilities the estimates made on page 68 are here repeated, with an additional figure based on the expectation that for at least another 800 feet of vertical depth the veins will prove equally productive.
Possible recoverable gold content of veins concealed
beneath the lava, above an altitude of 3,700 feet………………………..….. $10,000,000
Possible recoverable gold content of undiscovered shoots
in and near veins now being developed above 3,700………………….…..… $6,000,000
Possible recoverable gold content of all veins
between 2,900 and 3,700 feet……………………………..…………….....……. 40,000,000
Such estimates should not be taken too literally, but they nevertheless reflect the writer's considered opinion as to the productive possibilities of the district.
In closing there may perhaps be permitted a word of warning, applicable alike to active operators and to intending investors. The many failures in the district are believed to have been in large part due to the peculiar occurrence of the ore. It seems to be almost inevitable that the discovery of high-grade ore causes such a feeling of elation that the operator is too strongly tempted to cash in on his good fortune and does not reserve adequate funds for the inevitable period of un-productive exploration which follows the exhaustion of a high-grade shoot. Figures have been cited to show that the reward at Alleghany per ton of quartz mined or per foot of development driven is probably greater than for other California districts, but it must be kept in mind that in exploring veins of this type adequate initial capital is essential and that when the operator's perseverance is rewarded by the discovery of high-grade ore self-restraint must be exercised if a repetition of the first success is desired.
(1) Ferguson, H. G., Lode deposits of the Alleghany district, California; U.S. Geol. Survey Bull. 580, pp 153-181, 1914
(2) Trask, J. B., Report on the geology of the Coast Mountains and part of the Sierra Nevada. P 63, Sacramento, 1854.
H. G. Ferguson & R. W. Gannett