LODE MINING NEAR FAIRBANKS. By PHILIP S. SMITH. INTRODUCTION. The inevitable depletion of easily mined placer deposits near Fair­ banks and the consequent availability of both men and money for other enterprises has given an impetus to prospecting for lode de­ posits which has resulted in the opening of some productive mines. The writer visited the lode-mining district late in the open season of 1912 to study the conditions under which the veins occurred, and the results of that study are set forth in the accompanying report. Work in the field began September 7 and closed September 25, 1912. In spite of this short season the studies by Mr. Prindle and Mr. Katz furnished geologic data so complete that the investigations here recorded were devoted exclusively to mining developments. The writer is under deep obligations to both Mr. Prindle and Mr. Katz not only for their published data but also for the assistance they have given by consultation and advice. Grateful appreciation of the many courtesies and privileges received in the field is expressed to the various mine owners, operators, and residents of the Fairbanks region. Although specific mention is necessarily incomplete, thanks are especially due J. L. Sales, of Fairbanks; the Cook brothers, of Too Much Gold Creek; the members of the RexaU Mining Co., of Wolf Creek; Nels Giske and the Quemboe brothers, of Chatham Creek; Allan Cunningham and Antone Goessmann, of Bedrock Creek; C. S. Sargent, of Willow Creek; Sterling, Zimmerman, and Nightingale, of Twin Creek; W. L. Spaulding, of Dome Creek; and W. C. Harp, the Hudson brothers, and Louis Sagan, of Ester Creek. GENERAL GEOLOGY. The following summary of the bedrock geology of the Fairbanks district has been abstracted from a report by Prindle and Katz which should be consulted for more detailed descriptions.1 The unconsolidated deposits have not been discussed here, as they have little or no bearing on the subject of the present report. 1 Prindle, L. M., and Katz, F. J., Detailed description of the Fairbanks district: U. S. Geol. Survey Bull. 525, pp. 59-77,1913. 137 138 MINERAL RESOURCES OF ALASKA, 1912. The greater part of the bedrock of the Fairbanks district is formed of metamorphic rocks which have been grouped together and called the Birch Creek schist. These rocks include rather massive quartz- ites, quartzite schists, quartz-mica schists, hornblende schists (in part amphibolitic), carbonaceous schists, crystalline limestones, and altered calcareous rocks, with associated eclogitic rocks, andalusite hornfels, and a small amount of granitic gneiss derived from porphy- ritic intrusive granite. All of these rocks are closely folded. Recumbent minor folds are common, and so far as observed the direction of overturning is toward the northwest. In places the folding is so close that the limbs are nearly parallel and cleavage and bedding planes become also parallel. The dips, therefore, indicate rather the dip of the predominant struc­ tural planes, which may or may not be the true dip. The Birch Creek schist is unfossiliferous and its age has not been definitely determined. The earliest fossils found in passing from it into the overlying rocks in other parts of the Yukon-Tanana region are Ordovician in age, and therefore possibly the Birch Creek schist is in whole or in part Cambrian. The igneous rocks of the Fairbanks district include several varieties of intrusive granular rocks and a small amount of igneous material regarded as extrusive. Intrusive granular rocks that penetrated the schist at such depths below the contemporary surface of the earth as to have cooled with sufficient slowness to. become entirely crystal­ line are separable into quartz diorite, porphyritic biotite granite, light- colored persilicic granitic dikes, and altered porphyritic dikes related to granitic and dioritic rocks. All these have been shown on the map (PI. VI) in one pattern. The greater part of the intrusive rocks of the Fairbanks district belong to the granite and diorite group. The quartz diorite of Pedro Dome and the porphyritic biotite granite of the Smallwood-Gilmore ridge are distinct types, but there are several intermediate phases. Although the coarse porphyritic biotite granite was nowhere observed cutting the quartz diorite, fine-grained dikes like those which cut the biotite granite and which are closely related to it in composition and were only shortly subsequent to it in time of intrusion were observed to cut also the quartz diorite. Both the two main types are probably the products of the same general period of intrusion. The sericitized dikes of granite porphyry and quartz diorite porphyry are probably to be referred in origin to the main intrusive masses and have been in part altered by hydrothermal action. Passageways for various after-products of intrusion were afforded by an extensive shattering that took place in part at least subsequent to intrusion. No definite age can be assigned to these intrusives. Conglomerates regarded as Tertiary contain numerous bowlders of granite whose ori- LODE MINING NEAR FAIRBANKS. 139 gin is not definitely determined, but from what is known of Tertiary deposits in other parts of the Yukon-Tanana region probably these conglomerates are of Kenai age, which indicates that the granitic in­ trusion was earlier than the Kenai epoch. The nearest similar intru­ sive rocks whose age of intrusion is rather definitely known are those of the Rampart region, 90 miles to the northwest, which have in­ truded Upper Cretaceous rocks. PRESENT CONDITION OF LODE MINING. GENERAL FEATURE S. In the region adjacent to Fairbanks 2,000 lode claims have been recorded and of these probably 50 are being more or less extensively prospected. Practically all the veins that are being developed are free milling gold lodes, as deposits of this or other metals requiring metallurgical treatment for extraction can not be mined under exist­ ing conditions. Six of the properties have been developed to the producing stage and maintain their own mills. Six other mills are being constructed and should be in operation before the end of 1912, and still others are contemplated. The six mills now in operation have a total of 24 stamps, of which 22 are of the Joshua Hendy type (fourteen 1,000-pound stamps; five 500-pound stamps, and three 250-pound stamps) and two are Nissen stamps of 1,300 pounds each. These stamps have an estimated crush­ ing capacity of 1 to 5 tons of ore a day. Of the mills being built, two are to be equipped with seven stamps of the Hendy pattern, three with six Nissen stamps, and one with a Little Giant crusher having an estimated capacity of 6 to 15 tons of ore a day. According to these estimates the mills should be capable of crushing about 100 tons of rock a day. According to Brooks 1 about $64,100 was produced by the lode mines in 1911 and about $60,000 had been produced'previously. No reliable statements of the gold production from the lodes in 1912 have been received, but it is estimated to have been about $200,000. The discrepancy between this estimate and the amount that should have been produced as indicated by the theoretical capacity of the mills and the assumed tenor of the ore is due to three main causes, namely, many of the mills were built during the year and therefore lost more or less time on that account; few of the mines have been opened up sufficiently to yield a constant supply of mill rock; and many of the mills have been compelled to close down during the winter on account of the cold. The trouble with cold is serious and is felt most acutely in its effect on the water supply. It is relatively easy to keep a mill so 1 Brooks, A. H., The mining industry in 1911: U. S. Geol. Survey Bull. 520, p. 30,1912. J140 MINERAL RESOURCES OF ALASKA, 1&L2. warm that water will not freeze in the mortars or on the tables, but the cutting off of practically all sources of surface water during the six or seven whiter months presents'an almost insuperable difficulty. The larger streams, of course, afford ample water throughout the year, but these are at some distance from the mines and selection of a mill site on them involves additional charges for the transportation of ore. Attempts to gain a sufficient permanent water supply by boring holes a hundred or more feet in depth have so far met with indifferent success. At a few places seepage water pumped from the mine has been used at the mill, but the supply has not been ade­ quate. In fact, one mill where this was tried was forced to abandon the experiment and was moved near to a creek at a considerable dis­ tance from the mine. On the whole the camp has shown a rather healthy development, with less exaggeration and wildcatting than has been customary elsewhere. Most of the veins have been and are being opened by local men and capital, and some of the most productive have been made to pay from the start. In general this quiet, relatively inex­ pensive exploitation of the lodes has been beneficial, but the camp has had the disadvantage that many of the operators are not lode miners and consequently have made mistakes that might have been avoided. The need of proficient engineers and miners is already felt by the more enterprising operators and will become more urgent as mining increases. As a result of the absence of skilled manage­ ment none of the mines employ the careful methods of sampling, in­ vestigation of mining costs, scrutiny of geologic data, and precise measurements that are necessary to reduce mining from a speculative venture to a more or less exact business enterprise.