Geologists of the Geological Survey who collected data during this exploration work include Robert Chapman, Gordon Herreid, C. L. Sainsbury, and the writer. PRODUCTION The total production of antimony from the Stampede mine has been: 2 GEOLOGY AND PREVIOUS WORK The geology of the Stampede mine and the surrounding 25 square miles was mapped in 1941 by Donald E. White (1942) of the Geological Survey. The U.S. Bureau of Mines carried out an exploration project in 1942 at the Stampede mine; this project was supervised by N. Ebbley, Jr., and W. S. Wright. In their report (1948), they gave many valuable assay and beneficiation data and a discussion of the geology of the mine. During the exploration 1,520 cubic yards of trenching and 740 feet of stoping and crosscutting were done. The Swanson cross fault, the abrupt bend of the Mooney ore body to the north, and the Stampede fault in the Libbey crosscuts were discovered and mapped. (These features are described below.) Exploration work done under the Defense Minerals Exploration Administration contract consisted of 613.5 feet of drifting and crosscutting on the Lower Tunnel level, 1,397.5 feet of diamond drilling from the Lower Tunnel level, and 16,282 cubic yards of surface trenching north of the Stampede mine on the Kobuck and Pearl Harbor claims. White (1942) showed that the area which includes the Stampede mine is underlain by mica schist, schistose quartzite, and other rocks that are known collectively as the Birch Creek schist. The Stampede mine is near the nose of an east-northeastward-trending and plunging anticline and adjacent to a northeastward-striking high-angle fault, 'Figures furnished by Mr. Earl Pilgrim, owner and operator of the Stampede mine. the Stampede fault (White, 1942, pl. 49). Antimony-bearing veins have formed in a complex of faults in fine-grained quartz-sericite schist and impure quartzite immediately northwest of the Stampede fault. White (1942) determined that the following sequence of events took place: (1) formation of one(?) steeply-dipping fault striking N. 40° E. to N. 80° E., dipping 45° to 60° S., and located northwest of and at a low angle to the present Stampede fault; (2) normal faulting at nearly right angles to the initial high-angle fault, displacing it in four places with the northeast side (hanging wall) at each place dropped relative to the sourthwest side (footwall); (3) deposition, in order of decreasing abundance, of quartz, stibnite, pyrite, and arsenopyrite, mostly in the original steeply-dipping fault and partly in the cross faults; and (4) minor post-mineral movement along parts of the main fault. The Stampede fault, which branches eastward from the main vein fault at the Surface ore body, was exposed at the surface only in 1941, and its age relation to the mineralized faults could not be determined. The mine workings consist of the Upper Tunnel (altitude of portal about 2,190 feet) (White, 1942, pl. 59), the Lower Tunnel (altitude of portal about 2,035 feet), and the associated workings. The latter consist of a shaft from the Surface ore body to the Upper Tunnel level, the Nesse and Emil winzes from the Upper Tunnel level, several sublevel drifts from the Emil winze, the Mooney winze from the Lower Tunnel level, and minor openings from both levels. The portal of the Upper Tunnel is about 210 feet south and a little west of the Lower Tunnel portal (White, 1942, pl. 50), and the Surface ore body is at an altitude of about 2,290 feet and lies about 400 feet southwest of the Upper Tunnel portal. The small Kobuck adit was driven from a surface showing of stibnite, and a few tons of ore was mined from this body. The major ore bodies of the Stampede mine are: 1. The Surface ore body (now mined out), which was an irregular mass of almost pure stibnite with a maximum thickness of 26 feet that formed at the intersection of the Stampede fault and the northeastward-trending steep fault to the north (White, 1942, p. 342). 2. "The Nesse winze ore body, on the Upper Tunnel level consists largely of vein quartz and brecciated schist containing 10 percent or less of antimony. Six feet down in the Nesse winze, stibnite is a prominent constituent of the vein. The winze has been sunk to 20 feet below the level, where the vein consists of 1 foot of almost pure stibnite" (White, 1942, p. 345). This ore body is shown on plate 3. 3. The Emil winze ore body, also formed in the main vein fault, the richer parts of which are largely mined out, "is a definite vein, with 1 to 7 feet of medium- and high-grade ore on the Upper Tunnel and 50 foot (sub) levels," (White, 1942, p. 342). This ore body is not known to extend to the Lower Tunnel level. 4. The West and East Mooney ore bodies. (See pl. 3.) These also formed along the main vein fault. The West body "is at least 6 feet wide, *** consists of brecciated schist containing veinlets of stibnite," and "one persistent vein of almost pure stibnite about 2 inches wide extends" along the ore body (White, 1942, p. 344-345). The East body is 5 to 10 feet thick and contains veins and stringers of stibnite a fraction of an inch to about 1 foot thick in quartz and crushed wallrock. The ore bodies are similar to each other in their mineralogy. Stibnite (Sb2S3) is the chief antimony-bearing mineral. "It is commonly finely granular. Polished sections of the ore indicate an average grain size of about 0.01 millimeter in thickness and about 0.02 millimeter in the other two directions. Most of the crystals are oriented with the long dimensions parallel to the vein walls. A late generation of needlelike stibnite crystals cementing brecciated early stibnite is quantitively insignificant. The needlelike crystals are commonly several millimeters long" (White, 1942, p. 338). Small grains of pyrite (FeS2) are scattered in stibnite. Arsenopyrite (FeAsS) is "generally disseminated in quartz in the antimonypoor parts of the veins" (White, 1942, p. 339). The gangue consists of milky quartz with occasional pods, stringers, or individual grains of calcite. The ore is dense, and vugs are rare. GEOLOGIC STRUCTURE STAMPEDE FAULT This structure has been explored by the DMEA drift for 420 feet along its strike, as shown on plate 3, and a core drill hole from near the Mooney winze has been driven through the fault. The attitude of the fault is variable, the strike swings from about N. 85° E. south of the DMEA drift to N. 35° E. east of the DMEA crosscut and back to about N. 65° E. south of the Mooney drift and Chisholm crosscut. The western part of the fault dips about 73° to the south, and the eastern part appears to dip steeply to the north. The fault is marked by gray to dark-gray clay, slices of closely fractured wallrock, and crushed vein quartz. Traces of granulated pyrite and stibnite are present in much of the clay gouge. The fault zone may be as much as 33 feet thick south of the Mooney drift, as suggested by cores and driller's logs of hole 19, and at least 15 feet thick south of the DMEA drift. The sulfides and vein quartz in the Stampede fault were derived either from veins originally formed in the fault or from veins and stringers formed in wallrock that has since been crushed to gouge, or both. It is not known which alternative is correct. The close relation of small stringers and pods of stibnite and quartz in wallrock adjacent to the Stampede fault in the DMEA crosscut and drift suggests that ore deposition was localized by the fault. Premineral cross faults A and B apparently displace the Stampede fault and are therefore younger. Movement along the Stampede fault thus probably began prior to the mineralization. At least some of the thick gouge zone was formed after the mineralization, because it contains crushed sulfide grains. Neither the displacement nor direction of movement of the Stampede fault is known. From White's map (1942, pl. 49), it seems that the northwest block was dropped relative to the southeast block. CROSS FAULTS A AND B Cross faults A and B strike about N. 30° W. and dip 45° to 55° NE. and 75° to 85° NE., respectively. Cross fault A apparently was cut by drill hole 6, and it appears to extend upward toward the premineral cross fault mapped by White (1942, pl. 50) in the Upper Tunnel level 95 feet southwest of the Nesse winze. Cross fault B probably extends upward to the premineral steeply dipping faults that bound the southwestern edge of the Emil ore body. Both of these cross faults are concealed by lagging in the Lower Tunnel, where they are now expressed only by very weak ground. The Stampede fault appears to be offset 60 feet, measured horizontally, by cross faults A and B. It is possible, however, that the apparent offset is due to a sharp bend in the Stampede fault and that the Stampede fault is younger than the cross faults. CROSS FAULT C This fault zone strikes about N. 68° W. and dips steeply northeast. It consists of a 5-foot-thick zone of closely spaced parallel faults along which only minor amounts of gouge have formed. Very thin veinlets of stibnite have replaced quartz along several of these faults, and minor postmineral movement has formed slickensides on some of the stibnite veinlets. Cross fault C displaces the Mooney ore body, the westward continuation of which may be the slightly mineralized fault that parallels the main haulageway from just south of the Mooney drift to the Lower Libbey crosscut. This cross fault, however, apparently does not displace the Stampede fault, as shown by drill hole 19 and the Chisholm crosscut. The Stampede fault, therefore, is the younger. SWANSON CROSS FAULT The Swanson cross fault separates the East and West Mooney ore bodies. Vein quartz with disseminated stibnite has formed along the fault, which indicates that movement preceded mineralization. The Swanson fault is marked by a zone of quartz-stibnite vein rock and sheared schist with gouge that is 6 to 18 inches thick. The dip is 65° E. at two places, and 48° E. at the southernmost wall. Hole 5 was drilled to locate the West Mooney ore body just west of this cross fault, but only minor stibnite was found in the sludge. The attitude of this hole is too nearly parallel to the ore body for drilling to be significant, even if the ore body were intersected. FOLDS Attitudes of beds in the DMEA crosscut and drift are gentle, and most have northerly strikes and dips toward the east. White (1942) found similar attitudes at the surface in this general area. Several gently plunging, small drag folds were seen on the Lower Tunnel level. It is problematical whether the two drag folds in the Lower Libbey crosscut were formed in conjunction with regional folding or faulting. The folds in the main haulageway north of the Mooney drift and at the face of the blind crosscut from the east Mooney drift are markedly asymmetrical and may well be related to regional folding. EXPLORATION OF ORE BODIES NESSE WINZE CROSSCUT A stibnite-rich zone was found in drill holes 2 and 3. Assays of sludge by the Territorial Department of Mines at College, Alaska, are as follows: Drill hole 2 (core recovery 50 percent from 80-90 feet) 81-82. 82-83. 83-84. 84-85 Weight of sample (pounds) 0.875.. 1.25 1.0.. 1. 625. Percent Sb 10. 16 8. 51 20. 26 22. 56 This zone is the downward extension of the Nesse winze ore body, which here is a vein striking about N. 60° E. and dipping 60° SE. Hole 3 intersected the vein 78 feet below the Upper Tunnel level, as measured along the dip, and hole 2 struck the vein 102 feet from the Upper workings. The vein is five-eighths inch thick at the Upper Tunnel level and increases to 30 inches of high-grade ore at the bottom of the 35-foot Nesse winze (Pilgrim, oral communication). The ore |
