Occurrences of gold and other metals in the Upper Chulitna district, Alaska

Geological Survey

Gold and other metals occur in breccia pipes, veins, and disseminated deposits at several places in the upper Chulitna district, on the southern flank of the Alaska Range. The main purpose of this report, made as part of the U.S. Geological Survey's Heavy Metals program, is to call attention to four areas that contain deposits of gold or other metals. From north to south these are: Colorado Creek, Golden Zone, Blind Creek, and Long Creek. The main economic features of each area are:

Colorado Creek area. A stock of biotite quartz diorite porphyry about 1,500 feet wide by 2,000 feet long has associated arsenic-gold-antimony deposits which contain minor amounts of copper, bismuth, and tin. One sulfide-rich body appears to be a breccia pipe similar to that of the Golden Zone.

Golden Zone area. The main mineralization at the Old Golden Zone mine is in a breccia-pipe deposit that is 200 to 300 feet across and probably extends in depth to more than 1,000 feet. Although the grade of the deposit is uncertain, it is estimated to contain more than $10,000,000 worth of m etal s, principally gold, silver, and copper, but with anomalous amounts of tin, molybdenum, bismuth, cadmium, and boron.

Blind Creek. A major fault zone at Blind Creek contains quartz-rich breccia filling as much as 200 feet thick which contains a trace of gold, arsenic, lead, and zinc.

in the Upper Chulitna District, Alaska

Abstract Long Creek. Gold- and silver-bearing copper occurrences are widely distributed near Long Creek. The main copper concentration is at the old Copper King prospect, but very sparse amounts of chalcopyrite, molybdenite, and pyrite occur in quartz porphyry plugs nearby. An area of strongly hornfelsed rocks, centered around the main quartz porphyry plugs, possibly is underlain by a quartz porphyry stock at shallow depth.

Results of stream-sediment sampling in the district are shown in figure 10 and table 3.

The upper Chulitna district, southeast of Mount McKinley National Park (fig. 1), is only 8 to 10 miles from the Alaska Railroad, but at present is best reached by small fixed-wing aircraft or helicopter, because roads into the district from the railroad have been abandoned.

We gratefully acknowledge the aid and encouragement received from J. J. Mulligan and J. H. Herdlick, U.S. Bureau of Mines. Because the only underground mine workings of the area, those of the Golden Zone mine, are now inaccessible, much assay data is taken from maps contained in a report by Mulligan, Warfield, and Wells (1967) or from unpublished maps furnished by H. R. Beckwith, General Manager of Cemco, Inc., or C. H. Herbert.

We would also like to acknowledge the help of Jon Alan Benfer, who served as field assistant, and of Cliff Hudson, bush pilot of Talkeetna, Alaska, who furnished skilled transport to and from the area.

Arsenic-gold-antimony deposits in the Colorado Creek area (fig. 2) are associated with a small stock of hornblende-biotite quartz diorite porphyry. The stock is bisected by the deep canyon of Colorado Creek. The stock itself is almost inaccessible except at the bottom of the canyon (approximately 200 feet deep), but it appears generally bleached and locally contains disseminated chalcopyrite (as at the locality noted, fig. 3). The northern and southern contacts of the stock, as exposed in the canyon, are both iron stained, and the southern contact is marked by a zone of tactite, approximately 2 feet wide, containing abundant garnet, actinolite, and epidote. The surrounding rocks are hornfelsed and locally are strongly sulfidized.

The main sulfide occurrences are (1) at apreviously unprospected knob about 750 feet west-southwest of the Silver King prospect, and (2) at the Silver King prospect itself. The Silver King prospect pits are about 6,000 feet north of the location shown on the Healy A-6 quadrangle map (scale, 1:63,360). Another occurrence is about 750 feet southwest of the main Silver King prospect pits, where an arsenopyritestibnite-quartz lode as much as 10 feet wide crops out in Colorado Creek. The new occurrence (locality

1) is a highly sulfidized mass of altered porphyry

and hornfels at least 50 feet across that strongly resembles the highly broken and sulfidized rock of the Golden Zone pipe. Arsenopyrite is very abundant, but some chalcopyrite is present, and spectrographic analyses of samples show highly anomalous amounts of bismuth, cobalt, and tin in some samples (table 1). The gold content of samples (table 1) was generally less than 10 ppm (parts per million), but one sample (1A) showed 170 ppm gold by atomic absorption analysis.

The old pits at the Silver King prospect are in moderately to strongly sulfidized hornfels. Stibnite occurs in thin veins in the southernmost main pit and is accompanied locally by gold (sample 7A, table 1). Other samples from the Silver King pits contained from 0.08 to 6.2 ppm gold in addition to markedly anomalous amounts of arsenic and copper; copper ranges from about 150 ppm to 2 percent (20,000 ppm).

The composite vein or lode exposed in Colorado Creek south of the stock consists of distinct hanging and footwall veins in a 10-foot-wide shear zone. The largest vein fissure in the lode, the one sampled, is about 4 feet wide; it contains stibnite and a small amount of gold (sample 9, table 1). A nearby vein (sample 10, table 1) assayed about 20 ppm gold, more than 10 percent antimony, and some lead and zinc.

Except in the canyon of Colorado Creek, exposures are very poor, and therefore it is possible that sulfidized rocks are much more widely distributed than The Golden Zone mine area was described by Capps (1919, p. 226-227), Ross (1933, p. 321-325), and Hawley, Clark, and Benfer (1968), and the mine was sampled by the U.S. Bureau of Mines in 1950-51 (Mulligan and others, 1967). Claims were staked in the area as early as 1912, but production prior to 1941 is unknown. A little gold and silver were produced in 1941-42, but the mine has been inactive since.

The deposit is in and near a breccia pipe in a quartz diorite porphyry. Vein deposits of the area, not discussed in this report, include the Mayflower, East, Little, and Lindfors. The Golden Zone itself is held by assessment on the BOB group of claims by Cemco, Inc., of Anchorage, Alaska, whose General Manager, H. R. Beckwith, was helpful in furnishing information on ore grade and other data.

The Golden Zone area is largely underlain by volcanic siltstone and conglomerate of Permian or Triassic age (Ross, 1933) that were invaded by biotite quartz diorite porphyry, probably in early Tertiary time. The Golden Zone breccia-pipe deposit, is approximately in the center of a small, very steep-walled porphyry stock (figs. 4 and 5). The deposit is in the brecciated and shattered material of the pipe and in adjacent parts of the quartz diorite porphyry. In approximate order of abundance, the metallic minerals of the deposit are arsenopyrite, pyrite, chalcopyrite, sphalerite, and galena. Trace amounts of molybdenum and tin occur in the mineralized rock (table 2), and cassiterite was identified from the deposit by Walter Gnagy of the U.S. Bureau of Mines (written commun., 1967). The strongly mineralized rock is nearly white to very pale brown and has been strongly sericitized, silicified, and carbonatized. The spectrographic analyses show anomalous amounts of boron, probably reflecting tourmaline which was identified in thin sections of the altered rock.

The breccia-pipe deposit is exposed at the surface and has been explored by two adits (fig. 4) and by numerous diamond-drill holes drilled from the 200-footlevel adit. One steep-angle hole (DH-1) l,025feetlong was drilled by the U.S. Bureau of Mines in 1950-51 (fig. 5, section B-B'). A long crosscut adit, the 500- foot level (fig. 4) was begun in about 1939 but was abandoned about 900 feet from the deposit. None of the workings are presently accessible, but one segment of the contact of the breccia pipe is well exposed at the surface, and the drill core from the deep Bureau of Mines test has been preserved. Breccia Pipe and Mineralized Rock

The Golden Zone breccia pipe is exposed on a small knob north of BrynMawr Creek, particularly in a shallow opencut (fig. 4). At the surface, the pipe is approximately circular in plan and is slightly more than 200 feet across. Drill holes and mine maps indicate that it enlarges slightly with depth. At an altitude of about 3,400 feet (fig. 5), the pipe appears to be about 200 feet long by slightly more than 250 feet across; at an altitude of 3,200 feet, the width, based on one projected and one drilled contact, is as much as 300 feet across. Judged by other breccia pipes described in the geologic literature, it is probably more complex than shown in figure 5. Two distinct zones of concentric fracturing probably are present in the pipe on the 200- foot level one zone coinciding with the marginal ore zone, discussed later, and a second, outer zone nearly coinciding with the curving drift driven southward from a point about 240 feet from the adit portal (fig. 4). The one deep drill hole in the pipe (section B-B', fig. 5) shows that the pipe continues to a depth of at least 850 feet, and almost certainly to a much greater depth.

Exposures in the opencut and mine maps of the 100- foot level (fig. 6) show a concentric veinlike breccia zone that encircles the western part of the pipe and is locally separated from it by slightly altered rocks. The rocks in this concentric zone are subrounded to angular blocks of porphyry that are strongly silicified and sericitized and are separated from other blocks by arsenopyrite-quartz-chalcopyrite-rich fillings. Some blocks appear to have been slightly rotated. The main part of the pipe, as exposed in the cut, has a sharp boundary, but is less fractured than the concentric breccia zone; its fragments only locally show evidence of rotation. The sulfide minerals in the main body of the pipe form fracture fillings and blobs in shattered, highly altered rocks.

Economic Significance of the Deposit

The ore grade of the Golden Zone breccia pipe varies and has proved to be difficult to determine by drilling (Mulligan and others, 1967). It seems probable, however, that a relatively high-grade part approximately coincides with a strongly fractured zone that bounds the pipe. This zone, called the marginal zone, encloses a core of weakly mineralized rock and in turn is enclosed in a halo of altered and weakly mineralized rock.

The marginal zone is exposed in an opencut at the surface and is fairly well defined on the 100-foot level by assay values (fig. 6). On the 100-foot level, the outer part of the marginal zone is a breccia of veinlike form that appears to be from 15 to 25 feet wide and to average, on the basis of channel samples cut by other geologists and engineers, 0.30 oz (ounce) gold

On and near the 200-foot level, numerous assays from channel samples or sludge from low-angle drill holes suggest an oval-shaped marginal zone, with maximum dimensions of about 120 by 240 feet, enclosed in a weakly mineralized halo about 370 by 500 feet across (fig. 7). Assays in the marginal zone range from about 0.06 oz gold per ton to more than 0.6 oz per ton and average about 0.21 oz per ton; less abundant assay data suggest a value of from $1 to $3 for other metals, principally silver and copper. The core area, although penetrated by some drill holes, is essentially of unknown value. The average assay value of 228 samples in the halo area is about 0.04 oz gold per ton. Although most samples in this zone contain 0.01 oz gold or less per ton, widely scattered samples or groups of samples contain more, thereby raising the average significantly. As noted above, the oval-shaped marginal ore zone on the 200-foot level is probably surrounded in part by an arcuate zone of strong fracturing, and groups of assays from this zone, particularly in the curving drift south and west 240 feet from the portal (fig. 7), are higher in value than the general average of the halo. This suggests in turn, that more detailed work in the halo may delineate zones distinctly richer than the average.

The only check on the grade of the deposit as estimated from channel samples or drill core and sludge is from mining in 1941-42, when the Golden Zone produced 869 tons of concentrates that contained 1,581 oz gold, 8,617 oz silver, and 42,659 Ibs copper (Mulligan and others, 1967). According to H. R. Beckwith of Cemco, Inc. (written commun., 1967), these concentrates came from about 1,730 tons of ore yielding an average grade, from one lot of 1,480 tons, of approximately 0.99 oz gold per ton, 4.5 oz silver per ton, and 1.4 percent copper; a second lot of 250 tons was of lower grade, but both lots were appreciably higher than the grade determined by channel sampling. It seems apparent from the location of the opencut and of raises on the 100- and 200-foot levels (figs. 6,

7) that this ore was mined largely from the marginal

zone. The tonnage figure seems reasonable for the short period of mining activity, the size of the mill, and the amount of tailings, and so the grade of the deposit may have been underestimated by conventional sampling.

Although the grade and subsurface dimensions of the deposit are imperfectly known, rough calculations based on assays from the 200-foot level indicate a likelihood of about 260,000 tons of material, containing possibly 0.14 oz gold per ton, per 100 feet of depth. The fairly abundant assay data from surface, underground workings, and drill holes down to about 400 feet also permit a rough calculation of the hypothetical value of metals in the upper part of the deposit. Calculations from these data show about $10,000,000 worth of metals in the upper 400 feet of the deposit, including the marginal zone and the appreciably lower grade core and halo. The single deep U.S. Bureau of Mines drill hole shows that the pipe extends to at least 850 feet in depth and is at least weakly mineralized below 400 feet, so that the $10,000,000 figure is minimal. Furthermore, if the value of metals has been underestimated by conventional sampling, as data on the 1941-42production suggest, the $10,000,000 value is too small. The deposit appears to warrant further study and exploration.

Particular attention should be given to the possibilities of vertical or horizontal zoning in the pipe. The possibility of vertical zoning is suggested principally by the complex metal assemblage in the Golden Zone and nearby deposits. Horizontal zoning in the deposit itself is apparently indicated by data from U.S. Bureau of Mines drill hole 1, which shows an inverse correlation of copper with gold at the eastern edge of the pipe, with copper enriched relative to gold just outside the pipe.

The Blind Creek area (fig. 8) is about 1 mile west of the Golden Zone and is mainly underlain by volcanic siltstone and conglomerate. It is cut by a major fault zone striking north-northeast which records a late period of strike-slip deformation by nearly horizontal slickensides. Locally the fault zone contains quartz veins and quartz-cemented breccia zones as much as 200 feet across. The quartz ranges from white and massive to a faintly opalescent type with a colloform structure; locally it is limonite stained or contains pyrite.

Assays of the quartz veins and associated pyritic material show very minute amounts of gold, although none was seen. Two samples were collected; sample BC-1 (table 2) was a composite grab sample of quartz and limonitic quartz collected from residual fragments in the soil over a distance of several hundred feet. It contains slightly more gold (0.06 ppm) than the detection limit (0.02 ppm). Sample BC-2 (table 2) was a short (2.0-foot chip) sample collected over a strongly pyritic part of the vein; it contains slightly more gold (0.30 ppm) but still not in amounts approaching commerical value. The samples also contain measurable amounts of silver, trace or greater amounts of arsenic, and one sample shows 150 ppm lead and a trace of zinc (<200 ppm).

Although the Blind Creek occurrence has no apparent economic value, it is reported because of the large size of the quartz veins and because the controlling Blind Creek fault zone apparently is a major premineral throughgoing structure. LONG CREEK AREA The Long Creek area (fig. 9) is south of the Golden Zone and appears to have a slightly different type of mineralization. Copper sulfides occur widely in small amounts and are concentrated near the Copper King prospect. The copper minerals occur in veins, in massive replacement of favorable strata, and disseminated in bleached areas in predominantly red volcanic siltstone and conglomerate.

The Copper King prospect, once known as the Hector, is on land restaked by Mark Ringstad, Fairbanks, in April 1967. The workings consist of several shallow trenches, all of which expose copper minerals. Massive chalcopyrite and pyrrhotite are exposed as a lensoid mass about 5 feet thick in trench No. 3 (sample location 2 and 3).

The trenches are all in hornfelsed locally calcareous siltstone, which is cut by small dikes and plugs of quartz porphyry. The quartz porphyry and adjacent hornfels are cut by siliceous veinlets on several fracture trends, and the siliceous veinlets contain very sparse amounts of molybdenite, chalcopyrite, and pyrite. The molybdenite is seemingly more abundant in the quartz porphyry than in the hornfels, and the pattern of mineralization suggests a zonal distribution of copper around the quartz porphyry masses.

Samples from the main trenches at the Copper King (Nos. 1-4, Long Creek area, table 2) contain from about 0.5 to 2 percent copper, 1 to 3 oz silver per ton, gold worth about $1 to $8 per ton, and detectable amounts of bismuth, zinc, and tin. Mineralized rock from the prospect is also represented by sample 6, a grab sample of sacked ore that is probably from the same trench as samples 2 and 3. Molybdenum, visibly

A«2/ [Analyses, except of gold, are semiquantitative spectrographic by D. J. Grimes, 0,15, 0.2, 0.3, 0.5, 0.7, 1.0, 1.5, and so on, or by the following symbols: looked for. Results are given in parts per million, except for Fe, Mg, Ca, A. L. Meier, R. L. Miller, and T. A. Roemer. Sample localities are shown in

N N N N N N

N N N N N N N N N N Cr Cu La 'N 300 70 200 70 200 70 70 50 300 70

N L N samples, upper Chulitna district

E. E. Martinez, and Arnold Farley, Jr., and are reported in the series 0.1, N, not detected; L, detected but below limit of determination; -, not Ti, which are given in percent. Gold analyses are by atomic absorption by figure 10]

N 150 150 N N N 100 200 N N 150 150 N N 150 150 N

N 150 150 N N 100 150 N N 150 150 N N 150 150 N N 100 100 N Y Zn

ACF 084

of determination Limits Lab. No. Field No. Ag

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