Lead, zinc, and silver deposits at Bowser Creek, McGrath A-2 quadrangle, Alaska

Geological Survey

New occurrences of argentiferous galena-sphalerite and sphalerite-pyrrhotite were identified in the summer of 1967 during a field geologic evaluation of the southern Alaska Range as part of the U.S. Geological Survey's Heavy Metals program. The deposits occur in limestone as replacement bodies and fissure veins adjacent to granodiorite, igneous breccia, and felsite dikes, and in the breccia as fracture fillings. The deposits appear to be sporadic and discontinuous but locally are quite rich.

The Bowser Creek area is approximately 23 miles south of Farewell, a Federal Aviation Agency station that maintains a 5,000-foot gravel runway (fig. 1). McGrath, about 62 air miles northwest of Farewell, is the nearest source of gasoline and supplies. No roads or trails lead to the area. The deposits can best be reached by helicopter. Small, properly equipped aircraft can land on gravel bars along the Post River; the deposits can then be reached by walking westward about 3 miles up Bowser Creek (fig. 2).

Bowser Creek basin is in rugged terrain, with relief of about 3,000 feet. The valley floor is mantled by surficial deposits. Talus covers the lower part of the surrounding slopes. There is virtually no vegetative cover in the basin; the nearest trees are along the Post River.

Little geologic information is available about this remote rugged sector of Alaska, the only previous work being that of Brooks (1911) who traversed the Alaska

INTRODUCTION Range via the south fork of the Kuskokwim River in 1902. The present report is based on 2 days of examination and sampling. The area shown in figure 2 was mapped with the aid of a helicopter in about 5 hours, supplemented by aerial photointerpretation. The contacts and structural interpretations must be considered tentative; additional mapping of this and surrounding areas will be necessary before the geology and significance of the deposits can be fully understood.

The most conspicuous metamorphic effects, and those which appear to have the greatest potential economic significance, are skarn zones developed near felsite dikes and granodiorite porphyry. Ore minerals in the skarn are pyrrhotite and sphalerite (marmatite) with associated calc-silicate minerals,chieflyepidote and clinopyroxene. The skarn replaces silicated limestone. Generally within 10 feet of the sulfide-rich skarn bodies, the rock is altered to light-greenish-gray dense silicated marble composed of quartz, epidote, clinopyroxene, calcite, and chlorite.

The sedimentary strata have been deformed into folds which trend north-northeast and plunge to the south-southwest and north-northeast. The folds are locally overturned; axial planes are vertical or dip steeply to the east-southeast. Subsidiary asymmetric and disharmonic folds with wavelengths of a few inches to tens of feet were noted on the limbs of the larger folds. Faults, mapped from aerial photographs, are of two generations. The north-trending fault exposed south of Bowser Creek apparently accompanied the main period of folding. Cross faults of small apparent displacement (not shown in fig. 2) border the southern intrusive breccia.

Approximate scale Contour Interval 100 Feet The northeast-trending structural features are cut by intrusions of igneous breccia and associated felsite and subordinate mafic dikes. The southern breccia body shown near the center of figure 2 cuts the southwest end of an elongate locally overturned anticline. The northern breccia body cuts subparallel folds.

The mineral deposits are of two main types: (1) galena-sphalerite or sphalerite-phrrhotite deposits in limestone and (2) narrow pyrrhotite-sphalerite fracture fillings in igneous breccia. The deposits in limestone are in two areas, one southwest and the second northeast of Bowser Creek. The deposits in igneous breccia are near the headwaters of Bowser Creek. The locations of all three areas are indicated in figure 2. All deposits are closely associated with intrusive igneous bodies.

The galena-sphalerite or sphalerite-pyrrhotite deposits in limestone consist of replacement bodies and fissure veins. Sphalerite (marmatite) and pyrrhotite are the ore minerals in the replacement bodies; argentiferous galena, with or without other sulfide min-MINERAL DEPOSITS

Dashed where approximately located

Inclined Vertical Undulatory Strike and dip of beds

Number refers to tables 1 and 2 Limestone (partly covered)

Sample location erals (chiefly marmatite), characterizes the fissure veins. Pyrite and minor chalcopyrite are locally present. The replacement bodies range from a few inches to 10 feet in width. They are generally tabular or podlike and replace silicated limestone. Individual podlike bodies may consist of as much as 85 percent combined sphalerite and pyrrhotite. The long axes of the pods are generally parallel to the bedding of the limestone. Many bodies were observed to pinch out within a few feet.

Area southwest of Bowser Creek. Deposits of sphalerite-pyrrhotite and argentiferous galena are concentrated in an area on the southwest side of Bowser Creek (figs. 2, 3). Analyses for gold, silver, copper, lead, and zinc, and semiquantitative spectrographic analyses for the samples collected in this area are given in tables 1 and 2.

The largest replacement body examined (table 1, samples 3-5) has a vertical dip and an apparent width of 10 feet. Chip samples across this body indicate as much as 14.7 percent zinc and 6.1 ounces of silver per ton. Only the south side of the body is exposed; the contact is parallel to the limestone beds. Outcrops in talus downslope indicate a vertical extent of about 50 feet.

About 1,200 feet north of the limestone-breccia contact a group of tabular or veinlike bodies occurs in altered limestone and shows a close spatial relationship to felsite dikes (fig. 3). One vein (fig. 4), consisting of pyrite, galena, sphalerite (marmatite), and minor pyrrhotite inagangueofwhitemilky quartz, minor calcite, and unreplaced limestone fragments, follows the contact between limestone and a felsite dike. This vein is irregularly massive without marked banding and is oxidized to a dark chocolate brown. The vein pinches out upward into silicated limestone; the lower extension is covered by talus.

A few narrow discontinuous fissure veins consist of nearly solid argentiferous galena. These veins, however, pinch and swell irregularly and do not appear to have great continuity. The maximum width of massive galena in these veins is 10 inches, but most are between 2 and 6 inches across. A 20-foot chip sample (table 1, sample 10) across oxidized skarn (see fig. 3) containing narrow veins of galena assayed slightly more than 52 ounces of silver per ton. The walls of some of these veins are marked by brownish-yellow gouge. Preliminary examination of a polished section of sample 11 (see fig. 3; tables 1, 2), which contains 309.8 ounces of silver per ton, shows that approximately 85-90 percent of the sample is galena. Other primary minerals include sphalerite, pyrite, and minor amounts of tetrahedrite. Small grains of other minerals, perhaps members of the copper-arsenic sulfide group of sulfo-salts (Arthur Radtke, oral commun., 1968), also are present. Secondary minerals developed along galena cleavages include cerussite, hematite, and mimetite(?).

Area northeast of Bowser Creek. The deposits northeast of Bowser Creek (figs. 2, 5) show a close spatial association with the small granodiorite porphyry body or with felsite dikes. A description of the deposits examined and analyses for gold, silver, copper, lead, and zinc of samples collected from them are given in table 3.

\ Much of the area shown as limestone in figure 5 is covered by talus. However, mineralized skarn which appears to have potential economic significance locally crops out near the granodiorite porphyry and felsite dikes. The skarn typically consists of pyrrhotite and sphalerite (marmatite), associated with a variety of silicate minerals including epidote, clinopyroxene, quartz, and chlorite. The iron-bearing sulfide minerals weather to a deep reddish brown. There are scattered exposures of skarn in talus along the east side of the granodiorite porphyry. The skarn zones are as much as 25 feet across, and if they prove continuous beneath talus cover, then a north-trending skarn zone at least 1,000 feet long is indicated (fig. 5, samples 34-40).

Other deposits in this area occur along shear zones which border felsite dikes (table 3, samples 27-33). The sulfide minerals replace limestone, and locally the dikes are partly replaced. Most deposits are narrow and discontinuous and pinch out within a few feet (fig. 6). - Deposits in Breccia Small fractures filled with pyrrhotite and sphalerite occur in the northern igneous breccia body. Individual deposits appear to be small, but poor exposures prevent delineation of their true extent. The fracture fillings may represent only part of a more extensively mineralized breccia mass.

Area at head of Bowser Creek. Deposits in igneous breccia were examined only along the headwaters of Bowser Creek (fig. 7). Descriptions of the individual localities and analyses of samples from these localities are given in table 4 (samples 43-58). The deposits consist of small sulfide veins largely controlled by northwest-trending fractures in igneous breccia (fig. 7). Specks and clots of a sulfide mineral (pyrrhotite?) also are disseminated in the breccia. The veins have steep to vertical dips and strike N. 40°-60° W. Significant replacement of the breccia does not appear to have occurred, because in many places vein walls match. None of the veins exceed 2 feet in width, and some veins end abruptly. Surficial deposits prevent tracing the veins for more than 10 feet. The veins consist of pyrrhotite with minor sphalerite (marmatite), chalcopyrite, and a trace of galena. One 4- to 6-inch vein (table 4, sample 54) contains 85 percent of sphalerite. Although the pyrrhotite has little current economic value, further exploration in this area may find significant concentrations of sphalerite. Other Deposits The locations of other scattered occurrences north and west of Bowser Creek are shown in figure 2. Semiquantitative~spectrographic analyses of samples from these localities and a brief description of the samples are given in table 4 (samples 59-64). Of particular interest are samples 63 and 64 from the ridge west of Bowser Creek. Although the outcrops are small, and deposits are consequently of unknown extent, they do suggest that lead-zinc-silver mineralization extends for at least one-half mile southwest of the area shown in figure 3.

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