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U.S. Geological Survey Circular 559
Lead, Zinc, and Silver Deposits at Bowser Creek McGrath A-2 Quadrangle

By Reed & Elliott

... 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 across oxidized skarn 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, 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-ealts (Arthur Radtke, oral commun., 1968), also are present. Secondary minerals developed along galena cleavages include cerussite, hematite, and mimetite.

Bowser Creek Prospect
By W. M. Sharp, Consulting Geologist

...The most conspicuous and economically-significant alteration comprises the development of calc-silicate or skarn zones within the general zone of felsite-porphyry , intrusion. Typical skarn minerals are epidote, clinopyroxene, garnet, quartz, and calcite. The characteristically higher grade mineralization occurring in these aggregates is the result of partial to massive replacements by Fe-Zn-Pb-Cu sulphides with, locally, important amounts of silver.

...All of the Bowser Creek showings appear to be closely associated with intrusive igneous rocks. However, variations of mineralogy or metal content within individual zones suggest that metal-zoning operates at least locally. In this regard the higher temperature (Fe-Zn-Cu) assemblages would appear t o generally relate to zones of intrusive breccia, while those with lower-temperature (Ag-Pb) characteristics would appear to favour relatively unaltered limy host rocks; also, galena ( with silver ) appears to have been introduced during the later stages of mineralization. It occurs as massive seams in several showings and in quartz-calcite veinlets in the 3-1 zone.

Alaska Division of Mines & Geology Property Examination Report #74-2
By Robert Anderson, Mining Engineer

...Mineralization In this section occurs in veins which are associated with felsite dikes and which lie outside the area of metamorphism caused by the intrusions. The dikes follow the bedding trend of N 15' E and have altered the limestone in contact with the dikes to chert. Chert grades into a cherty limestone within 5 to 10 feet and except i n the "bowl" part of the structure the chert i s barren of mineralization. Mineralization occurs close to , or in contact with, the cherty limestone, probably on both sides of the felsite dike. As shown in the cross section, the most promising mineralized area of the southwest side of Bowser Creek is the small synclinal structure, forming a "bowl" at lower elevations just above the lower talus slope. The structure changes to a simple monocline within 150 feet above the talus slope.

No attempt has been made yet to expose the dike and related beds downstream on the left side of the "bowl" where beds are covered by talus. I would expect that mineralization there would be similar to that above the "bowl" concentration of sulfide minerals. Sulfide mineral concentrations in the veins vary from 2% sulfides up to nearly solid sulfide concentrations. The changes where observed are very abrupt. Regardless of the concentrations the r a t i o of sulfide minerals remained in fairly constant relationship.

Approximate ratio of the major sulfide minerals observed were:
Galena 60
Sphalerlte 30
Pyrrhotite 10

Roughly the same mineral relationships can be seen in the top trench at about 5000 feet elevation and the bottom trench at about 4200 feet elevation, indicating no major zoning trend with depth in this area.

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