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Geological Map, McGrath A-2 Quadrangle
Alaska Division of Geological & Geophysical Surveys

... Mineralization in the McGrath A-2 Quadrangle consists of lead-zinc-silver-copper skarn replacement bodies in limestone; veins and stockwork that contain zinc, tungsten, tin, and silver in igneous rocks; cobalt- and nickel-bearing massive pyrrhotite replacement deposits; and anomalous lead, zinc, and silver in shale (SOsh). The assay data in table 4 are not derived from channel samples and hence do not necessarily show average metal values of mineral deposits in the area.

Base-metal skarns at Bowser Creek (table 4) are typical of lowtemperature, fracture-controlled lead-zinc skarns described by Einaudi and Burt (1982). Reed and Elliott (1968a,b) described the Bowser Creek occurrences and presented assay and prospect map information in addition to that reported here. The main Bowser Creek deposit (table 4, map no. 2) consists of replacement deposits of pyrrhotite, sphalcritc, galena, and chalcopyrite in a hedenbergite-johannsenite skarn next to a felsite dike. The main pyrrhotite-sphalcritc occurrences are in a Johannsenite-rich exoskarn in the marble front; however, important fissure-controlled, silver-rich, galenatetrahedrite- pyrrhotitc-calcite mineralization occurs in the marble 5 to 25 m from the marble front. The fissure deposits have been mined for silver, and modest amounts of ore were marketed before 1973. Some metasomatic fluids have replaced felsic dikes with anhedral garnet and pyroxene and produced a small area of endoskarn. Skarn deposits northeast of Bowser Creek (table 4, map no. 3) are apparently associated with quartz morizonite porphyry. A sphaleritc-pyrrhotite-johannsenitc skarn zone 3 to 10 m thick continues at distances of 14 to 40 m within the marble away from the intrusive contact. Assay data (table 4) for both deposits show positive correlation of silver with lead content.

Veins and stockwork in igneous rocks occur at the head of Bowser Creek and in the Post River pluton. The Bowser mineralization (table 4, map no. 4, fig. 2, and map inset) occurs as 3- to 20-cm-thick veins of pyrrhotite, sphalerite, and minor chalcopyrite in a felsic intrusion (Tqp) that is similar in composition to the felsic dike exposed in the skarn described above. Sulfide-bearing veins in the intrusion are localized along steeply dipping joints that trend N, 65-70∫ W. Skarn mineralogy is notably absent. Occurrences in the Post River pluton (table 4, map no. 6a,b) consist of pyrite scheelite-chatcopyrite-quartz-calcite veinlets in horofelsed roof pendants, in fracture zones in the intrusion, or in the contact zones between intrusions and country rock. The mineralized areas are poorly exposed and of unknown extent. A sample of mineralized vein found in the rubble yielded 1.1 percent copper, 0.089 percent tin, 98 ppm silver, and 0.028 percent tungsten. Scheelite occurs in pan concentrates from ravines that drain the south side of the intrusion.

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