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The Upper chditna district, on the southern flank of the west-central Alaska Range, is the northern part of the ChulitnaYentna Mineral belt. I t is underlain mainly by sedimentary and volcanic rocks of pre-Permian to Paleocene ('7) age. Weakly metamorphosed pre-Permian rocks crop out along the southeast flank of the district and are succeeded stratigraphically-with unconformity-by nonmetamorphosed red beds, volcanic, and volcaniclastic rocks of Permian(?) and Early Triassic age. Limestone, calcareous siltstone, and argillite of Middle (?) and Late Triassic age, pillow basalt, limestone and questionably assigned to the Late Triassic, and dark argillaceous and arenaceovs rocks of Jurassic(?), Cretaceous, and Paleocene(?) age successively overlie the red-bed-rich sequence. Coal-bearing rocks of Oligocene age locally overlie the older strata, and glacial and fluviatile deposits cover large parts of the district. The sedimentary and volcanic sequence of Paleocene (?I and older age is cut by small stocks and dikes of hypabyssal intermediate to silicic rocks and by elongate masses of ultramafic rock, gabbro, and basalt. The intrusion of the igneous rocks followed folding and was contemporaneous with, or later than, major faulting of the Paleocene(?) and older rocks, but preceded deposition of the Oligocene unit and minor warping and faulting. Rock units, fold axes, and faults have a common northeasterly strike, giving the district a pronounced structural grain, also seen in the distribution of epigenetic mineral deposits and of small syngenetic deposits in serpentinite. The major epigenetic deposits are the combined vein-disseminated type, as a t Canyon and Partin Creeks, and .-le breccia pipe deposit at the Golden Zone mine; the minor types are the veins, lode zones, and massive sulfide bodies. Arsenopyrite characterizes the epigenetic deposits, and chalcopyrite; ~ ~ r r h o t i t e and pyrite are locally abundant. In detail the mineralogy of the deposits is complex. Minerals such as cassiterite, stibnite, and bismuthinite are known at one or more deposits or occurrences; metallic components of these minerals, such as arsenic, copper, and gold, characterize the lode deposits of the district and are locally found in anomalous concentration in stream-sediment samples. Relative abundance of copper, arsenic, and gold in different parts of the Golden Zone deposits suggests zoning on an ore body scale, and limited data from rock and stream-sediment samples of the district are consistent with zonal distribution of metals on a large scale. Alteration near mineral occurrences and deposits includes argillization of siltstones, propylitization of basaltic rocks, and quartz carbonate alteration of porphyries. Comparison of quantitative analyses of altered and unaltered rocks at the Golden suggest the of hydrogen, carbon dioxide, arsenic, and sulfur during alteration.

Geology and mineral deposits of the Upper Chulitna District, Alaska