STATE OF ALASKA William A. Egan - Governor DEPARTMENT OF NATURAL RESOURCES Phil R. Holdsworth - Commissioner DIVISION OF MINES AND MINERALS James A. Williams, - Director GEOLOGIC REPORT NO. 19 Geology of Part of the Amphitheatre Mountains, Mt. Hayes Quadrangle, Alaska BY Arthur W. Rose Juneau, Alaska February, 1966 TABLE OF CONTENTS Page ABSTRACT INTRODUCTION Amphitheatre formation Basalt and Andesite Silicic tuff and tuffaceous sediments Diabase and tuffaceous sediments Andes' te Agglomerate Interrelations of units in the Amnhitheatre formation Gabbra and nafic gabbro Granite Peridotite and dunite STRUCTURE ECONOMIC GEOLOGY GEOCHEMISTRY SUGGESTIONS FOR PROSPECTING BIBLIOGRAPHY ILLUSTRATIONS Table 1 Table 2 Table 3 Figure 1 Figure 2 GEOLOeY OF PART OF THE AMPHXTHWTRE MOUN'JSINS MT , HAYES QUmWGLE, ALASKA by Arthur W. Rose Reconnaissance geologic mapping in the Amphitheatre Mountains north of the Denali Highway shows that basalt and arradesite flowso silicis tuffs %nd tuffaceous sediments, and andesite agglomerate of the ~riassic(?) Amphi- theatre farmation are intruded by gabbxo, granite, and peridotite, Some of the gabbrs appears to occur as a thick layered sB,lP or lopo2itht other gabbrs and dizbase occur as sills in the tuffs and tuffaceous sediments, In general the sediments and sn1B dip gently northward, A layer of mafic gabbro about 150 feer khick eontams about 22% iron in magnetite and ilmenite, A magnetic concentrate contained 47% Iron and E109% TiOZo Stream sediment sampling detected several copper anomalies whish deserve follow-up, Geochemical analyses of stream sediments collected in 1964 showed weak to moderate anomalies for copper, lead, zinc, and molybdenum in part of the Amphltheatre Mountains just north of %he Denali Highway and about 10 miles ;ves%of the Richardson Highway a% Paxson (Rose and Samnders, 1965). In 2965, seven days were spent in the Amphitheatre lbuntains collecting addltaonal geochemical samples in the vneinity of the anomalies and mapping geology, One day was spent in the area near Landmark Gap (figure 2)@ in the M%, Hayes A-5 quadrangle ]us% west of the area shown on figure 1, Consi- derable snow was present on the north side of the range, so the geologic map is still ineomp9etee Except fo~a few sharp valleys and steep slopes, outcrops are not common in the area, especially in the higher partso and much of the mapping is based on frost-heaved blockse ROCK UNITS Amphitheatre Formation Basalt and Andesite The basalt and andes~teunft consists mainly of massive fine-grained dark green 'basalt, locally containing plagioclase phenocrysts, Minor 3rncunts sf medium gray-green porphyri-tlc andesite with a fine-grained to -1phanitie groundmass is also present- Individual flows can rarely be secognized, AmygduPes of chlorite and other minerals are sparsely developed in some zones, This is the same rock as described under the heading "%rnm-i;hieheatrebasa%tl<n the Paxson area (Rose and Saunders, 1965, po 4) SiLEsie tuff and tuf faceous sediments The tuff and tuffaceous sediments are typically hard, siliceouso and light gray to buff, Locallyo bedding ~s observed in units 118 to 1 inch thick defined by changes in grain size and composition, The grain size ranges from fine to coarse sand, with a silica cement that is difficult to distinguish from the grains, I?iineralspresent include quartz, feldspar, and chlorite, Calcite cement ns present in some 10calitles~ A pyroclast~c origin r'or at: Peasc part of these rocks is inferred from the ha.rd siliceous matrix plus the assseiatlsn with volcanic rocks, Sn the area near Landmark GapJ tuffs like those described above are interbedded with a thick sequence of coarse-grained massive andesitic or dacitic tuff, One limestone bed about %O inches thick was found an the tuffs of this area, Diabase and tuffaceous sediments The area just south of Sugarloaf Mountain is composed of thin zones of siliceous to andesitic tuff and tuffaceous sediments, mostly well-bedded, separated by more extensive masses of diabase which are believed to be sills, In addition to tuffaceous sediments filce those described above, subordinate mounts of andesite or basalt tuffe black siPiceous argillite, black slaty pyritlferous shale, and thin recrystallized limestone are present, Fine pyrite is present in most of the tuffs and tuffaceous sediments, but is especially zbundant in one black shale unit. Diabase appears to fozm about 75% of the unit, It is fine - to medium- grained, with skeletal pyroxene grains up to several millimeters in size loca91y present, In all cases where a contact could be found, the diabase was conformable with the tuffaceous sediments and appeared to be a sill, However, some diabase may have originated as thick flows or dikes. A thin section of a typical specimen indicates an original composition of about 35:, labradorite and 65% augite, plus traces of magnetite-ilmenite and pyxare, Partial alteration of augite to actinolite has oceusrred, The subhedral feldspars are partly enclosed in coarser skeaetsl augite, r~ndcsiteagglomerate The mountain about 2 mlles southeast of Sugarloaf Ivlountain (location of triangulation station angle) consists largely of agglomerate and some Luff containing rounded to subangular fragments of andesite up to several inches in size, The andesite of the fragments typically contains sparse plagioclase phenocrysts set in an aphanitie green groundmass- The matrix of the agglomerate appears to be finer-grained material of the same compos~tion,and may include some devitrified glass, along with small fragments of andesite altered to chlorxte, epidote, calcite, and other minerals, Inter-relations of units in the Amphitheatre formation The massive basalt at the eastern end of the map area is continuous .~:r"rl=the thick (more than 2000 feet) section of basalt exposed on Paxson -,-cuntainjust to the southeast (Rose and Saunders, 1965) Basalt on Sugarloaf Mountain appears lithologically identical. The latter basalt grades downward through several hundred feet of andesitic tuffs and flows into the tuffaceous sediment and diabase sequence, The existence of the fault east of the tuff-dlabase sequence 1s questionable, but no tuffs or Cuffaceous sediments were found to the east of this fault, l4ost exposures near geochen samples 41 and 42 are andesite agglomerate and few exposures are basalt or diabase, The fault is shown as the most likely alternative, but it is possible that the difference in rock type results from facies changes near a volcanic vent. The relations of the basalt north of the sgglomerate are not clear. Tuff and gabbxo south of the agglomerate appear to underlie the agglomerate, and may correlate with similar tuffs in the central part of the map area, Alternatavely, this tuff and gabbro may correlate wlth the tuff-diabase unit to the west, with a facies change instead of a fault between the agglomerate and the upper part of the tuff-diabase unit, The questionable andesite agglomerate east of the main body of agglomerate is underlain by black siliceous argillite, and this in turn by gabbro and buff to gray siliceous tuff, These relations further suggest that the siliceous tuffs and gabbros of the central part of the area may underlie the agglomerate, but detailed correlation of units was not possible. About 40 miles to the west In the drainage of Clearwater Creek, IvIoffit (1912) describes Triassic limestone associated wlth "banded slates, black slates, red-weathering slates or shales, graywackes or fine tuffs, tuffa- ceous conglomerates and diabase flows or intrusions" which overlie basalt or greenstone. The tuffs, tuffaceous sediments, limestone, and basalt of the map area are tentatively considered Triassic on the basis of similar~ty to tae rocks 3escribed by Moffit, and other relations previously described by Rase and Saunders (19651 , However, a Permf an aqe f s also possible fo~some of the rocks, and the diabase sills probably correlate with the gabb~odescribed below, Gabbro and mafic gabbro Gabbro and mafic-rich gabbro form a barge proportion of the central and eastern part of the map area, The gabb~sva~ies noticeably in grain size, composition, and texture, The major past. of the gabbro is medium-grained &naboak1-2 m) and contains augite (dialEage) and plagioclase (mostly An 55-65, Angs in one sample) wit31 accessory magnetite-ilrnenite, Minor alteration of plagioelase to saussusite and augite to actinolite is common, Some zones show vesy discinek fo%fation defined by preferred orien$ation of plagioclase and slight cmpositEsna$ layering, The observed foliation has shallow ta modexate daps to rhe north and south, W zone in the central pazt of the gaEbro complex, show3 as mafie gabbro, is considerably darker in color and finer grained [k/2 - % m], At least part of this zone consists sf norite (orthopyroxene and plagis@9ase, Table E, sample 5~24l.A)and local areas approach pyroxenite in composition, Some gabbro with both elinapyroxene and ~r~hopysoxeneis also present, One zone (dascusaed further under economic geology) contains between 25 amel 30% magnetite. The foliation and the compositianal variability suggest that at least pant of the gabbro complex is a differentiated sill or lopoli%h, The gabbro in %he north-western part of the compBex almost certainly occurs as thick sills separated by tuff units, However, the trend of some contacts, and the lack of persistence 0% some distinctive Sithologies suggests that multiple intrusion, faulting, and other processes have operated in addition to layering from crystal settling ax other