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DIVISION of MINES and MINERALS GEOLOGIC REPORT NO. 1 Juneau

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DIVISION of MINES and MINERALS GEOLOGIC REPORT NO. 1 Juneau STATE OF ALASKA William A, Egan - Governor DEPARTMENT OF NATURAfr RESOURCES phi1 R. HoPdswortk - Commissioner DIVISION OF MINES AND MINERALS James A. Williams - Director GEOLOGIC REPORT NO. 1 Preliminary Report on Geologic Mapping In The Coast Range Mineral Belt, Alaska BY G. Herreid Juneau, Alaska December 1962 NOTE OF EXPWATION: This geologic report was originally published in the Annual Report of the Division of Mines and Minerals for the Year 1962. The Annual Reports cover the status and progress of the mineral industries in Alaska, activities of the Division, etc., in addition to various geologic and other reports by members of the Division. The Annual Report for 1962 is now out of print, but the requests for Geologic Report No. 1 continue. To fill. the demand, we have adopted the expedient of "lifting" it from the Annual Report fox multilithing "as isv without retyping or redrafting except for changed page numbers. The following pages are the result. Since it is of interest and in the same axear we include a geochemi.ca1 report on Tracy and Endicott Arms by William H. Race, Mining Engineer of the Division. It was published in the same Annual Report . James A. Williams May 25, 1965 TABLE QF CONT'ENTS ANS) ILLUSTRATIONS Preliminary Report on Geoloqic Mzping in the Coast--." Range Mineral Belt, Alaska: Page Introduction Previous Investigations Scope of This Report Endico.t;t Arm-Tracy Arm Area General. Statement Rock Tmes Fold Structures Quartz Veins Structural Synthesis Tectonic Analysis Copper Zinc Prospect at Point Astley Portland. Group Prospect Neglected Prize Prospect Taku Inlet Alaslca Juneau Mine Genera.1 Summary References Cited Map 9 Geologic Map of Part of the NE Shore of Endicott and Tracy Arms Map 2 Point Astley Zn-Cu Prospect Map 3 Por.t.1and Group Map 4 Geologic Map of Part of Northwest Shore of Taku Inl.et Figure 1 Cross Joint Kink Band Figures 2, 3, 4, and 5 Equal Area Steragraphic plots of Structural Data Preliminary Geochemical Investigation - Tracy and Endicott Arm Area: Introduction Mines and Prospects in the Area Physi.ca1 Features Method 0bserva.tions Conclusions Explanation of Appendix Table 1 Soil Samples Map 5 May, of GeoehemicaL Investigation Tracy-Endicstt Arms Map 6 ~eologicMap of Jingle-Jangle Deposit PreJ i minary Report on Gnologic Mapping in tJhe Coast Range Mineral Belt by Gordon Iierreid , Mining Geologist Introduction Sou.t,lieast Alaska is an area of great geol.ogical regul.ari.l;y because of the s-t;rong con-trol by 1W trending geosynclines of rock types, folds and ore deposits. This regularity is pa.rticul.ar1.y we1.l shown along .the SW ma.rgin of t,he Coast Range batholith. Here, deformation and met;amorphisrn of NW trending belts have taken place at great depths to produce an elongated belt of metamorphic rock whose style of structure shows little change alorig the belt, but regular changes across the belt. 'This marginal bel-t; i:; the site of numerous ore showings and a few profitable mines. The elongatj-on of the mineral province pa.rallel -to th.e structural features indicates a close relation between the regional. geology and ore deposition. It seems probable that unexploited mineral. deposits of ec0nomi.c grade remain to be found along this narrow belt, and that these deposits are rela.ted to the structural and other geologic features of' the region. It is the object of the DMscM program in -t,liis area to study the tleposits, Lhc! regional. geology, and their interrelations. The classical explanati-on for the close spatial relationship of the ore de- posits of the Coast Range mi-neral belt (Juneau gold belt and extensions NW and SE) is that the ore is related in some way to the ranitic int,rusions in or along the Coast Range batholith. Recently, Forbes (19597 has shown that the Coast Range mineral belt at Juneau l.ies in a belt of progressive metamorphism and that the SW contac-1; of the Coas-t Range batholith is gradational wit,h these metamorphic rocks. Forbes has suggested (personal commu.nication t,o the author) that the origin of the ore deposits in the area may be related to the development of the metamorphic belt itself'. At the present time tJhe ].ow grade go:Ld. deposits mined formerly are no longer prof'itable and the main ore 1,argets along the belt are base metal sulfide deposits which have long been known in the area. Previous Investigations The founda-Lion, and still the most complete report on the geology of the Juneau gold belt, is A.C. Spencer's "The Juneau Gold Belt, Alaska," U.S.G.S. Bull. 2g7, published in 1906. Here, Spencer described with admirable simplicity the metamorphic and igneous rock:.; and t,lle mineral deposits.. His somewhat generalized geologic map of the Juneau area is t~lleonly one published to date. Spencer de- scribed the rock succession SW across the belt as diorite, a band of crystalline schist, and a band composed ma.i.nly of alt,ernating slates and greenstones. He recognized that tilting and developmr-,nt; of' the :;chiStosi'ty took place before in- trusion of thk diorit,e. He noted that "wherever observed" diorite contacts, in both the main mass and outlying "dli.liesu, are nearly always parallel to the ffstrikeu of the enclosing rocks. He recognized no major :Colding, but suggested that it might be present;. Concerning the ore deposit:;, he notes sfthe linear distribution of the mines of this part of' southeastern Alaska has been recognized for many years. He subdivided the ore clepos-i-L:; .into veins, sulfide disseminations ( lf im- pregnated masses of rockf'), and gradations between the two. The disseminated de- posits carry I.ittle gold, and Spencer devoted li1;tl.e attention to them for this reason. The veins ffquartz,, calcite, one or both", carry -varying am0un.t~of sulfi-- des along wi.th gold and silver, are di.sconl;:inuous; and i.n many areas are aggregated along certain I.ines "so that the cornbi.nation of many veins, each unimportant in itself, is traceabl-e with considerab1.e distinctness for long distances". These he termed "stringer leadsM. The mines on Gold and Sheep Creeks in the Juneau area are on extensive vein complexes of this nature and similar complexes "may be ob- served .....in the Windham Bay and Sumdum districtsf1. Spencer bel-ieved that the aniphibolite bodies and layers which occur in the slate i.n the Go1.d Creek area are intrusive "gabbrosYfwhich have been metamorphosed. These were termed 'metagabbros' by later writers. The:;e metagabbros are an impor- tant ore control: "one finds typi.ca1. skringer Leads composed of numerous nearly para]-lel vei-nlets occupyingirregular opening:; in the slates adjacent to the con- tactn or qfgash-li.keffstri.ngerleads may :form fin 1,he mctagabbro near the contact. Spencer believed that the vein openings were probably due to 1-ater vertical move- ments during uplift long after the period of' folding. necause diorite is the country rock of gold deposits at Rerners Ray and at the Treadwell deposits,.and because the fissure veins must have been emp:l.aced in solid rocks? Spencer places the da.te of mineralization as 1-ong aftel- emplacement of 1;he diorlte. He believed that the veins were deposited from hot aqueous solutions given off during crystal- lization of a buried body of magma. Spencer's ideas on the regional geology and ore deposits have been accepted with slight modifications and additions by all later workers who published reports on the area until. 1959 when Forbes' work appeared. Buddington and Chapin (1929) mapped the regional geology of Sfi; Alaska on a 1:500,000 scale and summetl up much of the early work. These authors recogni-zed that the rocks in the Juneau area arid all. along -Idhe SW margin of the Coast Range batholith have been isoclinally overturned tJo the SW. In general, they did not attempt t,o work out the details of the structure, but; where they did, they report- ed that minor folds were f'themselves acutely folded and in places pitch almost vertically". They also no.ted the increase i.n intensity of metamorphism from west to east as the batholith is approached. Livingston Wernecke, consulting geologist for the Alaska Juneau Gold Mining Company, described (1932) some of the outstanding features of the north and south ore bodies and presented a geologic map of the north ore body. Mixed'in with an account of the formation of the deposit is much descriptive information on the ore and wall rocks. He accepted Spencer's intrusive origin for the metagabbros which he believed. are surrounderl by contact metamorphic aureoles of spotted schist. He believed that the metamorphism has obliterated tlie evidence of folding in the wall rocks and that the ore was derived from a cooling magma at depth long after the period of metamorphism. Recently, Wayland (1960) has published a paper on the Alaska Juneau mine based on thesis work done in 1937. This paper provide:; a good summary of the mineralogic and structural features of the deposit. Wayland mentions folding of the rocks and considered that "fol-ding of some early quartz veins in phyllites is due to select- ive replacement by quarts of shear-folded bedsTf. He notes that the "longer dimen-. sions of a group oft uartz stringers seems a:? a rule to follow the regional plunge of major structures ? apparently metagabbro intrusives -GH) to the southeastff. Wayland follows Wernecke in describing the metagabbro as intrusive because of its "occa~iionalbranchi.rigff but notes that the spot,s o:C the spotted schists are augen formed during shearing.
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