STATE OF CALIFORNIA EDMUND G. BROWN, Governor DEPARTMENT OF NATURAL RESOURCES DeWITT NELSON, Director DIVISION OF MINES FERRY BUILDING. SAN FRANCISCO 11 IAN CAMPBELL, Chief SAN FRANCISCO SPECIAL REPORT 61 1959 GEOLOGICAL SECTION AND PETROGRAPHY ALONG THE POE TUNNEL BUTTE COUNTY, CALIFORNIA By PHILIP A. LYDON Price 50^^ GEOLOGIC SECTION AND PETROGRAPHY ALONG THE POE TUNNEL, BUTTE COUNTY, CALIFORNIA By Philip A. Lydon * OUTLINE OF REPORT Page Abstract 3 Introduction 4 Regional geology 4 Geology in the vicinity of the Poe Tunnel 5 Petrography 5 Serpentine 5 Undifferentiated metamorphic rocks 7 Schistose rocks 7 Massive rocks 9 Blastoporphyritie amphibolite 12 Limestone 12 Metagabbro 13 Granodiorite 13 Contact relations lo Metamorphism 16 Geological factors in tunneling 17 References 17 Illustrations Page Plate 1. Geological map of, and cross section along, the Poe Tunnel, Butte County In pocket Figure 1. Index map showing location of the Poe Tunnel area 3 2. Map showing rivers, streams, and principal roads near Poe Tunnel 4 3. Diagram showing mineral variation, north end Poe Tunnel 14 4. Graph showing effect of geology on rate of tunneling 17 Photo 1. Photomicrograph of serpentine 6 2. Photomicrograph of altered serpentine . 6 3. Photomicrograph of exsolved magnetite in ser- pentine 7 4. Photomicrograph of segregated layering in biotite- hornblende schist 7 5. Photomicrograph of biotite-hornblende schist 8 6. Photomicrograph of poorly foliated biotite-horn- blende schist 8 7. Photomicrograph of schist from near granodiorite contact 8 8. Photomicrograph of graphite schist 9 9. Photomicrograph of pelitic schist 10 10. Photomicrograph of undeformed amphibolite derived from igneous rock 11 11. Photomicrograph of amphibolite derived from cal- careous sedimentary rock 12 12. Photomicrograph of blastoporphyritie amphibolite 12 13. Photomicrograph of granodiorite 15 14. Photomicrograph of granodiorite 15 15. Photomicrograph of diorite dike 15 ABSTRACT The Poe Tunnel, situated on the Feather River north of Oroville, is part of a Pacific Gas and Electric Company water-power project. It is 6.4 miles long and has an aver- age unlined diameter of 24 feet. About 60 percent of its length is in serpentine, 35 percent in undifferentiated metamorphic rocks, and 5 percent in metagabbro. Grano- diorite, limestone, and blastoporphyritie amphibolite each account for less than 1 percent of the tunnel length. The undifferentiated metamorphic rocks are biotite- hornblende schist, greenschist, high-grade pelitic schist, impure quartzite, and amphibolites derived from sedi- mentary and basic igneous rocks. • Mining geologist, California Division of Mines. Serpentines here consist principally of antigorite, ser- pophite, and magnetite ; olivine occurs locally. The dif- ferent schists contain varying proportions of plagioclase, biotite, chlorite, hornblende, quartz, and calcite. Small idioblastic crystals of garnet were observed in one speci- men. Amphibolites derived from basic igneous rocks are composed chiefly of approximately equal amounts of plagioclase and hornblende, whereas those derived from sedimentary rocks contain plagioclase-ealcite-diopside and plagioclase-biotite. Mineralogical variations in rocks near the contact of granodiorite with biotite schist suggest that the grano- diorite might have been derived from a dioritic magma by a process involving differentiation of plagioclase and hornblende and introduction of silica. Metamorphic rocks along the Poe Tunnel were produced by low-grade regional metamorphism, modified locally to a relatively slight degree by later igneous intrusion and hydrother- mal alteration. Geological factors such as closely spaced joints, shear zones, and relatively heavy flows of water served to slow the rate of tunnel advance. Gross physical features within a rock type, however, generally exerted more in- fluence on the speed of tunneling and amount of support required than did the change from one rock type to another. SAN FRANCISCO Figure 1. Index map showing location of Poe Tunnel area, in northeastern Butte County. (3) California Division of Mines [Special Report 61 INTRODUCTION The Poe Tunnel lies in the northern Sierra Nevada in the western part of the 30-minute Bidwell Bar quad- rangle (figure 1). Its intake may be reached by traveling 29 miles north from Oroville along highway U. S. 40 Al- ternate. The topography of this area is characterized by deep, steep-sided canyons cut by the Feather River and its numerous tributaries. Big Bar Mountain, the highest point in the immediate vicinity of the tunnel, has an elevation of 4,419 feet, whereas the nearby town of Pulga, on the Feather River, has an elevation of only 380 feet. A heavy growth of trees and brush obscures surface geology except along road and railway cuts. The tunnel is part of the Poe Project, a Pacific Gas and Electric Company water-power project with a capac- ity of 106,000 kilowatts. This project also includes a concrete diversion dam 60 feet high near the tunnel in- take and a powerhouse with two Francis turbine gener- ators at the outlet. The tunnel intake is situated north of Mill Creek about 1 mile northea.st of Pulga, a station on the Western Pacific Railroad, and its outlet is about 2 miles northwest of the settlement of Big Bend. The tunnel is 6.4 miles long and has an average diameter of 24 feet, unlined, and 19 feet, lined. It has a loss of eleva- tion of about 120 feet from the intake to the top of a INTAKE Pulga \ # 7/ /-^ A Big Bar Mtn Lookout ALTERNATE J) \V Adi t No.l /f!^^y=^ j\ 1 1 iu / o '■■\ 4 I Adit No. 2 Feo' \\e r Scale in miles Figure 2. Map showing rivers, streams, and principal roads near the Poe Tunnel. Adapted from California Division of Forestry planimetric map of Butte County, revised 19.55. vertical shaft near the outlet; this shaft drops an ad- ditional 140 feet to the penstock tunnel. The tunnel trends north-northeast, thereby penetrating most re- gional and local structural features at almost right angles. According to periodic releases by the PG&E News Bureau (October 15, 1954; April 1, 1955; December 12, 1955), work on the project began when contracts for two bridges across the Feather River were awarded in Sep- tember 1954. The contract for the tunnel itself was awarded on April 1, 1955, to Utah Construction Com- pany and Bates and Rogers Construction Company. Adit No. 1 was driven to provide a working entrance approxi- mately midway along the route of the tunnel, and a second entrance (Adit No. 2) was driven about 3500 feet northeast of the outlet. Faces were advanced upstream and downstream from the two adits, and simultaneously from the intake and outlet, so that six working faces were in use. By the end of 1955, more than a mile of tunnel had been driven in this fashion. Tunneling was completed when the upstream heading of Adit No. 1 and the intake heading joined in February 1957, a month after headings from Adits No. 1 and 2 had joined. Ap- proximately 90 percent of the tunnel length was lined with concrete. The total cost of the project has been estimated at $37,900,000 (Nelson, 1957, p. 56). Geological mapping of the subsurface and portions of the surface was carried on intermittently from December 1956 to March 1957. Elmer Hall, Feather River develop- ment project engineer, and Roy Friedrichs, project office engineer, both of Pacific Gas and Electric Company, were helpful in providing tunnel maps, construction data, access to the tunnel, and other information and courtesies. H. Duane Woods of the California Depart- ment of Water Resources compiled data from project records that were used in graphing the rate of tunneling in figure 4. Previous geological studies in and near this area were made by H. W. Turner (1898), R. R. Compton (1950, 1954, 1955), and Anna M. Hietanen (1951). REGIONAL GEOLOGY The regional geology, as outlined by H. W. Turner, consists of a tightly folded, northwest-trending bedrock series that has been intruded by several masses of gran- itic rock. Schistose rocks are "wrapped around" each of the intrusive masses, so that the trend of bedding and folia often differs from the regional trend, ranging about all points of the compass. Turner attributed this feature to forcing aside of the schistose rocks by intru- sion of the granitic masses, although Compton (1954) regarded the emplacement of at least one such granitic body as concordant intrusion modified by assimilation. Metamorphic rocks of the bedrock series consist of inter- calated metasedimentary and metavolcanic rocks. Sedi- mentary rocks are represented by mica schist, quartzite, and limestone, and volcanic rocks include metamor- phosed basalt, rhyolite, and agglomerate. Rock types recognized by Turner (1898) are varied metasediments of the Calaveras group, slates of the Cedar formation, granitic rocks, and serpentine and amphibolite of un- certain age. No fossil or other direct evidence, however, has been presented to confirm the age of isolated rock masses mapped by Hietanen in the tunnel area that are 1959] PoE Tunnel, Butte County lithologically similar to rocks of the Calaveras forma- tion. They have previously been identified as equivalent to the Calaveras on the basis of lithologie similarity alone. A succession of Tertiary gravel, basalt, and andesite, remnants of which are still preserved on high ridges and hills, was deposited unconformably on the bedrock series. Rocks of Pleistocene age include scattered rem- nants of a coarse-grained basalt extruded locally during very early Pleistocene ( ?), glacial drift and moraines, lake sediments, and river bench gravels. GEOLOGY IN THE VICINITY OF THE POE TUNNEL According to Turner's map of the Bidwell Bar quad- rangle, the Poe Tunnel would be expected to lie almost entirely within serpentine and massive and schistose amphibolite. Lenses of rocks of the Calaveras group are indicated a mile west and east of the south end of the tunnel, and immediately west of Pulga ; and the southern border of the Bucks Mountain granodiorite forms the wallrock of the intake. Anna Hietanen's geologic map of the Merrimae area (1951, plate 1) shows the structural complexity and lithologic variation of surface outcrops in and near the site of the Poe Tunnel. At least nine separate serpen- tine bodies, from a few tens of feet to f mile wide, are shown cropping out over the surface projection of the tunnel line. These are separated chiefly by mica schists, metabasalts, and metarhyolites. Small lenses of hornblende schist, limestone, and chlorite schist also are noted. A body of metagabbro underlies Big Bar Mountain ; it is f mile wide and If miles long. A smaller lens of metagabbro is shown north of the tunnel outlet. The present study shows that serpentine, massive and schistose metamorphic rocks, and metagabbro are the principal rock types intersected by the tunnel (plate 1). Approximately 60 percent of its length lies in serpen- tine, 35 percent in metamorphic rocks, and 5 percent in metagabbro. Granodiorite, limestone, and a blastopor- phyritic amphibolite each account for less than 1 percent of the tunnel length. Serpentine occurs in the tunnel as four separate major bodies and several small lenses, although some of these probably are connected with each other (plate 1). The length of tunnel penetrating the major bodies ranges from about 1,300 to 9,200 feet, whereas that penetrating lenses ranges from 20 to 500 feet. The serpentine is megascopically divisible into two principal types : a dense, black, blocky, relatively unsheared type, and one that is thoroughly sheared and altered. "Blocky ser- pentine" usually contains some zones of shearing, and "sheared serpentine" usually contains minor masses of black, blocky serpentine. In the central portion of the tunnel, these two types are distinct enough to be map- pable units, but elsewhere they are so mixed as to be inseparable on a scale of 1 : 12,000. In addition, mapping of surface geology in railway cuts 1,000 to 4,000 feet west of the tunnel failed to show the presence of these readily distinguishable units, probably because of the effects of weathering. Metamorphic rocks in the tunnel consist largely of dark, fine-grained schists and greenstones. At the north- ern end of the tunnel, 1,800 feet of biotite-hornblende- quartz schists were penetrated. About 4,500 feet of am- pliibolites, quartzites, and sericite schists occur immedi- ately north of Adit No. 2, and an additional 500 feet of this same sequence are south of the adit. Another 4,400 feet of massive, sub-schistose metamorphic rocks com- posed of quartz, biotite, potash feldspar, and plagioclase occur just north of the central mass of serpentine. Other thinner sequences of amphibolite and schist occur throughout the tunnel. More than 1,700 feet of metagabbro were penetrated, beginning about 2,700 feet north of Adit No. 1. This rock, which is correlative with Hietanen's Big Bar Mountain metagabbro, is variable in texture and compo- sition, ranging in appearance from fine-grained horn- blendite to altered, coarse-grained diorite. Numerous veinlets of talc and serpentinous gouge were encoun- tered. The regional structural trend in the tunnel and the surrounding area ranges from northwest to west-north- west. Previous work in this part of the Bidwell Bar quadrangle indicates that this is caused by the intrusive action of several bodies of granitic rock. Approximately 45 feet of the tunnel at the intake lies within one of these bodies, the Buck granodiorite. Most jointing, bed- ding, shearing, and foliation in the tunnel conforms closely to the regional trend, although serpentine con- tacts often are transgressive, commonly striking more northward than do adjacent structures. Dips of struc- tural elements are almost always steep, ranging from 60" south to 60° north. Notable exceptions are flat-lying joints near the intake, and shear zones north of Adit No. 2. The shear zones north of Adit No. 2 serve as loci for small springs that produced heavy iron stains on fresh rock surfaces in the course of a few months. In February 1957, these springs had a flow ranging from 0.1 to 0.3 cubic foot per second. The earlier structural history of this area is indicated by the cross section along the tunnel (plate 1). A series of interbedded sedimentary and basic volcanic rocks was subjected to low-grade regional metamorphism, proba- bly during late Paleozoic and early Jurassic time, and then was severely deformed, intruded by granitic plu- tons, and further metamorphosed locally during the late Jurassic and early Cretaceous. Some of the small lenses and large irregular masses of serpentine that are so prominent in and near the tunnel appear to have been intruded prior to the granitic intrusions, inasmuch as their jointing, shearing, and contacts conform to the regional patterns of deformation which in turn have been conditioned by the granitic intrusions. In addition, one mass of serpentine appears to have been faulted after emplacement (plate 1). However, many serpentine bodies have transgressive contacts, and examination of olivine crystals in the few thin sections in which they are present shows a definite lack of deformation, so that some of the serpentine probably was intruded after most of the regional and local deformation had already occurred. PETROGRAPHY Serpentine Serpentine is the dominant and most variable rock type observed in the tunnel. Its color may be whitish, pale green, dark green, or almost black. Dark green and black serpentine generally is blocky, stands well with a California Division of Mines [Special Report 61 minimum of support, and has only a few shear zones that usually are weak and randomly oriented. It is fine- grained and contains flakes and minute seams of pale- green serpentine minerals. In dark, seemingly unaltered portions of hand specimens of this rock, cleavage faces of small, isolated, fibrous crystals of chrysotile( ?) are common. Light-green and whitish serpentine is charac- terized by veinlets of asbestos, wet zones, and abundant, well-developed shear zones that follow the regional struc- tural trend. These shear zones commonly contain talc, picrite, and minor amounts of calcite. Shearing in the serpentine becomes more pronounced as a contact is ap- proached, and talc, chrysotile asbestos, tremolite( ?), cal- cite, and clay-like gouge become important constituents of the rock. In thin section, most serpentines are seen to consist principally of antigorite, serpophite, and magnetite. Varying but smaller amounts of talc, sericite, chlorite (penninite), chrysotile, pyrite, chromite, leucoxene, bow- lingite( ?), hematite, and calcite also are present. Olivine is an important constituent in the serpentine just north of the gabbro, forming up to 25 percent of the rock, and a few remnants of corroded olivine were observed in a section from a thin serpentine dike in metamorphie rock 4,250 feet south of the intake. Relict crystals of pyroxene are relatively common. In most thin sections, a weak to moderate foliation is caused by streaks of chromite and magnetite, the par- allel arrangement of fibers of antigorite, and a gen- eral orientation of the axes of olivine crystals, when this mineral is present. Just north of the gabbro, olivine occurs in serpentine in groups of heavily fractured crystals separated by streaks and wide patches of poorly oriented antigorite (photo 1). Interstices between crystals and between Photo 1. Serpentine. Fractured olivine crystals in matrix of fibrous antigorite (white). Chrysotile occupies fractures in olivine. Scale in millimeters. remnants of the same crystal commonly are filled by magnetite and well-oriented fibrous antigorite, and less commonly by bowlingite( ?). There is very little undu- latory extinction in the olivine, and generally all rem- nants of any given crystal are optically continuous, even though they may be widely separated. At the south con- tact of the main central serpentine body, calcite has entirely replaced some olivine crystals, preserving the shape of the crystal faces. Disseminated magnetite, pos- sibly formed as a byproduct in the alteration of olivine to antigorite, occurs in halos 0.5 mm. wide around these calcite pseudomorphs (photo 2). »- : ' TH_. ..^. •-..». r-v ... - ■■w**»y--.» Photo 2. Altered serpentine (?). Crystal of olivine replaced by unoriented grains of calcite (note high relief of two of the several grains). Magnetite probably was exsolved during replace- ment of olivine. Groundmass consists of calcite, sericite, magnetite, and traces of talc. Scale in millimeters. Antigorite is the most common constituent of all serpentines in the tunnel. It occurs in aggregates form- ing elongate streaks, in unoriented aggregates, as a filling in micro-fractures, and as single fibers or small groups of parallel fibers arranged at right angles to each other in a lattice-like pattern. Certain parallel growths of antigorite appear to have been controlled by cleavage or parting planes of pyroxene crystals. Serpophite oc- curs sporadically as an anhedral groundmass mineral with indefinite extinction and very low birefringence. Chrysotile is neither widespread nor abundant. Where it does occur, it commonly fills micro-fractures that trend at right angles to the foliation, although a few such fractures have developed parallel to the foliation. Chry- sotile micro-veinlets cut antigorite and in one thin sec- tion separate aggregates of magnetite grains that have replaced remnants of olivine crystals. Sericite, calcite, and chlorite (penninite) are closely associated with each other. The abundance of any one of these minerals in a sample bears a general but direct relation to the abundance of all three. Because they are the dominant mineral constituents of the more thorough- ly sheared and obviously altered zones within serpentine, and because they are not consistently present in thin sections of serpentine, these minerals in all likelihood were not the result of regional metamorphism, but prob- ably originated through some form of local hydrother- mal alteration. Talc is relatively uncommon, occurring principally in thin, megascopically visible veinlets. Bow- lingite ( ? ) was observed in a thin section of serpentine 350 feet north of the gabbro. It occurs in microveinlets in olivine, and is strongly pleochroic from light olive- green to clear "wheat-yellow". Its maximum birefring- ence is lower second order, and its orientation is length- fast. Magnetite and chromite are present in all thin sections studied. They occur disseminated and in bands composed of single crystals, in aggregates, as a massive vein-fil- 1959] PoE Tunnel, Butte County ling, and replacing olivine cores and rims. White, trans- lucent leucoxene was common in a section from the northernmost serpentine body, but it was not spatially associated with dark oxide minerals, indicating a very complete alteration of ilmenite(?). Cubes of pyrite, modified by octahedral faces, were observed in a sample of completely altered serpentine south of Adit No. 1 that otherwise contained only talc, sericite, calcite, and magnetite. Photo 3. Serpentine. Magnetite exsolved during replacement of diallage (?) l)y mixture of antigorite and serpophite. Scale in millimeters. In the northernmost body of serpentine penetrated by the tunnel, the former presence of pyroxene, probably diallage, is indicated by close, parallel growths of anti- gorite, with serpophite filling the interstices. Most such relict crystals observed are single euhedral prisms, al- though some appear to have been corroded before re- placement, and others are joined in apparent twin growth. In the sample taken 350 feet north of the gabbro, magnetite occurs in a groundmass of unoriented anti- gorite and serpophite, in geometric patterns that suggest the former presence of pyroxene (photo 3). A sample of altered serpentine( ?) was taken north of Adit No. 2 from the small serpentine body just south of the main serpentine mass. It is composed of ap- proximately 50 percent calcite, 40 percent sericite-musco- vite, 10 percent magnetite, and traces of talc. Calcite consists of undeformed anhedral crystals the peripheries of which are penetrated by laths of sericite and musco- vite. Muscovite, commonly of small enough size to warrant use of the term sericite, is randomly oriented throughout the section in a groundmass mosaic of inter- locking laths. Magnetite occurs in clusters of anhedral grains the distribution of which suggests crystallization during the replacement of olivine and pyroxene by calcite. Undifferentiated Metamorphic Rocks At least ten distinct bodies of metamorphic rock have been cut by the Poe Tunnel, the intersected thicknesses of which range from 75 to 5,050 feet. Because of the difficulty in making accurate petrographic identifications of the different types of metamorphic rock while un- derground, distinction was based on the degree of schis- tosity exhibited by the rock. Metamorphic rocks were mapped as "dominantly massive" or "dominantly schisto.se" (plate 1). In addition to these units, a small lens of limestone, two small bodies of a porphyritic amphibolite, and a large mass of metagabbro were dis- tinguished, and are discussed separately in later sec- tions. Schistose Rocks Mappable units of schistose metamorphic rock are confined to the northern 4,200 feet of the tunnel. The largest such unit was penetrated for 1,300 feet. It is bounded on the north by the Bucks granodiorite, its con- tact with which is gradational, and on the south by a body of serpentine. The contact of this rock with the granodiorite is discussed further in the section on grano- diorite. All three bodies of schistose metamorphic rock are dark gray or greenish biotite-hornblende schist. Shear zones containing gouge are common. The rock gen- erally exhibits well-developed, near-vertical sehistosity, although bands of rock with poorly oriented biotite, hornblende, and quartz are encountered occasionally. The schist is not always less competent than massive metamorphic rocks, inasmuch as several hundred feet of the tunnel in schist stood well with a minimum of sup- port (fig. 4). However, as is the case with other rock types, badly sheared portions of the schist required heavy support. Photo 4. Biotite-hornblende schist. Segregated laj-ering of quartz (white) and dark ferromagnesian minerals (biotite and hornblende). Scale in millimeters. The schistose rocks are fine-grained biotite-horn- blende schists containing 35 to 55 percent quartz, 12 to 28 percent hornblende, 16 to 20 percent plagioclase, and 1 to 20 percent mica (biotite and chlorite). Minor con- stituents are potash feldspar (1 to 3 percent), magnetite (-^ to 2 percent), ilmenite, pyrite, chalcopyrite, clay minerals, sericite, allanite, and epidote. The mineral assemblage of these rocks and the anorthite content of their plagioclase indicate that they belong to a transition phase between the greenschist and albite-epidote amphib- olite facies as defined by Turner and Verhoogen (1951, pp. 460-473). Hietanen (1951, p. 577) considers these banded schists to represent a bedded formation in which layers of sedimentary and tuffaceous rock alter- nated, and that the even thickness of thin bands over relatively large distances suggests deposition in a shal- low water-filled basin. California Division of Mines [Special Report 61 Photo 5. Biotite-hornblende schist. Foliated laths of biotite and hornblende in groundmass of quartz (white). Magnetite (black) occurs in stringers and replacing ferromagnesian min- erals. Scale in millimeters. f: Photo 6. Sub-schistose biotite-horpblende schist. Laths of biotite (dark, low relief) and hornblende (dark, high relief) exhibit sub- parallel foliation, and enclose rounded aggre- gates of quartz grains. Scale in millimeters. Their texture ranges from strongly schistose near the granodiorite contact to moderately or weakly schistose several hundred feet from it. Where the texture is markedly schistose, alternate segregated layers of quartz and parallel laths of dark minerals (hornblende and biotite) are observed in thin sections (photos 4 and 5). Where sehistosity is not well-developed, this alterna- tion of light and dark minerals tends to be absent. In- stead, rounded and lensoid aggregates of relatively coarse quartz grains, around which laths of dark min- erals swirl, occur in a fine-grained matrix of quartz and plagioclase (photo 6). The degree of sehistosity appears to be controlled by the size of the aggregates of quartz, by whether the quartz occurs in globular aggregates or in bands, and by the size and orientation of individual laths of hornblende and biotite. Quartz is the most abundant constituent in all thin sections studied. It is characteristically fresh and un- strained. It occurs in micro-bands, in aggregates up to 2 mm. in largest dimension, and disseminated through- out a very fine-grained groundmass. In one section, many individual coarse quartz grains in irregular ag- gregates exhibit two or three normal crystalline faces, and in another the semi-regular exterior boundaries of a few aggregates themselves suggest an hexagonal form, although the constituent grains are xenoblastic. Hornblende is a prominent constituent of schistose rocks within a few hundred feet of the granodiorite con- tact, but becomes less abundant at distances beyond this. Hornblende crystals are pleochroic from light tan or yellowish-green to dark bluish-green in thin sections of samples taken throughout the schistose rocks, except within about 100 feet of the granodiorite contact. A thin section of a sample taken 70 feet from the contact eon- tains hornblende that is pleochroic from light brown to green. The change in color of the hornblendes from a bluish-green to green variety as the intrusive contact is approached indicates an increase in their alumina content. This is caused by the increase in temperature in rocks close to the granodiorite during its emplace- ment, and indicates a slightly higher grade of meta- morphism. In all thin sections studied, hornblende oc- curs in bands of oriented laths. Some of these bands alternate with bands of quartz grains or of biotite laths. Photo 7. Schist from granodiorite-schist contact zone. Quartz and corroded crystals of hornblende occur in segregated layers. Biotite is absent. Scale in millimeters. 1959] PoE Tunnel, Butte County and others occur alone in a matrix of fine-grained quartz and plagioclase (photo 7). In the less schistose rocks, hornblende bands swirl around single aggregates of quartz grains and between such aggregates when they occur in groups. Plagioclase may be divided into two types: 1) rela- tively large (up to 1 mm.), xenoblastic, embayed, vaguely twinned crystals, containing abundant blebs of quartz and variable amounts of sericite; 2) minute, fresh, sharply twinned xenoblastic grains in the ground- mass. The larger, altered crystals are common close to the grandiorite contact, but are completely absent sev- eral hundred feet from it. In spite of this, the plagioclase content of the schistose rocks varies only slightly (fig. 3). The anorthite content of the fresher crystals ranges from calcic oligoclase to sodic andesine. Biotite, the principal micaceous mineral, is an im- portant constituent except near the contact with gran- odiorite, where it is absent (photo 7). It is pleochroic from light brown to orange-brown several hundred feet from the contact, and from light brown to dark brown near it. Biotite occurs as oriented laths in segregated bands and swirls, and occasionally is seen replacing hornblende. Chlorite is a minor constituent of the rock, occurring as single laths and occasionally replacing bio- tite. Sericite occurs in the altered plagioclase crystals and, in one thin section, along micro-fractures and in small intersticial aggregates. Clay minerals occur spar- ingly in some plagioclase crystals and, in a few sec- tions, in minute shear zones. Potash feldspar occurs in small amounts as randomly distributed, xenoblastic crystals in the groundmass. Allanite was observed in one thin section as a thin streak of minute, xenoblastic, zoned crystals in a band of biotite. In another section, several corroded remnants of epidote were observed. Neither allanite nor epidote occurs in schists adjacent to the granodiorite contact. Opaque minerals inchide magnetite, ilmenite, pyrite, and chaleopyrite. Magnetite occurs as single octahedral crystals or xenoblastic grains, as elongate groups of grains (photo 5), and occasionally as a replacement of hornblende. The interstices and outer faces of groups of magnetite grains are lined with thin seams of ilmenite in one section. Pyrite is a common accessory mineral, and occurs in disseminated blebs, stringers, and striated cubes. Traces of chaleopyrite are associated M'ith pyrite in one hand specimen. In addition to the schistose rocks in the northern part of the tunnel, a layer of graphite schist too thin to map occurs interbedded with amphibolites 50 feet north of the outlet portal. This rock consists of folia of graphite enclosing stretched pebbles and sand grains (photo 8). In thin section, these rounded pebbles and grains are seen to consist largely of quartz, calcite, and sericite. Quartz occurs chiefly in aggregates of strained, cloudy, and embayed grains, although some quartz is fresh and unstrained. Anhedral, slightly strained quartz crystals occur in micro-veinlets that crosscut the sand grains and graphite folia. Calcite occurs in xenoblastic masses and stringers throughout the section, but is asso- ciated principally with quartz. Small aggregates of ran- domly oriented sericite are disseminated throughout the stretched pebbles and grains and in the matrix, and lo- cally is the dominant constituent of the rock. Identical flakes, coarse enough to identify as muscovite, occur sparingly in random orientation, and in slightly greater abundance in parallel orientation with flakes of graphite. Graphite appears to be dominant in the hand specimen and gives the rock its black color, but in thin section is seen to comprise only 10 or 15 percent of the rock. Photo 8. Graphite schist. Folia of graphite enclose stretched sand grains composed largely of quartz, sericite, and calcite (white, high relief). Discontinuous stringer of magnetite, upper right. Micro-veinlet of quartz, bottom. Scale in millimeters. Biotite, chlorite, plagioclase, magnetite, and leucoxene are minor constituents, and together probably total less than 2 percent of the rock. Tiny, fresh laths of biotite, pleochroic from colorless to brown, are scattered throughout the thin section. Only a small portion of tJie biotite is associated with chlorite, which occurs spar- ingly in large aggregates of crystals. A few tiny rem- nants of polysynthetically-twinmed plagioclase, some fresh and some altered, were observed. Magnetite occurs as single crystals and in stringers elongated parallel to the foliation. A few shapeless grains of leucoxene occur in quartz. Massive Rocks Bodies of massive metamorphic rocks are distributed throughout the tunnel. The three largest masses were penetrated for (1) 5,050 feet north and a short distance south of Adit No. 2; (2) 1,950 feet between Adit No. 2 and the vertical shaft; (3) 3,350 feet just south of the southernmost body of schistose rock (plate 1). Smaller bodies of massive metamorphic rock range from 75 to 550 feet in thickness. These massive rocks generally have weak joints that are randomly oriented. Megascopic foli- ation usually is absent, although the preferred orienta- tion of crystal laths and micro-zones of shearing becomes apparent when seen in thin section. Shear zones from 6 inches to 8 feet thick containing soft gouge or a hard, platy serpentinous material are not uncommon in the largest body of massive rock, north of Adit No. 2, but are almost entirely absent elsewhere. Portions of the tunnel in massive metamorphic rock generally require less support than portions of it in serpentine or schist, although occasional shear zones had to be heavily tim- bered. The unit, "undifferentiated massive metamorphic rocks", includes black and dark-green quartzite, green- schist, high-grade schist, and amphibolite. 10 California Division of Mines [Special Report 61 A specimen of quartzite, taken 5,100 feet north of Adit No. 2, consists of quartz and minor amounts of biotite, caleite, chlorite, magnetite, leucoxene, and pyrite. A vague schistosity, apparent in thin section, is caused by scattered, very thin, discontinuous layers of biotite, by micro-shearing, and by the elongation of rounded particles of coarse quartz sand. Quartz occurs in equant, xenoblastic grains less than 0.1 mm. in diameter in the groundmass, and in aggregates (up to 1.25 mm. in longest dimension) of sub-idioblastic and xenoblastic grains representing reconstituted quartz in the particles of quartz sand. Caleite occurs throughout the section as xenoblastic, embayed grains and stringers elongated parallel to the weak foliation. Biotite is pleochroic from light to dark brownish-green ; it is distributed in thin, discontinuous layers that "flow" around the recrystal- lized sand grains, and in micro-shear zones, where it is associated with chlorite and magnetite. Leucoxene, mag- netite, and pyrite occur^ as scattered single crystals, xenoblastic in the case of leucoxene, idioblastic and xeno- blastic in the case of magnetite and pyrite. A massive green rock just south of the tunnel outlet is a member of the greenschist facies, probably derived from a basic igneous rock that has been retrogressively metamorphosed without deformation. It contains approx- imately 65 percent andesine(?) plagioclase, 20 percent chlorite, 10 percent caleite, 5 percent sericite, ilmenite, and leucoxene, and traces of anthophyllite, muscovite, sphene, and chalcopyrite. Embayed, perforated, amoeba- or lace-like crystals of plagioclase and undeformed cal- eite are characteristic of this rock. Some crystals of plagioclase are clearly the corroded remnants of slender prisms, and commonly exhibit simple twinning. Pale- green chlorite generally occupies interstitial positions between the plagioclase and caleite. Sericite occurs in small aggregates of minute, unoriented fibers that, in a few instances, are large enough to warrant the name muscovite. Leucoxene surrounds a few xenoblastic grains of ilmenite, and also occurs in shapeless masses free of black oxide minerals. Disseminated blebs of chalcopyrite are common. A single, corroded crystal of sphene sur- rounded by caleite, and a sub-idioblastic microlath of colorless anthophyllite in caleite suggest the former existence of a mineral assemblage of higher metamorphic grade. A 100-foot thick body of black, massive, very fine- grained schist was penetrated in Adit No. 1, approxi- mately 450 feet from the portal. This almost-aphanitie rock consists principally of plagioclase and biotite, and contains small amounts of grossularite garnet, magnetite, pyrite, chlorite, caleite, and sericite (photo 9). It is a high-grade pelitic schist of the amphibolite facies as described by Williams, et al. (1954, pp. 231-235). A faint micro-sehistosity is caused by the elongation of aggregates of minute grains (less than 0.1 mm.) of plagi- oclase, which is slightly more abundant than biotite. It occurs in aggregates of very fine, xenoblastic grains, probably calcic oligoclase or sodic andesine in composi- tion and simply twinned, and also rarely in aggregates suggesting the replacement of a prismatic mineral. Xeno- blastic masses of poorly oriented biotite, pleochroic from colorless to reddish brown, are distributed evenly throughout the section. Colorless, idioblastic crystals of grossularite garnet, less than 0.3 mm. long and exhibit- B ■■HL.rr-* ^^" ^^^^HH9|^^I mm'^ ^^^^^^^^^^D^^^''^9IKmMb^^Sm^^'!^3 -?^d&^^^ ■ MMmfi^:^r- ^^m%.: J WKmf'^S* Mm- ■ -^'' fvi'al^H^H .^p.;- •- '''-"' i ^i^''(wl^^it^!^\i^^' '^^^ Photo 9. Pelitic schist. Crystals of grossularite garnet (white, high relief) in groundmass composed largely of plagioclase (white, low relief), biotite, sericite, chlorite, and magnetite (black). Mag- netite replaces core of one garnet crystal. Scale in millimeters. ing weak birefringence, are common. Cores of these crystals generally are i*eplaced wholly or partly by mag- netite (photo 9). Magnetite also occurs in clusters of crystals elongated parallel to the foliation, and as a fine dust in the groundmass. Pyrite, chlorite, caleite, and sericite are present in trace amounts. Amphibolite is the most common type of massive metamorphic rock penetrated by the tunnel. These rocks were formed by metamorphism and deformation of basic igneous rocks and calcareous and pelitic sediments. In addition, some amphibolites were formed by the meta- morphism of a basic igneous rock such as gabbro, with- out strong internal deformation, and thus are so weakly foliated that they resemble gabbro of igneous origin. Amphibolites are grouped and described in the para- graphs following according to their probable origin. Amphibolite Derived from Basic Igneous Rock, with Deformxition. Small lenses of this type of amphibolite occur in the serpentine south of Adit No. 1, and probably represent a gabbroic facies that was metamorphosed dur- ing serpentinization and subsequent deformation of the enclosing peridotite. Hornblende and plagioclase are present in approximately equal proportions in this rock. Small amounts of pyroxene, biotite, magnetite and cal- eite were noted, along with traces of antigorite, elinozoi- site, and pyrite. In thin section, the rock exhibits a vaguely schistose to moderately sheared fabric. In one section, hornblende crystals form poorly defined lamellae that enclose porphyroblasts of diopside, and in another, shear planes intersect and offset folia of hornblende micro-laths. Hornblende consists of large xenoblastic crystals and minute laths, the orientation of which is responsible for the micro-foliation of the rock. The large crystals com- monly are zoned, being pleochroic from colorless to light bluish green in the center and from light yellow green to dark bluish green around the exterior. Dense aggre- gates of small hornblende crystals commonly occur sur- rounding corroded remnants of large hornblende crystals, suggesting reconstitution of the mineral in place. Plagioclase. occurs as untwinned, xenoblastic, embayed crystals or groups of crystals occupying interstices be- 1959] PoE Tunnel, Butte County 11 tween laths of hornblende. Some aggregates of crystals suggest the prismatic outline of an euhedral plagioclase or pyroxene crystal, but are composed of optically discon- tinuous units of plagioclase joined by intricate, serrated boundaries. Xenoblastic crystals of diopside, pleo- chroic from colorless to pale green, were observed in one thin section. Plagioclase commonly fills portions of the center of these crystals. Xenoblastic masses of cal- cite occur throughout the thin sections studied, and are especially common in small amounts in plagioclase. Idioblastic crystals and stringers of magnetite are a common accessory mineral. Minute laths of pleochroie brown biotite were noted in one section, as was a single xenoblastic crystal of clinozoisite. In another, dissem- inated pyrite is present and antigorite occurs sparingly in microveinlets cutting hornblende and plagioclase. Amphibolite Derived from Basic Igneous Rock, With- out Deformation. Almost 2,000 feet of massive, dark green amphibolites of this type occur south of Adit No. 2, between the two southernmost bodies of serpentine. Of all the rocks in the tunnel, this sequence shows the least evidence of jointing, shearing, micro-foliation, or any other indication of having been acted upon by directional stress. Less than 100 feet of similar but coarser-grained amphibolite were observed immediately south of the first small exposure of serpentine south of the main serpentine mass (Station 247-|-34, about 5,530 feet north of Adit No. 2). In thin section, these rocks are seen to consist chiefly of hornblende and plagioclase (photo 10). Small amounts of epidote, clinozoisite, chlo- Photo 10. Undeformed amphibolite derived from basic igneous rock. Stubby, frayed crystals of hornblende, interstitial oligoclase (white, low relief), magnetite (black), and epidote (dark, high relief). Scale in millimeters. rite, caleite, ilmenite, and leucoxene, and traces of biotite, sericite, quartz, pyrite, and chalcopyrite, also are pres- ent. The most common texture suggests the igneous parentage of these rocks; this relict texture is coarse blastophitic or sub-blastophitic, in which unoriented, xenoblastic plates and stubby, frayed crystals of horn- blende partly enclose sub-idioblastic laths of oligoclase and, in some instances, are partly interstitial between them. This type of texture suggests that such rocks may have been derived from basalt. Sub-idioblastic crystals of plagioclase, up to 2 mm. in length, comprise between 35 and 50 percent of the rock. Carlsbad-albite twinning is common, and almost all crystals are cloudy and embayed and contain flakes of sericite. Stubby, frayed plates and small xenoblastic laths of untwinned hornblende comprise between 40 and 70 percent of the rock ; pleochroism is from yellow green to bluish green. Remnants of epidote crystals occur in random aggregates and along microfractures. South of Adit No. 2, epidote with colorless rims and pleochroie centers (colorless to pale brown) is common. Clinozoisite occurs sparingly in radial, fan-like groups of crystals associated with epidote. Aggregates of chlorite and single laths of pleochroie brown biotite are scattered through- out the thin sections studied. Xenoblastic masses of op- tically continuous, undeformed caleite are common. Chalcopyrite, cubes and stringers of pyrite, and xeno- blastic grains of ilmenite surrounded by leucoxene occur in trace amounts. A small amount of undeformed inter- stitial quartz occurs in the small body of amphibolite north of Adit No. 2. A sample of amphibolite taken from the southern margin of the metagabbro north of Adit No. 1 is de- scribed briefly in the section on metagabbro. Amphibolite Derived from Pelitic Sedimentary Rocks. Samples of this rock type .were taken 3,500 feet north of Adit No. 2, and 1,900 feet north of the northern contact of the main serpentine mass. Although these rocks are classed as amphibolites, hornblende is only a minor constituent. The absence of the critical stable assem- blage of the amphibolite facies (hornblende-plagioclase) in certain rocks of that facies has been noted before (Williams, et al., 1954, pp. 230-231). Plagioclase and biotite are the dominant constituents, accompanied by small amounts of hornblende, magnetite, and chlorite, and traces of caleite, zircon, pyrite, and chalcopyrite. Thin sections show vague micro-foliation. Oligoclase, which is more abundant than biotite, oc- curs as xenoblastic masses and sub-idioblastic laths that commonly exhibit simple Carlsbad or albite twinning; complex Carlsbad-albite twinning was not observed. Corroded and altered remnants of oligoclase commonly are invaded by biotite and chlorite. Many sub-idioblastic crystals are bent or shattered and separated by fractures filled with a mixture of xenoblastic grains of plagioclase and minute laths of biotite. A few large plagioclase crystals are as much as 2.5 mm. long, whereas xenoblas- tic laths in the groundmass average 0.1 mm. in length. Oriented, pleochroie, brown biotite is evenly distributed throughout the thin sections. Chlorite (penninite) oc- curs replacing biotite and in xenoblastic masses and stringers, and is especially common in micro-fractures and interstices between recrystallized plagioclase grains. Magnetite, pyrite, and chalcopyrite are common acces- sory minerals. Scattered minute prisms of zircon, recog- nizable by their pleochroie halo in biotite, were ob- served in the thin section of rock north of the main serpentine mass. The sample north of Adit No. 2 is characterized by occasional frayed laths of pleochroie (light green to dark green ; yellow green to bluish green) hornblende, xenoblastic masses of caleite, and magnetite replacing hornblende and biotite. Amphibolite Derived from Calcareous, Sedimentary Rocks. A sample of sheared and altered amphibolite consisting chiefly of caleite, plagioclase, and diopside 12 California Division of Mines [Special Report 61 was taken from 350 feet south of Adit No. 2. Horn- blende, clay, and biotite are minor constituents, and traces of clinozoisite, ilmenite, leucoxene, and pyrite are present (photo 11). Plagioclase consists of altered, embayed crystals with serrated edges. An iron-stained clay mineral, common throughout the thin section, is especially abundant in plagioclase. A few aggregates of optically discontinuous, serrated plagioclase have the outlines of a prism cross section. Calcite occurs abundantly as groups of xeno- blastic crystals elongated in the direction of shearing. Corroded, oval-shaped crystals of pleochroic (colorless to light green) diopside are common. Hornblende, pleo- chroic from yellow-green to bluish green, is less abun- dant than diopside, and occurs in folia of xenoblastic crystals associated with small amounts of pleochroic brown biotite. Clinozoisite, ilmenite surrounded by leu- coxene, and pyrite are randomly distributed in trace amounts throughout the thin section. Photo 11. Amphibolite derived from calcar- eous sedimentary rock. Corroded crystals of plagioclase in fine-grained groundmass composed principally of calcite, a clay mineral, diopside, hornblende, and biotite. Small, oval-shaped crystals with high relief (i.e., center of upper plagioclase crystal) are diopside. Scale in mil- limeters. Blastoporphyritic Amphibolite Approximately 4,300 feet south of Adit No. 1, two small bodies of blastoporphyritic amphibolite were mapped in a mass of sheared serpentine (plate 1). This massive, dark green rock is made prominent by the presence of numerous light-colored, small porphyroblasts of "plagioclase". Some thin sections of this rock ex- hibit a vague micro-foliation. The principal mineral constituents are antigorite, hornblende, zoisite, and al- tered plagioclase. Small amounts of calcite, chlorite, clinozoisite, biotite, magnetite, and leucoxene were ob- served, as were traces of pyrite, quartz, and hematite. Fibrous antigorite comprises 80 percent of one thin section, but is entirely absent from another. A third section contains about 85 percent of very fine laths (less than 0.05 mm. long) of material that probably also is antigorite. The thin sections composed largely of antig- orite contain up to 10 percent of "plagioclase" por- phyroblasts, which now consist chiefly of xenoblastic calcite, chlorite, iron-poor zoisite, and clinozoisite (photo 12). The former presence of plagioclase is detectable by means of a vague albite twinning that sometimes is present in serrated, altered remnants of prisms. Horn- blende occurs in one antigorite-rich thin section as pleo- choric (pale yellow-green to pale green), fibrous laths 0.5 mm. long. Calcite and chlorite are associated to- gether in micro-fractures, as well as in altered plagio- clase. Small laths of biotite, pleochroic from light brown to orange-brown, occur randomly throughout these thin sections. Minor amounts of interstitial, undeformed grains of quartz occur in one antigorite-rich section. Magnetite and pyrite are common minor accessory minerals. Photo 12. Blastoporphyritic amphibolite. "Plagioclase" pheno- cryst in groundmass of fine antigorite now consists chiefly of cal- cite, chlorite, iron-poor zoisite, and clinozoisite. Scale in milli- meters. One thin section of blastoporphyritic amphibolite con- sists of approximately 60 percent zoisite, 35 percent hornblende, and 5 percent calcite, leucoxene, magnetite, and biotite. The zoisite, arranged in fibrous radial groups, is a colorless, iron-poor variety that exhibits anomalous berlin-blue interference colors. Hornblende is pleochroic from pale yellow-green to green, and occurs in fibrous, equant crystals up to 0.2 mm. in length. Cal- cite is present as undeformed, amoeba-like, xenoblastic masses. Biotite is rare, whereas magnetite and leucoxene are common accessory minerals. Hietanen (1951, pp. 599- 600) describes a similar hornblende-zoisite rock, which she attributes to an early stage of metasomatism of serpentine to gabbro or diorite. Limestone A small, 20-foot thick lens of limestone in a sequence of massive amphibolites was mapped 3,000 feet north of Adit No. 2. It is a grayish-white, very fine-grained rock 1959] PoE Tunnel, Butte County 13 composed almost entirely of calcite. Traces of quartz, sericite, muscovite, magnetite, and hematite together total less than 2 percent of the rock. Elongate grains of calcite, many of which exhibit twin lamellae, have well- oriented major grain axes, although the optic axes are somewhat randomly oriented. Minute grains of quartz occur preferentially in a few microbands of finer- grained calcite (0.05 mm. average grain length, against 0.2 mm. average grain length for "ordinary" calcite). The other trace-amount minerals are scattered through- out the thin section. Metagabbro Beginning approximately 2,700 feet north of Adit No. 1 and progressing northward, the Poe Tunnel intersects 1,800 feet of metamorphosed gabbro. This rock has a varied appearance, ranging from a fine-grained black rock to a coarse-grained, greenish-colored rock with a color index of only 50 percent. The metamorphosed gab- bro is typically blocky and massive, but minor portions of it exhibit foliation or shearing. With one exception the minerals in the metagabbro also are found in the amphibolites. The mineralogy of the metagabbro is espe- cially similar to that of the amphibolites that were de- rived from basic igneous rocks without strong internal deformation, and this rock might properly be classified as belonging in that category. However, the physical appearance of the metagabbro is markedly different from that of any of the amphibolites, and in the field it is readily distinguished from other rock types. Because of the resemblance of hand specimens of this rock to gab- bro, and because study of thin sections suggests an origin from a basic igneous rock of gabbroic composition, the name "metagabbro" is assigned to it. This rock under- lies Big Bar Mountain, and is designated by the same name on Hietanen's map (1951, plate 1). The dominant constitutents of the metagabbro are amphibole (60 to 80 percent) and plagioclase (15 to 35 percent). The highest proportion of amphibole occurs in a sample taken from a lens of rock resembling igneous hornblendite. Sericite, muscovite, clinozoisite, zoisite, magnetite, stilbite, actinolite, and potash feldspar are minor constituents. In thin section, the texture of the rock is coarse and massive, consisting of randomly ori- ented interlocking crystals exhibiting little or no evi- dence of deformation. Amphibole in the metagabbro consists largely of frayed, stubby crystals of pleochroic hornblende or al- tered pale green actinolite. In addition, fresh, light green needles of actinolite in altered plagioclase, and fresh, slender laths of hornblende distributed randomly, occur in a sample taken near the center of the metagabbro. Refractive indices of the altered actinolite indicate that it is an iron-poor variety ; the fresh needles of actinolite, however, have a "normal" composition. Plagioclase com- monly consists of large altered and corroded plates with remnant albite twinning. In one section, plagioclase had been recrystallized to groups of tiny, serrated, interlock- ing untwinned crystals. In another, positions formerly occupied by large crystals of plagioclase now contain a mixture of sericite, actinolite, stilbite, and altered, vaguely twinned remnants of the plagioclase. Sericite is a common alteration product of hornblende, actinolite, and plagioclase. Zoisite occurs interstitially in one thin section, and clinozoisite occurs in two others as xeno- blastic, corroded crystal remnants. Minor amounts of stilbite (?) were observed in two thin sections from near the center of the metagabbro. The mineral occurs in colorless fan-like aggregates, some of which are twinned, and exhibits an undulatory but approximately parallel extinction. Its maximum birefringence is first- order white. Stilbite in this rock might have been derived by alteration of scapolite, which is a high-temperature alteration product of plagioclase. A sample of dark, very fine-grained rock at the south- ern margin of the metagabbro might represent a meta- morphosed, chilled border facies of the gabbro. This rock is an amphibolite in which hornblende and plagio- clase are present in approximately equal amounts. It is massive and its texture is sub-blastophitic. Untwinned plagioclase occurs interstitially between small laths of light- to dark-green pleochroic hornblende. A few cor- roded remnants of larger hornblende crystals are scat- tered throughout the thin section. Small laths of pleochroic brown biotite and a few grains of hematite also were observed. This rock is distinguished from metagabbro by the absence of the numerous alteration minerals and by its hornblende: plagioclase ratio. Granodiorite The northernmost 50 feet of the tunnel penetrates a fine-grained granodiorite that is separated from a series of biotite-hornblende schists immediately to the south by a gradational contact approximately 150 feet thick. This granodiorite is part of the southern margin of the Bucks intrusive mass, which consists principally of granodiorite but also includes diorite and gabbro. Sample Poe 1, taken just inside the intake, is a fine- grained, structureless, light-colored rock in which a brown stain in quartz is prominent. Sample Poe 2, taken 12 feet south of Poe 1, is coarse-grained, gray with a greenish cast, and has a poorly developed foliation. Sample Poe 3, a hornblende schist, was taken 81 feet south of this; it is a coarse, strongly foliated rock with a color index of about 30. Alternate light and dark streaks of segregated quartz and hornblende give the rock a gneissic appearance. Samples Poe 4a and Poe 8 lack this segregation, and in appearance are typical of almost all the biotite-hornblende schists. The mineralogical variations occurring among the foregoing samples and in an intrusive diorite dike (Poe 6) are shown diagrammatically in figure 3. The analysis of the dike is included because its composition may approximate that of a magma from which the granodio- rite was derived, although it almost certainly does not represent the ultimate parent magma. Mineralogical variations within the schists are discussed in more detail in the section on schistose metamorphic rocks. The Bucks granodiorite, within Poe Tunnel, consists principally of plagioclase, quartz, biotite, and potash feldspar (photos 13 and 14). Adjacent to the contact, there is little mica, but hornblende is a prominent con- stituent instead. Minor mineral components are musco- vite, chlorite, sericite, clay minerals, epidote, zoisite, clinozoisite, magnetite, sphene, apatite, zircon, pyrite, and calcite. The texture of the granodiorite is hypidio- morphic granular, but near the contact this massive rock becomes somewhat foliated. 14 California Division of Mines [Special Report 61 I- z llJ o o > Figure 3. Mineral variation diagram, north end Poe Tunnel. Data obtained with Chayes' Point Counter, using approximately 650 counts per thin section. Samples to left of contact are from border of large body of granodiorite, except sample "Poe 6," which is from a basic intrusive dike at Station 9+50. Shown across the top are sample numbers; shown across the bottom are tunnel stations. Plagioelase consists of calcic oligoclase and sodic andesine. It is twinned according to the albite and Carls- bad-albite laws, and exhibits normal zoning and minor amounts of oscillatory zoning. Alteration of the centers of plagioelase crystals to sericite, clay minerals, and muscovite is common. Thin fringes of untwinned albite (?), in which minute blebs of clear quartz occur, also are common. Myrmekite occurs sparingly between plagio- elase and potash feldspar. Quartz occurs principally as anhedral plates that contain abundant dust inclusions and exhibit minor undulatory extinction. The quartz blebs in plagioelase are unstrained and free of dust. Pleochroic brown biotite occurs principally as a late- crystallizing interstitial mineral, but also in parallel 1959] PoE Tunnel, Butte County 15 Photo 13. Granodiorite. Plagioclase (pi), quartz (qu), biotite (b), and muscovite (m). Crossed nicols. Sealt in millimeters. Photo 14. Granodiorite. Plagioclase (pi), quartz (qu), pot- ash feldspar (kf), biotite (b), and myrmekite (m). Crossed nicols. Scale in millimeters. growth with muscovite and in association with epidote. Chlorite is associated with epidote and is rare as a replacement of biotite. Muscovite consists of scattered remnants of frayed crystals. Very minor amounts of sericite and a kaolin-type clay mineral occur as altera- tion products of plagioclase. Orthoclase is the dominant potash feldspar mineral. Minerals present in very small amounts include corroded remnants of weakly pleochroie epidote and- colorless zoisite, anhedral crystals of sphene, stubby prisms of apatite and zircon, and tiny euhedral crystals of magnetite. Near the contact with hornblende schist, certain changes occur in the minerals comprising the granodior- ite, aside from their changes in quantity. Zoning and complex twinning in plagioclase are less common, whereas the intensity of alteration of the plagioclase crystals is markedly increased. Unstrained, amoeba-like masses of calcite, together with muscovite, chlorite, elinozoisite, zoi- site, and a clay mineral, occupy the centers of many plagioclase crystals. Quartz occurs in larger plates, contains few or no inclusions, and is entirely unstrained. A small amount of quartz occurs as small blebs scattered throughout altered crystals of plagioclase. Biotite is absent ; a few percent of corroded laths of muscovite and traces of chlorite are the only mica minerals. Hornblende, which is absent in sample Poe 1, constitutes about 11^ percent of sample Poe 2, adjacent to the contact. It occurs in small clots of crystals and in semi-oriented bands, and is pleochroie from light brown to yellow or blue-green. Potash feldspar is only slightly more abun- dant, and consists principally of orthoclase. As in sample Poe 1, small amounts of epidote, chlorite, apatite, and magnetite are scattered throughout the thin section. Zircon, sphene, and sericite are absent. Approximately 825 feet south of the intake portal, an intrusive diorite dike strikes west and dips steeply south. This rock (sample Poe 6) may represent a magma from which the granodiorite was derived. It is a dark, fine-grained, massive rock containing thin, megascopic- ally visible laths of hornblende. Its texture in thin section is sub-foliated. Large crystals of zoned horn- blende tend to be somewhat oriented, but there are so few that the effect is lost. Small laths of biotite also are poorly oriented (photo 15). Most of the remainder of the thin section consists of unoriented anhedral crystals of plagioclase. Minor mineral constituents include ortho- clase, quartz, chlorite, epidote, elinozoisite, zircon, mag- netite, and apatite. •*< •, f*y ^f^\ ■■ t~ ;» .jT "^"^e ^ ?. # t '1 >-f J '. ■^•r« Photo 15. Diorite dike. Euhedral crystals of zoned hornblende and fine laths of biotite against a light-colored background of plagioclase, quartz, and potash feldspar. Scale in millimeters. Plagioclase consists of calcic andesine. Residual rem- nants of large, altered plagioclase crystals usually are complexly twinned, whereas smaller crystals (approxi- mately 0.1 mm. long) are untwinned. Longitudinal sec- tions of pleochroie (light tan to dark green) hornblende commonly are replaced by biotite in parallel and trans- verse growth, whereas cross sections of the mineral just as commonly exhibit corroded centers replaced by un- twinned plagioclase. Orthoclase and unstrained quartz occur interstitially. Remnants of chlorite, epidote, and elinozoisite are scattered throughout the thin section in trace amounts. Minor accessory minerals include zircon, magnetite, and apatite. Contact Relations Using idealized chemical formulas for the principal mineral copiponents, the approximate percentages of 16 California Division of Mines [Special Report 61 eight oxides were computed for each of the rock samples feldspar were derived from the plagioclase, and the soda represented in figure 3. Samples Poe 4a and Poe 8 thus released (about 0.5 percent) was probably absorbed are chemically very similar in spite of a marked varia- in solid solution by the crystallizing orthoelase. An even tion in the proportions of several minerals. This indicates smaller amount of lime, released at the same time, may that the chemical homogeneity imposed on the schists have been consumed by the crystallization of epidote and by metamorphism was not disturbed by intrusion of zoisite. the granodiorite more than 180 feet from its contact. METAMORPHISM Sample Poe 3 has less alumina and iron oxide and more Metamorphic rocks intersected by the Poe Tunnel are silica than the other schists, probably reflecting in part products of the lowest grade of regional metamorphism introduction of silica during intrusion of the Buck within the amphibolite facies, except for the biotite- granodiorite. The granodiorite itself (sample Poe 1) has hornblende schists at the north end of the tunnel, which more alumina and soda and less magnesia and iron oxide belong to a transition phase between the greenschist than do the schists. The migration of ions both ways and albite-epidote amphibolite facies, and a thin stratum across the contact during and slightly after intrusion of massive greenschist immediately south of the outlet, is apparent in that sample Poe 2 represents an attempt Both of these rock types may have been formed by retro- at mineralogical and chemical adjustment between sam- gressive metamorphism of a higher grade metamorphic pies Poe 1 and Poe 3. The range within which these ions rock. The mechanism by which this could have occurred, migrated away from the granodiorite was rather re- in the case of the greenschist south of the tunnel, is not stricted, however, as they did not affect sample Poe 4a, clear, although the observed reconstitution of plagioclase only 180 feet from the contact. Migration of ions from and replacement of sphene and anthophyllite by calcite the schists into the granodiorite was even more re- is evidence that such a process was active. There is no stricted, inasmuch as sample Poe 1 (23 feet from the direct evidence (such as the presence of relict high-grade contact) was not affected, as indicated below by com- metamorphic minerals) that the biotite-hornblende parison with a sample of granodiorite collected at schists at the north end of the tunnel have been retro- Grizzly Creek, almost 5 miles to the northeast and more gressively metamorphosed. However, these rocks appear than half a mile from a contact with metabasalt (Hiet- to be of slightly lower metamorphic grade than the am- anen, 1951, table 1, sample 119) : phibolites that are encountered elsewhere in the tunnel, Poel Hietanen i*^ Spite of being close to the Bucks granodiorite, where Percent Percent higher temperatures would be expected to produce Plagioclase 48.7 (Anas) 42.8 (An^i) higher grade metamorphic rocks. The petrographic de- Quartz 30.3 27.7 scriptions of the granodiorite and schist adjacent to the Potash TeWsjar'""*^^ ^^'^ Contact, together with the data presented in figure 3 ^(mostly orthoelase) 9.3 8.4 and the chemical activity inferred therefrom, indicate Hornblende 6.8 that high temperature and pressure did not accompany Magnetite and sphene less than 0.5 1.2 the intrusion, and that the amount of water and other Epidote less than 0.1 18 volatile components discharged from the granodiorite .^^^^ ^j^^ adjacent wallrock was relatively small. The Computations with the idealized chemical analyses of incomplete readjustment of the schists to a lower-grade samples Poe 6 and Poe 1 suggest the course of crystalli- metamorphic rock probably occurred very slowly as the zation and differentiation taken by the magma repre- result of the presence of small amounts of volatile sented by sample Poe 6. The use of idealized "analyses" fluids and a gradual decrease in the elevated tempera- is justified by the fact that they are similar to actual ture that accompanied intrusion, sustained over a rela- analyses of granodiorite and diorite given by Hietanen tively long period of time. (1951, table 1, samples 118 and 119). Using these Metagabbro and the various amphibolites belong to idealized analyses and mineral formulas, the chemical the lowest grade of the amphibolite facies. The miner- composition of sample Poe 1 can be produced from a alogy and texture of some of these rocks are only with magma with the composition of sample Poe 6 by re- difficulty distinguished from those brought about by moving approximately 18 percent each of plagioclase deuteric alteration of basic igneous rock. Reconstitution and hornblende, and introducing 25 percent quartz. If and resorption of hornblende and plagioclase crystals allowance is then made for reaction of the diorite plagio- are common, and aggregates of grains of plagioclase clase (Auss) to Auso, and for introduction of 1 percent suggest replacement of pyroxene crystals. Alteration epidote as a late magmatic mineral, the chemical com- minerals commonly found in deuterically altered plu- position of sample Poe 1 is reproduced with an accuracy tonic rocks, such as epidote, sericite, clay minerals, and of d= 1 percent. However, if the modal proportions of calcite, are common minor constituents of these amphi- this ideal rock are then computed, it is seen to differ bolites. Such rocks are identified as metamorphic be- from sample Poe 1 by having 6 percent more plagioclase cause they have a higher proportion of hornblende than and 7 percent less potash feldspar. This discrepancy igneous diorites or gabbros; plagioclase commonly is can be adjusted by assuming that the potash which recrystallized, and has a relatively sodic composition; normally would have been consumed by crystallizing bio- and euhedral plagioclase crystals are absent. It is likely tite went instead into orthoelase and microcline, because that some form of hydrothermal alteration occurred dur- of the lack of iron and magnesia in the magma following ing and after metamorphism of these amphibolites. removal of the hornblende. (Note the very slight in- A high-grade pelitic schist and a quartzite containing crease in modal biotite between samples Poe 6 and Poe 1, minor amounts of biotite, chlorite, and calcite also are fig. 3). The necessary silica and alumina for the potash members of the amphibolite facies. 1959] PoE Tunnel, Butte County 17 o in L NORTH o If) + O in + in O m + O in + o lO _J BIOTITE- HORNBLENDE SCHIST SUPPORTED 4 SOUTH •f»- SERPENTINE *\ SUPPORTED STATION NUMBER ROCK TYPE I- "- / z < — _j 3 — O -I o o o o z UJ < => LiJ O I "J in - 2 UJ < o o — X UJ I- 5; I- u. 2 u. ^ O O ^ rsi ro rsi 2 < O — rsl O >_ 5 UJ S UJ CD X '"uT UJ iS < 1 1- Q- S <^ UJ H (5 (9 < — =) S Q 10 10 1 H-'x Ol St K ^* U ? 2 O H O UJ to I in if) o (T O r) FiGURE 4. Graph showing relations between local geological features, rock type, rate of tunneling, and amount of support required in tunnel. Tunnel intake portal is 1,000 feet north of Station 11 + 50. The metamorphic rocks of the Poe Tunnel thus appear to have been produced by low-grade regional metamor- phism, presumably contemporaneous with folding of the sedimentary and volcanic rocks, and modified locally to a slight degree by later igneous intrusion and hydro- thermal alteration. GEOLOGICAL FACTORS IN TUNNELING Experience in driving the Poe Tunnel indicates that jointing and shear zones, in particular, have controlled the rate of tunneling, inasmuch as strongly jointed or sheared rock almost always required closer and stronger support than massive rock. In addition, jointed and sheared rock, or rock from which a large quantity of water flowed, was difficult to drill and did not break well when blasted, and a great deal of hand labor gen- erally was required to move it. As a result, headings moved slowly when in this type of rock (fig. 4). Ser- pentine generally required more support than other rock types and in the Poe Tunnel all serpentine was sup- ported. Certain portions in metamorphic rock also re- quired very heavy support. In general, the gross physical features within a rock type exerted more influence on the amount of support necessary than did the change from one rock type to another. Figure 4 demonstrates the relations existing between the rate of tunneling, local physical features of the rock (shearing, water flow, etc.), the areas requiring support, and rock type. In mid-1956, two separate rock falls killed four men. The first accident occurred at the intersection of Adit No. 1 and the tunnel itself. Seven timber sections col- lapsed, causing a portion of the sheared serpentine roof to cave in. Vibration from rail cars removing rock was suggested as a possible cause of the collapse, inasmuch as the nearest working face was 3,600 feet distant (Eng. News-Record, July 12, 1956, p. 24). The collapsed area was retimbered in 2 days and work at the headings was resumed in 6 days. A second accident 10 days later was caused by the fall of a 15-ton slab of serpentine that had slipped from the upper quadrant of the working face. (Electrical World, July 23, 1956, p. 204). These acci- dents demonstrate the hazard of tunneling in heavy, sheared rock. Aerial photographs show seven fairly large creeks, the courses of which cross the tunnel line. Geological map- ping demonstrates that the courses of three of these streams are controlled by serpentine contacts, and one by a shear zone in serpentine. Inside the tunnel, two unusually wet zones occur directly beneath streams, and in addition, a troublesome 40-foot wide zone of "bleed- ing" ground at the south contact of the central mass of serpentine also is directly beneath a stream. These wet zones slowed the rate of tunneling. REFERENCES Compton, R. R., November 30, 1950, Petrology of the southwest part of the Bidwell Bar (30 minute) quadrangle, California (abst.) : Abst. of Dissertations, 1949-50, vol. 25, pp. 265-267, Stanford University. Compton, R. R., 1954, Pattern of structure and composition in a small Sierra Nevada batholith (abst.) : Geol. Soc. America Bull., vol. 65, p. 1337. Compton, R. R., 1955, Trondhjemite batholith near Bidwell Bar, California : Geol. Soc. America Bull., vol. 66, pp. 9-44. Hietanen, A. M., 1951, Metamorphic and igneous rocks of the Merrimac area, Plumas National Forest, California : Geol. Soc. America Bull., vol. 62, pp. 565-607. Nelson, Paul, December 1957, Feather River : Pacific Builder and Engineer, vol. 68, no. 12, pp. 53-59. 18 California Division op Mines [Special Report 61 PG & E News Bureau, 1954, PG & E awards contract for two bridges: Pacific Gas and Electric Co. (San Francisco), October 15, news release. PG & E News Bureau, 1955, PG & E awards Feather River tun- nel work: Pacific Gas and Electric Co. (San Francisco), April 1, news release. PG & E News Bureau, 1955, PG & E Feather River project employs 600: Pacific Gas and Electric Co. (San Francisco), De- cember 12, news release. Turner, H. W., 1898, Bidwell Bar folio, California : U. S. Geol. Survey Geol. Atlas of the U. S., folio 43, 8 pp. Turner, F. J., and Verhoogen, Jean, 1951, Igneous and meta- morphic petrology, McGraw-Hill Book Company, Inc., New York, pp. 446-473. Williams, Howel, et al., 1954, Petrography, W. H. Freeman and Company, San Francisco, pp. 209-247. 99470 7-59 3,500 prilled in California state pkintinc office Ne ,»»i»»r«f; ^ <**^fc^^'- ;?/]?- 500 1000 zooo 3000 feot H'^ 1 I 1 "^ft^i^ , <(' , J. INTAKE EXPLANATION GEOLOGIC MAP AND CROSS SECTION OF THE POE TUNNEL. BUTTE COUNTY CALIFORNIA [um\ amphiboliles', dominonlty schisloje schiil. ^o Bloitoporpft/rilie amphibolile. [S ^— IV:°'/l "••-"■"■• ■+ J. ^ 4I S 0,;... ,,,,,.,„., _.,...,., ^ Str.ie Old dip of beds ^ SI. ike ond d.p ot tol.dlion J^ 5l,,»d dnd «,,ol ...l.collo ^ SI.U, end dip of 101..1. <^ /V^ leclion (oMiludli observed m ' 0-i2G-t-i3 Poe Tunnel ttoiion nu