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Table 4.--Potassi ma [Constants used In calculati of accuracy are estimates Hap Field Lat N. No. (fig. 17) long Y 0' Hor 16: les: +4,=1 3.581~10-~~ yr'l; ~~-4.962~10-~~ yr-l; 40~/~=1 .167x10-~ mol/mol . Lfmi ts ltlcal prec :f sf on at the 68-percent-conffdence 1 eve1 1 tl neral - - Race-metal anomalies mxx~~Iat& w ~ stlicification n and ar@Uc alteration in a rhyolite flow&rne mmplex ia vo~eanic mdre of ttte N River area, Medm w-e, By Miles L. SIlberman, Ridmw u OZeetg, Le-da Seth Greg, and William I. Patton, Jr. Several small 6ess than 5 a) rhyolite flowdome complexes of Late Cretaceous and (or) early Tertiary age intrude and overlie dominantly tmdesitic volcanic rocks of the Nowitne River area (area 4, fig. 13) in the northwestern part of the Medfra quadrangle (Petton and others, 1980). The rhyolitic rocks are exposed as flows and tuff within the more mafia volcanic rocks, and as domes consisting of phty rhyolite and turf containing sparse quartz and partially altered feldspar crystaIs. ?he matrix consists of alkali feldspar and quartz (Moll and others, 1981). Alteration, quite common, consists largely of deposition of minor amounts of quartz in open spaces, oxidation of sulfides, devitrification of the matrix, and formation of spherulites. The most intense alteration observed is restricted to a small dome at the confluence of the Susulatno River and Sunrise Creek (sample traverse E, fig. I%), informally called Sunrise dome, which has a mineral nssemblwe dominated bv quartz and kaolinite containing significant amounts of i ron oxide 5, evidence of hydrothermal brceciation, and anornaiot E- amounts of mercury, arsenic, and antimony. PETROGRAPHY AND PHYSICAL CHARACTERISTICS The rocks of Sunrise dome wear to consist of crystalpaor flow-banded rhyolite and tuff; the original textures are largely obscured by alteration. The rocks are mostly light tan, brown, or reddish, with mottied and enalytlcal data for samples from the Katyuh Mountains h P Mean K 0 40Arrad 40~r,,d Calculated age Unit (wt ~ $ 1 (1010 allg) ipcrtcnt) (m.y.) gray streaks from addition of fine-grained quartz veinlets and wisw. Hvdrotherrnal brecciation is common. and the breccia fkments are largely angular, free: tured, and surrounded by a matrix of iron oxfde or iron-oxidestained fine-grained to chalcedonic silica. Some areas have stockwork fractures coated with Iron oxide, ineluding hematite, and, locally, alined vugs are fiLled with botryoidal hematite. The rocks are fine grained and largely recrystallized. The most common mineral is quartz, present as fine-grained to very fine grained aggregates,. patches, wispy veins, and vug and vesicle filings. Oriainal felds~ars are. in most places. completely &placedby kaolinite, andspatches of knol linite occur in the recrvstallized moundmas as well. 1 No sulfides were seen in-hand spechen or thin section, 1 although heavy disseminations of limonite and hematite are common, as are the numerous thin fracture caatings and veinlets of iron oxides mentioned previously. The large amaunt of iron oxide (Fe content of samples ranges as high as 10-20 percent by spectrographic analyses) suggests the prior occurrenee of sulfides in the rocks. A single sample (39, table 5) contained disseminated tourmaline and minor sericite as well as quartz and knolinite. The mineral assemblage of the mcks would be considered argillic, rather than advanced argillic, because of the absence of other minerals commonly found in advanced argillic suites, such as pyrophyllite, diaspore, alunite, etc. (Meyer and Hemley, 1967; Hudson, 1977). We suggest that the kaolinite formed late, probably by alteration of feldspars and the matrix that were not replaced by silica during early stages of alteration, as a response to acid+ulfate waters generated by the weathering of pyrite and other sulfides. This part of Alaska has been exposed to weathering for much of the Tertiary and has not been glaciated at the elevations of this occurrence. An alternative origin for the kaolhite could be solfataric alteration from
!omlensed iring vapors generated by siderably higher than the cwtal a1 boiling above a srtclllow water table. We believe that Parker (1967). Two ."^ ----I SUI~IVL~S contah~ ..." -7- CUI~J~~UI~US an. \-.. LJU he absence of other advanced argillic minerals, how- Cu and Pb contel nis are ql ilte low. Two otht wer, favors a cool, supergene-type origin for Vle kao- contain detectabl Le Mo, bul t only at 1 *e 5-ppm inite. 155'00' 0s EXPLANArnN Slrrliclal d Isposits (Ouarternary) TKr Rhyolite d ornes, tulls.and breccia fflTertlary and Cretacaws) TKt Truh,,mrl ,,,u,.,,,ieslte, basaltic andeslfe. basalt flows, and mlnor rhyolite (1 rertlary and 1 Cretxmus .) MRs v Sedimanta ~ry and volca inic mcks (Mezazolc and Paleozoic) 4 Samnla -- ,. tn . 3 v m fautr, doti !mi whwe co - Syncllns Rgum 1B.--GeneraIlzed geologic map of Sunrise dome area, Medfra quadrangle, west-central Alaska (mcdified from Patton and others, 1980). Table 5 lists partrtu spectrographic and selected hemica1 analyses of the samples from Sunrise dome. n contrast to tho generally low values of trace metals ound In the partially dtered and unaltered volaenic rocks of the Nowitnn River area, the samples from the severely altered Sunrise dome (table 5) are strongly anom~ous fn M (avg 2.1 pprn), As (8vg 57 ppm), and Sb (avg 21 ppm? relative to the crustal aversger for elsic rocks (Parker, 1967). In the Sunrise &me amles, Au and Ag are below the limits of detectabwty or the analytical methods used (0.05 and 0.5 pprn, repectively), although these limits themselves are con- 2por ted b! Tabla ~.--sr.lu-td trace-element p;nmlitry of alter@ Pock% fm Sunrise nr -- [All values dn parts pcr m+)lfon. and Sb dat*dfi& by atmic-abrorptio~. ~cLW. fig by fnstrunm~mI nethod. 4s and Zn by colortmetrlc method. and kq. Cu. m. and ~b by rm~qurnt~ta~!vs Ipectroprrphie method. n. not d~qt@r.t& at dnalytlcat llmlt (In parpntheres): L. detected. but &lor annlytlcal limlt] 37 38 3U 29 40 41 42 (3 431 44 4 5 176 Average-. Comldering the very Urnlted geologic and gea chemical work to date at Sunrise dome, r Ne state that this is a mineralized system, utnougn ... canno . - I- does satisfy four of the important criteria that heve been identified as exploration guides for hot-spring precious-metal systems (M. L. Silberman, unpub. data, 1981): (1) Silicificcltlon-addition of chalcedonic silica in Large amounts In the altered zones; (2) hydrothermal brecciation-indicatlve of repeated boiling episodes, which appear to be important in mheralfeing processes (Buchanan, 1981); (3) anomalous contents of trace elements (AS, Sb, tIg), which are commonly associated with thermalspring systems and used as guides to these deposits (Badtke and others, 1980; M. L. Silberman, unpub. dsta, 1981); (4) ~rgillic alteration, associated with partid silIcificatlon. The argillic assemblage and partial silicificatiol are typical of the upper zones of several hot-sprln( and stockwork gold deposits where they overlie denseb silicified and mineralized rocks. At Cinola (Kimbach and others, 1981) and in other deposits of this type, Au and Ag contents are very low in the upper parts of the system. The apparent absence of silver at Sunrise dome might be explainable If the pracess of formation of kaolinite wes as we have suggested. Silver could have ken transported downwards by supergene solutions and enriched at depth, as oacurced at Pueblo Vie* (Kessler and others, 1981). Gold, though general. Ly less mobile than silver in an oxidizing environment can and does migrate, particubly if it was depositer withh sulfides or as very fine grained native golc (Boyle, 1079). It is equally Ukely, however, that golc was not present in significant amount3 at this level 01 the system. Analytical techniques with lower limits 01 detectabiuty, such as instrumental neutronactivatlor.
Page 1 and 2: n Alas Accompli: ' I.S. GE OLOGI CA
Page 3 and 4: I CONTENTS Page A bst raet ........
Page 5 and 6: -- --A I ALASEU i nued rral expluru
Page 7 and 8: ........ igure .JB. ;5~ercn map ox
Page 9 and 10: . , ".,k a. ,.e ,\a 1 y U11 IIIUUII
Page 11: THE UNITED STATES GEOLOGICAL SURVEY
Page 15 and 16: ~ t I r n t S i m ~ ~ Alarr a ~ man
Page 17 and 18: I -1977, RelImlnary documentat!on l
Page 19 and 20: I I MaCenn, W. R., Perez, 0. J., an
Page 21 and 22: were deslgned to impMve the accurac
Page 23 and 24: Noatak Sandstom and is overlaln con
Page 25 and 26: I (Nilsen and others, 1981a); (2) f
Page 27 and 28: I that contains the Upper Devonian
Page 29 and 30: who found Westeqaardodina sp., posb
Page 31 and 32: Table 2 lists the means rtnd for th
Page 33 and 34: ' Noatak Vdley (fig. 129. This ice
Page 35 and 36: 3 EUMN OF MAP UNITS WAmWARY OUAERNA
Page 37 and 38: -om displacement of the cc tween th
Page 39 and 40: I US I Surlicial dcnrrua,ts [~dater
Page 41: I Plafker, George, Hudson, Travis,
Page 45 and 46: and the thinning. -upward cycles .,
Page 47 and 48: Kellum, L. B., Devless, S. N., and
Page 49 and 50: 1912 sample (a mediumwey pumice blo
Page 51 and 52: various Utholagic units present Thu
Page 53 and 54: fault, and Its depositional basemen
Page 55 and 56: suggested by coplanar foUaticm and
Page 57 and 58: 1 slgnlficarrtly more umnlum (73&1,
Page 59 and 60: ecrSigtallized catadastic matrix of
Page 61 and 62: analyzed to determine whether Immob
Page 63 and 64: are tgplcd of both ocean-floor basa
Page 65 and 66: & Fclsic in~rutirt rucks 0 Eio~ite
Page 67 and 68: are Lrdlcated by coexisting @&ite+q
Page 69 and 70: (Mg3.09 pe2+ 0.69 pe 0.~1~0.01~~0.9
Page 71 and 72: westward into a narrow band that ex
Page 73 and 74: EXF'lANATIOW 66600' Contan-Apprnimn
Page 75 and 76: ! few fold closures are preserved.
Page 77 and 78: even thickness and conform to irreg
Page 79 and 80: (Alnus ap.), heaths (Ericaceae, + E
Page 81 and 82: terrane extends at least 300 krn to
Page 83 and 84: Table 19.--6tneral petrography of M
Page 85 and 86: were measured on 8 12-in. mass spec
Page 87 and 88: Thin lenses of cabonate packtone to
Page 89 and 90: The cantwell(?) Pormation south of
Page 91 and 92: in the 18 lava flows b thermoremane
Page 93 and 94:
Upper Cretaceous shale in contact w
Page 95 and 96:
Gran tz, Arthur, 1960, Generalized
Page 97 and 98:
at 15 to 20 percent. Primary Inolus
Page 99 and 100:
addition, this factor generally def
Page 101 and 102:
Joreskog, K. G., Klovan, J. E., and
Page 103 and 104:
Mineral qItWation end r ~ k t k W e
Page 105 and 106:
1 "~_liO-/ 200 1000 B roo C E % A B
Page 107 and 108:
Smaller placer mines ere active on
Page 109 and 110:
Figme 62.-Plant fassils from the co
Page 111 and 112:
Hallam, Anthony, 1975, Alfred Wegen
Page 113 and 114:
u ALASKA Figure 65.--Sketch map of
Page 115 and 116:
F ' i a?.-Intertidal bluffs compose
Page 117 and 118:
C-s - - Figure 70.-Products of eros
Page 119 and 120:
curve is based indicates that 6.1 c
Page 121 and 122:
sampled is related to widespread ma
Page 123 and 124:
marble is alsa locally present In t
Page 125 and 126:
Bedding in the conglomerate ranges
Page 127 and 128:
1 purpose of this study wes to dete
Page 129 and 130:
The secona k~~-~ri?tation mmes that
Page 131 and 132:
epizonb'l granitic Miss, mixed with
Page 133 and 134:
40 40 Forbes, R. B., and Engels, J.
Page 135 and 136:
foliated and inequigranular end con
Page 137 and 138:
I I thy euheclra. Sphene anhedra an
Page 139 and 140:
Quartz Alkali feldspar Plagioclase
Page 141 and 142:
F'@m 85.--Sketch map of Juneau area
Page 143 and 144:
HoUister, L. 8, 1966, Garnet zoning
Page 145 and 146:
! ~ bI8oO, (IN PERMIL) ~lqp~e 87.-8
Page 147 and 148:
Figure 90.-Offshore ereas discussed
Page 149 and 150:
Preliminerg analpsis of miemfauna f
Page 151 and 152:
Survey tn Alaekai AccompUshments hh
Page 153 and 154:
Gecllogical Survey Open-File Report
Page 155 and 156:
I provenance, and tectonia sIgnific
Page 157 and 158:
Keith, T. E. C, Barnes, Ivan, afrd
Page 159 and 160:
Open-Flle Report 8P811C, 19 p. + 2
Page 161 and 162:
United States Qeological Survey in
Page 163 and 164:
, ,. nldslra poblisbe!d by Prior, D
Page 165 and 166:
60, Alaska: Offshore Technology Con
Page 167:
Author Index [Page number underscor
Page 170 and 171:
Parks. Bruce ......................
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