n Alas - Alaska Division of Geological & Geophysical Surveys - State ...

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