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

development of axialplane cleavage in which the dom-
inantly sleep east-west-trending structures were over-
turned slightly to the south (Brooks, 1012; Berg and
others, 1972). Locally the rocks have been metamor-
phosed to lower greenschist faciea
Gold deposits in the Port Valdez district occur as
ffsgure veins in faults, fractures, and shear zones,
principally dong the axes and upper limbs of over-
turned anticlinai structures (Brooks, 1912). Thus, the
mlneraI occurrences are located in distinct local belts
(fig. 54). The occurrence and grade of gold minerali-
zatjon appear to be unrelated to host-rock Lithology
but may correlate with the more disturbed and sheared
areas.
Two fairly distinct tfles of quartz veins are
found in the Port Valdee area. The first type consists
of thin (generally less than 10 em thick) irregular veins
that principally parallel regional cleavege. These
veins, and similar cleav e-localized quartz velns in
the Hopesunrise dktriet?Mitchell, 1918; Mitehell and
others, ZBSl), are considered to be a result of meta-
morphicsegregation processes (Robin, 1979). me
metamorphic quartz is white, massive, tine grained,
1 and tightly frozen to the wallrock. mite, the only
sulfide mineral observed, in some places is found along
the edges of veins and in wallrock fragments within
the veins. Calcite is a common vein constituent. Most
often these veins are found along cleavage or bedding
planes, and seldom as joint and fracture fillings.
The second type of veins are those associated
with gold mineralization. Such veins are coarse
grained, vuggy, iron stained, and commonly shattered,
with weU4eveloped shears on one or both walls.
These veins, r8ngIng from sever& centimetars to a
meter in width, are discontinuous (corn rnonly separated
along strike by gteat distances of barren shattered
country rock), podifarm, and most commonly occur as
fracture flUings in joints and faults that crosscut and,
in many places, offset metamorphic-segregation veins
and regional cleavage. Figure 55 illustrates the dif-
ference in orientation for the mineralized veins and
the cleavagdocalized metamorphic-segregation
quartz veins. A few mineralized veins are also found
in shear zones along bedding planes and contacts. The
mineralized veins are all postfoldlng. Structural
features suggest that mineralization occurred along
joints and faults which opened during relaxation of the
compressive forces and (or) uplift, es was the ease at
Hope-Sunrise (Mitchell and others, 1981). The absence
of wallrock alteration or mineralization suggests that
the mineralized veins formed at a temperature below
or equal to that of metamorphism.
I
Vein mineralogy consists of quartz, chloritiaed
waUrock inclusions, calcite, sulfides, free gold, and
minor albite (Brooks, 1912; Johnson, 1915). The sulfide
minerals pyrite, galena, sphalerite, chalcopyrite, stlb-
nite, pyrrhotlte, and menopyrite (Bmh, 1912;
Johnson, 191 51, as well as free gold, are found essoci-
ated with late-stage quartz, in rnicrolractwes, and in
open cavities. Together, the sulfides generally make
up less than 3 percent of the ore. Mine reports indi-
cate that mineralfeation is discontinuous along strike
but commonly b found in steeply dipping pipelike
shoots.
Fluid inclwions in the metamorphi c-segregation
and mineralized veins have different characteristics.
A, Minerallzed quartz veins t
PERCENT
PERCENT
Figure 55.-Rase diagrams showing orientatlon and
percentage of measured occurrences within each 10'
of azimuth lor mineralized quartz veins (A) and
bedding/foliation (8) for mines in Port Valdez gold
district, Alaska. Bata from Brooks (19121, Johnson
(1915), and this study.
In the metamorphic quartz, prlmar inclusions are
small (generdllg Las than 5 urn diarn?, mcl nde Uquid
domlnatd; the volume of the vapor phase is estimated