USGS Professional Paper 1697 - Alaska Resources Library

margin or island-arc tectonic environments. Subsequent to
formation, most of these metallogenic belts and their host rocks
migrated along the margin of or were accreted to the North
Asian Craton Margin or the North American Craton Margin.
Accretionary Metallogenic Belts
Several major and minor accretionary metallogenic
belts occur in the Russian Far East, Alaska, and the Canadian
Cordillera. The principal deposit types Au polymetallic
vein, Au quartz vein, Au skarn, Cu-Ag quartz vein, Cu skarn,
granitoid-related Au, Kennecott Cu, porphyry Cu-Mo, Sn
quartz vein, Sn greisen, Sn skarn, Sn vein, Sn-W polymetallic
vein, talc, W quartz vein, and W skarn. These deposits are
generally interpreted as forming from metamorphic-hydrothermal
and (or) magmatic-hydrothermal fluids that formed
in collisional zones between terranes or between terranes and
craton margin units. In some regions, the deposits are interpreted
as forming in lower-grade, greenschist-facies extensional
zones immediately after the major period of accretion
and thrusting. The various felsic-magmatism-related deposits
are interpreted as forming during major periods of anatexis
that occurred during or immediately after accretion. The
accretionary metallogenic belts generally occur along both
sides of major fault boundaries between adjacent terranes, or
between terranes and craton margins.
Postaccretionary Metallogenic Belts
Several major and minor metallogenic belts of igneous
arc-related deposits, mainly porphyry, polymetallic vein,
epithermal, skarn, and related deposits, occur in the Russian
Far East, Alaska, and the Canadian Cordillera. The principal
deposit types are Au-Ag epithermal vein, B skarn, clastic
sediment-hosted Hg, Cu skarn, Cu-Mo skarn, disseminated
Au-sulfide, Fe-Au skarn, felsic plutonic U, granitoid-related
Au, hot-spring Hg, kuroko massive sulfide, Mo-Cu skarn,
Pb-Zn skarn, polymetallic vein, porphyry Au, porphyry Cu,
porphyry Cu-Au, porphyry Cu-Mo, porphyry Mo, porphyry
Sn, rhyolite-hosted Sn, sandstone U, Sb-Au vein, silica-carbonate
Hg, sulfur-sulfide (volcanic S), Sn greisen, Sn quartz
vein, Sn silicate-sulfide vein, Sn skarn, volcanic-hosted Hg,
and W skarn.
These deposits are mainly hosted in or near granitoid
plutonic, hypabyssal siliceous, and volcanic rocks that formed
mainly during younger, mainly Cretaceous and Cenozoic
continental-margin-arc igneous activity. These postaccretionary,
igneous arc deposits commonly transect several adjacent
terranes and continental-margin units. A general geographic
and temporal pattern exists in that the older metallogenic belts
and associated igneous rock belts, mainly of Cretaceous age,
occur inboard and the younger, Cenozoic belts occurring progressively
outboard towards active continental margins. These
metallogenic belts contain a most diverse spectrum of lode
deposits but also have a high potential for discovery of new
deposits. Particularly in the Russian Far East, the metallogeny
of postaccretionary, igneous-arc-related deposits depends to
a great extent on the lithology and composition of the host
rocks of the basement terranes. Throughout the region, in the
Russian Far East, Alaska, and the Canadian Cordillera, the
Cretaceous and Cenozoic the postaccretionary, igneous-arcrelated
deposits generally exhibit a general metallogenic zoning
typical of continental-margin arcs.
Conclusions
Conclusions 283
The Phanerozoic metallogenic and tectonic evolution of
the Circum-North Pacific can be explained as a succession
of arcs and tectonically paired subduction zones that formed
along the margins of the Northeast Asian and North American
plates above the subducting oceanic lithosphere of mainly the
Mongol-Okhotsk, Cache Creek, ancestral Pacific, and Pacific
Oceans. In both Northeast Asia and in the North American
Cordillera, most of the arcs formed in island arcs near continental
margins or along the continental margins. With respect
to Northeast Asia and North America, the paleolocations of
those arcs, which occur oceanward of coeval accretionary
complexes, are highly suspect in the Paleozoic but are successively
less so in the Mesozoic.
The complex metallogenesis and tectonics of this region
are analyzed by the following steps: (1) The notable or significant
lode deposits are described and classified according
to defined mineral deposit models. (2) Metallogenic belts are
delineated. (3) Tectonic environments for the cratons, craton
margins, orogenic collages of terranes, overlap assemblages,
and contained metallogenic belts are assigned from regional
compilation and synthesis of stratigraphic, structural, metamorphic,
and faunal data. The tectonic environments include
cratonal, passive continental margin, metamorphosed continental
margin, continental-margin arc, island arc, oceanic
crust, seamount, ophiolite, accretionary-wedge, subduction
zone, turbidite basin, and metamorphic. (4) Correlations are
made between terranes, fragments of overlap assemblages, and
fragments of metallogenic belts. (5) Coeval terranes and their
contained metallogenic belts are grouped into a single metallogenic
and tectonic origin, for instance, a single island arc or
subduction zone. (6) Igneous-arc and subduction-zone terranes,
which are interpreted as being tectonically linked, and their
contained metallogenic belts are grouped into coeval, curvilinear
arc-subduction-zone-complexes. (7) By use of geologic,
faunal, and paleomagnetic data, the original positions of terranes
and their metallogenic belts are interpreted. (8) The paths
of tectonic migration of terranes and contained metallogenic
belts are constructed. (9) The timings and nature of accretions of
terranes and contained metallogenic belts are determined from
geologic, age, and structural data. (10) The nature of collisionrelated
geologic units and their contained metallogenic belts are
determined from geologic data. (11) The nature and timing of
postaccretionary overlap assemblages and contained metallogenic
belts are determined from geologic and age data.