DATE TBA AGS Luncheon Topic:
"Revisiting the Ruby Terrane: Insights from Geologic Mapping in the Moran Area, Central-Interior Alaska"
Noon Luncheon 11:30-1:00 pm
In 2011, the Alaska Division of Geological & Geophysical Surveys (DGGS) conducted 300 mi2 of 1:63,360-scale geologic mapping in the Moran Dome area of Central-Interior Alaska. The Moran project map area is located 150 miles west of Fairbanks, in the eastern Kokrines Hills, immediately north of the Yukon River, which follows the Kaltag, regional-scale, right-lateral, strike-slip fault system. DGGS geologists have distinguished eight mappable units, two regional metamorphic events, at least four ductile deformation events, and a single intrusive event followed by brittle deformation; at least three metallogenetic events overlap the area.
The Proterozoic to Paleozoic Ruby Terrane (RT) underlies most of the Moran area. DGGS subdivides the RT into five meta-sedimentary units, which spatially correspond with interpretations of the DGGS Moran airborne geophysical data. From structurally lowest to highest they are paragneiss and pelitic schist, calcareous-mica-albite-quartz schist, massive to foliated graphitic quartzite, pelitic schist and metagrit, and an interlayered graphitic quartz schist and quartzite unit. Detrital zircon U/Pb ages in three samples of the RT are predominantly Paleozoic with a minimum interpreted age of middle Triassic. In contrast, a fourth sample from the pelitic schist-metagrit unit is characterized by abundant Paleoproterozoic detrital zircons, similar to samples from the Wickersham grit and the Yukon-Tanana terrane. Shallow-dipping S2 foliation is axial planar to recumbent F2 isoclinal folds, and refolded by upright, open-to-close-profile NE- and SW-plunging F3 folds and lesser upright, open-to-close-profile NW- and SE-plunging F4 folds. Stretching lineations plunge shallowly to the NW and SE. A Kaltag-parallel, steep, E–W-trending, S3 foliation cuts S2 along the Yukon River. Intermixed within the meta-sedimentary units are outcrop- to map-scale pods of mafic amphibole-feldspar schist, with local relict magmatic augite and hornblende. Trace-element geochemistry indicates diverse tectonic settings: (1) two distinct groups of volcanic arc basalts (VAB), (2) within-plate basalt (WPB), and (3) a low Ti, P, and Zr group of unknown affinity.
The allochthonous Jurassic Tozitna Terrane (TT) structurally overlies the RT, presumably in thrust contact. Meta-mafic rocks have a trace-element-indicated mid-ocean-ridge basalt setting, there are only minor intercalated metasediments and ultramafic rocks in the map area. Shallow NE- and SW-dipping foliation is folded about an upright NW-trending axis, which is parallel to shallow-plunging L1 stretching lineations. Shear-sense indicators parallel to L1 are mixed, with six samples showing top-to-the-NW, two samples showing top-to-the-SE, and two samples with mixed sense.
RT mafic schists record a high-pressure epidote-amphibolite±garnet M1 event strongly overprinted by an M2 greenschist-facies albite+chlorite+actinolite assemblage. Relict late-magmatic hornblende replacing augite in mafic cumulate indicates an even earlier hydrothermal metamorphism. In pelitic rocks abundant chloritoid and paragonite suggests high-P M1 conditions; mineral compositional geobarometry indicates P of 9-11 kb. M1 syn-S1 garnet porphyroblasts are wrapped by S2 biotite and overprinted by an M2 greenschist-facies event. In contrast, microprobe analyses of TT rocks confirm the presence of pumpellyite and blue amphibole species riebeckite, crossite, and compositionally zoned ferro-glaucophane to ferro-hornblende. Late magmatic to early-post-magmatic hornblende in clinopyroxenite cumulate indicates pre-regional metamorphic P of 4-6 kb. TT rocks typically contain sub-to-euhedral augite porphyroclasts within a lower-greenschist-facies mylonitic foliation defined by chlorite+albite±riebekite±actinolite±pumpellyite.
The Early Cretaceous Melozitna pluton (MP) intrudes the RT. It is predominantly a very coarse-grained, porphyritic, K-feldspar-megacrystic, biotite monzogranite, with lesser medium-grained, equigranular, two-mica granite and granite porphyry dikes. Locally abrupt textural and compositional changes suggest multiple levels of the pluton are juxtaposed by late, brittle faulting. Rocks of the MP contain 67-76 wt.% SiO2 (average ~74 wt.%), display slightly peraluminous composition (average Al Saturation Index ~1.2), and generally plot in the high-K calc-alkaline field (average K2O wt.% = 4.93). Tectonomagmatic plots indicate a within-plate granite signature, with elevated Rb, Nb, and locally Y, suggesting extensional or transtensional emplacement processes.
Lode-gold accompanied by strongly anomalous arsenic occurs in inclined to steeply dipping veins and breccia zones that appear to be unrelated to the Melozitna pluton. The identified lodes are not the direct sources of most of the historically mined placer gold, however. Microprobe analyses of placer gold grains from the Moran area show a wide range of fineness (<750 to >950) and core Hg contents (<0.1 to >1.2 wt% Hg), indicating multiple lode sources types (both igneous and metamorphic), and suggest prolonged residence in the surface environment. Metamorphic lode deposits may be coeval with and related to metamorphic fluids necessary for the massive greenschist overprint in the RT. Tin and tungsten-bearing skarns and tourmaline veins are present in and adjacent to the Melozitna pluton. Polymetallic cassiterite, argentifereous galena, and sphalerite occur peripherally to and are coeval with the Melozitna Pluton, and are a likely source of abundant cassiterite pebbles and an identified placer-tin resource in Tozimoran Creek. Igneous lode-gold at Monday Creek in the RT is associated with quartz-arsenopyrite veins with an Ar-Ar reset age (white mica) of 70 Ma. An extension of the northeastly trend of 65 Ma plutons associated with placer gold-tin in the Ruby –Poorman district may represent an igneous heat source for the Monday Creek veins after right-lateral Kaltag offset is restored.
The RT represents a metamorphosed composite terrane comprised of sedimentary protoliths with diverse provenance and mafic protoliths with multiple tectonic affinities. Some protoliths are analogous to the Casadepaga Schist and Mixed unit on the Seward Peninsula while others are similar to rocks of the Yukon-Tanana terrane. All rocks within the RT in the Moran area have undergone similar tectonic and metamorphic histories with a synchronous high-grade D1/M1 and isoclinal F2 folding and development of S2 foliation. Widespread greenschist metamorphism overprints the early metamorphic history. Subsequent upright F3 folds record moderate NW-SE compression, which predates intrusion of the Melozitna pluton. Both the RT and TT exhibit NE-SW compressional D4 features, shallow NW- and SE-plunging F4 fold axes and stretching lineations, suggesting they were juxtaposed to each other by this time. This metamorphic history, similar in timing to that recorded elsewhere in the RT, varies in the metamorphic grade recorded, suggesting that blueschist metamorphism in the RT is not ubiquitous, and that blue amphiboles in the footwall of the TT are not direct indicators of high pressure metamorphism unless positively identified as glaucophane. This work will published as a DGGS Report of Investigations geologic map with companion text and data files.
Larry Freeman, Geologist
Larry Freeman is the Section Chief of the Mineral Resources Section of the Alaska Division of Geological & Geophysical Surveys. He has a B.S. in Geology from Western Washington University and an M.S. in Geology from Oregon State University. He joined DGGS in 2000, following 23 years in the minerals industry covering much of Alaska on projects ranging from grass-roots exploration to mining. He is a coauthor of the Annual Alaska Minerals Report which documents mineral industry activity and mineral resources of Alaska, and leads a team of geologists that complete mineral resource assessments and geologic mapping in potential mineralized areas of the state. The Mineral Resources Section completes geologic mapping covering 300 square miles at 1:63360 scale and publishes airborne geophysical surveys of more than 1000 square miles annually. www.dggs.alaska.gov/sections/minerals