By Sergey M. Rodionov1, Alexander A. Obolenskiy2
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MAASTRICHTIAN THROUGH OLIOGOCENEMETALLOGENIC BELTS (72 to 24 Ma)Popigay Metallogenic Belt ofImpact Diamond Occurrences(Belt PP) (Russia, northern North Asian Craton)This Eocene metallogenic belt is related to an astrobleme or impact ring structure developed on Early Precambrian crystalline basement and Phanerozoic sedimentary rock of the North Asian Craton. The age of the belt is interpreted as Eocene. The belt occurs in the Popigay ring structure. Popigay Impact Diamond DepositThis deposit (Masaitis and others, 1975, 1998) occurs in an impact structures that is about 80 km in diameter on the northeastern margin of the Anabar shield. The ring structure forms a round basin with a floor that is 200 to 300 m lower relative to the surrounding plateau. The basin contains a specific rock complex, including volcanic-like rock. Masaitis and others (1975, 1998) identified an impactite with varying amounts of glass that chemically corresponds to andesite and dacite, rock and mineral fragments; explosive allogenic breccia that fell after the explosion in, or beyond the limits of the crater; and authigenic breccis formed from brecciated material at the bottom of the crater and that underwent high-grade shock metamorphism with melting and formation of pseudotachylite. The impactite is classified as a massive lava-like tagamite and glassy-clastic suevite. The tagamite and impact glasses have an 40Ar/39Ar isotopic age of 35.7 ± 0.2 Ma. Diamond occurs in graphite gneiss and tagamite that undersent shock metamorphism (Masaitis and others, 1998). Diamond crystals range from 0.05 to 2.0 mm diameter. Adjacent placer deposits contain diamonds up to 8 to 10 mm. Most abundant are yellow; colourless, transparent, grey, and black crystals are rare. Diamonds from the gneiss retain morphological and structural features inherited from crystalline graphite. Common are tabular crystals with a characteristic striation of basal planes due to repeated twinning, parallel intergrowths, irregular intergrowths, and aggregates. Origin and Tectonic Controls for Popigay Metallogenic BeltThe belt is hosted in the Popigay ring structure for which two origins are proposed, either meteoritic impact, or cryptovolcanic eruption. Most popular is a meteoritic origin idea (Masaitis and others, 1975, 1998) who interpret the structure as resulting from the impact of a giant meteorite. They note numerous indications of shock metamorphism and partial melting of Early Precambrian crystalline rock. The other cryptovolcanic hypothesis consists of explosion during a volcanic eruption with the ring structure being the stage of kimberlite formation, both alnoite kimberlite and carbonatite, and cryptoexplosion. REFERENCES: Masaitis and others, 1975, 1998. Lower Amur Metallogenic Belt ofAu-Ag Epithermal Vein, EpithermalQuartz-Alunite, Porphyry Au,Porphyry Cu (±Au), Sn-W Greisen,Stockwork, and Quartz Vein Deposits(Belt LAM) (Russia, Far East)This Late Cretaceous and Paleocene metallogenic belt is related to veins in the East Sikhote-Alin volcanic-plutonic belt that intrudes or overlies the Amur River and Kiselyovka-Manoma accretionary-wedge terranes. The Au-Ag epithermal vein deposits, as at Mnogovershinnoe, range from medium to large and are generally hosted in Paleocene alkaline granitoids that are closely related to coeval andesite to dacite volcanic rock. A few Au-Ag epithermal vein deposits are related to Eocene and Oligocene volcanism. The Au-Ag epithermal vein deposits, as at Belaya Gora and Bukhtyanskoe, are closely associated with rhyolite and trachyrhyolite flows and vent rocks that are commonly hydrothermally-altered to siliceous and adularia phases. Au is either disseminated throughout the hydrothermally-altered rock or is concentrated in small quartz veins. The adularia phases also locally contain Au. Placer Au deposits, as at Kolchanskoe, Ulskoe, and Oemku, are derived from Au-Ag epithermal vein deposits. In addition to the Au-Ag epithermal vein deposits, the Lower Amur metallogenic belt contains a few, small porphyry Cu (±Au) deposits that are all hosted in, or near Paleogene alkaline granitoids. The major Au-Ag epithermal vein deposits are at Belaya Gora, Bukhtyanskoe, and Mnogovershinnoe; a porphyry Cu (±Au) deposit is at Tyrskoe; and a major and large quartz-alunite deposit is at Iskinskoe. Mnogovershinnoe Au-Ag Epithermal Vein DepositThis large deposit (Zalishchak and others, 1978; Ivanov and others, 1989) consists of hydrothermally altered, adularia-sericite-quartz vein zones that range up to 800 m long and contain a series of adularia-quartz veins and veinlets. Some deposits consist of rhodonite-carbonate veins, and lenses of skarn and sulfides. The ore minerals are pyrite, marcasite, gold, argentite, Au-and Ag-tellurides, galena, sphalerite, chalcopyrite, and freibergite. The ore minerals comprise up to 1% veins and the Au:Ag ratio is 1:1. The deposit is hosted in Paleocene andesite and dacite that are genetically related to a multiphase intrusion of highly alkaline granitoids. K-Ar isotopic studies indicate an age of deposits of 49 to 69 Ma. During formation of local Au-bearing skarn, that presumably formed during intrusion of Paleogene subalkaline granite, Au was remobilized. The deposit is medium size. Belaya Gora Au-Ag Epithermal Vein DepositThe deposit (Mel'nikov, 1978) consists of disseminations and stockworks that occur in extrusive bodies of subalkalic rhyolite and dacite, and in explosive breccia of an Eocene-Oligocene igneous complex. Alteration minerals are quartz (50 to 90%), kaolinite, dickite, sericite, hydromica, and adularia. The ore minerals are gold, silver, argentite, pyrite, marcasite, chalcopyrite, sphalerite, galena, hematite, and cinnabar. The ore assemblages are gold-quartz and Au-sulfosalts-sulfide-quartz. Gold distribution is highly irregular and the deposits have gradational boundaries. The deposit is medium-size. NOTE: Need latitude and longitude. Iskinskoe Epithermal Quartz-Alunite DepositThe deposit (Onikhimovsky and Belomestnykh, 1996) is hosted in intensively altered Tertiary dacite and rhyolite. The deposit consists of a etasomatic body of quartz-alunite surrounded by a concentric zone of quartz-sericite alteration, and an outer zone of propylitic alteration. The deposit is 2.3 km long, 1.2 km wide, and up to 360 m deep. The ore minerals are: 29.4-32.0% alunite; 60.0-66.0% quartz; 2.0% halloysite; 1.5-5.0% Fe oxides; 1.5-1.6% kaolinite; and up to 2.0% beudantite. Rare minerals are pyrite, diaspore, andalusite, dickite, montmorillonite, and kaolinite. Pure alunite occurs in descrete masses ranging up to 8 to 10 cm in diameter. The deposit is large with reserves of 336,581,000 tonnes alunite ore with an average of grade of 26.1% alunite. Origin and Tectonic Controls for Lower Amur Metallogenic BeltThe belt is interpreted as forming during generation of granitoids along a continental-margin arc related to subduction of the ancestral Pacific Ocean Plate. The granitoids hosting the belt are part of the Late Cretaceous and early Tertiary East Sikhote-Alin volcanic-plutonic belt. This belt consists chiefly of five major units: (1) Early Cenomanian rhyolite and dacite; (2) Cenomanian basalt and andesite; (3) thick Turonian to Santonian ignimbrite sequences; (4) Maastrichtian basalt and andesitc; and (5) Maastrichtian to Danian (early Paleocene) rhyolite. The East Sikhote-Alin belt also contains coeval, mainly intermediate-composition granitoid plutons that in the frontal (eastern) part of the belt are predominantly Early Cretaceous magnetite-series granitoids. The East-Sikhote-Alin belt is correlative with the Okhotsk-Chukotka volcanic-plutonic belt on strike to the north in the Russian Northeast, and is tectonically linked to the Aniva, Hidaka, and Nabilsky accretionary wedge and subduction-zone terranes. REFERENCES: Mel'nikov, 1978; Zaiishchak and others, 1978; Ivanov and others, 1989; Khomich and others, 1989; Nokleberg and others 1994, 1997, 1998, 2000, 2003. |