The Falémé Iron District, Senegal
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The Faleme iron district in the Kedougou-Kenieba inlier of the Paleoproterozoic Birimian Supergroup of West Africa consists of nine major and 19 minor orebodies, distributed in a belt 65 km long and 15 km wide. Two major exoskarn orebodies have total reserves of 320 million metric tons (Mt) magnetite ore with 42 percent Fe, and seven major supergene-enriched orebodies, which overlie endoskarn, have reserves of 310 Mt with 59 percent Fe. Previous workers advocated an (exhalative)-sedimentary origin for the primary ore. In this paper, however, we present strong evidence for contact-metasomatic skarn mineralization associated with microdiorite intrusions, with an emphasis on the Karakaene-Ndi and Goto deposits. The ore deposits of the Faleme district are the only major Precambrian magnetite skarn deposits worldwide that have not been subjected to postore metamorphism. The deposits of the Faleme district have many similarities to Phanerozoic magnetite skarn deposits that are associated with dioritic intrusions and also lack postore metamorphism. The endoskarn of the Karakaene-Ndi deposit is hosted by microdiorite with pervasive albitization. Garnet (And 56 –100 Grs 0 –43 Alm 0 –2 Sps 0 –1 Pyr 0 –1 ) and clinopyroxene (Di 58 –100 Hed 2 –42 ) are early phases. Individual grains of garnet and clinopyroxene commonly are inhomogeneous, with Fe being concentrated at the margin of the grains rather than in the core. Fe-rich zones in clinopyroxene are also enriched in Na. Biotite and magnetite precipitated relatively late, followed by pyrite (2 vol %) ± chalcopyrite ± pyrrhotite ± iss. Aqueous-carbonic fluid inclusions in quartz from one sample have CO 2 concentrations of 5 to 14 mol percent and indicate pressures of 300 to 900 bars and temperatures of 281° to 373°C at total homogenization. The magnesian exoskarn of the Goto deposit is hosted by dolomitic and calcitic marble, which locally contains graphite. Magnetite is associated with prograde clinopyroxene (Di 89 –100 Hed 0 –11 ) and phlogopite-rich biotite (Mg/(Mg + Fe)~0.89) as well as retrograde serpentine. Garnet is scarce. Sulfur concentrations average 1 wt percent. Pyrrhotite and subordinate pyrite ± chalcopyrite ± arsenopyrite ± pentlandite ± cobaltite postdate most of the magnetite. The exoskarn formed under more reducing conditions and at lower temperatures than the endoskarn. Magnetite in the exoskarn has trace element concentrations similar to those of magnetite in the endoskarn but is distinguished from igneous magnetite in granodiorite and magnetite in andesite (possibly of igneous origin) by low concentrations of Cr 2 O 3 and V 2 O 5 . Microdiorite is the most likely source of iron although no igneous magnetite in microdiorite could be identified. Microdiorite with only moderate hydrothermal alteration has lower iron concentrations (1.6–4.5% Fe 2 O 3(total) ) than unaltered granodiorite and weakly altered andesite in the area (avg ~7% Fe 2 O 3(total) ). Iron was probably leached from the moderately altered microdiorite and transferred into zones of intense alteration in microdiorite and host-rock marble. The low degree of hydrothermal alteration of granodiorite and small areal extent of andesite argue against a granodioritic or andesitic source of iron. The supergene ore is derived from endoskarn with about 30 percent Fe and consists of hematite and hydrous iron oxides. The largest orebodies have a thickness of 100 m and are located in the central part of hills that rise 250 m above the surrounding peneplain; the bottom of the enrichment zone usually is centered deep below the top of the hills. The absence of supergene enrichment in exoskarn orebodies probably is due to unfavorable structural conditions. Supergene enrichment of endoskarn was probably not exclusively by residual enrichment, in which Ca, Mg, Na, and Si were removed from the orebody, but also involved addition of Fe from descending solutions to the site of enrichment. Exploration for new supergene iron ore should be focused outside the exoskarn belt between Goto and Safa, as the deposits in this area are unlikely to have experienced major supergene enrichment. The Faleme district has some similarities to the geologic setting of iron oxide-copper-gold districts in which the deposits are hosted by dioritic rocks and may be representative of an iron oxide-rich end member of the iron oxide-copper-gold class. However, only about 3 t of gold have been mined from alluvial deposits in the area and the district is relatively poor in copper.Research Article| January 01, 1971 Precambrian Rocks of the Lake Hopatcong Area, New Jersey DAVIS A YOUNG DAVIS A YOUNG Department of Geology, Washington Square College of Arts and Science, New York University, New York, New York 10003 Search for other works by this author on: GSW Google Scholar Author and Article Information DAVIS A YOUNG Department of Geology, Washington Square College of Arts and Science, New York University, New York, New York 10003 Publisher: Geological Society of America Received: 22 Jun 1970 Revision Received: 18 Aug 1970 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1971, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1971) 82 (1): 143–158. https://doi.org/10.1130/0016-7606(1971)82[143:PROTLH]2.0.CO;2 Article history Received: 22 Jun 1970 Revision Received: 18 Aug 1970 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DAVIS A YOUNG; Precambrian Rocks of the Lake Hopatcong Area, New Jersey. GSA Bulletin 1971;; 82 (1): 143–158. doi: https://doi.org/10.1130/0016-7606(1971)82[143:PROTLH]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Precambrian rocks near Lake Hopatcong, New Jersey, form a part of the intensely deformed and metamorphosed Reading Prong. The Lake Hopatcong area is divisible into several northeast-trending fault blocks, each of which contains a mappable stratigraphic sequence of paragneisses and granitic or syenitic rocks.The paragneisses generally are well foliated and well layered. They consist chiefly of biotite-feldspar-quartz gneisses and quartz-oligoclase leucogneisses that are interpreted as metamorphosed potassium-rich sandstones and quartz keratophyres, respectively. A thin well-foliated unit of biotite-plagioclase gneiss is thought to be a metamorphosed sill of gabbroic anorthosite.The granitic and syenitic rocks generally form thick, regionally concordant sheets. They are typically foliated and are composed chiefly of microcline microperthite and plagioclase (or mesoperthite), quartz, and iron-rich hornblende and clinopyroxene. These foliated granitic and syenitic rocks are viewed as syntectonic magmatic intrusives. One regionally discordant, unfoliated sheet of clinopyroxene quartz syenite is probably a late tectonic magmatic intrusive.Mineral assemblages in Lake Hopatcong paragneisses may be assigned to the hornblende granulite subfacies of metamorphism. The presence of Ca-bearing mesoperthite in biotite-feldspar-quartz gneiss indicates that metamorphic temperatures exceeded 700° C, and the assemblage garnet-sillimanite-quartz without cordierite indicates that load pressure was greater than 2.5 kb. The rocks have thus probably been buried to depths in excess of 10 km. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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大兴安岭北部韩家园镇一带的古元古界兴华渡口群中分布有眼球状花岗片麻岩和条带状片麻岩,以往将其作为兴华渡口群的下部地层兴华组,本次研究表明它们是受变质和变形改造的正片麻岩.采用SHRIMP和LA-ICP-MS技术对其中的2件花岗质片麻岩进行了锆石U-Pb年龄测定,其中21TW29-1样品用SHRIMP测得的207Pb/206Pb年龄为1837Ma±5Ma,该年龄被解释为花岗质岩浆的结晶年龄.21TW29-2样品用LA-ICP-MS测得的207Pb/206Pb年龄有1741Ma±30Ma和1854Ma±20Ma2组,其中1741Ma被解释为花岗质岩浆的结晶年龄.21TW29-2样品的锆石微区Hf同位素测定结果显示其eHf(t)值为-3.9～-8.5,地壳模式年龄为2.78～3.01Ga,低的锆石eHf(t)值表明其岩浆主要来自古老太古宙地壳物质的部分熔融.1.8Ga左右花岗质片麻岩的发现证实,额尔古纳地块存在早前寒武纪变质基底,为该地块的构造属性提供了明确的证据,对认识区域构造演化及构建区域构造格架具有重要的地质意义.
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