Mineralna sestava in vzroki za magnetnost limonitnih železovih rud v Julijskih Alpah

Loboda, Jernej (author), Dolenec, Matej (mentor)

Limonitne železove rude se pojavljajo na območju Julijskih Alp in v njihovem predgorju, kot netopni ostanek v kraški preperini karbonatnih kamnin, nakopičene v kraških depresijah, v jamskih sedimentih, sedimentih brezstropih jam, zakraselih razpokah in v breznih, na drugotnem mestu so med glacialnimi in fluvialnimi sedimenti. Osredotočili smo se na raziskovanje mineralne sestave magnetnih različkov limonitnih železovih rud, na vseh raziskovanih lokacijah in na interpretacijo procesa, ki je omogočal njihov nastanek. Nastanek nemagnetnih - goethitnih skorjastih in pizolitnih limonitnih rud je značilen za močvirska, jezerska ali jamska okolja nastanka na redukcijsko-oksidacijski geokemični barieri. Ugotovili smo, da magnetne limonitne železove rude tvorijo magnetit in/ali hematit, v mnogo manjši meri tudi ilmenit, ter goethit in amorfni limonit. Magnetni različki limonitne rude so nastali z izomorfnim nadomeščanjem – s psevdomorfozo predhodnega železovega sulfida (pirita in/ali markazita) z železovimi oksidi (v prvi fazi z magnetitom, ki ga nato v drugi fazi nadomesti hematit). Hematit je v tretji fazi lahko nadomeščen z železovimi hidroksidi (goethit in/ali lepidokrokit). Kadar proces nadomeščanja hematita in magnetita z goethitom in lepidokrokitom še ni potekel do konca, se je ohranila magnetnost. Nastanek železovih oksidov in železovih oksid hidroksidov v procesih kraškega preperevanja in postopne oksidacije iz prvotnih železovih sulfidov nam je potrdila zelo redka ohranjenost primarnega sulfida (pirita in/ali markazita) v jedru kristalov, prisotnost žvepla v analizi slednih prvin, pridobljenih z rentgensko fluorescenčno geokemično analizo, ter euhedralni preseki kristalnih ploskev pirita v poliranih obrusih magnetnih rud. Iz oblike nekaterih kosov rude lahko tudi na površini rude prepoznamo ploskve in robove predhodnih kristalov pirita s ploskvami heksaedra in oktaedra, ki so jih limonitni minerali psevdomorfno nadomestili. Prisotnost magnetnih železovih oksidov je torej posledica kraškega preperevanja in kopičenja netopnega ostanka nepopolno oksidiranih in hidroksiliranih izvornih predhodnih železovih disulfidov v kraških depresijah in s še nedokončanim procesom psevdomorfoze magnetnih mineralov z nemagnetnim goethitom in/ali amorfnim limonitom.

• Language: Slovenian
• Keywords: limonit, magnetit, hematit, železova ruda, magnetnost, izomorfna psevdomorfoza
• Work type: Bachelor thesis/paper
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2018
• PID: 20.500.12556/RUL-103580
• COBISS.SI-ID: 1418590
• Publication date in RUL: 20.09.2018

Secondary language

• Language: English
• Title: Mineralogy and cause of magnetism of limonite iron ores in Julian Alps

Abstract

Limonite iron ores occur in the area of Julian Alps and their foreland. They are found as insoluble residue in karst eluvium of carbonate rocks, accumulated in karst depressions, cave sediments, sediments of unroofed caves, in karst fissures and shafts. Secondary places of occurrence are glacial sediments and fluvial sediments. We focused on researching mineral composition of magnetic samples of limonite iron ores on every researched location, and on interpretation of process that has enabled their creation. Creation of nonmagnetic – goethite crustifrom and pisolitic limonite ores is typical for bog, lake, or cave environments on reduction-oxidation geochemical barrier. We found out that magnetic limonite iron ores are formed by magnetite and/or hematite and in much smaller quantities ilmenite, goethite and amorphous limonite. Magnetic versions of limonite ore were created by isomorphic replacements - with pseudomorphosis of preceding iron sulphide (pyrite and/or marcasite) with iron oxides (in first phase with magnetite, which has been replaced with hematite in secondary phase). In tertiary phase hematite can be replaced with iron hydroxides (goethite and/or lepidocrocite) or amorphic limonite. When the process of replacement of hematite and magnetite with goethite and lepidocrocite has not yet been concluded, magnetism was preserved. Origin of iron oxides and iron oxy hydroxides in processes of karst weathering and gradual oxidation of primary iron sulphides was confirmed by extraordinary rare conservation of primary sulphide (pyrite and/or marcasite) in the cores of crystals, presence of sulphur in trace elements analysis, gained by x-ray fluorescence geochemical analysis, and euhedral cross sections of crystal faces of pyrite in polished magnetic ore samples. From the shapes on the surface of some ore samples we can recognize some crystal faces and edges of preceding cubic crystals of pyrite with hexahedral and octahedral crystal faces, which have been replaced by limonite minerals. Presence of magnetic iron oxides is therefore a consequence of karst weathering and accumulation of insoluble residue of imperfectly oxidized and hydroxylated primary preceding iron disulphides in karst depressions, and with incomplete process of pseudomorphosis of magnetic minerals with nonmagnetic goethite and amorphic limonite.

• Keywords: limonite, magnetite, hematite, iron ore, magnetism, isomorphic pseudomorphosis