NGU - BULL 427, 1995 Bjorn Bolviken, Siv Kjeldsen & Rolf Tore Ottesen 9

On the composition , distribution and genesisof continental oxidate sedimentsBJ0 RN B0LVIKEN, SIV KJELDS EN & ROLF TORE OTIESEN

Bjern Bolviken1, Siv Kjeldsen1,2 & Roll Tore Oltesen 1,3

1 Geological Survey of Norway, P.O. Box 3006 Lade, N-7002 Trondheim2 Present address: Durudk ollen 10, N-1344 Haslum3 Present address: Trondheim Municipality, Department of Environment, P,O. Box 2904, N-7002 Trondheim

IntroductionThe term oxidate sediments was introduced byGoldschmidt (1954, p, 639-641) for sedimentsformed by oxidation of solute ferrous and man-ganous ions to the tri- and quadrivalent state,respectively, Oxidate sediments can be groupedinto continental (freshwater) and marine (saltwater) varieties, The continental type, which istreated here, occurs as coatings, crusts andnodules in soils, springs , bogs, lakes and stre-ams. Such deposits were referred to very early inthe literature (e.g. Vogt 1906), due to their utilisa-tion in the production of bloomery iron (Espelund1991). Oxidate sediments can be efficient sinksfor heavy metals others than Fe and Mn, Thishas been regarded as a problem in geochemicalexploration , because varying amounts of Fe andMn oxides in stream sediments may causeheavy metal anomalies that are not related tomineralisation (Ottesen & Theoba ld 1994),However , the scaveng ing effect may also betaken as an advantage in geochem ical explorati -on, since it indicates that oxidate sediments maybe sensitive indicators of heavy metals,

Although the formation of secondary oxides ofFe and Mn has long been considered to be avery important geochem ical process (Gold-schmidt 1954, p. 639-642), the present knowled-ge about oxidate sediments is rather incomplete.This contribution reviews some data recentlyobtained by NGU (Ottesen & Volden 1983,Ottesen et al. 1985, 1994, Belviken et al. 1994,Ottesen & Kjeldsen 1986, and Kjeldsen et. al.1994).

Materials and methods125 samples of oxidate sediment nodules, crustsand coatings were collected from stream bedsand lake bottoms in southeastern Norway. Thesamples were analysed for up to 52 chemicalelements by a variety of methods, Mineral com-ponents were identified in 33 of the samples by

X-ray diffraction spectrometry and microprobebacks-cattered electron image and secondaryelectron image analysis , Carbon-14 dating wasperformed on 2 samples,

ResultsComposition and age of oxidate sedimentsThe main Fe minerals are goethite (a-FeO(OH)),ferrihydr ite (Fe507(OH)4H20) , lepidocrocite(y- FeOOH) and hercynite (FeAI20 4), The mainMn minerals are vernadite (MnO(OHh) and bir-nessite (Na4Mn14027 9H20), AI-bearing silicatesand psilomelane ((Ba,H20)Mn80 16) occur asminor minerals. Amorphous substances contai-ning Fe, Mn, C, AI and Si are intermixed witheach other and with the minerals,

Table 1 shows that a variety of chemical ele-ments occur in wide concentrat ion ranges in oxi-date sediments. Depending on location , the ele-ments are enriched or depleted relative to their

Tab le 1, Contents 01hydrochlori c acid-so lub le elements in oxi -date sedi me nts, Norw ay, and ave rage enric hme nt lac tors rela -tive to the total contents in granites (Vinogradov 1962).

Element Arlthm. Rang e N Enrichmentmean tactor

AI (%) 2.1 0.4 - 7.9 114 0.3As (ppm) 27.5 1- 110 109 18Ba (ppm) 2973.7 29 -31390 11 4 4Cd (ppm) 16.4

10 Bjorn Botvtken. Siv Kjeldsen & Rolf Tore Ottesen NGU • BULL 427.1995

normal contents in granites (Vinogradov 1962).For the HCI soluble fraction of the trace ele-ments , the four highest average enrichment fac-tors are 175 (Mo), 164 (Cd), 77 (U) and 63 (W).Maximum enrichment factors for single valuesmay be up to five times higher. These resultsconfirm that determination of the contents of tra-ce elements in samples of oxidate sedimentscan be used as a sensitive method in regionalgeochemical exploration.

Stream nodules from the northern OsloGraben were found to have an age of 1220 :;:120 years S.P., while corresponding figures forconcret ions from lake Storsjoen in the Solar regi-on were 2760 :;: 200.

Geographical distribution of oxidatesedimentsOccurrences of oxidate sediments tend to clusterwithin large provinces. In agreement with earlierobservat ions (Vogt 1906, Ljungren 1953, 1955,Ottesen & Volden 1983, Ottesen et al. 1985)such provinces in Norway were found to beassociated with granites and other acidic rocks.Moreover, our observations indicate that oxidatesediment-bearing granites are in genera l magne-tic and connected with major tectonic zones. TheOslo Graben and the Protogene zone in Solor(Gaal & Gorbatshev 1987) are outstand ingexamples of such granites.

Suggested genesis of oxidate sedimentsLocal occurrences of oxidate sediments will formin situations such as that indicated in Fig. 1. Dueto low redox potential (Eh) in a bog, Fe3+ andMn4+ (which both are ubiquitous at the Earth'ssurface) are reduced to the more soluble formsFe2+ and Mn2+. When entering water with higherEh and higher pH further downstream, the dis-solved bivalent ions are reoxidised to the lesssoluble forms, which are precipitated onto mate-rial in the stream bed. Organic matter in the bogis the reducing agent , while air in the streamwater is the oxidation agent.

We suggest that the format ion of all types ofoxidate sediments follows the same sequences ,namely 1) reduction of Fe3 + and Mn4+ , 2) disso-lution of the bivalent ions and subsequent migra-tion, and 3) reoxidation and precipitation if thedissolved species encounter oxidising environ-ments. The ultimate oxidation agent will beatmospher ic oxygen. The reducing agent willvary with the circumstances; the existence of oxi-date sediment provinces implies that it must beactive over large areas. The following five alter-natives for such reducing agents fulfil this requi-rement:

. . . . . . . TILL · .

Fig. 1. Formation of oxidate sediments in a stream that is aera·ted after draining a bog area.

1) Organic matter in bogs . In regions with a tem-perate climate, bogs may be very frequent andextensive. Their reducing effect is illustrated inFig. 1.2) Sulphide mineralisations. Sulphides of Fe andother heavy metals occur both locally and regio-nally as disseminations or massive layers insevera l types of bedrock. Such reducing mine-rals may also have been brought into the over-burden by mechanical processes .3) Carboniferous black shales. Occurrences ofCarboniferous black shales are common overlarge areas in a variety of geological environ-ments. These shales have reducing propert iesdue to their high contents of C and sulphides.4) Reducing gases of deep-seated origin.Emissions of reducing gases that have migratedthrough the lithosphere have been suggested bymany authors, see e.g. Gold & Soter (1980),Malmqvist & Kristiansson (1981, 1984, 1985),Mac Donald (1983), Kozlovsky (1984) and Oliver(1986). The gases may be biogenic (due to buri-ed organ ic material) or abiogenic (mantle derivedor of other deep-seated origin).5) Cathodes in natural galvan ic cells . In connec-tion with differences in electrical conductance inthe bedrock (e.g. at occurrences of graphite hori-zons or sulphide bodies), natural potential diffe-rences (Self Potentials, SP) may be recorded atthe surface or in drill holes. SP is a manifestationof a flow of natural electric currents in the lithos-phere. The potential sources are thought to begalvanic cells caused by good conducting bodies(electrodes) that penetrate redox potential gradi-

NGU - BULL 427,1995 Bjorn Bolviken, Siv Kjeldsen & Rolf Tore Ottesen 11

ents in a less conduct ive country-rock (electroly-te). Such cells have reducing properties at theircathodic upper end (Belviken 1979).

Our work has shown that there are no univer-sal associat ions between oxidate sediment pro-vinces and features such as bogs, sulphidemineralisations and Carboniferous shales.Therefore, life processes or one or both of theremaining two more obscure reducing agentswould also play a role in the reduction of Fe3+and Mn4+. Gas emanat ions are probably morecommon than supposed earlier (Malmqvist &Kristiansson 1981, 1984, 1985), Natural galvaniccells may also provide an explanation why oxida-te sediments more often are associated withmagnetic than with non-magnet ic granites.Magnetite contains both Fe2+ and Fe3+ and is agood electric conductor indicating that magneticstructures in a granite may form Fe2+ producingcathodes.

Future workFurther research on oxidate sediments - in parti-cular the role of different reducing agents in theirformat ion - should be carried out for several rea-sons:

Areas with common occurrences of oxidatesediments may be manifestations of 1) gasemanations over oil and mineral deposits or 2)electrically conducting structures beneath thesurface. Both point to the possibility of usingsurveys of oxidate sediments as a tool inexploration.Large reservoirs of oxidate sediments mayhave a buffering effect when exposed to acidrain, and precipitating oxidate sediments pro-bably function as traps for heavy metal pollu-tants in surface waters. On the other hand,the heavy metals in oxidate sediments couldbe in a form that may be easily mobilised byexcessive amounts of acid rain and therebycause harmful effects.Deposits of oxidate sediments are probablyassociated with characteristic flora and faunawhich are adapted to the specific geochemicalconditions, Local and regional occurrences ofoxidate sediments are, therefore, of interestin environmental health and geomedicine.

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