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									Implication of halmyrolysis in migration of REE during formation of
ferruginous sedimentary rocks in Uzelga massive sulphide deposits,
Southern Urals (Russia)
V.V. Maslennikov          N.R. Ayupova
Institute of Mineralogy, Urals Branch of Russian Academy of Science, Miass, Russia

R.E. Herrington
Natural History Museum, London, Great Britain

L.V. Danyushevsky
Centre for Ore Deposit Research at the University of Tasmania, Hobart, Australia

Keywords: VHMS, Southern Urals, ferruginous sediments, halmyrolysis

ABSTRACT: Ferruginous sedimentary rocks associated with the Devonian Uzelga VHMS deposits of the
southern Urals formed by cold seawater interaction (halmyrolysis) of hyaloclastic material with intercalated
carbonates and sulphides. These units have been previously interpreted as either “jasperites”, “gossanites” and
“umbers”. They have been investigated using REE geochemistry in both bulk samples and laser ablation in-
ductively coupled plasma mass-spectrometry. In carbonate-rich hyaloclastic sediments the REE contents de-
crease in the following order: hyaloclastites  partially hematitized hyaloclastites     hematite-quartz aggre-
gates. The REE contents in jasperites are therefore always much lower than in primary hyaloclastites. The
REE patterns in most gossanites and umbers were preserved with the exception of a pronounced negative Ce-
anomalies. The REE-behaviour is dependent on the presence of anion complexes available during sea-
water/rock interaction. The presence of CO32– and HCO3– in carbonate-bearing hyaloclastites promotes the
formation of soluble carbonate REE-complexes and the migration of the REE into the subalkaline seawater
during interaction of seawater with carbonaceous hyaloclastites.


1   INTRODUCTION                                                  2   GEOLOGY

Ferruginous sediments associated with the Devonian                The Uzelga VHMS deposits are situated in the
Uzelga VHMS deposits of the southern Urals are                    northern part of the West-Magnitogorsk back-arc
similar to modern metalliferous sediments (Zaykov                 zone of the Southern Urals. The ferruginous sedi-
& Zaykova, 1994). The processes of catagenesis and                mentary rocks form a halo around the massive sul-
early metagenesis have little effect on texture, struc-           phide bodies in depressions between the volcanic
ture and geochemical evidence for halmyrolytic                    domes (Fig. 1). The effusive-extrusive domes, sul-
genesis of many varieties of these rocks (Zlotnic-                phide ore and limestones were the likely sources of
Hotkevich, 1989, Maslennikov, 1999) to which jas-                 material for the ferruginous sedimentary rocks.
perites, gossanites and umbers belong. We believe                    Jasperites occur in the lower horizon of the Tal-
that jasperites were formed by cold seawater interac-             ganskoe VHMS deposit, where jasperites, fine-
tion (halmyrolysis) acting on hyaloclastites in the               grained hyaoclastite and carbonates are replaced by
presence of carbonaceous material. Besides hyalo-                 hematite and quartz. Gossanites and umbers are lo-
clastic material the oxidized sulphide fractions took             calized in the boundaries of foot wall dacites and
part in formation of gossanites. Umbers were formed               hanging wall limestones at the wedging out of the
in the top of gossanites and/or jasperites horizons by            sulphide bodies (Fig. 2). In gossanites and umbers
diagenetic fractionation of iron and manganese.                   numerous oxidized clasts of sulphides and barite
   The modern data show low mobility of REE-                      have been observed.
elements related to the halmyrolysis of effusives and                Abundant lithified gossans as products of subma-
their hyaloclastites (Guy et al., 1999). However, the             rine oxidation of massive-sulphide ores have been
low contents of Al and Ti in some siliceous-                      identified in the ore body top and the flank of
ferruginous sediments suggest REE-migration in                    Molodezhnoe VHMS deposit (Fig. 3). Hematite and
specific physical-chemical conditions during sea-                 chlorite-hematite cement contains hematitized chal-
water interaction with hyaloclastic sediments.                    copyrite, pyrite, sphalerite, galena, bornite fragments
                                                                  and hyaloclastic fragments [Maslennikov, 1999].



                                                                                                                     147
                                                               Figure 2. The setting of metalliferous sediments in section of
                                                               the Talganskoe massive copper-zinc sulphide deposit (see sec-
                                                               tion A-A in fig.1).
                                                                   1 – rhyolites; 2 – disintegrated rhyolites ignimbritic like; 3
                                                               – jasperites; 4 – xenolavaclastites; 5 – gossanites and umbers; 6
                                                               – massive copper-zinc sulphide ores; 7 – limestones; 8 – brec-
                                                               cias of limestones; 9 – breccias of andesite-basalts and lime-
                                                               stones; 10 – cherts; 11 – dike of diabase.




Figure 1. Location of siliceous-ferruginous sediments in the
Uzelga massive-sulphide bearing area (South Urals).


3     METHODS

REE analyses of the rocks were performed by induc-
tively coupled plasma mass spectrometry (VG                    Figure 3. Submarine gossanites in the hanging wall of the
                                                               Molodezhnoe copper-massive sulphide deposit. 1 – dacites; 2 –
Plasma-Quad–2c ICP-MS) with analyses of solu-                  rhyolites; 3 – copper-zinc-massive sulphide ores; 4 –
tions (Natural History Museum, Great Britain).                 interlayers of sulphide sandstones; 5-8 – clasts of: 5 – of
   The laser ablation inductively coupled plasma               dacites, 6 – of chlorite-hematite rocks, 7 – of massive sulphide
mass spectrometer (LA-ICP-MS) consisted of a laser             ores, 8 – of hematite-quartz rocks; 9 – sandstones; 10 – gos-
ablation sampler UP-213 coupled to a plasma–mass               sanites.
spectrometer HP-4500 (Hewlett Packard) at the Cen-
tre for Ore Deposit Research, University of Tasma-
nia (Hobart). Measurements were made with a Q-                 4   RESULTS
switched ND–YAG UV source, frequency quadru-
pled (wavelength of 213 nm), using He and Ar as the            The results of analyses show wide variations of
carrier gas. Single point laser shots over a 100 sec-          REE-contents in different lithotypes of siliceous-
ond period, created a crater 80 m across and ap-               ferruginous sediments.
proximately 100 m in depth. Inter-laboratory                      The REE contents in jasperites are always much
analysis of various types of standards developed by            lower than in primary hyaloclastite (Fig. 4a). Results
other LA-ICP-MS users is currently underway to                 of LA-ICP-MS analyses confirm a decrease of REE-
check the ultimate accuracy of this analytical tech-           contents in the order: primary hyaloclast – partial
nique (Norman et al., 1998).                                   hematitized hyaloclast – pseudomorph hematite-
                                                               quartz aggregates (Fig. 4b).




148                                       V.V. Maslennikov, N.R. Ayupova, R.E. Herrington & L.V. Danyushevsky
Figure 4. Chondrite-normalized REE patterns of ferruginous sedimentary rocks from Uzelga massive-sulphide-bearing area of the
Urals paleoocean.
a) 1–4 – jasperites, 5 – hyaloclastite of dacite composition (ICP MS analyses).
b) 1– hyaloclasts of dacite composition; 2 – partially replacement of hyaloclasts by hematite-quartz aggregates; 3 – hematite-quartz
aggregates (LA-ICP MS analyses).
c) 1–3 – chlorite-hematite gossanites; 4–6 – barite-bearing hematite-quartz gossanites; 7 – sulphide ore (ICP MS analyses).
d) 1 – manganiferous gossanite, 2–4 –carbonate umbers (ICP MS analyses).


   REE-contents decrease from chlorite-hematite
                                                                    5 DISCUSSION
gossanites to hematite-quartz varieties. Many REE-
patterns of gossanites have negative Ce-anomalies
                                                                    The absence of significant direct correlations of
(Fig. 4c). Positive Eu-anomalies occur in baritic and               REE-elements with Fe and Mn and a positive corre-
sulfide-bearing samples. They could also simply be
                                                                    lation with Al, Ti, Mg, K, Zr testifies to the preva-
an artefact of the Ba-content in the samples, since                 lence of a lithogenic contribution of REE concentra-
barite takes preferably Eu into its structure. The
                                                                    tion rather than sorption. Similar ratios of REE in
negative Eu anomalies typical for jasperites are de-                lavas and in hyaloclastites testify to the general im-
tected only in gossanites with siliceous material.
                                                                    mobility of REE during seawater and volcanic glass
   The REE patterns of umbers (Fig. 4d) are similar                 alteration. However, in carbonate-rich hyaloclastic
to jasperites and/or gossanites that are associated
                                                                    sediments, REE contents decrease in the following
with carbonate material, however, the negative Eu-
                                                                    order: hyaloclastites to partially hematitized hyalo-
and Ce-anomalies are less well pronounced than in
                                                                    clastites to hematite-quartz aggregates. The REE
jasperites.                                                         contents in jasperites are always much lower than in
                                                                    primary hyaloclastites.
                                                                       We suggest that the REE-behaviour is depending
                                                                    on the formation of anion complexes and their avail-




Implication of halmyrolysis in migration of REE during formation                                                               149
ability is determined by the composition of the lyers    REFERENCES
intercalated with the hyaloclastic sediments.
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al., 1999]. The presence of CO32– and HCO3 in car-          The brief reference book on chemistry. Kiev: Naukova
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are only stable under acidic conditions. Therefore,      Maslennikov, V.V., 1999. Sedimentogenesis, halmyrolysis and
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of such REE sulphate complexes during gossan for-           fields (after example of the South Urals). Miass, Geotur: 348
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cold seawater/rock interaction with carbonate-              geochemistry of metalliferous sediments. In Humphris S. E.
sulphide-rich hyaloclastites under alkaline condi-          et al. (eds), Seafloor hydrothermal systems: physical,
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cordance with the known range of solubility of              physical monograph, 91: 392-407.
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Eu and La can lead to positive La- and Eu-anomalies         elements and yttrium. Chemical Geology, 82. 159 –186.
in some gossanites. The latter is consistent with data   Zaykov, V.V., Zaykova, E.V., 1994. The metalliferous deposits
on REE patterns published for modern metalliferous          of oceanic rifts. Ekaterinburg, UrO RAN:117 p. (in Rus-
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sediments formed by seafloor oxidation of massive        Zlotnic-Hotkevich,        .G. 1989. Ferruginous and quartz-
sulfides (Barret et al., 1990; Mills & Elderfield,          ferruginouse sediments of massive-sulphide deposits In
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6     CONCLUSIONS

The REE patterns of gossans and jasperites in the vi-
cinity of the Uzelga massive sulfide deposits (south-
ern Urals) may be a result of cold seawater interac-
tion with carbonate-sulphide-rich hyaloclastites
under alkaline conditions (halmyrolysis). The
chemical coherence of rare earth elements makes
them potentially useful for the reconstruction of the
physico-chemical conditions of halmyrolysis of hya-
loclastites and sulphide sediments.


ACKNOWLEDGEMENTS

Researches are supported by RFBR (project 02-05-
64821), program «Universities of Russia» (UR-09
01.028), Russian federal program «Intergration»
(project -0035). The ICP-MS research in Natural
Hystory Museum (London) were funded by Eu-
ropian Communities, Cordis-RTD projects, 5th Fre-
imwork Programme INCO-2, project number ICA2-
CT-2000-10011. LA- ICP-MS analyses were carried
out during a visiting program funded by the ARC
Special Research Centre grant to CODES, Univer-
sity of Tasmania.




150                                   V.V. Maslennikov, N.R. Ayupova, R.E. Herrington & L.V. Danyushevsky

								
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