『Abstract
Proximal brecciform ferruginous and manganiferous rocks related
to VMS deposits of the Urals are subdivided into jasperites, gossanites,
and umbers, in addition to thin-bedded jaspers and cherts. The
coherence of host rock composition and Mn-Fe-fertility of the
sediments have been established. Fe-poor pink hematitic and gray
sulphidic chert are typical of the felsic class of VMS deposits.
In contrast the contents of Fe vary from high to moderate in ferruginous
rocks enclosed in basaltic units associated with VMS deposits.
Fe- and Mn-rich ferruginous rocks and umbers occur in association
with limestones and calcareous sedimentary rocks in both types
of volcanic sequences. A common feature of jasperites and umbers
is the abundance of replacement textures of hyaloclastites and
carbonates by hematite and silica. In addition, replacement of
clastic sulphides by hematite and magnetite is a characteristic
genetic feature of gossanites. All of these sedimentary rocks
are accompanied by pseudomorphs of hematite and quartz formed
after bacterial filaments. The abundance of replacement textures
are supportive of the halmyrolysis model, in addition to hydrothermal
sedimentary and sub-seafloor hydrothermal replacement theories.
Study of chemical zonation of altered hyaloclasts shows depletion
of their rims, not only in mobile Na, K, Mg, but also in immobile
Al, Ti, and REE; whereas Si and Fe are concentrated in situ. The
halmyrolysis model presented here, involving organic-rich calcareous
hyaloclastic sediments, resolves the problem of subtraction of
Al, Ti, REE and other elements, which are commonly immobile under
hydrothermal conditions. The evolution of the halmyrolysis process
from acidic reducing to alkaline oxidized conditions infers a
possible range in transformation from FeII-Mg smectites
to Fe-silicates and Fe-Si oxides as precursors of brecciform jasperite
and thin-bedded jasper. The higher acidic, initial stage, or gossanite
formation seems to be required for oxidation of organic matter
and/or pyrite. The acidic condition facilitates the temporal preservation
of “immobile” elements (Al, Ti, REE) in “immature” chlorite-hematite
gossanites. Another peculiarity of the gossanite-forming processes
is the likely sorption of P, U and V by iron hydroxides displacing
sulphides. The general evolution of all ferruginous sediments
results in complete Fe2+ oxidation and silicification
accompanied by subtraction of other elements. The vertical diagenetic
differentiation leads to concentration of Mn-carbonates, silicates
and oxyhydroxides into the tops of jasperite and gossanite layers.
Mn oxyhydroxides scavenge positively charged hydrated cations
like Co and Ni. Near-vent bacterial communities may activate the
processes of volcanic glass and sulphide degradation. The proposed
processes of halmyrolysis followed by silicification, in situ,
may resolve the enigma of silica-rich sediment formation in a
silica undersaturated ocean. The discrimination between gossanite
and jasperite is useful for elaboration of new criteria for local
exploration of VMS- and Mn-deposits. Halo dispersion of gossanites
covering an area about to three times that of the massive sulphide
deposit is a good vector for ore body discovery. Proximal gossanites
can be differentiated from jasperites by presence of relic sulphide
clasts or elevated contents of chalcophile elements (Cu, Fe, Zn,
Pb, Bi, Te, As, Sb, Ba), noble metals (Au, Ag) and distinct REE
patterns with La and Eu positive anomalies. The development of
halmyrolysis and biomineralization models merit further elaboration
and testing in on-going research, in order to add or revise theories
of iron and manganese deposit formation.
Keywords: VMS deposits; Haloes; Halmyrolysis; Jasperite; Gossanite;
Umber; biomineralization』
1. Introduction
2. Methods
2.1. Geological setting
2.2. Types of ferruginous and manganiferous rocks
2.2.1. Jasperite
2.2.2. Gossanite
2.2.3. Umber
2.2.4. Jasper and sulphidic chert
2.3. Biomorphic signatures
2.4. Chemical composition
2.4.1. Si, Fe, Mn and Ca
2.4.2. Al and Ti
2.4.3. Mg, Na and K
2.4.4. Co, Ni, and Cr
2.4.5. Cu, Bi, Te and Se
2.4.6. Zn, Cd, Pb, As, Sb, Ba, and Sr
2.4.7. Au and Ag
2.4.8. Mo, W, and Sn
2.4.9. P
2.4.10. Ga, Ge, Sc, Zr, Nb, Hf, Ta, and Th
2.4.11. REE and Y
3. Discussion
3.1. Jasperite
3.2. Gossanite
3.3. Umber
3.4. Microbial influences
4. Conclusions
Acknowledgments
Appendix 1. Major (wt.%) and trace elements (ppm) analyses of
ferruginous and manganiferous rocks from VMS deposits, the South
Urals
Appendix 2. Correlation-coefficient matrix of elements in the
ferruginous and manganiferous rocks, the South Urals
Appendix 3. REE (ppm) analyses of hyaloclasts and hematite-quartz
pseudomorphs (LA-ISP analyses) from Mezhoserniy district, the
South Urals
References