Mineral Deposits At The Beginning of the 21st Century

The Joint 6th Biennial SGA - SEG Meeting

August 26-29, 2001 Kraków, Poland

J. Cabała

Faculty of Earth Sciences, University of Silesia, Katowice, Poland

 

 Development of oxidation in Zn-Pb deposits in Olkusz area

 The richest deposits of oxidized ores were extracted in open pits until the mid 1980s. Heavily oxidized segments of the ore deposits can be found in horst faults that are not isolated by impermeable roof sediments of Keuper. Such oxidized ores in the horst areas were analysed, documented and extracted in open pit in Krążek, Ujków, Jaworzno, Matylda.

Mapolza.jpg (69882 bytes)Utl_olkuski.gif (149387 bytes)Oxidized areas in Olkusz-region--->

<---Geological map showing Zn-Pb ore deposits in Olkusz-Zawiercie region

 

Unfortunately, there is little information about oxidized pockets existing in the bound of sulphide ore.

The systems of tectonic fissures are essential determinants of the degree of water permeability in Triassic sediments and as such they play a key role in the development of zones of oxidation. Faults and accompanying fissure systems create paths of hydraulic contact through which vertical and/or horizontal permeation of descending oxidized water occur.

Occurrence of oxidation zones depends on directions of water flow including oxygen.

Main direction of oxidation development in ore deposits:

Rys2A_sga.jpg (55045 bytes)A - in fault zone,

Rys2B_sga.jpg (87327 bytes)B- in collapse breccia,

Rys2C_sga.jpg (73904 bytes)C- in erosion gaps and sinkholes

Rys2D_sga.jpg (69679 bytes)D-in surrounding of karstic cavern

Karstic cavern in oxidized  ore-bearing dolmiteKRAS.JPG (58378 bytes)

Orebodies within upthrown blocks of faults are more likely to occur within aeration zones where the process of oxidation develop much faster than in zones of saturation.

Within the Pomorzany deposit, the majority of the isolated oxidized zones can be found in upthrown blocks of normal faults.

Oxidized ores can also be found underneath impermeable Keuper formations (Sikorka, Pomorzany) or at considerable depth in Roethian and Devonian formations (Klucze, Zawiercie). Their origin is associated with horizontal flows (fissures, interlayer joints, karst systems) of oxidizing water, and sometimes with ascent of these waters in a regime of the confined acquifer.

Rys3_sga.jpg (45837 bytes)Distribution setting of oxidation degree

of zinc sulphides in profiles of ores

Fig4_sga.jpg (46735 bytes)Distribution setting of oxidation degree

of iron sulphides in profiles of ores

Mineral composition of oxidized ores

Zinc, lead, and iron sulphides in relic amount are present among oxidized mineral components. The proportion of content of zinc, lead, and iron sulphides in oxidized ores are different that in unchanged ore sulphides.

oxidiz4.gif (10846 bytes)oxidiz3.gif (10573 bytes)

Accompanying elements

The most important of them are silver and cadmium, both of which can be recovered during processing.

Silver does not create its own mineral phases. It occurs in the form of substitutions in the crystal lattice of zinc, lead, and iron sulphides. Average content of silver in oxidized ores is slightly higher than in ore sulphides. This is due to the increased content of galena.

Cadmium is present mainly in the crystal structure of zinc sulphides. Only rarely does it create (in combination with sphalerite) its own isostructural sulphide - greenockite. During the process of ore oxidation cadmium is dissolved and then it can enter crystal lattice of zinc carbonate - smithsonite. This relation is presented in chart below.

Cadmium concentration in oxidized ores (Olkusz Mine) 

Most often in well oxidized ores the concentration of cadmium can be slightly diminished due to the possibility of removal of some of the Cd2+ ions by the waters of the oxidation zone.

Arsenic is a typical element in the ore deposits from Silesia-Cracow area. The arsenic is absorbed by the secondary iron oxides. Because in the ore dressing all iron minerals are treated as waste, all of the elements that are associated with them (including arsenic) become flotation tailings.

Conclusions

Depletion of ore resources in the central parts of deposits is the main reason for undertaking the exploitation of workable reserves on the peripheries.

Deposits such as Klucze, Sikorka and neighbouring the already exploited Pomorzany Mine show higher oxidation and  considerable variability of zinc and lead sulphides oxidation.

The identification and development of oxidation zones allow for more accurate recognition of metal reserves and estimation of  deposits value.

The change of rock tint associated with iron oxidation facilitates to follow tectonic zones in difficult mining conditions.

Based on oxidation levels it is possible to assess whether tectonic zones were permeable to water passing through.