Mezica, Topla


Main commodities: Zn Pb
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The Mezica carbonate hosted Pb-Zn deposits are located 55 km northeast of Ljubljana in northern Slovenia (#Location: 46°30' N, 14°46' E).

The Mezica ore field comprised more than 350 orebodies within an area of 64 sq. km, which were mined over a period of 350 years to 1994, with total production plus reserves of about 19 Mt @ 5.3% Pb and 2.7% Zn.   Topla is a smaller, high grade deposit, which is located 2 km to the west to the Mala Peca orebody of the Mezica ore field, and comprised 0.250 Mt @ 10% Zn and 3.3% Pb that was mined between 1974 and 1988.

The Mezica ore field lies within the Middle to Upper Triassic platform carbonate rocks of the northern Karavanke/Drau Range of the Eastern Alps which host a large number of zinc and lead deposits, ranging from small, uneconomic prospects, to large deposits. Other significant examples include Bleiberg-Kreuth in Austria and Raibl and Salafossa in Italy. Both Bleiberg and Mezica are hosted by reef and lagoonal carbonate rocks of the Ladinian Wetterstein Formation and Late Triassic of the northern Karavanke nappe, while Topla occurs within the Anisian strata of the higher Peca nappe.

Mezica is hosted within the upper 600 m of the >1200 m of platform carbonates that make up the Ladinian to early Carnian (upper-middle to lower-late Triassic) Wetterstein Formation. This part of the section comprises a fore-reef, reef, and prevailing lagoonal sedimentary facies of the carbonate platform characterised by cyclothemic sedimentation of supratidal breccias, stromatolites and biomicritic limestones formed by sea level oscillation, and subsequent shallow karstification of the emergent sediments. Mineralisation occurs as zebra ore, replacements and open-space filling in shallow paleokarst and hydrothermal karst. The principal ore minerals are sphalerite and galena, with traces of pyrite and marcasite. White sparry dolomite and blocky calcite are the main syn- and post-ore hydrothermal carbonate cements.

The smaller Topla Zn-Pb deposit is hosted by supratidal pond facies in the older Anisian (lower-middle Triassic) strata of the higher Peca nappe, 2 km to the west of the Mala Peca orebody of the main Mezica deposit. It is distinguished from the Mezica and Bleiberg deposits by the different tectonic position, the age of the host rocks, the stratabound nature of the ore and early diagenetic microtextures, zinc/lead concentration ratios, different sphalerite stratigraphy, and different ore trace element and lead isotope compositions. The deposit formed on the karst topography after the partial emergence of the carbonate platform.

The 410 m thick Anisian host carbonate succession at Topla has been divided into three sequences, namely:   i). The lower horizon of mainly very fine crystalline, commonly bioturbated dark grey limestone with intercalations of banded dolomite.   ii). The middle horizon, hosting the mineralisation, of micritic to sparry, mostly laminated, dark grey dolostone interbedded with marly dolostone which has up to 15% detrital fine-grained quartz, clay minerals, and scarce relics of terrigenous organic matter. Load-casts, slumps and folds are common. Very rare bioturbation within slumps, were subsequently diagenetically impregnated with sulphides.   iii). The upper horizon is composed of layered, frequently bioturbated, fine crystalline dark grey limestone with thin marly intercalations.

Sulphide mineralisation at Topla occurs as three irregular, lens-shaped orebodies, the Old, Eastern and Western respectively, each of which is 200 to 250 x 20 to 50 x up to 7 m, with a generally flat, horizontal hanging wall and irregular footwall. The mineralisation is largely within finely laminated dark grey dolomite. Rare breccias have barren and mineralised clasts, up to a few centimetres across, set in a barren or mineralised matrix. The principal ore minerals are sphalerite and galena, with trace pyrite, marcasite and melnikovite. Different generations of ore and gangue minerals are recognised, from:

i). An early, shallow-water diagenetic assemblage, hosted by laminated dolomicrite which is rich in organic matter, occurring as very fine (20 to 40 µm) crystalline sphalerite globules intergrown with framboidal pyrite and coarser (100 to 300 µm) galena. This form is only found at Topla;
ii). A late diagenetic after deep burial assemblage, formed by recrystallisation of the majority of the initial sphalerite I to form sub- to euhedral (60 to 100 µm) sphalerite II. This form matches the initial stage sphalerite at Mezica and
iii). Epigenetic, related to upwelling of hydrothermal waters to produce sphalerite III at Topla, formed after iron sulphides and galena and prior to the late generation of void-filling saddle dolomite III. This form is absent in the Mezica and Bleiberg deposits.

The most recent source geological information used to prepare this summary was dated: 2006.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Herlec U, Spangenberg J E and Lavric J V,  2010 - Sulfur isotope variations from orebody to hand-specimen scale at the Mežica lead–zinc deposit, Slovenia: a predominantly biogenic pattern: in    Mineralium Deposita   v.45 pp. 531-547
Kuhlemann J, Vennemann T, Herlec U, Zeeh S, Bechstadt T  2001 - Variations of Sulfur isotopes, trace element compositions, and cathodoluminescence of Mississippi Valley-type Pb-Zn ores from the Drau Range, Eastern Alps (Slovenia-Austria): implications for ore deposition on a regional versus microscale: in    Econ. Geol.   v96 pp 1931-1941
Kuhlemann J, Zeeh S,  1995 - Sphalerite stratigraphy and trace element composition of East Alpine Pb-Zn deposit (Drau Range, Austria-Slovenia): in    Econ. Geol.   v 90 pp 2073-2080
Spangenberg J E and Herlec U,  2006 - Hydrocarbon Biomarkers in the Topla-Mezica Zinc-Lead Deposits, Northern Karavanke/Drau Range, Slovenia: Palaeoenvironment at the Site of Ore Formation: in    Econ. Geol.   v101 pp 997-1021

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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