Magnesite-bearing fracture zones of the Zlatibor ultrabasic massif ( Serbia ) as a discrete structural – morphological type of magnesite deposits in ultrabasites

In this paper, a discrete structural–morphological type of magnesite deposits in ultrabasites, i.e., in magnesite-bearing fracture zones, is presented. The most prominent occurrences of such zones in Serbia are in the Zlatibor ultrabasic massif and they are economically very significant because they contain large reserves of high-quality magnesite, as well as of the accompanying sepiolite.


Introduction
A discrete structural-morphological type of vein magnesite deposits, magnesite-bearing fracture zones, which has not hitherto been recognized, is presented in this paper.It should be added to the already known types: magnesite veins -single or systems; brecciated veins, both lenticular and irregular magnesite bodies, and magnesite stockwork (ILIĆ 1969a;ILIĆ & RUBE-ŽANIN 1978;POPEVIĆ et al. 1996).These zones occur at numerous locations in Serbia, the most prominent ones being in the Zlatibor ultrabasic massif (Fig. 1) (ILIĆ et al. 2005).Thus, they will be the topic of further consideration.

Magnesite-bearing fracture zones
Magnesite-bearing fracture zones are complex disjunctive deformations in ultrabasites (ILIĆ 1969b;KARAMATA & POPEVIĆ 1996), having hectometre to kilometre length, mineralized by magnesite (and often by accompanying sepiolite; Figs. 1 and 2).Although, considered partially, they contain all the above-mentioned known structural-morphological types of magnesite deposits (and constitutive orebodies) in ultrabasites, they can be recognized, integrally considered, as a discrete complex structural-morphological type, based on their particular structural, morphological, mineragenetic and economic-geological features.
So far, seven magnesite-bearing fracture zones have been discovered in the Zlatibor ultrabasic massif: four in the Ribnica-Donja Jablanica ore field (Čavlovac, Masnica II, Masnica III and Rasevac), one in the Stublo ore field (Marin Izvor), one in the Slovići ore field (Slovići), and one in the Gola Brda ore field (Rasadnik) (Fig. 1).Their general strike is W-E (up to WNW-ESE) and dip towards S (up to SSW) at 20-50°.
The length of these zones ranges from several hundred metres to about two kilometres; their width ranges from several metres to several dozen metres (Figs.2a and b), while the extension to depth along the dip ranges from 100 m to 300 m (Fig. 3).They pinch out gradually along the strike and dip, but there are also transitions into magnesite veins.They usually have clear salbands (on the footwall and/or hanging wall) towards the neighbouring ultrabasites, while, in their interior, ultrabasites are intensively cataclased, serpentinized, nontronized and limonitized (Figs. 3 and  4).The magnesite in them most often occurs in the form of parallel or sub-parallel veins (simple or complex ones -with apophyses, locally brecciated), of the same orientation as the whole zone, of lenticular and irregular bodies, and stockwork.The degree of miner-alization of these zones by magnesite substance ranges from 30 to 40 %.In addition to magnesite, sepiolite also occurs in these zones in the form of veins in magnesite or discrete veins, and as cement in magnesite breccias (Figs. 3 and 4).
Magnesite either follows the whole dislocation or occurs only in some of its parts.The former is the Čavlovac magnesite-bearing fracture zone (Fig. 2a), and the latter is the Donja Jablanica-Bakića Kolibe dislocation, in which magnesite accumulation occurs in its two parts (which are treated as separate magnesitebearing fracture zones): Masnica III and Rasevac (Fig. 2b).Partial accumulation of magnesite within some dislocations can be explained by specific features of pre-mineralization and mineralization tectonics; postmineralization tectonics, which however, led only to a change of the position of particular parts of the zone (owing to differential movement of separate blocks).
Structurologically considered, the magnesite-bearing fracture zones and the fractures where the largest independent magnesite veins occur represent shear fractures, namely h0l ruptures.Thus, magnesite veins, within magnesite-bearing fracture zones (which have the same position as the zones, but are of smaller size), and independent magnesite veins (in other parts of the Zlatibor ultrabasic massif) have mainly similar elements of dip; the statistical maximum of the dip elements of magnesite-bearing fracture zones is 178/22, while in case of independent magnesite veins there are two maxima of dip elements, i.e., 14/82 and 183/68.The apophyses of magnesite veins (both those in magnesite-bearing fracture zones and in independent ones) mainly represents mineralized feather fissures accompanying the main h0l ore-bearing ruptures.
The rupture structure, formed in the pre-mineralization period, impacted decisively magnesite mineralization, which occurred in the Upper Oligocene-Mio-cene (related to strong disjunctive tectonics and accompanying hydrothermal activity).These ruptures served both for introduction of hydrothermal solutions and for localization of magnesite mineralization.In the mineralization period, the existing fissures were activated many times, there were movements in various directions along them and they were opened and closed periodically, in accordance with the development of regional tectonic movements.These movements caused shattering and crushing of the magnesite substance and neighbouring rocks, while the magnesite and accompanying minerals (dolomite, calcite, quartz, chalcedony, opal and sepiolite) of younger generations were deposited in the newly formed empty spaces.
In the post-mineralization period, strong tectonic movements occurred, finally forming magnesite orebodies, including their fragmentation and differential movement of separate blocks along fault systems.
From a mineralogical point of view, the magnesite from the magnesite-bearing fracture zones is identical to the magnesite from independent veins: it is dense (microcrystalline to cryptocrystalline), white, and exhibits conchoidal fracture.With regards to the magnesite from independent veins, it differs only in a somewhat higher content of other carbonates (dolomite, calcite), while the content of other accompanying minerals, mainly silica (quartz, chalcedony, opal), is very similar.
In accordance with its mineral composition, the magnesite from the magnesite-bearing fracture zones differs from the magnesite from independent veins in a higher content of lime (CaO), i.e., 1.5-3 % in the former and less than 1 % in the latter.
From a mineragenetic point of view, the magnesitebearing fracture zones and the independent magnesite veins are syngenetic and synchronous formations.Both these structural-morphologic types of magnesite deposits occur in ultrabasites as host rocks and were formed in the same mineralization cycle bound to the Upper Oligocene-Miocene fracture tectonics and accompanying intermediate volcanism, namely its hydrothermal activity.Then hydrothermal solutions (which, in fact, represented a mixture of genuine juvenile solutions and prevailing ground waters of meteoric origin, heated by volcanic chamber), rich in CO 2 , on their ascending movement leached magnesium out of deeper lying ultrabasites, transported it in the form of bicarbonate and deposited it in the form of magnesite in ruptures (fracture zones, faults, fissures) in higher parts of the ultrabasite massifs (ILIĆ 1969a).Differences between these two types, thus, are only related to some structural and morphological features, but not to their genesis.
From an economic-geological point of view, magnesite-bearing fracture zones are large deposits of magnesite and accompanying sepiolite, having a complex constitution.In them are magnesite orebodies of varied structural-morphological types (veins, brecciated veins, lenticular and irregular bodies and stockwork), as well as accompanying sepiolite orebodies, which altogether, from an economic-geological point of view, form deposits.These deposits contain significant reserves (from several dozen thousand tonnes to several hundred thousand tonnes) of magnesite substance of good quality for application in the fireproof materials industry, as well as significant reserves of accompanying sepiolite, which has wide industrial application based on its sorbent and catalytic properties.Mining of such large and complex deposits should be preformed on the whole and completely (of all magnesite and accompanying sepiolite orebodies), not only of the largest and highquality magnesite orebodies.
The Čavlovac, Rasevac, and Masnice III zones in the Zlatibor ultrabasite massif (Figs. 1, 2 and 3) can be considered as typical examples of magnesitebearing fracture zones.The zones have been noticed in some other ultrabasite massifs (e.g., in the Goleš and Maljen-Suvobor ones) but they have not been sufficiently explored.

Conclusions
Magnesite-bearing fracture zones represent specific dislocations in ultrabasites, mineralized by magnesite (and often accompanied by sepiolite), having hectometre to kilometre length.Although, they contain all known structural-morphological types of magnesite deposits (and constitutive orebodies) in ultrabasites (veinssingle or systems, brecciated veins, lenticular and irregular bodies and stockwork -ILIĆ 1969a), they can be classified into a discrete complex structural-morphologic type, based on their distinct structural, morphological, mineragenetic and economic-geological features.These zones are the most prominent in the Zlatibor ultrabasite massif and economically very significant, as they contain large reserves of high quality magnesite, as well as of the accompanying sepiolite.