ABOUT GENETIC LINK OF MAGNETITE SOLONGO DEPOSIT MINERALIZATION WITH DOME-LIKE INTRUSIONS OF GRANITES

Nefedjev M.A.

Geological Institute SB RAS, Ulan-Ude, Russia

The deposit occurs in the south of the Vitim plateau, the Eravna ore knot of Buryatia. It localizes in the central, most uplifted block of the Ozeornoe ore knot, confined to the steeply dipping eastern contact of submeridional granite dome-like intrusion and represented by the scarn-ore bed-like deposits that occur in the eastern limb of big anticline of Low Cambrian sedimentary-volcanogenic rocks of south-east dipping. The intrusion of granites is outcropped in erosional section and divides the deposit into the north-eastern and south-western parts (Fig.1-A).

The zinc ores compose 23 lens-like bodies that mainly occur within the scarns of southern flank, inheriting their shape and conditions of occurrence. The contacts of ore bodies are gradational, ores are massive and coarse-grained that are typical of the nazarov’s type ores.

Eight iron ore bodies lie in the eastern limb of anticline along the exocontact of granite massif. The ores are naturally alloyed (4% Mn). The bed-like bodies occur at base of scarn-ore deposit that repeats the bend of granite massif roof and are observed for 6 km along strike and 600m by dip.

A common genetic link of mineralization with dome-like intrusion of granites is established for the deposit (Vinogradov, 1972). Active granitization of host rocks, interrelation of granites with metasomatites and mineralization of contact halo, regular zonation of contact-metasomatic formations and ores, dependence of ore body structure on roof morphology of granite massif testify to it. Processes of scarn- and ore formation are observed on their interrelations on the background of inheriting the basic structural plan of deformation, conditions of localization, their internal structure and geochemical behaviour of boron, manganese and other elements. The formation of ore-bearing scarns, iron ore and zinc bodies is genetically associated with hypothetical intrusion of granites. The magmatic rocks are referred to the Middle-Upper Paleozoic (granites and granosyenite-porphyries) Zaza and Mesozoic (dolerites) complexes (Litvinovsky, Zanvilevich, 1976). Leucocratic, massive, porphyry-like granites frame the Eravna relics, host ore knot and compose the crest-like massif that cuts sedimentary-metamorphic rocks of the roof (Fig.1A). Their absolute age is 260 Ma that corresponds to the end of magmatic activity in the Lower Permian. Dykes of granosyenite-porphyries crosscut granites, contact-metasomatic formations and ores. Their formation is associated with formation of granites. Magnesial metasomatosis resulted in biotization and amphibolitization of lavas, tuffs, hornfels and limestones. Distribution of scarn-ore bodies is controlled by submeridional faults and lithological composition of rocks in no dependence on their stratigraphical position. The massif of granites and faults that bound them formed there in the Upper Carboniferous-Upper Permian. Limestone scarns, greisenization, boron, iron ore, zinc ore bodies and molybdenum zones that re[eat the main bends of granite roof by strike and dip are developed in the contact halo of granites and submeridional faults. Dykes of granosyenite-porphyries formed in the Upper Permian-Middle Triassic. They crosscut scarns, greisens and ores and are not replaced by them. Dykes of dolerite porphyrites crosscut al rocks and ores of the deposit.

Fig.1. Results of detailed geological and geophysical studies at the Magnetite-Solongo deposit:

A – Geological map; B – Map of combined favourable features; C – Plan of gravitational local anomalies; D – Plan of magnetic field isodynamic lines. (1) limestones; (2) hornfelsed felsite-porphyries; (3) granites; (4) syenites and granosyenites; (5) magnetic ores; (6) mineralized scarns; (7) scarns; (8) calciphyres; (9) dislocations with break in continuity: a) main faults (1 – Magnetite, 2 – Contact, 3 – Granosyenite, 4 – Transversal); b) secondary faults; (10) conductivity zones of the combined electric profiling (CEP); (11) anomalies of NF (natural electric field, in 1 cm: 400 mV); (12) anomalies of PP (provoked polarization; in 1 cm: 20%); (13) axes of variometric anomalies; (14) isolines of local anomalies ∆g (a – positive, b – zero); (15) isolines of magnetic field in thous. gammas (a – positive, b – zero, c – negative); (16) numbers of main faults; (17) the prospecting lines, drill holes and their numbers.

Granites formed in the postorogenic hypabyssal conditions being the product of tectono-magmatic activization in the Upper Paleozoic. Intensive magmatic replacement of host rocks with wide occurrence of metasomatic processes in the magmatic stage and gradual transition of melt into host rocks took place simultaneously with introduction of granites.

The structure of the deposit is complex. It is divided into blocks of various size and vertical thickness (0-1,5-2,5 km) by numerous faults. The rocks are intensely crushed and smashed. Geophysical techniques (Fig.1B, C, D) are of invaluable help in studying the peculiarities of the deposit structure in those conditions. The structure of the deposit, its position in the plan, block texture and ore bodies are clearly reflected in anomalies of gravity and magnetic field. Elements of block-rupture tectonics are clearly mapped in field of the seeming resistances. In addition, ore bodies are distinctly shaped by intensive anomalies of PP, NF and Vxz (Nefedjev, 2003).

Granite massif is localized in the tectonic block. Its form is determined by the three large faults, i.e. Magnetite (1), Contact (2) and Granosyenite (3) ones. In south-western part, the massif is sharply pinched out to 100 m, subsides to depth 600-700 m and widens to 4,5 km to the north. These faults controlled the location, form of Magnetite structural block and contact surface and determined the complex block structure of Magnetite ore field.Thus, the genetic link of mineralization with dome-like intrusion of granites is established for the Magnetite-Solongo deposit. The processes of scarn- and ore formation are observed through their interrelations on the background of inheriting the basic structural plan of deformation, conditions of localization, their internal structure and geochemical behaviour of boron, manganese and other elements.

References

Litvinovsky B.A., Zanvilevich A.N. (1976) Paleozoic granitioid magmatism of West Transbaikalia. Novosibirsk. Nauka. 141p.

Nefedjev M.A. (2003) Structure and estimate of ore field and deposit perspectives of Buryatia by geophysical data. Izd-vo of Buryat Research Centre SB RAS. Ulan-Ude. 206p.

Nefedjev M.A., Vinogradov B.K. (1982) Complexing the methods in predicting and searching the deposits. Novosibirsk. Nauka. 168p.

Vinogradov B.K. (1972) About relation of granite intrusion to scarns and minealization in the Solongo deposit (West Transbaikalia) // Materials on geology and deposits of Buryat ASSR. Ulan-Ude. Issue XV. P.120-125.