MAGNETISM OF GOLDBEARING GRANITOIDS

Erofeev L.Ya.

Tomsk Polytechnic University, Tomsk, Russia, geof@tpu.ru

Nowadays Ore Concentration – Granitoid Magnetism connections have been regionally studied in a general way by the examples of the granitoids in the Far-East and North-East Russia only (Romanovsky, 1976; Rozental, 1977).

This paper analyzes the results of a detailed study of the magnetic parameters of two ore-containing granodiorite massifs with a common type of ore concentration: Kuznetsky Alatau (Tsentralny batholite, Tsentralnoe field), and Vostochnoe Zabaikalye (Tegenguisky pluto, Aprelkovo field).

The Tsentralnoe gold-quartz-vein field is fully located within the massif, which includes Middle Cambrian volcanogenic rocks of the matrix and medium composition. The periphery part of the massif is composed of diorites, the central part of granodiorites. The batholite belongs to Pre-Lower Devonian period.

The Aprelkovo ore field is also located within a granodiorite massif, which penetrates the metamorphic complex of proterozoic rocks (shale, gneiss, marbled limestone). In modern erosive cross section this massif represents an intruded body without distinct signs of assimilation and melting of the host («frame») rock. The massif in Aprelkovo field has a clear contact with the host rocks while the massif in Tsentralny batholite is composed of hybrid rocks in the pre-contact zone.

Quartz-vein bodies of the deposits are geometrically equal and have nearly common mineral composition: Tsentralnoe ore concentration was formed during the final stage of the batholite formation; Aprelkovo ores occurred much later – during the Cimmerian cycle of tectogenesis, much later than the granitization processes occurred.

However the two massifs have different magnetic properties. Firstly, Tsentralny batholite of Martaiga has lower magnetic response values compared to the formations it is embedded in, while Telenguisky Pluto has higher values of magnetic susceptibility. Secondly, the pre-contact zone of the first batholite is a large (several tens of meters and several thousand meters) complex anomaly of highly magnetic rocks bordering the massif; the second one represents a simple step-like anomaly. Thirdly, the magnetic response of Tsentralny massif is twice as high as that of the Telenguisky.

The difference in granitoid magnetic properties and the identity of ore specialization are easy to explain: these properties reveal the character and conditions of granitoid rock formation. Granitoid rock formation finishes at a stage of relatively high temperature, when most oxide ferromagnets (mainly titanium-magnetite series minerals – magnetite and ulvospinel massif solutions) reach Curie point. Hydrothermal ore-bearing development processes occur at a lower temperature; that is why they do not change the general granitoid magnetic properties. Hence it may be concluded that early potential “ore-bearingness” has no impact on the general formation features of granitoid magnetic properties.

Further to this many common features related to the magnetic properties of rocks can be observed.

Both Tsentralny and Aprelkovo granitoids are characterized by a positive correlation between magnetic response and the value of remanent magnetization; the divergence angle of the remanent and induced magnetization vectors is minor (3° max.); rocks with practically equal dispersion value of its change have the magnetic response values of a distinct double-model (at the first approximation).

The studies of numerous gold concentrations (Erofeev, 1989) showed that hydrothermal ore-bearing processes influenced the granitoid properties. Hydrotherms impact revealed itself in two aspects: areal metamorphism increased the rock magnetic response contrast mainly by its decrease in melanocratic differentials; and local near-vein changes (decrease also) of rock response led to the occurrence of a number of practically non-magnetic formations. Its presence does not reveal itself in the common range of magnetic response distribution; however it is distinctly fixed: spectrum density dispersion of induction increases and its anisotropy of change increases either during the detailed analysis of the magnetic field structure over the gold-bearing granitoid massifs (Erofeev, 2003). The nature of these magnetic response transformations is due to the fact that the basic ferromagnet-titaniummagnet is destroyed during mobilization, migration and ore-deposition. Practically non-magnetic pyrite forms instead of ferromagnet-titaniummagnet.

Thus magnetic parameters, characterizing common features of granitoid massifs magnetism and ore area of concentration, are not attributed to the area of concentration. The magnetism peculiarities reveal the conditions of occurrence and formation of massifs.

The detailed analysis of the changes in massif rocks magnetic response shows the importance of that the ore-forming processes are identical for different granitoids. The whole range of granitoid massif magnetization features can be effectively used in the interpretation of magnetic fields results, in genetic classification of granitoids and the study of ore-fields, localized in them (Erofeev, 1989; Shoplo, 1977).

References

Romanovsky N.P. Magnetic response and some metallogenic peculiarities of granitoids in the East USSR // Soviet Geology. 1976. № 12. P.64-74.

Dubinchik E.Ya., Rozental I.V. Petromagnetic research upon the studies of granitoid complexes. Leningrad: Nedra, 1980. 104p.

Erofeev L.Ya. About the character and the nature of granitoid magnetic response change // Geophysics, 2003. № 5. P.63-66.

Shoplo L.Ye. The use of rock magnetism for the solution of geologic problems. Leningrad: Nedra, 1977. 182p.

Erofeev L.Ya. Magnetic field and the nature of anomalies in gold-ore deposits. Tomsk: Tomsk State University, 1989.