Magnetic Properties of Rocks of the Evgrafovsky Gold Deposit (Central Transbaikalia)
Abstract Generated abstract
This paper examines the magnetic properties of rocks at the Evgrafovskoye gold-ore deposit in Central Transbaikalia in order to assess whether magnetic surveying can help map quartz veins and intrusive contacts. Magnetic susceptibility and remanent magnetization were measured for 400 samples taken along mine sections through shale, quartz veins, and granodiorite, followed by statistical treatment and profile analysis. The study finds generally weak magnetization but identifies systematic zonation near quartz veins, including central and marginal zones of reduced magnetization bordered by higher magnetization zones, as well as differentiated magnetic behavior near granodiorite contacts. These patterns suggest that detailed magnetic surveys of sufficient accuracy may help distinguish vein-bearing shales from barren shales and trace quartz-vein related magnetic minima.
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Magnetic Properties of Rocks of the Evgrafovskoye Gold-Ore Deposit (Central Transbaikalia)
Z. A. Myshko, B. G. Mordanov, V. M. Volkov
(Presented by the Department of Geophysical Exploration Methods)
The Evgrafovskoye gold-ore deposit is located in the central part of the Lyubavinskoye ore field, composed of Permian–Triassic sandy-shale deposits, which are intruded by stock-like bodies of granitoids of the Kyrinsk intrusive complex and by a complex of dike rocks of acidic and intermediate composition. The deposit is confined to a tectonic zone of deep-seated origin, having a sublatitudinal strike with a dip to the south at an angle of \(50\text{–}70^\circ\). Within the deposit, disjunctive disturbances of latitudinal, northwestern, and meridional strike are widely developed. The ore bodies are represented by quartz veins of small extent and thickness with a poor content of sulfides.
To determine the character of the magnetic field over the quartz veins, for the purpose of mapping them, the magnetic properties of 400 rock samples were studied; the samples were taken every 3–5 meters along the entire section from a mine. On an MA-21 astatic magnetometer, the magnetic susceptibility \(\chi\) and residual magnetization \(I_n\) were measured; statistical processing of the measurement results was carried out, and graphs of the variation of these parameters along the geological section were constructed (where aleurolite-clayey shales, quartz veins, and granodiorites are exposed).
The rocks studied proved to be weakly magnetic; the average values of the parameters of magnetic susceptibility and residual magnetization are:
1) aleurolite-clayey shales — \(\chi = 10 \cdot 10^{-6}\) cgs, \(I_n = 3 \cdot 10^{-6}\) cgs;
2) granodiorites \(\chi = 60 \cdot 10^{-6}\) cgs, \(I_n = 80 \cdot 10^{-6}\) cgs;
3) vein quartz — \(\chi = 3 \cdot 10^{-6}\) cgs, \(I_n\) close to zero. However, analysis of the graphs of \(\chi\) and \(I_n\) along the sections gives grounds to note the following important features:
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Over a zone of shales not containing quartz veins, the graphs of residual magnetization and magnetic susceptibility are weakly dissected, and the values of \(I_n\) fluctuate within \((1\text{–}3)\cdot 10^{-6}\) cgs, while those of magnetic susceptibility are \((1\text{–}12)\cdot 10^{-6}\) cgs.
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In the area of development of quartz veins, the graphs of \(\chi\) and \(I_n\) acquire a regularly differentiated character: over the vein and the rocks immediately enclosing it (2–3 m wide), \(I_n\) amounts to \((1\text{–}5)\cdot 10^{-6}\) cgs, and \(\chi = 2 \cdot 10^{-6}\) cgs. Beyond this zone, the appearance is characteristic of two marginal maxima of \(\chi\) and \(I_n\) on the hanging-wall and footwall sides of the vein, reaching values up to \((80\text{–}100)\cdot 10^{-6}\) cgs, with the largest maximum
in the roof. Thereafter the maximum values give way to lower ones, and then again level off to values above the shales devoid of vein formations.
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A direct relationship is observed between magnetic susceptibility and remanent magnetization (the correlation radius is 0.7), which makes it possible to judge magnetization from a single parameter—$\chi$.
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In the zones of minima of $\chi$ and $I_n$, a certain decrease in density is indicated, though not everywhere.
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Above the granodiorites of the near-contact area, over a distance of 25 m, the graphs of $\chi$ and $I_n$ are sharply differentiated: zones of maxima on the order of $180 \cdot 10^{-6}$ cgs are replaced at equal intervals (4–5 m) by zones of minima of $(1\text{–}10)\cdot 10^{-6}$ cgs. Beginning 30 m from the contact, within 12 m, $\chi$ and $I_n$ of the granodiorites amount to $(10\text{–}15)\cdot 10^{-6}$ cgs and then again increase to $(30\text{–}60)\cdot 10^{-6}$ cgs.
Thus, the investigations carried out made it possible to obtain a number of new data on the distribution of the magnetic properties of the rocks of the deposit. The revealed zonation in the magnetization of the rocks of the near-ore space (three zones of reduced magnetization—the central one and two marginal ones—and two zones of increased magnetization bordering the central one) has also been established at the Darasun (G. G. Nomokonova and L. Ya. Erofeev, 1969) and Nikolaevskoye (Z. A. Myshko et al.) gold-ore deposits.
The regularities revealed above give grounds for considering it expedient to carry out detailed magnetic surveying with an accuracy of 3–5 gamma for the purpose of mapping the contact of granodiorites with shales and distinguishing, by the structure of the magnetic field, aleurolite–clayey shales bearing quartz veins from shales devoid of vein formations. In addition, according to the revealed zonation in the magnetization of the rocks, it will apparently be possible to trace minima of the magnetic field associated with quartz veins, since the expected changes in the magnetic field will be no less than 40–50 gamma. The zonation in the magnetization of the rocks is closely connected with a definite stadial character of mineralization and with the zonation of near-vein metasomatic aureoles.