Rhenium distribution in molybdenite: Results of microprobe scanning (Copper Porphyry Deposits, the Urals)

Abstract

The geochemistry of rhenium in ores of copper porphyry deposits from various regions has been con� sidered in numerous papers presented in the reviews (1-3 and others). We have recently systematized anal� yses for the Urals (4). Estimations of this element in different parts of separate molybdenite grains are very scarce (3, 5, 6, and others). Data on rhenium scanning in molybdenite grains are not available. However, the pattern of rhenium distribution in molybdenite grains is very important for understanding of the conditions of its precipitation and further redistribution within deposits especially those that have undergone low� temperature modifications and deformations. Patterns of rhenium distribution in separate molybdenite grains were obtained by scanning on the SX�100 microprobe at the Institute of Geology and Geochemistry, Ural Division, Russian Academy of Sciences. The diameter of the beam was 1 μm, U = 25 kV, I = 30 nA, and the detection limit of rhenium is 0.02 wt %. The standards were presented by metallic rhenium, molybdenum, and pyrite. The ReMα line was analytical. The neighboring CaK α line becomes gently sloping near the ReM α line and practically does not influence the measured rhenium concentration, but clearly fixes calcite grains (Fig. 1) because of the duration of significant exposure. Backscattered elec� tron study of the polygon and scanning using the CaK α spectral line provides evidence for the absence of cal� cite in the areas of high rhenium concentrations. In the preliminary stage of this study, rhenium concen� trations were measured in single microareas of molybdenite grains. Grains with high rhenium con� centrations were subjected to scanning. The scanning polygon was approximately limited by the contours of the molybdenite grains. The exposure time for the two samples described here was 16 and 6 h. Most copper porphyry deposits of the Urals are paragenetically controlled (7) by lowpotassium small intrusions of (gabbro)-diorite-plagiogranodiorite composition with an absolute prevalence of quartz diorite or quartz diorite porphyry related to mesoand hypabyssal depth facies ("diorite" model). The age varies from S to C2. Plagiogranitoids are hydrother� mally altered everywhere. Less serecitized and propyl� itized quartz diorites are characterized by low values of the ( 87 Sr/ 86 Sr)t ratio (from 0.7038-0.7048 to 0.7052), high eNd(T) values (4.1-7.5, in single samples decreas� ing to 0.9-2), and low TR concentrations (29- 67 ppm), without the Eu anomaly (8). These data evi� dence that orebearing granitoids contain a prevailing portion of the mantle component; they should be related to the island arc geochemical type being formed at the expense of the metabasic source in the lower crust-upper mantle or in the upper mantle. Judging from the low 87 Sr/ 86 Sr ratio (0.704-0.705) in vein carbonates, δ 18 О (7.4…8.5‰, SMOW), δD (-49…-61‰, SMOW), and δ 34 S (0 ± 2‰, CDT), CDT) in other hydrothermal minerals, the initial fluid had a magmatic nature and inherited the geochemical characteristics of orebearing granitoids. Thus, rhe� nium may also have a mantle primary source. The magmatic parameters of the fluid are also typical for many other hydrothermal deposits (9 and others). Common values of the Cu/Mo ratio in deposits of the Urals with a dioritoid substrate are 70-400, with cop� per contents of 0.3-0.8 wt %, molybdenum contents of 5-27 (up to 80 and higher) ppm, rhenium contents of up to 0.3-2 ppm (4). The concentration of rhenium in molybdenite varies significantly reaching 0.63 wt %, as is evident from our recent measurements (see below). Deposits of the "diorite" model are consid� ered to be very favorable for rhenium accumulation in ores (1 and others). However, sometimes significant rhenium concentrations (0.1-0.3 wt % and higher) are also observed in molybdenites from deposits of "monzonite" and "granodiorite" models, for which