Origin of the Laleaua Albă dacite (Baia Sprie volcanic area and Au-Pb-Zn ore district, Romania): evidence from study of melt inclusions

Crystal inclusions (plagioclase, biotite, magnetite) and melt inclusions were studied in minerals of the Laleaua Albă dacite (Baia Sprie, Romania). Electron microprobe analysis of 29 melt inclusions in the plagioclase, K-feldspar, and quartz confirm that crystallization of these minerals took place from typical silicic melts enriched in potassium relative to sodium (K2O/Na2O = 1.5). The sum of the petrogenic components is 92–99 wt%. This points to a possible change in water content from 8 to 1 wt% during crystallization of phenocrysts. According to ion microprobe analysis of 11 melt inclusions, the minimum water content is 0.5 wt%, and the maximum water content is 6.1 wt%. The presence of high-density water fluid segregation in one of the melt inclusions suggests that the primary water content in the melt could reach 8.4 wt%. Ion microprobe data revealed a high concentration of Cu (up to 1260 ppm) as well as higher U content (from 5.0 to 14.3 ppm; average 11.5 ppm) in some melt inclusions as compared to the average U contents in silicic melts (2.7 ppm in island-arc settings and 7.9 ppm in continental rift settings). Chondrite-normalized trace-element patterns in melt inclusions suggest a complex genesis of the studied magmatic melts. Contents of some elements (for instance Sr and Ba) are close to those in island-arc melts, while others (for instance Th, U, and Eu) resemble those in melts of continental settings

(Junoite and pekoite - first occurrences in the USSR.

These two Bi-Pb-Cu sulphides were found in the Bi mineralization of the Kochbulak deposit in eastern Uzbekistan, in which vein and pipe-like bodies of sulphide-quartz ore are enclosed in andesite-dacites. The junoite and pekoite typically occur together, in association with tetradymite, aikinite, gladite and sometimes cassiterite and Cu-Sn sulphides. Microprobe analyses are tabulated for 9 specimens; on a Cu2S-Bi2S3-Pb2S2 diagram, the analyses of junoite plot close to the theoretical composition (Cu2Pb3Bi8S16) whereas those for pekoite are scattered in the upper part of a line between the bismuthinite (Bi2S3) apex and gladite (CuPbBi5S9). The junoite XRD powder data can be indexed for a monoclinic cell, space group C2/m, with a 26.66, b 4.06, c 17.03 A, beta 127.20o; the patterns for pekoite are similar to those for the type material (M.A. 78-856) and give an orthorhombic cell with space group Pb21m.-R.A.H

(Junoite and pekoite - first occurrences in the USSR.

These two Bi-Pb-Cu sulphides were found in the Bi mineralization of the Kochbulak deposit in eastern Uzbekistan, in which vein and pipe-like bodies of sulphide-quartz ore are enclosed in andesite-dacites. The junoite and pekoite typically occur together, in association with tetradymite, aikinite, gladite and sometimes cassiterite and Cu-Sn sulphides. Microprobe analyses are tabulated for 9 specimens; on a Cu2S-Bi2S3-Pb2S2 diagram, the analyses of junoite plot close to the theoretical composition (Cu2Pb3Bi8S16) whereas those for pekoite are scattered in the upper part of a line between the bismuthinite (Bi2S3) apex and gladite (CuPbBi5S9). The junoite XRD powder data can be indexed for a monoclinic cell, space group C2/m, with a 26.66, b 4.06, c 17.03 A, beta 127.20o; the patterns for pekoite are similar to those for the type material (M.A. 78-856) and give an orthorhombic cell with space group Pb21m.-R.A.H