V. B. Naumova, V. A. Kovalenkerb, V. Yu. Prokofievb, M. L. Tolstykha, G. Damianc, and F. Damianc
aVernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences,
ul. Kosygina 19, Moscow, 119991 Russia
e-mail: naumov@geokhi.ru
bInstitute of the Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of
Sciences, Staromonetnyi per. 35, Moscow, 109017 Russia
e-mail: kva@igem.ru
c Universitatea de Nord Baia Mare, Dr. Victor Babes street, 430083 Baia Mare, Romania
Received January 19, 2012; in final form, June 15, 2012
Abstract—Crystalline and melt inclusions were studied in large (up to 2 cm across) dipyramidal quartz phe-
nocrysts from Miocene dacites in the area of the Rosia Montana Au–Ag deposit in Romania. Data were
obtained on the homogenization of fluid inclusions and the composition of crystalline inclusions and glasses in
more than 40 melt inclusions, which were analyzed on a electron microprobe. The minerals identified in the
crystalline inclusions are plagioclase (An 51–62), orthoclase, micas (biotite and phengite), zircon, magnetite
(TiO2 = 2.8 wt %), and Fe sulfide. Two types of the melts were distinguished when studying the glasses of the
melt inclusions. Type 1 of the melts is unusual in composition. The average composition of 20 inclusions is as
follows (wt %): 76.1 SiO2, 0.39 TiO2, 6.23 Al2O3, 4.61 FeO, 0.09 MnO, 1.64 MgO, 3.04 CaO, 2.79 Na2O, 3.79
K2O (Na2O/K2O = 0.74), 0.07P2O5, 0.02 Cl. The composition of type 2 of the melts is typical of acid magmas.
The average of 23 inclusion analyses is (wt %) 79.3 SiO2, 0.16 TiO2, 10.27 Al2O3, 0.63 FeO, 0.08 MnO, 0.29
MgO, 1.83 CaO, 3.56 Na2O, 2.79 K2O (Na2O/K2O = 1.28), 0.08 P2O5, 0.05 Cl. The compositions of these melts
significantly differ in concentrations of Ti, Al, Fe, Mg, Ca, Na, and K. The high analytical totals of the analyses
(close to 100 wt %, more specifically 98.9 and 99.0 wt %, respectively) testify that the melts were generally
poor in water. Two inclusions of type 1 and two inclusions of type 2 were analyzed on an ion probe, and their
analyses show remarkable differences in the concentrations of certain trace elements. These concentrations (in
ppm) are for the melts of types 1 and 2, respectively, as follows: 10.0 and 0.69 for Be, 29.3 and 5.7 for B, 6.4
and 1.4 for Cr, 146 and 6.9 for V, 74 and 18 for Cu, 92 and 29 for Rb, 45 and 15 for Zr, 1.7 and 0.6 for Hf, 10.3
and 2.3 for Pb, and 52 and 1.3 for U. The Th/U ratio of these two melt types are also notably different: 0.04 and
0.19 for type 1 and 2.0 and 2.9 for type 2. These data led us to conclude that the magmatic melts were derived
from two different sources. Our data on the melts of type 1 testify that the magmatic chamber was contaminated
with compositionally unusual crustal rocks (perhaps, sedimentary, metamorphic, or hydrothermal rocks
enriched in Si, Fe, Mg, U, and some other components). This can explain the ore-forming specifics of magmatic
chambers in the area.
Keywords: melt inclusions, trace elements, Romania
DOI: 10.1134/S0016702913110050
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