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UPb zircon age of the gold-bearing Seredovina Massif, Middle Urals

Authors:
  • Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences

Abstract

The Seredovina gold-bearing massif is located in the Middle Urals, 2 km southeast of Nev’yansk in the well-known Nev’yansk gold district and is confined to the outer contact of the large Verkhisetsk tonalite‐granodiorite batholith (Fig. 1). The massif is a typical minor granitoid intrusion localized in the apical part of batholith-shaped bodies, which usually contains dike fields with gold-bearing quartz veins and wallrocks composed of listwaenite‐beresite metasomatites [1‐3]. The massif hosts the Nev’yansk Seredovina quartz‐sulfide gold deposit. The Seredovina Massif is composed of hornblende‐ biotite granodiorites crosscut by thin (up to 0.5 m thick) aplite dikes. The hornblende‐biotite granodiorites are light gray medium-grained rocks with a vague gneissic structure and porphyritic hypidiomorphic texture. The rock contains plagioclase, quartz, biotite, hornblende, and K-feldspar. Epidote, titanite, apatite, allanite, and zircon are accessory minerals. Plagioclase forms elongated prismatic and tabular grains with distinct rhythmic zoning. Potassium feldspar occurs as irregularly shaped interstitial grains of a cross-hatched microcline containing oikocrysts of tabular plagioclase and occasional amphibole. Quartz is observed as irregular grains. Amphibole is represented by common hornblende with brownish green pleochroism along Np and Nm to dark green along Ng . Biotite forms tabular and elongated plates with brownish green pleochroism. Biotite flakes often contain inclusions of apatite, zircon, and titanite, and often grow together with epidote. Zircons for isotope dating were extracted by the routine procedure: crushing of typical granodiorite sample (30 kg) of the massif to a fraction of ‐0.5 mm; extraction of the heavy fraction using the Wifley table and heavy liquid technique; and separation of zircon grains under binocular microscope. U‐Pb analyses were performed on a SHRIMP-II ion microprobe at the Center of Isotope Research, Karpinskii All-Russia Research Institute of Geology, using the standard technique [4, 5].
693
ISSN 1028-334X, Doklady Earth Sciences, 2008, Vol. 420, No. 4, pp. 693–696. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © E.A. Zinkova, V.V. Kholodnov, G.B. Fershatater, 2008, published in Doklady Akademii Nauk, 2008, Vol. 420, No. 4, pp. 516–519.
The Seredovina gold-bearing massif is located in
the Middle Urals, 2 km southeast of Nev’yansk in the
well-known Nev’yansk gold district and is confined to
the outer contact of the large Verkhisetsk tonalite–gra-
nodiorite batholith (Fig. 1). The massif is a typical
minor granitoid intrusion localized in the apical part of
batholith-shaped bodies, which usually contains dike
fields with gold-bearing quartz veins and wallrocks
composed of listwaenite–beresite metasomatites [1–3].
The massif hosts the Nev’yansk Seredovina quartz–sul-
fide gold deposit.
The Seredovina Massif is composed of hornblende–
biotite granodiorites crosscut by thin (up to 0.5 m thick)
aplite dikes. The hornblende–biotite granodiorites are
light gray medium-grained rocks with a vague gneissic
structure and porphyritic hypidiomorphic texture. The
rock contains plagioclase, quartz, biotite, hornblende,
and K-feldspar. Epidote, titanite, apatite, allanite, and
zircon are accessory minerals.
Plagioclase
forms elon-
gated prismatic and tabular grains with distinct rhyth-
mic zoning.
Potassium feldspar
occurs as irregularly
shaped interstitial grains of a cross-hatched microcline
containing oikocrysts of tabular plagioclase and occa-
sional amphibole.
Quartz
is observed as irregular
grains. Amphibole is represented by common horn-
blende with brownish green pleochroism along
Np
and
Nm
to dark green along
Ng
. Biotite forms tabular and
elongated plates with brownish green pleochroism.
Biotite flakes often contain inclusions of apatite, zir-
con, and titanite, and often grow together with epidote.
Zircons for isotope dating were extracted by the rou-
tine procedure: crushing of typical granodiorite sample
(30 kg) of the massif to a fraction of –0.5 mm; extrac-
tion of the heavy fraction using the Wifley table and
heavy liquid technique; and separation of zircon grains
under binocular microscope. U–Pb analyses were per-
formed on a SHRIMP-II ion microprobe at the Center of
Isotope Research, Karpinskii All-Russia Research Insti-
tute of Geology, using the standard technique [4, 5].
U–Pb Zircon Age of the Gold-Bearing Seredovina Massif,
Middle Urals
E. A. Zinkova, V. V. Kholodnov, and G. B. Fershatater
Presented by Academician V.A. Koroteev May 4, 2007
Received May 16, 2007
DOI:
10.1134/S1028334X08040387
Zavaritskii Institute of Geology and Geochemistry,
Ural Division, Russian Academy of Sciences,
Pochtovyi per. 7, Yekaterinburg, 620219 Russia;
e-mail: zinkova@yandex.ru
GEOCHEMISTRY
Fig. 1.
Schematic geological map of the Verkhisetsk Massif.
Compiled after materials of D.A. Dvoeglazov, G.N. Kuzovkov,
and D.S. Vashgal (1972–1979) and original data. (
1
) Host
rocks of the Verkhisetsk Massif (Silurian–Early Devonian vol-
canogenic and volcanosedimentary rocks); (
2–6
) rocks of the
Verkhisetsk batholith: (2) gabbroid xenoliths, (
3
) diorites and
tonalities, (
4
) low-K series rocks, (
5
) K–Na series rocks,
(
6
) adamellite–granite series.
Yekaterinburg
Nev'yansk
Seredovina Massif
N
S
v
e
r
d
l
o
v
s
k
F
a
u
l
t
S
e
r
o
v
M
a
u
k
F
a
u
l
t
1
2
3
4
5
6
10 km
M
a
in
U
ra
lia
n
D
e
e
p
F
a
u
lt
694
DOKLADY EARTH SCIENCES
Vol. 420
No. 4
2008
ZINKOVA et al.
Zircons are represented by euhedral elongated pris-
matic crystals. Cathodoluminescent images (Fig. 2)
show rhythmic zoning parallel to crystal outlines. The
increase of U and Th contents from core to rim (table)
is related to crystallization differentiation of the grano-
diorite melt. This fact indicates that the obtained zircon
age corresponds to the crystallization age of the grano-
diorites. U–Pb zircon study of granodiorites of the
Seredovina Massif showed that the ages obtained on the
core and the rim of the zircon grain are practically iden-
tical; the marginal parts of other grains also yield close
ages (Fig. 2, table). The average value based on four
data points is 314
±
5.4 Ma (Fig. 3). Thus, U–Pb
SHRIMP zircon dating showed that the Seredovina
Massif was crystallized in the Middle Carboniferous.
The Verkhisetsk Massif, including its satellite
Seredovina Massif, was formed in two stages [6, 7].
The first stage produced low-K and K–Na tonalite–gra-
nodiorite series. The second stage was responsible for
the formation of massive granite bodies in the central
part of the massif. Low-K granodiorites have a Rb–Sr
age of 316
±
6 Ma, whereas K–Na granodiorites yield
320
±
12 Ma. The massive granites of the central bodies
define a Rb–Sr age of 276
±
5 Ma. The
207
Pb/
206
Pb dat-
ing of single zircons from different series of the
Verkhisetsk Massif using the Kober technique [8]
yielded an average value of 318
±
4 Ma, which coin-
cided with their Rb–Sr age. Detailed study of zircons
from young granites revealed their isotopic heterogene-
ity: the
207
Pb/
206
Pb age of cores varies from 314 to
295 Ma, whereas the age of rims (275–280 Ma) coin-
cides with the Rb–Sr age, indicating their relation with
older tonalite–granodiorite series of the massif. Com-
parison of the data obtained indicates that zircon dat-
ings on the Seredovina granodiorite massif are plotted
in the age range of tonalite–granodiorite series of the
0.2 mm
1.2
2.1
3.1
1.1
Fig. 2.
Cathodoluminescent images of individual zircons
from granodiorite (sample Ser 16) of the Seredovina Mas-
sif. Circles show the measurement points; numbers corre-
spond to the analysis numbers in the table.
0.33
0.048
0.35 0.37 0.39 0.41 0.430.31
207
Pb/
235
U
0.046
0.050
0.052
0.054
206
Pb/
238
U
330
310
290
N
= 4
T
= 314 ± 6 Ma
MSWD = 0.0079
Fig. 3.
U–Pb concordia diagram for zircons from granodior-
ite (sample Ser 16) of the Seredovina Massif. The probabil-
ity of concordance is 0.93.
Results of U–Pb SHRIMP study of zircons from granodiorites (sample Ser 16) of the Seredovina Massif
Measurement
point
206
Pb
c
,
%
UTh
232
Th/
238
U,
g/t
206
Pb*
Age, Ma
g/t
206
Pb/
238
U
207
Pb/
206
Pb
1.1 0.36 303 71 0.24 13.1 314.9
±
5.8 379
±
95
1.2 0.27 650 138 0.22 27.8 312
±
5.3 349
±
50
2.1 0.19 774 281 0.38 33.9 320.4
±
5.4 283
±
45
3.1 0.32 813 290 0.37 34.6 310.3
±
5.2 286
±
59
Note: Errors are quoted at 1
σ
level; (Pb
c
, Pb*) common and radiogenic lead, respectively. Error in standard calibration was 0.68%.
DOKLADY EARTH SCIENCES
Vol. 420
No. 4
2008
U–Pb ZIRCON AGE OF THE GOLD-BEARING SEREDOVINA MASSIF 695
Verkhisetsk Massif. The granodiorites of the Seredov-
ina Massif are geochemically similar to the granodior-
ites of the K–Na series of the Verkhisetsk Massif (Fig. 4).
Thus, the obtained U–Pb zircon age of the grano-
diorites of the Seredovina Massif and new geochemical
data indicate that they were intruded and crystallized in
the Middle Carboniferous at the final stages of the for-
mation of K–Na tonalite–granodiorite series of the
Verkhisetsk Massif.
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research (project no. 05-05–64079).
REFERENCES
1. A. A. Ivanov,
Geology of the Native Gold Deposits in the
Urals
, Tr. Gorn.-Geol. Inst. UFAN USSR, Issue 16
(1948).
Cs
10
La
Rb Ba Th U K Nb La Ce Sr Nd Hf Zr SmEu Ti Gd Dy Y Er Yb Lu
(b)
0.1
1
100
1000
K–Na granodiorite series
Rock/Chondrite
K–Na granodiorite series
Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
(a)
Low-K granodiorite series
12
10
100
1
Rock/Chondrite
Fig. 4.
(a) Chondrite-normalized REE distribution and (b) PM-normalized trace element abundance in the granodiorites of the
Seredovina Massif. (
1
) Sample Ser 19, (
2
) Sample Ser 16.
696
DOKLADY EARTH SCIENCES
Vol. 420
No. 4
2008
ZINKOVA et al.
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Batholiths from the accreted terranes in the Urals were generated by repeated episodes of melting and intrusion. Verkhisetsk, the largest and one of the most complex subduction-related batholiths from the Urals, comprises an outer envelope of older tonalites, trondhjemites and granodiorites dated at 315–320 Ma and equilibrated at 6 kbar, intruded by an inner core of younger granodiorites, adamellites and granites dated at 275–290 Ma and equilibrated at 4 kbar. Older rocks have a high-A1 TTD/adakite chemistry, ϵ320Machur(ND) ≈ 2–5, and initial , with pronounced lateral zoning marked by the increase of LREE/HREE, Cr, Ni, and Mg eastwards. They were originated by melting of metabasalts in a subducted slab of young hot oceanic lithosphere with a temperature/depth trajectory that intersected the garnet-in univariant at ≈ 1050°C and 13 kbar, thus causing the lateral zoning. We believe that such a ‘warm’ trajectory can be attributed to the oblique subduction of a young lithosphere. Younger rocks range from Na-rich metaluminous granodiorites to K-rich peraluminous two-mica granites with relict epidote crystals, and have almost the same isotopic signature as older rocks, from which they were generated by anatexis as a consequence of a melting event that occurred throughout the Urals at 275–290 Ma and produced most of the huge continental-type batholiths on the eastern side. This melting event is tentatively supposed to be the result of underplating by mafic magmas, which also caused the growth of the Urals crust from the Moho downwards until it reached the increased thickness revealed by seismological studies.
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Tests derived from time series analysis play an important role in many empirical studies. These tests are frequently applied to the residuals obtained by fitting an econometric model using some standard estimator. We focus attention here on tests developed for univariate time series models. Various approaches to testing the adequacy of such models are discussed and compared. The validity and sefulness of applying these tests to econometric residuals are then examined and some Monte Carlo evidence is reported.
  • V N Smirnov
  • F Bea
  • P Montero
V. N. Smirnov, F. Bea, P. Montero, et al., Dokl. Earth Sci. 363A, 1254 (1998) [Dokl. Akad. Nauk 363, 389 (1998)].
Geology of the Native Gold Deposits in the Urals
  • A A Ivanov
A. A. Ivanov, Geology of the Native Gold Deposits in the Urals, Tr. Gorn.-Geol. Inst. UFAN USSR, Issue 16 (1948).
  • I S Williams
I. S. Williams, Rev. Econ. Geol. 7, 1-35 (1998).
  • A N Larionov
  • V A Andreichev
  • D G Gee
  • Mem
A. N. Larionov, V. A. Andreichev, and D. G. Gee, Mem. Geol. Soc. London, No. 30, 69–74 (2004).
  • V N Sazonov
  • V N Ogorodnikov
  • V A Koroteev
V. N. Sazonov, V. N. Ogorodnikov, V. A. Koroteev, and Yu. A. Polenov, Gold Deposits of the Urals (UGGA, Yekaterinburg, 2001) [in Russian].
  • A N Larionov
  • V A Andreichev
  • D G Gee
A. N. Larionov, V. A. Andreichev, and D. G. Gee, Mem. Geol. Soc. London, No. 30, 69-74 (2004).