TIMING OF GRANITE MAGMATISM IN THE CENTRAL KYZYL KUM
(UZBEKISTAN) AND ITS RELATION TO THE FORMATION OF GIANT
GOLD DEPOSITS
U. Kempe1, R. Seltmann2, T.Graupner3, S.A. Sergeev4
1
Institute of Mineralogy, TU Bergakademie Freiberg, Germany; 2Centre for Russian and Central
EurAsian Mineral Studies, Natural History Museum, London, UK; 3Bundesanstalt für
Geowissenschaften und Rohstoffe, Hannover, Germany; 4Center of Isotopic Research, AllRussian Geological Research Institute, St. Petersburg, Russia.
Introduction
Word-class gold, silver, tungsten, and uranium deposits occur in the Central Kyzyl Kum
metallogenetic province of the Southern Tien Shan belt. Recently, the genesis and economic
significance of large to giant gold deposits in this area (e.g., Muruntau, Kokpatas) related to
Hercynian hydrothermal systems received increased attention in the literature (e.g., Berger et al.
1994, Graupner et al. 2001, Wilde et al. 2001). The age of gold and tungsten mineralization at
Muruntau and Sarytau has been constrained to a time interval between 275 and 290 Ma using KAr, Rb-Sr, Sm-Nd, and Re-Os isotope methods (Kostitsyn 1996, Kempe et al. 2001, Konopelko et
al. 2003, Morelli et al. 2004, 2007). Noble gas analysis and other available isotope and
geochemical data indicate intense fluid – wall rock interaction and a probably small but
significant mantle input into the Muruntau hydrothermal system (Kempe et al. 2001, Graupner et
al. 2006, Morelli et al. 2007). However, the presence of Hercynian granite magmatism and its
possible role in the formation of Hercynian ore deposits has not attracted adequate attention
although several authors have suggested a direct magmatic influence on the ore-forming fluid
system (Yudalevich & Levchenko 1981, Kotov & Porickaya 1991, Shayakubov and Dalinov
1998, Konstantinov et al. 2000, Wall et al. 2004). The present communication summarizes our
published (Kempe et al. 2004) and recently obtained new data which resulted from a study
directed on determination of precise absolute zircon ages for the granites within and in the
surroundings of the Muruntau ore field.
Geological setting and sampling
The suite of Hercynian magmatic rocks in the region ranges in composition from diorite to
leucogranite and is late- to postorogenic in tectonic setting (see Shayakubov and Dalinov 1998 for
review).
Fig. 1. Location of granite outcrops used for sampling
in the present study.
Six granite samples from outcrops in and around the
Muruntau ore field were included in our study. Only
granite bodies without observable deformation features
were considered here for dating (Fig. 1). The rocks
sampled may be subdivided into two groups: (1)
melanocratic,
amphibole-bearing
granites
and
monzonites and (2) leucocratic granites. The
Aristantau granite sample Ari 1-99 (50 km south from
Muruntau) represents a type 1 quartz monzonite with
secondary sericitization and chloritization. In the
Tamdytau area (30 km to the northwest from Muruntau), a rock of similar type is exposed
(sample MT 45) but with pronounced albitization as the leading alteration process. A dike sample
from the Muruntau deposit (MT 168) may be added to this group using chemical criteria and
zircon morphology. Primary characteristics are obliterated by intense albitization, chloritization,
sericitization, and sulfide mineralization. The type 2 granites are represented by two samples from
the Tamdytau granite massif (MT 44 and 43) with intense albitization (both samples) and
sericitization (MT 43). These rocks show intrusive relations to the melanocratic granite (sample
MT 45). A sample from the deep drill hole SG-10 south of Muruntau (MT 42318) represents the
Murun leucogranite discovered more than 4004 m below surface. This granite is intensely altered
by K-feldspatization, albitization, and (local) sulfide mineralization but may be recognized as a
type 2 rock according to whole rock characteristics.
Methods
Rock samples were investigated by conventional chemical whole rock analysis (XRF, ICP-MS,
INA) and a combination of optical (OM) and analytical scanning electron microscopy (SE, BSE,
CL, EDX). Zircon separates were obtained by standard techniques except sample MT 42318 for
which sample volume was too small. Zircon morphology was investigated using SE imaging and
applying criteria proposed by Pupin (1980) for dominant prisms and pyramids. Pb-Pb dating of
zircon was performed for zircon from samples Ari 1-99 and MT 168 using the evaporation
technique after Kober (1986, 1987). SHRIMP dating of zircon was carried out using SHRIMP-II
facility at the CIR (VSEGEI, St. Petersburg, Russia) for zircon from samples Ari 1-99, MT 45,
MT 44, MT 43 and MT 42318 (direct dating of zircon in a thin section of the rock for the latter
case). Internal structure of zircon was checked using SE, BSE, and CL imaging of polished grains
and EDX analysis.
Results
Pb-Pb and U-Pb dating of zircon yields consistent ages between 288 and 296 Ma which are, in
general, indistinguishable from each other within error (Fig. 2). Only most precise measurements
were used for age calculation excluding data related to older “cores” in each case. On first glass,
these ages may be understood as intrusion ages of the related granites. Note that our U-Pb and PbPb zircon ages are somewhat higher than reported Rb-Sr isochron ages from the literature (e.g.,
Kostitsyn 1996). This is not surprisingly because it is well known that the Rb-Sr system of
granitic rocks is often somewhat disturbed by later alteration processes yielding too young
intrusion “ages”. There are, however, some serious difficulties with such a straight forward
interpretation of our data. First, it turns out that the geological younger Tamdytau leucogranite
(MT 44) yields a significant older apparent “intrusion age” (393.3±2.1 Ma, MSWD 0.55, n=6)
than the adjacent geological older melanogranite (MT 45; 387.7±1.4 Ma, MSWD 0.83, n=9).
Moreover, Nemchin (2002, written communication) reported a highly precise, concordant U-Pb
zircon age of 236.5±1.4 Ma (MSWD 1.07, n=3) for an altered granitic dike from Muruntau using
conventional U-Pb dating techniques.
A r i 1- 9 9
206
P b/
207
P b e v a por a t i on
206
P b/
238
U S HRIMP
206
P b/
207
P b e v a por a t i on
A r i 1- 9 9
M T 18 6
M T 45
c onc or di a a ge S H R I M P
M T 44
c onc or di a a ge S H R I M P
M T 43
c onc or di a a ge S H R I M P
M T 4 2 3 18
si ngl e
280
285
290
295
age (M a)
300
206
305
P b/
238
U
310
Fig. 2. Comparision of Pb-Pb
evaporation and U-Pb SHRIMP
ages of zircons from granitoids
from the Muruntau area.
Closer inspection of SHRIMP
results
as
well
as
of
mineralogical and petrologic
data
clearly
shows
that
albitization of granitoid rocks is
accompanied by growth of U-
rich hydrothermal zircon with up to 3 wt % U mainly forming overgrowth on older zircon crystals
(MT 45, MT 44, MT 43) but also separate new grains (MT 42318). On the other hand, this new,
U-rich zircon is most easily affected by later alteration with a related appearance of direct
(sericitization of plagioclase and biotite) and reverse (replacement of zircon by thorite-coffinite)
U-Pb age discordances. There is, therefore, good evidence that dating of U-rich, weakly altered
zircon constraints the age of albitization at a narrow time interval between 290 and 293 Ma.
It is important to note here that apparently concordant U-Pb SHRIMP ages obtained for zircon
may – as in our case - represent minimum intrusion ages but not necessarily the “true” intrusion
age of the granite and that even meaningless, apparently concordant U-Pb ages may be obtained
for altered zircon. Possibly, this may resolve the apparent discrepancy between our SHRIMP and
evaporation age data and those obtained by Nemchin (2002, written communication) using
common U-Pb techniques. Our data shows that the intrusion age of the Hercynian granitoids
should be somewhat higher than 290-293 Ma. However, only a crude estimate of 293-300 Ma
may be given here.
Relation to gold mineralization
The time intervals constrained for the intrusion of Hercynian granites and formation of gold
mineralization in the Muruntau area overlap within analytical errors with each other (Fig. 3).
( 1) gr a ni t oi ds
P b/ P b a nd U / P b z i r c on
R b - Sr w h o l e r o c k
R b - Sr w h o l e r o c k
R b - Sr mi n e r a l i s o c h r o n
U / P b z i r c on
( 2 ) A u m i ne r a l i sa t i on
R e - Os a r s e nopy r i t e
Sm- N d s c h e e l i t e 2
R b - Sr mi n e r a l i s o c h r o n
R b - Sr mi n e r a l i s o c h r o n
K - A r bi ot i t e
K - A r bi ot i t e
Fig. 3. Compilation of age
data
for
granite
magmatism and gold
mineralization
in
the
Muruntau area (literature
data from Kostitsyn 1996,
Kempe et al. 2001, Wilde
et al. 2001, Nemchin
2002).
K - A r mi c r o c l i n e
K - A r a mp h i b o l e
Ar -Ar ser i ci te
We obtained somewhat
older ages for granite
200
220
240
260
280
300
320
340
magmatism than for gold
age (Ma)
mineralization. Whether or
not there is a significant
time gap between magmatism and mineralization should be a topic of further studies.
Nevertheless, our results demonstrate that there should be at least a general genetic link between
granite magmatism and ore formation both related to a common tectonic process in the upper
mantle/lower crust region of the Southern Tien Shan during Hercynian collision.