Download Nedialkov, R., Zartova, A., Moritz, R., Von Quadt, A., Peytcheva, I

Magmatic petrology and exsolution of ore bearing fluid in the magmatic center Assarel, Central
Srednogorie, Bulgaria.
Nedialkov R.1, Zartova A.1, Moritz R.2, Bussy F.3, von Quadt A.4, Peycheva I.5, Fontignie D.2
Sofia University “St. Kl. Ohridski”, Sofia, 15 Bd. Tzar Osvoboditel, [email protected]
Geneva University, Rue des Maraichers 13, 1205 Geneva, Switzerland, [email protected]
3 University of Lausanne, Faculty of Geosciencies, Humense, CH-1015 Lausanne, [email protected]
4 ETH – Zurich, [email protected]
5 CLMC – BAS, Sofia, [email protected]
1
2
Abtract. The Assarel district is situated in the Central Srednogorie zone (CSZ). The magmatism builds up a volcanoplutonic edifice embedding the porphyry copper deposit. Magmatism is Calc-alkaline to High K Calc-alkaline. Magmatic
evolution and element behavior are governed essentially by mineral fractionation and less by crustal contamination and
magma mixing. The magmatism of the Assarel magmatic center is Turonian – 90  1,5 Ma for volcanites and 90  0,24
Ma for the Assarel porphyritic intrusion. Isotopic data suggests magma derived from a slightly enriched mantle combined
with crustal contamination. During the evolution of the upper Cretaceous hydrous magma (5-6 wt. % H2O) Cu has a
moderately incompatible behavior concentrating in the more evolved volcanites (Bi-Hb andesites to dacites) – 50 ppm.
The synsubdutional, upper Cretaceous, calc-alkaline magma is the source for the Cu extracting fluids (up to 80 % of Cu
from the magma) forming the orthomagmatic hydrotherms.
Key words: Porphyry copper deposit, petrology, fluid exsolution.
The Asarel magmatic center is disposed 7 km NW
from the town of Panagurishte in the Central Srednogorie.
The Srednogorie zone is a part of the Banat-Srednogorie
ore-magmatic upper Cretaceous belt. It is characterized by
the calk-alkaline to shoshonitic magmatism with
disposition of numerous copper and copper-gold deposits
(Majdanpek, Bor, Chelopech, Elatzite, Assarel and others
– HEINRICH & NEUBAUER, 2002, VON QUADT et al., 2005)
with major importance for copper and gold ore
production. The Panagurishte ore district is composed of
five volcanic stripes (VS) (Chelopech VS, Assarel VS,
Krassen-Petelovo VS, Pesovetz VS, Radka VS)
subparallel one to another with direction at 110 to 130o.
The volcanic stripes are interpreted as strike-slip basins
formed in transtensional tectonic regime. The epithermal
and porphyry copper deposits are aligned in a
submeridional zone called the Panagurishte corridor
representing probably a deep setted fault.
The geology of the Panagurishte region consists in a
basement composed of Paleozoic metamorphic rocks
(biotite and two mica gneisses, amphibolites and schists),
variscan granitoid plutons (Smilovene, Strelcha, Poibrene
and Koprivshtitza) intruded in them and a cover of
Triassic sediments and volcanic, subvolcanic and
intrusive rocks of the Upper Cretaceous. The Assarel
magmatic center represents a volcano-plutonic edifice
disposed at the most eastern part of the volcanic stripe.
The volcanism activity successively form 1) Hb andesites
to latites, 2) Hb-Px to Px-Hb basaltic andesites and 3/ Bi-Hb
andesites, quartz andesites to dacites. The volcanites and the
Paleozoic granitoides of the Smilovene pluton are intruded
by the comagmatic to the volcanites porphyritic plutonites
of the Assarel intrusion. The intrusion is formed in three
impulses: 1) Fine to medium porphyritic quartz-diorite to
quartz-monzodiorites porphyries; 2) fine to medium
porphyritic quartz-diorite quartz-monzonite to granodiorite;
3) granite porphyry (simultaneous to post ore) (ZARTOVA et
al, 2004; NEDIALKOV et al., 2006). The hydrothermal
alteration related to the formation of the Assarel porphyry
copper deposit consists in K-silicate and K-silicate propilitic
alterations of the Pz granitoides and the cretaceous
porphyrites. Propylitization, propylite-argillic, sericite
advanced argillic, and advanced argillic alteration of two
subtypes: acid-chlorine and acid-sulfate are also established
in the cretaceous magmatites (KANAZIRSKI et al., 1995,
2002, POPOV et al., 2000, STRASHIMIROV et al., 2002).
The ore mineralization affects both the rocks of the
volcano-plutonic edifice and the basement rocks (the
Smilovene pluton granitoides and the metamorphics). The
ore component is irregularly distributed in the space of the
porphyry copper deposit controlled by the WNW and the
submeridional fault systems. The shape of the Assarel
porphyry copper ore deposit is an ellipsoidal cone with a
long axis in N-S direction, deeping in south to south-west
by 80-85o (STRASHIMIROV et al., 2002). As a whole the ores
of the deposit are estimated at 354 mln. t. with average
content of Cu – 0,44% and about 0.7-1 g/t Au
(STRASHIMIROV et al., 2002).
Hb andesites to latites are presented mainly by
epiclastic rocks and a subordinate quantity of lava flows
and pyroclastic rocks. These rocks build up the major part
of the Assarel volcanic stripe. The second volcanic event
is related to the formation of several neck-like isometric
or oval in cross section, column bodies of cPx-Hb to Hbtwo pyroxenes basaltic andesites with small mafic
rounded enclaves. Rare pyroclastic (psamitic tuffs),
epiclastic rocks and basaltic andesite dikes are also
present. The small mafic enclaves, evidences for magma
mixing, are two types: 1) built up essentially by euhedral
clynopyroxene and; 2) composed of hornblende,
clinopyroxene and plagioclase (andesine – labradorite)
with rare small biotite crystals. The third volcanic event is
related to the formation of subvolcanic bodies (isometric
or elongated in WNW direction) of Bi-Hb andesite, Qandesites to dacites with enclaves and xenoliths.
In the porphyritic rocks of the pluton ocassional
small (up to 1-2 mm) miaroles filled with later
hydrothermal minerals are also established (fig. 1). Their
formation is related to the fluid exsolution from the
crystallizing water saturated magma as suggested for
granitoids by CANDELA (1997). Their small size, low
abundance and hydrothermal alteration suggest that the
intensive magma degasing processes took place in deeper
levels of the plutonic body. In the upper parts of the
intrusion are also established eutectic micropegmatitic
textures, characteristic for water saturated crystallization
of acid magmas.
Fig. 1. Miarolitic cavity in the groundmass of a
granodiorite porphyry of the Assarel intrusion bordered with
potassic feldspar, illed with secondary hydrothermal quartz.
Crossed Nicols. Scale bar – 1 mm.
Petrology
The magmatic evolution characterized by the
decreasing in TiO2, Al2O3, FeO, Fe2O3, MgO and CaO is
interpreted as a result of clinopyroxene, amphibole and
titanomagnetite fractionation of (ZARTOVA et al., 2004;
NEDIALKOV et al., 2006). This is also confirmed by the trace
elements behavior showing decrease in Co, Ni, Zn, Y, Sr,
Ga, V and Sc content and increasing in Rb, Ba, Th and Hf
during the magmatic evolution. The contents of Nb, Zr, Cr,
Ce, La, Nd, Pb, and Cu are relatively constant but Cu
demonstrates a slight upward inflexion at the end of the
evolution.
The hondrite-normalized distribution patterns of the
REE are characterized by a slope from Light to Heavy REE
(enrichment in light REE), lack of negative Eu anomaly and
increasing of Lan/Ybn ratio during the magmatic evolution:
(for Hb andesites and latites - Lan/Ybn = 6.8; Px-Hb
basaltic andesites - Lan/Ybn = 7.4 – 8.7; Bi-Hb andesites Lan/Ybn = 9,8 – 10,2 and porphyritic intrusive rocks Lan/Ybn = 14.2). That could be explained with processes of
pyroxene and amphibole. Plagioclase was probably not
involved in the fractionation.
The magmatism is subduction related derived from
enriched in LILE spinel-lherzolite source by higher melting
temperature (KAMENOV et al., 2004).
Mineralogy
The mafic minerals in volcanites are represented by
pyroxenes, amphiboles and biotites. Pyroxenes are
established in basaltic andesites and ocassionally (single
grains) in Hb andesites. Clinopyroxene is much more
abundant than orthopyroxene. Mg/Mg+Fe2+ in pyroxenes is
0.7 – 0.8. Amphiboles are present in all upper cretaceous
rocks (tschermakites, magnesio-hornblende, magnesiohastingsite and edenite) with Mg/Mg+Fe2+ = 0.64 - 1.00.
This amplitude is probably due to the mixing of the different
magmas. Biotite is partly or entirely altered in vulcanite and
in porphyritic intrusive rocks. Plagioclase is presented by
zonally arranged euhedral grains of two generations. The
first generation plagioclase crystals in the basaltic andesites
are with sieved texture (magmatic corrosion due to the
disequilibrium, resulting from magma mixing). In mafic
phenocrysts (amphiboles and clinopyroxenes) from all the
volcanic rock varieties are established small (up to 35
microns) sulfide melt inclusions (pyrhotite and rare
chalcopyrite) indicating relatively high S potential of the
magma and its relatively reduction conditions at the
beginning of the crystallization in depth (fig 2). Sulfide melt
inclusions could extract > 95% of ore metals from the
silicate melt (HALTER et al., 2005).
The mineral geochemistry shows that clinopyroxenes
concentrate Cr, Ni, Co, Sc, amphiboles concentrate Ni, V,
Co, Sc, and not so clearly Zn, Nb and Ga, plagioclase
concentrates Sr, Ba and less Ga. Rock normalized patterns
of REE in plagioclase show well pronounced positive Eu
anomaly decreasing with magmatic evolution. If plagioclase
fractionation took place, then the rock patterns should have
Eu negative anomaly that is not the case. The decreasing of
the positive Eu anomaly in plagioclases indicates increasing
of the oxidation conditions in the magma with the evolution.
Fig. 2. Melt inclusion with bubble and sulfide inclusion
with fish tail morphology in amphibole phenocryst from basaltic
andesites. Parallel Nicols, scale bare – 100 microns
The
thermo-barometric
estimates
for
the
crystallization process indicate pressures within 4 and 9
kilo bars (highest in basaltic andesites and lowest in BiHb andesites) and temperature interval 730 – 910oC. The
water content estimated after the geohydrometer of
MERZBACHER & EGGLER (1984) is about 5-6 %.
According to the Mt-Ilm geothermometer-oxybarometer
of SPENCER & LINDSLEY (1981) the crystallization
temperature of the coexisting magnetite and ilmenite is
770oC, 2 unites below the magnetite-hematite buffer,
indicating oxidizing conditions for the magmatic
cristallization.
The isotopic characteristics of the volcanic rocks are
slightly enriched with respect to 87Sr/86Sr and lower than
CHUR 143Nd/144Nd values. This could be interpreted as a
slightly enriched mantle source combined with crustal
contamination.
Discussion
There are three important events governing the oregenerating capability of the magmatism: 1) partial melting
and source type, producing the primary magma rich or
poor in ore elements; 2) the different processes of
magmatic evolution leading to the enrichment of
incompatible ore elements; 3) the fluid/melt partitioning
responsible for the transfer of ore elements from melt to
the exsolved fluid (HANNAH & STEIN, 1990).
The more primitive magma derived from the mantle
is probably of basic composition close to the first type of
enclaves with pyroxenitic composition (SiO2 = 50 – 54 wt
% and FeO = 8-10 wt %). According to the investigations
of WALLACE and CARMICHAEL, (1992) and MAUGHAN D.
et al. (2002) this basic magma could have sulfur content
approximately 800 – 1000 ppm, even more. The discrete
relation of the intermediate crustal chamber of evolved
magma with the source of the primitive magma (the
magmatic enclaves evidences) is the main way to increase
the S budget of the magmatic ore system (HATTORY, 1993;
HATTORY & KEITH, 2001 HALTER et al., 2005)
As demonstrated the leading processes of magmatic
evolution at the Assarel magmatic center are crystal
fractionation, crustal contamination and magma mixing. The
role of crustal contamination for the ore generating
capability of the magma is not clear enough yet. The
crystallization fractionation is prime for the enrichment in
ore components with incompatible behavior. The mixing of
the evolved andesitic magma with mantle derived primitive
melt has an important role for the supply of S in the ore
magmatic system. With the crystallization progress the
magma reaches rapidly water saturation which is favorable
for the fluid exsolution in the shallow magmatic chamber of
the porphyritic intrusion (small miaroles). The increasing
oxygen fugacity and the aqueous fluid exsolution lead to the
destruction of the sulfide segregates concentrating ore
elements and the transition of S (as SO2) and the ore
elements into the fluid.
The copper contents in the more evolved volcanites are
50 ppm and those in the post ore granite porphyry – 11 ppm.
We could suppose that the efficiency of the copper
exsolution from the magma is about 80 %. Regarding such
conditions, a magmatic chamber 15 km3 in volume and Cu
concentration 50 ppm is sufficient for the formation of the
Assarel porphyry copper deposit.
Acknowledgments This study was financed by the
Swiss National Science Foundation – SCOPES project
7BUPJ02276.00/1 and the National Fund of Scientific
Research of the Ministry of Education of Bulgaria – project
NZ - 1405/2004 and VU NZ-02/05. We would like to thank
Lushka Koprivshka from the geologic survey of the Assarel
deposit for her help during the field work.
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