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Metamorphosed gold deposit from Orlik near Humpolec,
Czech Republic
Zbyněk Buřival, Lenka Losertová
Institute of Geological Sciences, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic
Abstract: The paper presents mineralogic considerations
about gold ores in the Orlik deposit (Czech Republic).
Data have been obtained on samples coming from
medieval pit. Studies of sulfidic ores revealed typical
pyrrhotite-löllingite-arsenopyrite and gold textures. These
aggregates were recently linked to certain conditions of
LP/MP-HT metamorphic evolution. Documented textures
confirm the premetamorphic origin of gold-bearing ores
and serious remobilisation of gold. The conditions of
older (~329 Ma) migmatite metamorphism with
temperatures up to 730 oC and pressure about 6 kbar
were sufficient to decompose premetamorphic gold
bearing arsenopyrite and form löllingite-pyrrhotite
assemblages with the younger generation of arsenopyrite
and gold. Also significant remobilisation of gold and
sulfides into wall rock was observed. Consequent
periplutonic metamorphosis (~327 Ma) with only 500 oC
and 2,5 kbars was too weak to significantly affect
sulphidic textures, but might be connected with the the
decomposition of maldonite into the gold-bismuth
myrmekite structures and secondary Bi-minerals.
Keywords: Humpolec, Orlik, gold, deposit, metamorphic,
sulfides
1 Introduction
The Orlik gold deposit is located near the town
Humpolec in the central Czech Republic. The deposit
itself is probably the most mineralized part of the
Humpolec-Pacov gold-bearing zone which extends 25
km to south.
Gold has been mined at this locality since the
beginning of the 13th century. Together with the nearby
area of Trucbaba-Valcha deposit, the whole gold-
producing region is dotted with hundreds of shallow
diggings and placer-mining relics. The richest part of the
deposit was opened by the small open pit in the length
of nearly 120 m and up to 8 m deep (Losertová et al.
2011). The total yield of gold is expected to be around
100 kg (Luna et al. 1988).
The deposit was abandoned for centuries, until 1980-
1988, when modern prospecting was conducted by the
state. Despite the discovery of very high gold grade (up
to 20 g/t) the whole deposit was considered to be too
small and to present too many complications to be
mined (Luna et al. 1988). Extremely irregular ore grade
and distribution of gold within unpredictable quartz
blocks and lenses are considered to be the most
significant of these obstacles.
The still unsolved puzzle is the origin of the gold
mineralization and evolution of the deposit. Some
authors considered a variscan hydrothermal origin
(Ďurišová et al. 1992) while others consider the
stratiform origin (Litochleb et al. 1982, Morávek et al.
1992). However, the latest research suggests that the
evolution was more complicated and the deposit itself
was metamorphosed at least twice. This paper provides
the evidence of MP-HT metamorphic event and its
influence on gold mobility.
2 Geological Setting
2.1 Geology of the area
The Orlik deposit is located in the Moldanubian zone –
an area built of long N-S variscan granitic plutonic
complex with large surroundings built by metamorphic
rocks of amphibolite-to-granulite facies. Most of the area
consists of paragneisses in various stage of
migmatitization with local lenses of amphibolite,
quartzite, marble and calc-silica rocks (Litochleb et al.
1982). Several larger bodies of orthogenisses of
uncertain origin are emplaced in the paragneiss. The
whole area is assumed to be of marine origin and of
proterosoic age exhibiting evidence of at least one
regional metamorphic event as well as subsequent
periplutonic metamorphosis (Žák et al. 2011).
Figure 1. Geology map and cross-cut of the Orlik deposit
Legend: 1 Migmatitized biotite paragneiss 2 Paragneiss with
calc-silica rocks and quartzites 3 Massive calc-silica rocks 4
Pegmatites 5 Main open pit 6 Old mine workings 7 Cross-cut
profile
2.2 Ore minerals of the Orlik deposit
Ore mineralization is connected with irregular quartz
lenses forming long streak in the W-E direction. The ore-
bearing zone is 1.2-2.5 m thick dipping 80o to the NW
(Luna et al. 1988). Because of serious metamorphic
remobilization, part of the gold was disseminated into
the migmatites, quartzites and calc-silica rocks. The gold
is often interspersed with maldonite and native bismuth
or included within arsenopyrite and löllingite (Litochleb
et al. 1982). Less common sulfides include pyrite,
marcasite, chalcopyrite, sphalerite, galena and
molybdenite. Ilmenite, haematite and rutile-common
accessories of metamorphic rocks-can be also be found
in association with the gold (Litochleb et al. 2001).
Three generations of gold were identified. The gold I
type is probably the original gold in the form of solitary
grains with up to 6 % Ag. The gold II type forms
aggregates with maldonite, and contains up to 8 % Ag
and formed by expulsion from the sulfides. The gold III
type is extremely pure gold (0.2 % Ag) which formed by
maldonite decomposition into the gold-bismuth
myrmekite (Morávek et al. 1992, Litochleb et al. 2001).
3 Experimental
Samples with sulfides and gold were collected from the
main medieval mining pit and prepared into polished
pellets. The WDX analyses were performed on Cameca
SX100 with these settings: accelerating voltage 25 keV,
beam current 20 nA and beam diameter <1m.
4 Results
4.1 Mineral assemblage
Small quartz veinlets in the migmatized gneiss were
studied. The texture consists of quartz dominated bands
up to 1 mm thick and fine-grained silicate mineral bands.
The quartz bands contain sulfide aggregates as large as 3
mm and consisting of pyrrhotite, arsenopyrite and
löllingite with minor gold inclusions. Similar aggregates
up to 1 mm occur also in the fine grained bands. The
mineral assemblage also contains plagioclase, biotite, K-
feldspar, titanite, apatite, rutile, ilmenite, zircon,
monazite and small occurrences of sillimanite.
4.2 Sulfides and gold
Pyrrhotite forms subhedral or anhedral grains up to 1.5
mm. It often occurs as solitary grains in quartz, or
together with rutile, titanite and rarely also sillimanite,
apatite and monazite. Sulfide aggregates usually contain
pyrrhotite together with arsenopyrite and löllingite.
Pyrrhotite has almost ideal composition with trace
content of As (0.009 apfu) and Se (0.006 apfu).
Arsenopyrite is present in two generations.
Arsenopyrite I is present only in the form of rare solitary
euhedral grains. Much more interesting is the
arsenopyrite II forming the reaction rims up to several
hundred m wide between pyrrhotite and löllingite. The
boundary between pyrrhotite and arsenopyrite II is
always sharp while the boundary between arsenoyprite II
and löllingite is diffusive. Tiny grains of Au-rich silver
(up to 30 % gold) form on the border between
arsenopyrite and rock-forming minerals. The
arsenopyrite II is very pure with trace amount of Co
(0.002 apfu) and Ni (0.001 apfu).
Löllingite occurs largely in the form of anhedral
grains 0.2-0.6 mm big, rimmed by arsenopyrite II.
Rarely, it is also found as subhedral grains in quartz. The
löllingite contains elevated content of S (0.115-0.125
apfu) and trace amounts of Ni (0.004-0.013 apfu) and Co
(0.002-0.004 apfu).
The gold I is likely of the same age as old
unmetamorphed minerals like arsenopyrite I and many
silicates. The gold II formed by the decomposition of
sulfides and is syngenetic with younger arsenopyrite II
and older then most silicate minerals. The gold III is the
youngest and formed by decomposition of maldonite.
5 Discussion
5.1 Ore textures
Observed ore textures were previously described from
other metamorphosed deposits and were interpreted as a
synmetamorphic (Barnicoat et al. 1991, Neumayr et al.
1993). Later, the excessive field and experimental study
of these textures by Tomkins and Mavrogenes (2001)
revealed their formation sequence during prograde and
retrograde metamorphism.
Arsenopyrite I becomes unstable during prograde
metamorphism and decomposes into aggregates of
löllingite and pyrrhotite: complete decomposition
requires 702 oC (Tomkins and Grundy 2009). The gold
often dissolved in arsenopyrite I in the form of
submicroscopic inclusions becomes insoluble under high
temperature (Cook et al. 2013) and is mostly remobilised
into the löllingite.
During retrograde metamorphism, the secondary
arsenopyrite II often forms. Arsenopyrite II grows from
the original border of löllingite and pyrrhotite towards
the löllingite. Because of the high temperature, the gold
is insoluble in the arsenopyrite and is expelled from
decomposed löllingite as gold grains. The observed
textures from Orlik deposit suggest an evolution similar
to experiments performed by Tomkins and Mavrogenes
(2001).
Figure 2. Sulfide aggregate The grain of primary
arsenopyrite (not present) was decomposed into aggregate of
pyrrhotite (Po) and löllingite (Lo). During subsequent
retrograde metamorphism, the löllingite became unstable and
was partially replaced by secondary arsenopyrite (Apy) and
gold (Au) was expelled from löllingite.
The Orlik deposit contains also significant amount of
Bi (Litochleb et al. 1982) in the form of native Bi grains
and inclusions in sulfides or rock forming minerals.
Maldonite is often present, sometimes even as grains in
the löllingite-pyrrhotite aggregates. Maldonite usually
decomposes into the gold-bismuth myrmekites: at Orlik
both maldonite and myrmekites are present. Some of the
myrmekites with bismuth were altered by secondary
processes and contain bismuth carbonates and oxides
instead of native bismuth (Litochleb et al. 1982).
Calc-silicate rocks contain also significant amount of
scheelite in the form of impregnation or up to 5 cm thick
quartz-scheelite veinlets (Morávek et al. 1992).
Figure 3. Evolution of sulfide aggregates (modified after
Tomkins and Mavrogenes 2001) a) Replacement of primary
arsenopyrite (Apy) by pyrrhotite (Po) and löllingite (Lo) during
prograde metamorphism b) start of the replacement of löllingite
(Lo) by secondary arsenopyrite (Apy) during retrograde
metamorphism c) evolved replacement of löllingite (Lo) by
secondary arsenopyrite (Apy) with newly formed gold grains
(Au).
5.2 Gold size and mobility
The results of the metamorphic evolution of the gold
deposits have also significant impact for the mining
industry. If the metamorphism is weak, the gold remains
in the form of submicroscopic inclusions within
arsenopyrite I and/or löllingite. With the evolution of
retrograde metamorphism and formation of secondary
arsenopyrite II, the gold is expelled from these minerals
and forms bigger gold grains in the sulfides or on their
rims. The size of gold grains at Orlik ranges from 0.1 to
1 mm and can, in exceptional cases, reach up to 1 cm.
Observed features from the Orlik deposit also suggest
the possibility of serious migration of both sulfidic melt
and gold into the wall rocks of the original ore
structures. This might result into gold concentrated in
the newly formed microstructures in the country rock.
The significant remobilisation at Orlik is obvious from
amount of newly formed sulfides and gold grains filling
the tiny fissures in biotite and diopside, as well as gold
grains covering the older sillimanite and gold inclusions
in the anorthite leucosomes (Litochleb et al. 1982).
5.3 PT-conditions
The observed sulfide textures suggest temperature well
above 702 oC. The general metamorphic conditions
during formation of migmatites in the Orlik area are
estimated to be about 730 oC with pressure about 6 kbars
(Žák et al. 2011). This means significantly higher
temperature and pressure then is estimated for other gold
deposits: 550-450 oC at 2-4 kbar in Mokrsko (Boiron et
al. 2001), 480-390 oC at 2 kbar in Kašperské Hory
(Strnad et al. 2012), 350-250 oC at 2 kbar in Roudný
(Zachariáš et al. 2009). Also most of the other deposits
reveal the age between 340-345 Ma except Roudný with
less then 300 Ma. Orlik deposit does not fit in any group
with the age of gold remobilisation around 329 Ma.
The younger (~327 Ma) periplutonic metamorphic
event at Orlik deposit reached only about 500 oC and 2.5
kbar (Žák et al. 2011) - a temperature too low to form
observed sulfide textures. However, the periplutonic
metamorphosis probably affected late decomposition of
maldonite, which is stable only up to 373 oC (Litochleb
and Malec 1995).
Acknowledgements
We would like to emphasize the awesome work of Jiří
Litocheb on this deposit. Unfortunately, our cooperation
on this topic came to an unexpected halt. Zdeněk Losos
and Sophia K. Shultz kindly helped to improve this
paper.
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