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Goldschmidt2015 Abstracts
Timing mismatch between VMS
deposits and footwall epidosite
alteration in the Semail Ophiolite
L. W. DIAMOND1* AND S.GILGEN1
1
Institute of Geological Sciences, University of Bern,
Baltzerstrasse 3, CH-3012, Switzerland (*correspondence
[email protected])
Epidosites (rocks metasomatically transformed to epidote
+ quartz + accessory hematite or magnetite) are thought to be
products of extreme rock–water interaction and metal leaching
deep in hydrothermal convection cells under active spreading
ridges in mafic oceanic lithosphere [1] [2]. Discharge of the
hydrothermal fluid on the seafloor produces volcanogenic
massive sulfide (VMS) deposits. Thus, according to this
genetic model, every VMS deposit is under-lain by epidosites
at the base of the sheeted-dike complex.
The eastern margin of the Semail Ophiolite, Oman,
constitutes a tilted section through a thick Sheeted Dike
Complex (SDC) overlain by comagmatic ridge-related basalts
(Geotimes and Lasail Units), in turn capped by younger suprasubduction zone basalts erupted in a nascent forearc setting
(Alley and Boninitic Alley Units [3] [4]). All four volcanic
units host VMS deposits [4]. Three small epidosites were
previously known in the ophiolite [5], all situated at the base of
the SDC in accord with the genetic model. However, our
recent field work has identified dozens of other epidosite
bodies up to 1 km2 in extent, situated throughout the SDC and
the Geotimes and Lasail lavas. Their cross-cutting relations
with respect to dated magmatic and tectonic features show that
all but one of them formed during the supra-subduction-zone
volcanism at 95–94 Ma, some 1.5 Ma after cessation of ridgerelated volcanism and formation of the SDC. The one
exceptional epidosite formed during off-axis, transitional latespreading (Lasail) volcanism. Thus, none of the epidosites is
demonstrably synchronous with the main oceanic spreading
event marked by formation of the SDC and the comagmatic
Geotimes lavas, which host abundant VMS deposits. This clear
mismatch in timing conflicts with the current genetic model
and therefore questions the genetic relationship between
footwall epidosites and the source rocks for metals in basalthosted VMS deposits.
[1] Richardson et al. (1987) EPSL 84, 243-253. [2] Alt JC
(1995) Geophys Monograph 91, Am. Geophys. Union, pp. 85114. [3] Rioux et al. (2013) J. Geophys. Res. Solid Earth 118,
2085-2101. [4] Gilgen et al. (2014) Econ Geol 109: 15851610. [5] Nehlig et al. (1994) J. Geophys. Res. Solid Earth
99(B3), 4703-4713.
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Goldschmidt2015 Abstracts
Late Jurassic ocean anoxic event:
Evidence from voluminous sulfide
deposition and preservation in the
Panthalassa Ocean
T. NOZAKI12, Y. KATO12 AND K. SUZUKI1
1
R&D CSR/JAMSTEC, 2-15 Natsushima-cho, Yokosuka,
Kanagawa 237-0061, Japan ([email protected])
2
Univ. of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656,
Japan ([email protected])
Basement rock of the Japanese Island is mainly composed
of accretionary complexes since Paleozoic periods. There are
several types of ore deposits such as Besshi-type sulfide
deposit, Mn carbonate deposit in the bedded chert succession
and Mn oxide deposit on seamount basalt, which were derived
from seafloor mineralization and are now observed on land.
Constituent minerals of Besshi-type sulfide and Mn carbonate
deposits are stable under the reducing condition, whereas Mn
oxides precipitated contrastingly under the oxic condition.
Since the distribution of these three types of deposits is
curiously uneven in the Japanese accretionary complexes [1],
the redox history of the Panthalassa Ocean might be unraveled
based on their depositional ages.
The historically productive copper-bearing Besshi-type
sulfide deposits in the Japanese accretionary complex were
formed as volcanogenic massive sulfide deposits on the deepsea floor of the Panthalassa Ocean. Here we report that eleven
typical Besshi-type deposits yielded Re-Os isochron ages
around 150 Ma (148.4 ± 1.4 Ma from the composite isochron)
in Late Jurassic time [2] [3]. This date coincides with the
lowest marine 87Sr/86Sr ratio and highest atmospheric CO2
concentration of the past 300 million years. We infer that
intense mid-ocean ridge hydrothermal and volcanic activity in
the Late Jurassic produced huge sulfide deposits and large
emissions of CO2 gas, leading to global warming and a
stratified Panthalassa Ocean with anoxic deep seas that favored
preservation of sulfides in the pelagic environment. The
emergence of ocean anoxia triggered by seafloor volcanism is
also consistent with a positive δ13C excursion and widespread
deposition of petroleum source rocks and black shales.
[1] Sato, K. and Kase, K. (1996) Island Arc, 5, 216-228. [2]
Nozaki, T. et al. (2010) Geochim. Cosmochim. Acta, 74, 43224331. [3] Nozaki, T. et al. (2013) Sci. Rep., 3, 1889, doi:
10.1038/srep01889.
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2301
Goldschmidt2015 Abstracts
Pyrite Re-Os geochronology: Lessons
from the Irish orefield
D. HNATYSHIN1*, R. A. CREASER1, J. J. WILKINSON2,
R. A. STERN1 AND S. A. GLEESON1
1
Department of Earth & Atmospheric Sciences, University of
Alberta, Edmonton, AB, Canada (*[email protected],
[email protected], [email protected],
[email protected])
2
Department of Earth Sciences, Natural History Museum,
Cromwell Road, London SW7 5BD, UK
([email protected])
The Irish Zn-Pb orefield is one of the world’s largest
hydrothermal Zn-Pb ore districts, but uncertainty in the timing
of mineralization made distinguishing between different
potential genetic models for ore formation difficult. Three
main proposals for the timing of mineralization have been
suggested. The first model is that the deposits formed in the
shallow subsurface shortly after or during deposition of the
host rocks (~350Ma). The second is that these deposits formed
after significant burial (<340Ma), and the third is that they
formed during the Variscan orogeny (310-290 Ma).
Distinguishing between these different possibilities is possible
using pyrite Re-Os isochron methods. Hnatyshin et al. (2015)
[1] determined robust Re-Os ages for the Lisheen Main Zone
(346.6 ± 3.0 Ma, MSWD = 1.6 ) and the Silvermines B-zone
(334 ± 6.1 Ma, MSWD = 19).
However, some pyrite samples fromt the Irish deposits
produced isochrons that show a greater amount of complexity.
In particular, the high quality results produced by the Lisheen
Main Zone are not replicated using samples from the Lisheen
Bog Zone. Whereas a very similar age is produced
(345 ± 11 Ma) the scatter in the data is much higher
(MSWD = 80). To determine the source of the scatter a variety
of different techniques were used to characterize both sets of
samples, including petrographic imaging, back-scattered
electron imaging, trace element mapping, in-situ sulfur isotope
measurements, as well as acid leaching experiments. The
results show that the Bog Zone samples contain a much more
complicated suite of intergrown sulphide minerals and contain
multigenerational pyrite showing distinct textural and chemical
differences, likely resulting in the excess scatter observed in
the Re-Os data. Ultimately, these results suggest that screening
out excessively complicated samples is required when trying to
produce precise and accurate ages using pyrite Re-Os
geochronology.
[1] Hnatyshin. et al. 2015, Geology, v. 43, p. 143-146.
1278
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Goldschmidt2015 Abstracts
Re-Os age of late bornite-chalcopyrite
vein ores, Kupferschiefer, SW Poland
S. Z. MIKULSKI1 AND H. J. STEIN23
1
Mineral Resources Program, Polish Geological Inst. National
Research Inst., Poland (stanislaw.mikulski @pgi.gov.pl)
2
AIRIE Program, Colorado State University, USA
([email protected])
3
CEED, University of Oslo, 0316 Oslo, Norway
The age of the world-class Cu-Ag stratabound
mineralization from the southern margin of the Upper
Zechstein basin in Poland has been addressed using different
geochronological methods. In general the ages of the
Kupferschiefer’s mineralized samples span from Lower
Triassic to Lower Cretaceous [1] [3-5] [7-8]. Economic Cu
mineralization transgresses sedimentary sequences and is
hosted by Kupferschiefer black shale, underlying sandstone
(Rotliegendes) and overlapping limestone of the Upper
Permian marine sequence (lower Zechstein). Cu-Ag ores are
represented mainly by Cu sulphides such as chalcocite, bornite,
chalcopyrite and covellite which are commonly associated
with silver admixtures and/or minerals. Four samples from the
Lubin (-610 m b.s.l.) and Polkowice (-740 m b.s.l.) operating
mines were acquired for Re-Os analyses. Analyzed samples
consist of economic Cu ores characterized by 1-10 mm bornite
± chalcopyrite veinlets in calcareous shale (5-10 cm thick) rich
in organic matter. In any single veinlet, bornite and
chalcopyrite may occur in variable proportion, and penetrate
along shale laminations as fine-grained (1-30 μm in diameter)
disseminations and small aggregates (<50 µm). Within
macroscopic veinlets where chalcopyrite is present, it forms
symmetrical margins to a bornite interior. Bornite, bornitechalcopyrite and/or chalcopyrite veinlets may cross-cut
bedding or be nearly parallel to lamination in black shales. We
report a Re-Os isochron age for bornite ± chalcopyrite veinlets
that exhibit shallow cross-cutting features to bedding in black
shale. A Model 1 regression yields 212 ± 7 Ma, with an initial
187
Os/188Os ratio of 2.13 ± 0.31 (MSWD = 1.3). The analyzed
bornite ± chalcopyrite veinlets have a Re concentration ranging
from 5.7 to 12.1 ppb, and a total Os concentration ranging
from 27-52 ppt. Significant common Os is present in all of the
analyzed samples. The current Model 1 age suggests a higher
initial 187Os/188Os than our previous result [4]. The origin of the
Kupferschiefer mineralization is commonly attributed to
multiple flow events of low-temperature oxidizing
metalliferous fluids triggered by tectonic activation of basinal
sediments (e.g. [2] [6]). Our results strongly suggest that the
main Cu-mineralization event took place in the Late Triassic
(Norian), ca. 212 ± 7 Ma. The work was supported by grant
No. N525 393739 from the Ministry of Science and Higher
Education to SM.
[1] Bechtel et al. (1999) Econ. Geol. 94, 261 [2] Blundell et al.
(2003) Econ. Geol. 98, 1487. [3] Jowett et al. (1987) J.
Geophys. Res. 92, 581. [4] Mikulski and Stein (2010) GCA,
A708. [5] Nawrocki (2000) Econ. Geol. 95, 241. [6]
Oszczepalski (1999) Min. Deposita 34, 599. [7] Pasava et al.
(2007a) GCA, A763. [8] Pasava et al. (2010) Min. Deposita
45, 189.
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Goldschmidt2015 Abstracts
Multiple IOCG-forming events in the
Carajás Province, Brazil
CAROLINA P. N. MORETO1*, LENA V. S. MONTEIRO2,
ROBERTO P. XAVIER1, GUSTAVO H. C. MELO1,
MARCO A. DELINARDO DA SILVA1 AND
ROBERT A. CREASER3
1
Geoscience Institute, University of Campinas, Rua João
Pandiá Calógeras 51, Campinas, Brazil (*correspondence:
[email protected])
2
Geoscience Institute, University of São Paulo, Rua do Lago
562, São Paulo, Brazil
3
Department of Earth and Atmospheric Sciences, University of
Alberta, Edmonton, AB T6G 2E3, Canada
World-class iron oxide-copper-gold (IOCG) deposits (e.g.,
Sossego, Salobo, Igarapé Bahia) comprise the most important
Cu resources in the Carajás Province, Brazil. The deposits are
hosted by: i) Mesoarchean granites, gneisses and greenstone
belts; and ii) Neoarchean metavolcanic-sedimentary units,
bimodal intrusive rocks and gneisses.
Isotope data combined with field evidence suggest a
multistage evolution of IOCG mineralization in the Carajás
Province, with recurrence of ore-forming systems during the
Neoarchean and the Paleoproterozoic. These included:
i) Syngenetic to diagenetic chalcopyrite associated with
rhythmites at the Igarapé Bahia deposit, and fluid circulation at
the Bacuri deposit at 2.76 Ga;
ii) A major episode of IOCG formation at 2.72-2.68 Ga
related to basin inversion coupled with Neoarchean
magmatism (e.g., Sequeirinho-Pista ore bodies at the Sossego
deposit; Bacaba, Castanha, Bacuri, Visconde and Cristalino
deposits);
iii) A ca. 2.5-2.4 Ga hydrothermal and/or remobilization
events synchronous with shear zone reactivation and
responsible for the Sabolo and Igarapé Bahia deposits;
iv) Paleoproterozoic (1.90-1.87 Ga) IOCG mineralization
related to the emplacment of A-type granites and represented
by the Sossego-Curral ore bodies (Sossego deposit) and the
Alvo 118 deposit.
The deep-seated Neoarchean IOCG systems are
characterized by breccia and replacement bodies associated
with
albite–scapolite,
biotite–scapolite–tourmaline
or
almandine–grunerite, and magnetite– (apatite–actinolite)
formation. Shallow-emplaced Paleoproterozoic IOCG
systems, formed under brittle-dominated regime, have
characteristic potassic and chlorite alteration zones. The
reccurence of hydrothermal systems in time and space
contributed to the complex hydrothermal overprint observed in
the Neoarchean deposits, as well as Cu enrichment in the
Paleoproterozoic.
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Goldschmidt2015 Abstracts
High spatial resolution SHRIMP and
LA-ICPMS U-Pb geochronology of
Pea Ridge Fe-REE-Au deposit, USA
L. A. NEYMARK1, J. N. ALEINIKOFF2,
C. S. HOLM-DENOMA3, A. J. PIETRUSZKA4,
R. M. PILLERS5 AND R. J. MOSCATI6
1
[email protected]
[email protected]
3
[email protected]
4
[email protected]
5
[email protected]
6
[email protected]
2
Precise and accurate determination of the timing and
duration of ore-forming processes is crucial for understanding
the origin of deposits and placing them in a regional geologic
context. The Pea Ridge iron oxide - apatite deposit in the
~1.47-1.44 Ga St. Francois Mountains terrane, southeast
Missouri, USA is an endmember type in the global spectrum of
iron oxide – copper - gold deposits. Abundant monazite and
xenotime occur in REE-rich breccia pipes that cut the host
rhyolite, magnetite ore, and alteration zones associated with
the iron ore system. As revealed by CL and BSE imagery,
most dateable minerals from this deposit are intergrown in a
fine-grained matrix, or have numerous inclusions and/or
overgrowths, thereby requiring high spatial resolution
geochronology to obtain accurate age constraints on ore
formation.
SHRIMP U-Pb zircon ages of 1473.6 ± 8.0 (2σ) and
1472.7 ± 5.6 Ma were obtained for the host rhyolite. Two
dissolved bulk apatite samples from magnetite ore yielded
TIMS upper intercept ages of 1461.3±8.3 and 1466.2±4.0 Ma
and showed normal age discordance due to minor Pb loss.
Micron-sized equant monazite inclusions in the apatite yielded
a LA-ICPMS age of 1442±12 Ma. LA-ICPMS analysis of
inclusion-free portions of pyrite (with U/Pb~0) from magnetite
ore yield high radiogenic Pb isotopic values (206Pb/204Pb up to
60), indicating redistribution of Pb during a process that was
likely much younger than 1.4 Ga. Monazite and xenotime in
the REE-rich breccia pipes have variable morphologies, but
yielded nearly identical SHRIMP ages of 1462.5 ± 1.5 and
1462.7 ± 9.3 Ma, respectively.
Although textural evidence suggests the possibility of
multiple hydrothermal events, better analytical precision than
is currently achieved by LA-ICPMS and SHRIMP is required
to resolve potential age variability at a finer time scale.
However, our high spatial resolution dating results indicate
that the hydrothermal activity responsible for mineralization at
the Pea Ridge deposit post-dated volcanism by up to 10 Ma
and lasted for tens of millions of years.
2264
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Goldschmidt2015 Abstracts
Temporal constraints on magma
dynamics resulting in porphyry
copper deposit formation
S. TAPSTER1*, D. J. SMITH2, J. A. NADEN3 AND
D. J. CONDON1
1
NERC Isotope Geosciences Laboratory, British Geological
Survey, Keyworth, Nottingham, NG12 5GG, U.K.
*correspondence: [email protected])
2
Department of Geology, University of Leicester, University
Road, Leicester, LE1 7RH, U.K
3
British Geological Survey, Keyworth, Nottingham, NG12
5GG, U.K.
The timing and duration of volatile exsolution repsonsible
for porphyry-style copper mineralisation are ultimately
controlled by the dynamics within the long-lived magmatic
systems that supply the metals and ore-forming fluids. Volatile
saturation occurs in response to changes in pressure,
temperature and crystallinity of the source magma. We
examine the temporally constrained relationships between
mineralisation and magma dynamics at depth in one of the
world’s youngest exposed porphyry systems, the Koloula
Poprhyry Prospect, Solomon Islands.
Geological relationships and high precision ID-TIMS U-Pb
zircon dating constrain assembly of the shallow level pluton to
<150 kyr. Mineralising poprhyry intrusions followed within ca.
50 kyr. Discrete mineralising intrusive events are separated by
34 ± 24 kyr, also constraining the duration of the first, oreforming hydrothermal event. Dates define both protracted
zircon crystallisation at depth and rapid recycling to the crustal
level of porphyries.
We apply a multifaceted approach combining: textural
analysis of zircons; U-Pb geochronology; Ti-in zircon
thermometry; and geochemical modelling of zircon dissolution
rates and magma crystallinity-temperature relationships for
suitable approximations of conditions in the underlying
plutonic system. Results show that magmas resided in a highly
crystallised (>50%), volatile saturated, immobile state for
extended periods at depth preceeding porphyry formation.
Liberated mobile melt fractions that transported the requisite
components for mineralisation to shallow levels were transient,
existing for <10–20 kyr.
Intrusion of andesitic magmatism under the highly
crystallised silicic pluton provided the energy for thermal
rejuvenation. Porphyry formation was likely promoted by
rejuvenation following the addition and percolation of hot
volatiles, during low fluxes of andesitic magma; rather than
extensive magma mixing, which occurred during high fluxes.
3079
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Goldschmidt2015 Abstracts
Duration of ore formation: Grasberg
porphyry copper deposit, Papua,
Indonesia
S. WAFFORN1*, M. CLOOS1 AND D. F. STOCKLI1
1
University of Texas at Austin, Austin, TX 78713
*correspondance: [email protected]
Isotopic dating of intrusions and hydrothermal alteration
from porphyry copper deposits worldwide is rarely able to
constrain the duration of ore formation with a resolution better
than one million years. Zircon U/Pb dating of intrusions that
host and cross-cut ore grade mineralization at the supergiant
Grasberg deposit, located in Papua, Indonesia, provides a
constraint on the maximum duration of hydrothermal fluid
flow. Porphyry copper-type mineralization is hosted in the
Grasberg Igneous Complex (GIC), which comprises three
pulses of magmatism: the Dalam Phase, the Main Grasberg
Intrusion (MGI), and the Kali Dikes. Main phase copper
mineralization initiated following intrusion of the MGI
(3.07±0.05 Ma, n=107) and predates the Late Kali Dike
(2.99±0.05 Ma, n=90). Based on these ages the Grasberg
deposit formed in less than 180 k.y, and perhaps less than
80 k.y. The oldest intrusion dated in the Ertsberg-Grasberg
district is the Wanagon Sill (3.43±0.07 Ma, n=52) and the
youngest intrusion is a dike cutting the Ertsberg pluton
(2.71±0.07 Ma, n=32). These data constrain the duration of
magmatism in the district to less than 900k.y.
Apatite and zircon (U-Th)/He (aHe and ZHe) ages provide
additional insight into the low-temperature thermal history
associated with ore formation. Samples were collected from a
vertical profile in the Kali Dikes spanning 2 km. Near-surface
samples cooled almost immediately following crystallization
(3.1±0.2 Ma zHe age), whereas samples at 2 km depth cooled
more slowly (2.1±0.3 Ma zHe age). Throughout the vertical
profile aHe ages are less 0.6 m.y. younger than the than the
zHe ages. Based on these ages the calculated cooling rate from
750-180°C was 150°C/10 k.y. near the surface, 11°C/10 k.y. at
1 km depth, and 4°C/k.y. at 2 km depth. The cooling rate from
180-70°C was 11°C/k.y. Collectively these results indicate
Grasberg ore formation occurred immediately following MGI
emplacement, was short-lived, and the system rapidly cooled.
The high cooling rates to temperatures below 70°C at 2 km
depth indicate the wall rock was cold and preclude the
presence of a 2 km tall volcanic structure over the orebody.
High cooling rates and steep thermal gradients along the edges
of the stock would cause rapid deep-seated crystallization of
quartz and feldspars. This led to the formation of copper-rich
fluid bubbles in mobile magma that rose to collect beneath a
cupola before ascending to form the Grasberg orebody.
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Goldschmidt2015 Abstracts
Using zircon petrochronology to
constrain timescales of porphyry Cu
formation: an example from Bajo de
la Alumbrera, NW Argentina
Y. BURET1*, A. VON QUADT1, C. A. HEINRICH1 AND
I. PEYTCHEVA12
1
Department of Earth Sciences, Institute of Geochemistry and
Petrology, ETH Zurich, Switzerland
(*correspondence: [email protected])
2
Bulgarian Academy of Science, Geological Institute, Sofia,
Bulgaria.
Using high-precision U-Pb dating we are able to determine
timescales of porphyry emplacement and ore formation.
Previous studies have suggested timescales of porphyry Cu
formation ranging from <100 yr [1], to as much as 1 Ma [2]. In
contrast recent numerical simulations suggest Cu precipitation
occurs in the range of 50-100 ka [3]. Therefore in order to
better constrain timescales of porphyry Cu formation, we apply
high precision U-Pb zircon geochronology to estimate
porphyry emplacement ages. Furthermore, high precision
zircon U-Pb dates combined with trace element and Hf isotope
analyses of zircons can provide useful insights into upper
crustal magmatic processes which immediately precede the
formation of porphyry Cu deposits.
This study focuses on the ~7 Ma Bajo de la Alumbrera CuAu deposit, NW Argentina. The deposit consists of a
composite stock of dacitic porphyries. The relative timing of
each porphyry intrusion is established based on clear crosscutting field relationships between different porphyry
intrusions, which include the pre-mineralisation P2 porphyry,
pre-syn-minerlisation EP3 porphyry, and the postmineralisation LP3 and P4 porphyries.
Single zircon crystals from individual porphyry intrusions
(P2, EP3, LP3, P4) in the Alumbrera deposit have been dated
using CA-ID-TIMS, employing the ET2535 tracer solution for
maximum precision and accuracy. All porphyries display
protracted zircon crystal growth over 100-200 ka timescales.
Using the youngest zircon population from each of the
porphyry intrusions, we conclude that Cu mineralisation
occurred on 10 ka timescales, similar to those proposed by
recent numerical predictions [3]. Trace element analyses from
the dated zircons suggest that all of the dated porphyries are
derived from the same body of underlying magma and show
non-systematic trace element and εHf trends with time.
[1] Cathles and Shannon (2007) EPSL 262: 92-108. [2] Ballard
et al. (2001) Geology 29: 383-386. [3] Weis et al. (2012)
Science 338: 1613-1616.
425
425
Goldschmidt2015 Abstracts
Cooling and mineralisation history of
Karakartal porphyry system,
Erzincan, Turkey
OĞUZHAN GÜMRÜK1*, MIĞRAÇ AKÇAY1,
NESLIHAN ASLAN1, BRENT MCıNNES2, NOREEN EVANS2
2
AND FRED JOURDAN
1
Karadeniz Technical University, Trabzon, Turkey
John de Laeter Centre, Curtin University, Perth, Western
Australia
(*correspondence: [email protected])
2
The Karakartal porphyry deposit is located near the
Kabataş village (Kemaliye-Erzincan) in a metallogenic zone
covering Divriği (Sivas)-İliç-Kemaliye (Erzincan) and Tunceli
provinces of east central Anatolia. The region has a complex
geological setting with the presence of Mesozoic to Tertiary
rocks. Early-Middle Eocene subvolcanic rocks (SVR) intrude
Jurasic-Crataceous limestones and Early Eocene clastic and
volcanic rocks. Field observations on cross-cutting
relationships between magmatic phases show at least four (preand syn- mineralization, post mineralization dykes and post
mineralization volcanic rocks) different magmatic episodes.
These magmatic phases have gabbroic to granodioritic
compositions and are geochemically similar to volcanic arc
granites.
This study aims at exhibiting mineralisation and cooling
history of the Karakartal porphyry system based on U-Pb,
Ar/Ar and U-Th/He geochronology. Zircon U-Pb data from
SVRs, (potassicly altered) syn-mineralization- and postmineralization dykes, and post mineralization basalts indicate
emplacement ages of 49.2±1.5 Ma, 45.81±0.44 Ma and
43.2±1.2 Ma, respectively. Ar/Ar ages from biotites and Kfeldspars of K-silicate alteration are determined to be
49.86±0.32 Ma and 47.32±0.57 Ma, respectively. (U-Th)/He
thermochronology on zircons from potassicly altered SVR’s,
however, give an age data of 45.8±0.8 Ma. These age
constraints from magmatic rocks and potassic alteration zones
within them indicate that magmatism was initiated in the
region at around 50 Ma and continued till around 43 Ma,
lasting for around 7 Ma. Commencement of porphyry system
is nearly coeval with the initial magmatism and went on till
about 45 Ma, characterised by zircon thermochronology.
This project was financially supported by the Turkish
Science Foundation (Tübitak) through the 110Y308 project.
1122
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Goldschmidt2015 Abstracts
The optimal pathways leading to
earthquake-enhanced gold
precipitation in the epithermal
environment
P. SÁNCHEZ-ALFARO12*, M. REICH12, T. DRIESNER3,
G. ARANCIBIA24, P. PÉREZ-FLORES24, J. CEMBRANO24,
J. ROWLAND5 AND C. HEINRICH3
1
Dept. of Geology, Universidad de Chile, Santiago, Chile
(*correspondence: [email protected])
2
Andean Geothermal Center of Excellence (CEGA),
Universidad de Chile, Santiago
3
ETH Zürich, Isotope Geochemistry and Mineral Resources,
Zürich, Switzerland
4
Dept. of Structural Engineering, Pontificia Universidad
Católica de Chile, Santiago, Chile
5
School of Environment, The University of Auckland,
Auckland, New Zealand
Hydrothermal ore deposits result from the combination of
a sustained flux of metal-rich fluids and an efficient
precipitation mechanism. Earthquakes may trigger gold
precipitation in the epithermal environment but its efficiency
and time-integrated contribution is poorly quantified.
In order to quantify the feedbacks between earthquakedriven fracturing and metal precipitation in the shallow crust,
we have constrained the past and present physico-chemical
conditions of a geothermal system in the highly seismic
Chilean Andes. We combined temperature measurements in
the deep wells with geochemical analyses of fluid samples
retrieved from the reservoir. In addition, we reconstructed the
paleo-fluid conditions using microthermometry and LA-ICPMS data of fluid inclusions from a deep borehole core. The
effect of pressure and enthalpy changes on precipitation was
evaluated by calculating the solubility of Au in P-H space, and
the impact of externally-forced, seismic perturbations on fluid
parameters was constrained using a thermo-mechanical piston
model for a “suction pump” mechanism. The reconstructed PT-H-X fluid trajectories indicate that fluids feeding the
hydrothermal reservoir reach boiling conditions with a high
gold budget (~1-5 ppb) at saturated liquid pressures between
50 and 120 bar.
Our results show that if hydrothermal ore fluids reach this
optimal threshold for metal precipitation, small adiabatic
pressure changes (~50 bar) triggered by transient fault-rupture
can drop gold solubility by up to two orders of magnitude. We
conclude that such externally-forced perturbations, equivalent
to low magnitude earthquakes (Mw<2) significantly enhance
gold precipitation efficiency.
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Goldschmidt2015 Abstracts
Hydrological and geological
constraints on timescales of
magmatic-hydrothermal ore deposits
PHILIPP WEIS12
1
Institute of Geochemistry and Petrology, ETH Zurich,
Switzerland ([email protected])
2
GFZ Potsdam, Germany
Chemical enrichment of metals in magmatic-hydrpthermal
ore deposits majorly depends on the physical hydrology of
fluids flowing through rocks. The respective geological setting
and associated physical hydrology play a decisive role in
forming distinct ore deposit types, including volcanogenic
massive sulfide deposits at mid-ocean ridges or submarine arc
volcanos, porphyry deposits in continental arcs, and epithermal
deposits. Simulation results from a numerical process model
for thermohaline convection in conjunction with a dynamic
permeability model are used to constrain the timescales of ore
formation. Thermal convection, volatile expulsion, and saltwater dynamics are first-order hydrological components with
different intrinsic timescales, and different combinations or
successions of these general patterns can help to constrain the
timing and duration of particular ore-forming systems.
The physical behavior of hydrothermal systems can be
counterintuitive, because of the nonlinear properties of fluids
and rocks as a function of pressure, temperature and
composition. In porphyry copper systems, mineralization is
localized by a self-stabilizing hydrological front, located at the
transition from brittle to ductile rock behavior and controlled
by the heat balance between an external convective cooling
engine and an overpressured magmatic fluid plume. Above this
hydrological divide, magmatic and meteoric fluids mix on
ascent to the surface, providing a mechanism for the transition
from porphyry to epithermal deposits. In mid-ocean ridge
hydrothermal systems, focused warm downflow in the
immediate vicinity of hot upflow zones may be a more
efficient mechanism for metal leaching than broad-scale lateral
infiltration of seawater, promoting ore-formation in Cyprustype massive sulfide deposits. In submarine magmatichydrothermal systems, phase separation can lead to a
decoupling of vapor-dominated venting during relatively short
periods of magmatic fluid expulsion, leading to the formation
of Au-rich chimneys, and brine-dominated venting during
convection in an extended waning stage, leading to the
formation of base-metal deposits.
All simulations of magmatic-hydrothermal ore formation
indicate timescales in the order of a few 102 to 105 years for
single mineralization events.
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