كليدواژه :
δD , δ^18 O , fluid , rock alteration , iron deposits , east , central Turkey
چكيده فارسي :
The origin and evolution of ore-forming fluids is commonly inferred from a combination of the
oxygen isotopes of quartz and the hydrogen isotopic composition of fluid inclusions trapped within
gangue or ore minerals. Both are bulk techniques and the validity of data requires there to be only a
single generation of inclusions or quartz both of which represent the ore forming process. Incorrect
analytical procedures frequently produce data that are incorrect for D. Frequently these criteria are
not met and the results are an average of the different fluid populations or quartz generations.
Different P-T-X characteristics of fluid inclusion generations can be identified by
microthermometry and in many instances the data does indicate a mixture of magmatic fluids with
local meteoric waters. In most cases it can be impossible to definitively assign a particular fluid
inclusion generation with mineralization and the products of fluid-rock interactions. These newly
formed alteration minerals are clearly linked to the hydrothermal fluids and their isotopic signature,
combined with conventional δD and δ^18 O measurements and fluid inclusion data provide a better
appreciation of the hydrothermal fluids.
An alternative, or addition, to the use of fluid inclusions is to determine the D and 18O of
clay/phyllosilicate minerals, which form as hydrothermal alteration products. The effect of fluid
composition and the fluid/rock ratio is more significant and the crystallinity of clay minerals is
largely controlled by fluid temperature. If clay minerals precipitated in equilibrium with the fluid
the H- and O-isotope composition of the clays can be used to infer the fluid’s isotopic composition
(if the fluid temperature is known) or temperature (if the fluid’s isotopic composition is known). In
addition to ore-forming fluid composition, radiometric dating of K-bearing clay minerals (illite,
mixed-layered illite-smectite) also provides information on the timing and duration of the
hydrothermal activity. Illite that formed in hydrothermal alteration environments and was not
affected by later fluid flow and thermal events remains stable, even at the conditions of porphyry
copper mineralization, therefore it can be used to understand the formation conditions, fluid source
and water/rock ratio.
In volcanic-hosted base-metal (Pb-Zn-Cu) deposits (Koru, Tesbihdere and Kumarlar) from the
Biga Peninsula, NW Turkey, the stable and radiogenic isotope data of illites from the hydrothermal
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6
alteration zones shows that illites can be used to better understand some aspects (i.e., origin of
fluids, temperature conditions, age of hydrothermal activity etc.) and supports evidence from other
techniques primarily related to the fluids of the mineralizing system (Bozkaya et al., 2106). The
stable isotope data, δD and δ18O, of illites indicates a hypogene origin and plots close to the
magmatic water box indicating that the hydrothermal fluids producing these alteration products
were predominantly magmatic water. The values are also consistent with the δD and δ18O values
obtained from fluid inclusions and host quartz that also plot close to the mineral data and indicate a
dominantly magmatic fluid. The ability to determine the age of the alteration, by using Rb/Sr data
of the illites, is advantageous as in this case a clear link to the age of mineralization, determined by
other methods or on different samples can be made. The latest Oligocene and lowest Miocene age
of illites indicates plutonic intrusions related to extensional tectonic regime in the Biga Peninsula,
are the main cause of hydrothermal activities that led to the numerous mineral deposits.
In the Bizmisen skarn-type iron (magnetite, hematite) deposit in east-central Turkey, kaolinite,
I-S and smectite from argillic alteration zones in the contact zone between Eocene dioritic
intrusions and Carboniferous-Early Cretaceous crystallized limestones, indicates a hypogene origin
for kaolinite and I-S, and a supergene origin for smectite (Bozkaya et al., 2017a). These clay
mineral associations infer low-temperature conditions. Stable isotope data of clay minerals indicate
100-200 ºC for smectite in the low-grade argillic zone, whereas in excess of 200 ºC for I-S and
kaolinite in the high-grade argillic zone. Fluids in isotopic equilibrium with the measured values,
using the average values for fluid-inclusion homogenization temperatures, indicate magmatic water
dominance (Bozkaya et al., 2017b). This magmatic association is further reflected in calculated
fluid δ18O and δD data in quartz, δ13C and δ18O data of calcite and δ34SV-CDT data of pyrite and
chalcopyrite, all of which reflect a strong magmatic influence compatible with a dominantly
magmatic source. Ar/Ar age data of I-S is consistent with argillic alteration starting in the Upper
Eocene, 10 Ma after the plutonic intrusion, and continuing to the uppermost Oligocene, with
duration of approximately 12 Ma. The obtained data indicate that hydrothermal clays were formed
from magmatic fluids in a hydrothermal system that persisted for an extended period well after the
intrusion of the igneous body.
It has shown that utilizing isotopic data from hydrothermal clays (kaolinite, illite, mixed-layered
illite-smectite) in combination with data from fluid inclusions and their host quartz has some
important advantages over interpreting fluid inclusion data. Therefore clays can be of use for better
understanding the age and conditions of hydrothermal ore fluids.
