Illite occurrences related to volcanic-hosted hydrothermal mineralization in the Biga Peninsula, NW Turkey: Implications for the age and origin of fluids

• Illite isotopic data provides a direct link to the hydrothermal fluids forming the mineralization.
• δD & δ18O isotopic data of illites are comparable to fluid inclusion isotopic data in quartz.
• δD & δ18O data between the MWL and magmatic box are due to WRI at lower temperature, not necessarily fluid mixing.
• The Rb–Sr age of illites indicate the heat and source of the hydrothermal fluids is primarily related to plutonic intrusions.
• Illites in this study have similar properties to hydrothermal illites from several deposits around the world.

A different approach to investigate the origin of fluids, temperature conditions, age of hydrothermal activity of mineralization in the Biga Peninsula, (Koru, Tesbihdere and Kumarlar) employed mineralogical (illite Kübler Index, b cell dimension, polytype) and geochemical (major, trace/REE, O–H stable isotope and Rb/Sr dating) methods. The Kübler Index (KI) values of illites indicate different temperature conditions, such as low temperature (high-grade diagenesis) for Koru deposit, and high temperature (anchizone) for the Tesbihdere and Kumarlar deposits. The textural, mineralogical and geochemical data from illites show that these have potential for estimating the age of hydrothermal activity and fluid characteristics. Both mineralogical (high grade diagenetic to anchizonal KI, 1M polytype, low d060 values) and geochemical (similar major and trace element composition to host-rocks, low octahedral Mg + Fe contents, oxygen and hydrogen isotope composition) data are compatible with commonly known hydrothermal illites. Stable isotope data of illites are well matched to similar data from fluid inclusions, which indicate mainly magmatic fluids. The Rb/Sr age (22.4 ± 2.3 Ma: latest Oligocene and lowest Miocene) of the illites coincides with plutonic intrusions that are the main instigators of hydrothermal activities related to the extensional tectonic regime in the Biga Peninsula. The mineralogical and geochemical data of illites have some important advantages with respect to the use of fluid inclusions in determining δD of hydrothermal fluids thereby leading to better understanding ore-forming hydrothermal conditions.

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