Genesis of alunite-bearing kaolin deposit in Mudamköy member of the Miocene Göbel Formation, Mustafakemalpaşa (Bursa), Turkey

Tectonic-controlled hydrothermal alteration of Lower Miocene volcanic rocks in the Mustafakemalpaşa (Bursa) region resulted in the development of kaolin deposit, which have an economical potential. This study will be important for the mineralogical, geochemical, and genetic contributions to the ongoing mining operation of this kaolin deposit and its possible application fields. The upward mineralogical and geochemical zonation of the kaolin deposit is as follows: kaolin, alunite-bearing kaolin, silicified kaolin, and silcrete; associated with Fe-oxide bearing veins and silicification. The characteristics of the kaolinized and alunitized units and their parent rocks were examined using polarized-light microscopy, X-ray powder diffractometry (XRD), scanning electron microscopy (SEM–EDX), chemical (ICP–AES and –MS), and isotopic (H–O, S–O) methods. Kaolinite and alunite are abundant and generally associated with quartz, feldspar, volcanic glass, and mica, locally smectite, accessory halloysite, and goethite. The kaolinite content increases up to 75% in the kaolin zone. Kaolinite shows both ideal hexagonal book-like and non-idiomorphic (initial stage) crystal forms associated with rhombohedral blocky alunite are formed authigenically on altered volcanic materials and locally covered by accessory star-like goethite and tube-like halloysite crystals. Under the open hydrological system and acidic conditions, the Al ± Fe/Si ratio and Al + K + S values increased compared to the parent rocks, favoring the formation of kaolinite and alunite, respectively. The enrichment of Sr, Ba, and depletion of Fe, Rb, Ce, Y, and La, and the negative Eu anomaly shows that the fractionation of feldspar and mica originated from Miocene volcano-sedimentary components during the magmatic and hydrothermal alteration process(es) were the main sources for the formation of kaolinite and alunite. The δD and δ¹⁸O composition and formation temperatures of kaolinite are 106–167 °C, suggesting an epithermal alteration process(es). The negative δ³⁴S and positive δ¹⁸O values for alunite exhibit local contribution of meteoric water mixed with magmatic fluid during hydrothermal activity.