Tourmaline Composition of the Kışladağ Porphyry Au Deposit, Western Turkey: Implication of Epithermal Overprint

Abstract

The Kışladağ porphyry Au deposit occurs in a middle Miocene magmatic complex comprising three different intrusions and magmatic-hydrothermal brecciation related to the multiphase effects of the different intrusions. Tourmaline occurrences are common throughout the deposit, mostly as an outer alteration rim around the veins with lesser amounts disseminated in the intrusions, and are associated with every phase of mineralization. Tourmaline mineralization has developed as a tourmaline-rich matrix in brecciated zones and tourmaline-quartz and/or tourmaline-sulfide veinlets within the different intrusive rocks. Tourmaline was identified in the tourmaline-bearing breccia zone (TBZ) and intrusive rocks that had undergone potassic, phyllic, and advanced argillic alteration. The tourmaline is present as two morphological varieties, aggregates of fine crystals (rosettes, fan-shaped) and larger isolated crystals and their aggregates. Four tourmaline generations (tourmaline I to IV) have different compositions and substitutions. Tourmaline I in TBZ and INT#1 is distinguished by the highest Fetot and enriched in Fe3+. Tourmalines II and III occur as fine aggregates, accompanied by the formation of isolated crystals and are characterized by lower Fetot and Fe3+. Tourmaline IV is characterized by the lowest Fetot, enriched in Cl, and has the highest proportion of X-site vacancy among all the tourmalines. Tourmaline I may be attributed to the potassic stage in INT#1 and early tourmaline in TBZ. Tourmalines II and III from INT#1 and the TBZ could be referred to the phyllic stage. The low Fe content in tourmaline is caused by the simultaneous deposition of sulfide minerals. Tourmaline IV from the TBZ and tourmaline II from INT#3 are distinguished by the high X-site vacancy proportion up to the formation of X-site vacant species as well as enriched in Cl; they can be attributed to the argillic stage of the hydrothermal process. The textural and especially chemical data of the tourmaline from the Kışladağ Au deposit provide information on the physico-chemical conditions during the porphyry to epithermal transition and subsequent epithermal overprinting.

Global Occurrence, Geology and Characteristics of Hydrothermal-Origin Kaolin Deposits

“…Based on their mineralogical assemblages of alterations and chemical compositions, these rocks have been subdivided [197] as follows: (i) Advanced argillic alteration is represented by quartz-alunite and dickite-pyrophyllite-pyrite mineral assemblages [196]. (ii) Argillic alteration is dominated by the development of kaolinite, halloysite and smectite [196,198]. The argillic alteration overprints all other alteration assemblages.…”

Tourmaline occurrence and gold mineralization at a granitoid-metasediment contact in the Upper Lom Basin, east Cameroon

“…The Mg-Fe 2+ substitution is indicative of an insufficient proportion of Fe 3+ in the tourmaline composition [66]. However, low Fe content in tourmaline has been demonstrated to reflect high abundance of Fe-oxides and sulfides, and indicates greater ore-forming potentials [68], or an evolution from an Fe-rich to Mg-rich variety resulting from late-stage sulphide deposition [69]. So, this maybe suggesting that lode gold mineralization in the Upper Lom Series is most likely associated with Fe-sulfides and oxides as is the case in the Lower Lom Series [26,52].…”