The physical–chemical regime of argillaceous interseam sediments in the Emet borate district, Turkey: A transition from non-metallic volcano-sedimentary to metallic epithermal deposits

Abstract

Among the world-class borate deposits, the borate deposits at Emet Espey and Hisarık in Western Anatolia, Turkey, stand out by their enrichment in As-bearing minerals realgar and orpiment. In terms of abundance these Turkish deposits are on a par with the world-class borate deposits in North America. In Turkey As-bearing base and precious metal deposits of epithermal low-sulfidation type developed adjacent to these non-metallic deposits under consideration. The borate deposits formed during the Miocene under arid climatic conditions within intermontane basins. The latter were filled with volcano-sedimentary series, which host a playa lake at the center. If we use the non-borate minerals, mainly phyllosilicates, as marker minerals and as a link to the equivalent shallow metal deposits, we can distinguish six paragenetic mineral associations (PAS). They reflect the following intrabasinal processes responsible for the formation of the deposits: (1) siliciclastic detrital input into the depocenter, (2) basic volcanic input into the depocenter, (3) felsic volcanic input into the depocenter, (4) evaporation in the depocenter, (5) hot brine activity in the depocenter and at its rim, and (6) oxidation. The physical–chemical regime of the various stages, including the temperature of formation, and the Eh and pH conditions are discussed in the current study indicating the interaction of intrabasinal and extrabasinal fluids:PAS-1: The alluvial–fluvial drainage systems delivered siliciclastics into the playa lake, which is a result of the weathering and denudation from the crystalline footwall rocks. The phyllosilicate association may be described as “kaolinite-in” and “pyrophyllite-out” bearing witness of a true supergene siallitic weathering, unaffected by hydrothermal activity (Eh>0, pH<7, T≈30°C).PAS-2: The phyllosilicate association which features chlorite and saponite is indicative of a reducing environment during smectitization. A moderate activity of basinal hot brines can be recognized, affecting the mineralogical input derived from basic magmatic rocks into the Neogene borate basin (Eh<0, pH6–7, T=30°–100°C).PAS-3: The alteration within the felsic tuffaceous rocks is marked by two discrete trends, the Mg trend with smectite as the diagnostic mineral (pH>7, T<100°C), and the K trend with sanidine and zeolite (clinoptilolite) being typical of this trend (pH>5.5, T<100°C). The temperature of alteration in the tuffaceous interseam pyroclastic deposits was higher than during alteration of stage (2).PAS-4: The borate minerals resulted from the combined action of evaporation and hydrothermal activity. The temperature of formation increased relative to the afore-mentioned processes. As far as the phyllosilicates are concerned, muscovite can be considered a marker mineral of this stage (pH=7–13, T≪100°C) (≪ means significantly lower than).PAS-5: Although dominated by As sulfides, the sheet silicate muscovite can be referred to as a marker for stage 5. This mineral association most strikingly shows the genetic relationship between the low-sulfidation epithermal metal and volcano-sedimentary non-metallic deposits, which developed within an ephemeral lacustrine environment of deposition (Eh<0⇒>0, pH=3–7, T≈100°C).PAS-6: A side-effect of a rising Eh is also observed among the Fe-bearing phyllosilicates. Nontronite, the diagnostic element acts like an umbrella, which covers the low-sulfidation epithermal metal and volcano-sedimentary non-metallic deposits (Eh>0, pH>5.5, T<100°C).A comparison of non-metallic and metallic shallow or epithermal systems reveals that the borate–smectite–muscovite system is the distal or non-metallic part of the low-sulfidation-type epithermal system when it comes to the accumulation of precious and base metals in a region under semi(arid) climatic conditions. Both systems show the largest overlap in stages (5) and (6). At the opposite end of this metallic–non-metallic catena, the alunite (APS mineral)–pyrophyllite–kaolinite system takes a position similar to the borate–smectite–muscovite system for the high-sulfidation metallic epithermal system.