Kaleb S. Boucher, MSc, 2016
Abstract: The Western Tethyan Metallogenic Belt in Turkey hosts numerous hydrothermal mineral deposits. The Efemçukuru epithermal gold deposit, southwest of Izmir contains a resource of 1.69 Moz Au (at 8.39 g/t) and comprises two mineralized NW-trending quartz-rhodochrosite veins (Kestanebeleni and Kokarpınar). These structures are hosted in carbonate-rich phyllites of the Bornova Flysch; an Upper Cretaceous ophiolitic mélange sequence. The veins are pre-dated by similarly striking rhyolite dikes, and a 200 to 500 m wide halo of silicification and chloritization. Six main stages of veining were identified based on mineralogy, textures, and crosscutting relationships. Early veins of quartz, chlorite, and calc-silicates (Stage I) are cut by two stages of brecciating and banded veins of quartz, rhodochrosite, rhodonite, and pyrite (II, III). Later veins with disseminated (IV) and massive base-metal sulfides (V) cut previous phases, and subsequently by late quartz-carbonate veinlets (VI). Gold occurs as electrum and with pyrite and galena in Stages III, IV, and V. Kestanebeleni and Kokarpınar veins are controlled by moderately to steeply dipping NW- to NNW-trending faults. An analysis of the fault patterns, vein and dike orientations, combined with rock and soil geochemistry indicate that the highest and most prolific gold grades are associated with the steeply dipping NNW-trending structures. These vein segments are dominated by the vein stages of IV and V and represent “fault-linkage zones” where hydrothermal fluids were focused. Carbon and oxygen isotope analysis of vein carbonates indicate a mixed meteoric and magmatic source for the hydrothermal fluids. Variations in δ¹³C and δ¹⁸O values reveal two trends; one corresponding to interaction of meteoric fluids with host rocks (Trend 1), and the second from strong degassing (i.e. boiling) of magmatic fluids (Trend 2). Trend 1 samples are common in both Kestanebeleni and Kokarpınar, while Trend 2 samples occur as localized segments in Kestanebeleni. The latter also correspond to the thickest and highest gold grade regions of Kestanebeleni, indicating degassing was an important process for high grade gold deposition. The fault linkage zones provided the structural permeability that facilitated rapid degassing of auriferous fluids from Stages IV and V.