Ore and gangue mineral textures, fluid inclusions, mesoscopically structured quartz and pyrite, and their bearing on the genesis of hydrothermal breccias in the low-sulfidation Surnak gold deposit, SE Bulgaria

Abstract

This paper provides comprehensive analyses of mineral microtextures, nanoparticulate electrum, defective crystal structures of key primary hydrothermal minerals - quartz and pyrite, the bulk sulfur isotopic composition of pyrite and marcasite, and fluid inclusions in hydrothermal quartz and calcite, all aimed at characterizing ore mineralization. The study primarily focuses on samples collected from a steep normal fault and its damage zone, which formed during hydrothermal brecciation, while also incorporating samples from other thinner brecciation zones. The data utilized in this study originate from the Surnak (or Sarnak) gold deposit located in the Eastern Rhodope Mountains of Southeast Bulgaria. This deposit, characterized as low-sulfidation, offers a distinctive geological context for exploring the hydrothermal processes associated with hydrothermal brecciation, colloidal, and mesocrystal formation. The unique microtextures and mesocrystal structures observed in quartz and pyrite crystal lattices offer valuable insights into the colloidal stage that the paleohydrothermal solution experienced during hydrothermal brecciation, pressure drop, and subsequent boiling. Bladed-textured calcite crystals, containing both vapor-rich and liquid-rich inclusions, provide direct evidence of fluid boiling. Fluid inclusion data from hydrothermal quartz further suggest the involvement of two distinct fluid types, each with different temperatures and salinities. Our findings point to a causal relationship between brecciation episodes, fluid boiling, nanoparticle nucleation, the colloidal stage, and the subsequent formation of mesocrystals.