Mineral chemistry and phase relations of Co–Ni arsenides and sulfarsenides from the Baycheh-Bagh deposit, Zanjan province, Iran

Abstract

The polymetallic hydrothermal veins of Baycheh-Bagh deposit in NW Iran occur in quartz-carbonate veins which are hosted by Oligocene to Miocene volcanic (mainly andesite) and volcaniclastic (agglomerate, tuff) rocks unconformably overlying metamorphic basement rocks. Three transitional paragentic stages of hypogene mineralization have been identified. Stage 1 includes base metal (Cu, Zn, Pb) sulfides and gold and is succeeded by Co-Ni-Fe-As minerals and bismuth (Stage 2) and Cu-Bi sulfides/sulfosalts (Stage 3). Stage 2 Ni-Co-Fe mineralization is complex and is subdivided into three substages starting with sulfarsenides (arsenopyrite, alloclasite I, II), followed by native bismuth, triarsenides (skutterudite I, II, III) and diarsenides (rammelsbergite, safflorite) and terminated by sulfarsenides (gersdorffite-cobaltite solid solution, alloclacite III). Skutterudite and alloclasite are present in several generations and preserve delicate growth zoning reflecting a multi-stage episodic growth. The sulfarsenides and arsenides reveal significant substitution of Ni, Co, Fe as well as As and S reflecting compositional changes of the ore forming fluid (e.g., activities of metals, As/S) as well as changes in pH and oxidation state. The mineral assemblages and comparison with similar As-dominated vein-type Ni-Co mineralization elsewhere, collectively referred to as “five element veins”, suggest that the complex arsenide/sulfarsenide assemblages formed at temperatures below 300 °C. Baycheh-Bagh can be regarded as a member of this heterogeneous group of Bi-Co-Ni-As deposits. A peculiarity of hydrothermal vein-type mineralization at Baycheh-Bagh is that initial epithermal (?) Cu-(Au-Zn-Pb) mineralization evolved into (Bi)–Co-Ni-As mineralization with time.