Abstract

The Tavreh mercury prospect, a listwaenite-type alteration/mineralization system, is located c. 90 km west of Khoy in northwestern Iran. Tavreh is hosted within the Khoy ophiolite zone. Three types of listwaenites have been recognized in the Khoy ophiolite: silica, silica-carbonate and carbonate. Of these three, Hg mineralization at Tavreh is spatially and genetically associated with the silica-type listwaenite, also known as berberite. Alteration and mineralization at Tavreh are restricted to a faulted contact between shale and serpentinite. The Tavreh listwaenite is inferred to form from the hydrothermal alteration of brecciated serpentinite. Major mineralogical changes resulting from this alteration include the decomposition of serpentine-group minerals and the formation of silica phases. In this study, the mass changes of 18 listwaenites from Tavreh were assessed relative to the least altered serpentinites. To illustrate these changes quantitatively a comparative analysis of three different methods of calculating mass change was undertaken using Grant's isocon analysis, MacLean's equation and Gresens’ equation. Results from the three methods are similar. Listwaenite alteration was associated with a large increase in SiO₂ (44.4, 36.2, 63.9%, respectively). MgO and loss on ignition were depleted (−34.8, −36.9, −36.6; −8.5, −9.3, −8.3%, respectively) and Al₂O₃ was relatively unchanged (0.7, 0.6, 0.9%). Mercury is the most enriched rare element in altered rock (375.1, 346.8, 474.6 ppm). Arsenic, Pb, Au and Sb were also enriched. The intensity of mass changes of the various alteration components increases significantly from the serpentinite wall rock towards the listwaenite alteration and the ore-bearing zone. Therefore, the mass balance method can probably be used to locate mineral deposits from a few hundred metres and to explore for blind mineral deposits.