Interaction of carbonate host rock with ore-forming fluid in mineralization process in Nakhlak lead-(silver) deposit, Isfahan

Nakhlak Pb-(Ag) deposit, one of the oldest and largest Iranian lead deposits, is located at the Nakhlak Mountain about 55 km NE of Anarak, a town in Isfahan Province, Iran. Lead ores are mainly hosted by open space of fractures and normal faults in the Upper Cretaceous carbonate (Sadr unit). The style of mineralization is stratabound and epigenetic, as shown by steeply east-west veins. Ore deposition in the Nakhlak deposit was controlled by three main factors: 1) lithology, 2) stratigraphy, and 3) structure. These can be viewed as fundamental controls of fluid transmissivity, either at the district or mine scale, that allowed the focusing of fluid flow and created opportunities for depositional processes to occur (Leach et al., 2005).

The Nakhlak Pb-(Ag) deposit is situated in the northwest corner of the Central Iran Structural Zone, which contains the Anarak region. The Nakhlak deposit is on the eastern fringe of Nakhlak Mountain. This mountain consists of pre-Triassic? ophiolites (Holzer and Ghasemipour, 1973; Alavi et al., 1997) and a Triassic sedimentary succession (Nakhlak Group, which comprises the Alam, Baqoroq, and Ashin formations), Upper Cretaceous (Sadr unit), and Paleocene (Khaaled unit) sedimentary cover. The Upper Cretaceous carbonate rocks (Cenomanian to Campanian), crop out across a large area of Nakhlak and reach a thickness of 258 m (Khosrow-Tehrani, 1977; Vaziri et al., 2005; Vaziri et al., 2012). The Sadr unit consists of conglomerates, sandy limestones, calcareous sandstones, sandy dolostones, sandy-argillaceous limestones, sandy dolomitic limestones, and reefal limestones that have been subdivided into five subunits based on their facies characteristics (Rasa, 1987). The Sadr unit rocks exposed on Nakhlak Mountain represent marginal-marine, shallow-shelf, and moderately deep marine environments (Vaziri et al., 2012).

The Sadr unit displays suitable characteristics for mineralization such as the presence of reef and dolostone facies and limestone to dolostone transition. The alteration of wall rock in the Nakhlak deposit is represented by the dissolution of carbonate and hydrothermal carbonates deposition that widespread close to ore-body and occurred from pre to syn- mineralization. There are three types of dolomite concerning mineralization, which saddle dolomite is the most important type of them. MgO, Fe2O3(Fe total), MnO as well as Pb, Ba, and Zn are enriched in altered wall rock and contrast with CaO is depleted. The positive relationship observed between the intensity of the dolomitization process, concentration of ore- metals, and sulfide mineralization in carbonate host rocks; suggesting that the migration of metal-bearing fluids was linked to hydrothermal processes. The estimate of hydrothermal fluid nature of the Nakhlak Pb-(Ag) deposits hydrothermal dolomite indicate a basinal brines (average 14.11 wt% NaCl eq.) and low temperatures (average 174 C), relatively equivalent with ore-forming fluid. In the Nakhlak deposit, interaction of carbonate host rock with the ore-forming fluid of mineralization process performed by feedback mechanisms. The suitable carbonate host rock (chemically and physically) is main factor entering of hydrothermal fluid into the host rocks and mineralization causing the dissolution of carbonate host rocks and increased formation of hydrothermal dolomite in the host rock, and consequently, that dissolution and dolomitization increased permeability of the host rock for ore fluids and allowed the deposition of additional ore minerals.