FLUID INCLUSION STUDY OF THE MORRO AGUDO PB-ZN DEPOSIT, MINAS GERAIS, BRAZIL

Abstract

The Morro Agudo is the only Zn-Pb mine in Brazil, with an ore reserve of 9,470,000 tonnes, at average grades of 6.13% Zn and 2.00% Pb. The host rocks are dolomites from the Morro do Calcário facies of the Vazante Formation, Bambuí Group. The ore is composed of sphalerite and galena. Gangue minerals include dolomite, quartz, barite and calcite. The Morro Agudo mineralization is closely associated with a N-S trending fault zone, which dips from 20 to 70 degrees towards the west. The orebodies are stratabound and sometimes displaced by late faults. Shallow-water sedimentary structures such as teepee, laminated stromatolites and microcrystalline quartz nodules (length-slow type) suggest an evaporitic paleoenvironment. The structural and textural nature of the mineralization, as well as some replacement features of the ore, indicate that the main mineralization stage occurred in the early stages of diagenesis. Fluid inclusion (FI) studies of primary and pseudo-secondary FI in sphalerite crystals indicated moderate saline solutions (around 14%wt Eq. NaCl). The wide distribution of salinity values within the orebodies could be related to the higher salt concentration in the fluid near the fault zone. Total homogenization temperature and salinity data indicate a zoning pattern closely related to the main fault zone. Paleotemperatures are relatively higher in the 'A' Block, closer to the main fault zone. Away from the fault, temperatures reduced gradually. The stratiform ore of the 'N' Body exhibits the lowest temperature interval.