Weisheng Zang

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


One of the most fascinating problems in economic geology is to understand the behaviour of surface ore deposits under conditions of intense tropical weathering. The Igarape Bahia lateritic gold deposit in the Carajas region of the Amazon has developed on a primary Cu(Au) mineralization zone hosted by chlorite schists which are hydrothermally altered basalts, pyroclastic and clastic sedimentary rocks in an Archean rift basin. From country rock towards the mineralization zone, both Fe/(Mg + Fe) ratio in chlorite and Ca/Na ratio in inclusion fluids increase, suggesting that the primary Cu(Au) mineralization was formed from the convection of high salinity fluids. Freezing temperatures of fluid inclusions in quartz veins (veinlets) suggest a salinity of {dollar}>{dollar}24 wt% NaCl or 23-26 wt% CaCl{dollar}\sb2{dollar}.;The intense weathering resulted in accumulation of Fe-, Mn- and Al-oxides/hydroxides, and kaolinite in laterite profiles. From base to top the laterite profile consists of saprolite, pallid, mottled, ferruginous zones, and top soil with progressive enrichment of Al-correlated elements (Al, Ti, Zr, V, Ga, Cl), Mn-correlated elements (Mn, Sc, Co, Ni, Cu, Zn, Cd, Ba), Fe-correlated elements (Fe, Nb, Mo, W, Pb, Au, Ag, Pt, Pd, Sr, Y, La, Ce, P), and depletion of Si-correlated elements (Si, Mg, K, S, Cr). A general REE enrichment with positive Ce and Gd anomalies was observed in the weathering profile. The minerals of laterites are coated with C and N, suggesting that organisms are involved in the weathering processes at depths of up to 200 meters.;Gold orebodies occur in the ferruginous zone. Secondary gold grains are associated with Fe-oxide/hydroxide nodules. Fineness of gold grains increases downwards in the profile, suggesting a three-stage genetic model. Weathering of sulfides in parent rocks favoured migration of both Au and Ag as transient thiosulfate complexes to saprolite and mottled zones, which is responsible for formation of electrum or Ag-rich gold. Electrum or Ag-rich gold grains were incorporated into the ferruginous zone as erosion surface lowered. Chemical purification of gold grains is due to Cl{dollar}\sp-{dollar} leaching of Ag after incision of the profiles. Organic matter near the surface can lower redox potentials to protect silver from leaching. This speculation is also confirmed by related thermodynamic calculations.



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