Thin-skinned thrust mineralization in the Bras�lia fold belt: the example of the old Luzi�nia gold deposit

Abstract

The Luzigmia gold deposits in southern Goi/ts lie within the Late Proterozoic Brasilia fold belt. The rocks that host the gold mineralization are a monotonous series of hydrothermally altered phyllites that have been subject to low grade regional metamorphism. The major controls on the gold mineralization are northeast trending and gently northwest dipping ductile-brittle, dextralreverse shear zones associated with regional thin-skinned thrusting of the Canastra Group. From a preliminary fluid inclusion study it is deduced that low salinity, < 7 eq. wt% NaC1, moderately dense, H20-CO 2 + C H 4 o r e fluids deposited gold at temperatures of 300 + 75 ~ and pressures of 1.5 to 3 kb in the filling stage of the vein formation. Post-filling stage gold deposition probably occured by mixing of fluids at higher crustal levels (1.5-2 kb). During thrusting, prograde metamorphism released pore water which penetrated along thrust planes that acted as high permeability zones for the ponding and release, by hydraulic fracturing, of overpressured fluids. Later in the tectonic evolution and at shallower crustal levels, there was likely an incursion of near suface water into the fault zone.Although thin-skinned thrust related gold deposits in Proterozoic metasedimentary rocks have recently become important exploration targets, their structural setting, mode of emplacement and geochemical signatures are poorly understood and widely accepted genetic models are generally lacking. For this reason, an integrated and continuing study of a Proterozoic thin-skinned thrust related gold deposit in the central part of the Brasilia fold belt has been undertaken. The goals of the study are to: (1) identify structural processes that may have controlled and influenced the development of the mineralized veins, (2) determine the timing of gold deposition, (3) reconstruct the P-T conditions during the rain-* Present address:

Local and Regional Mass Transfer During Thrusting, Veining, and Boudinage in the Genesis of the Giant Shale-Hosted Paracatu Gold Deposit, Minas Gerais, Brazil

“…Applying these 350° to 400°C temperatures to the assemblage ferri-ilmenite-magnetite-rutile-siderite, together with the lack of amphibole, constrains the metamorphic pressures to between 150 and 200 MPa (Schwartz et al, 1992;Connolly, 1995). Although we are not reporting details of the fluid inclusion analyses of Freitas-Silva (1996) here, the fluids analyzed from sulfide-bearing quartz veins were low salinity, H 2O-CO2-CH4-N2 dominant, and trapped at between 300° and 400°C and 150 and 250 MPa, consistent with the geothermometry, mineral parageneses, and some published work from the Luziânia deposit in the same belt some 150 km to the northwest (Hagemann et al, 1992). The underlying Vazante Group rocks have mineral associations (Table 3) consistent with conditions of anchimetamorphism, P ≤ 100 to 150 MPa and T <150° to 200°C (Freitas-Silva, 1996).…”

Fluid inclusion characteristics of hematite-bearing quartz vein from the Daenam mine, Republic of Korea

“…Also note the clustering by composition and molar volume. and methane clathrates (CO 2 Á5.75H 2 O and CH 4 Á7H 2 O) (Hagemann et al, 1992;Yao et al, 2001).…”

Mineralogy, fluid inclusion and stable isotope constraints on the genesis of the Namseong Au-Ag deposit, Republic of Korea

“…7). They may result from a mixture of CO 2 and CH 4 clathrates (CO 2 •5.75H 2 O and CH 4 •7H 2 O) (Hagemann et al, 1992;Yao et al, 2001). Collins (1979) suggested that Tm clathrate values of >10°C for H 2 O-CO 2 inclusions indicate the presence of minor amounts of other volatiles such as CH 4 or N 2 .…”