Geology and metallogenetic potential for gold of an to the area north of the city of Salgueiro (PE)

Abstract

The region of the city of Salgueiro, west of the state of Pernambuco, is known for the presence of gold-bearing quartz veins. Geological studies in the region took place in the 1980s and have intensified in recent years due to the high price of Au metal on the market. The study area is located in the Borborema Province, in the Piancó-Alto Brígida Domain, where supracrustal rocks of the Salgueiro Supergroup, of Tonian age, and various Ediacaran granite intrusions such as the Serrita Intrusive Suite and the Barra Verde Stock are found. The geological mapping of an area of approximately 36 km² on a 1:20.000 scale resulted in the characterization of 4 units of metasedimentary rocks called: G_I, G_II, G_III and G_IV. In general, these units have rocks with colors varying from reddish to greenish-gray, fine-medium grained, exhibiting schistosity and lepidoblastic texture. A metarhyolite occurs in the form of a sill in the schists of the G_IV Unit, representing the felsic volcanism of the Salgueiro Supergroup. Quartz veins crop up in practically all portions of the studied area, and can be divided into 2 groups: type V₁ formed by saccharoidal quartz veins, millimeters to centimeters, with a preferential NW-SE direction, filling fractures and foliation axial plane of folds intrafolial; and type V₂, with a milky appearance, has centimeter to metric dimensions and fills fractures in a WNW-ESE direction. The structural geology of the area is marked by a main foliation/schistosity (S_n) parallel to the sedimentary bedding (S₀) in a NW-SE direction with a weak to moderate dip to the NE. Almost perpendicular to Sn, crenulation cleavages form axial planar folds oblique to the schistosity (S_n+1) and are affected by open folds with vertical axial planes (S_n+2) WNW-ESE and an incipient strike-slip at the end of the Brasilian orogeny. Fracturing and faulting in the main NE-SW direction allowed the passage of hydrothermal fluids and consequent generation of biotite schist with carbonate-muscovite (hydrothermalite). Geochemical analyzes by RXFp indicate that there is an increase in the amount of SiO₂ (from 40% to 48%) from the base to the top of the supracrustal rock units (from G_I to G_IV), with an enrichment of K₂O (up to 118%) given by hydrothermalism affecting biotite schist with carbonate-muscovite, in addition to a relative increase in Al₂O₃ (up to 39%), in relation to metasedimentary rocks not altered by this process. The quartz veins are formed by SiO₂ (>96%) with contents of Cr (150-240 ppm), Cu (40-80 ppm), S (62.5-70 ppm) and V (17.5-30 ppm), in addition to Zn, As, Rb, Sr, Zr, Nb, Pb and Bi, all detected, but in concentrations below the analysis detection limits. The presence of paragonite [NaAl₃Si₃O₁₀(OH)₂] in textural equilibrium with the ore minerals complements the hydrothermal mineral association under the influence of magmatic hydrothermal fluids. There is still carbonation, potassification, sericitization, chloritization and oxidation. The presence of gold-bearing pyrite (Au around 8% by weight) associated with the quartz veins is indicative of a fertile environment with prospective possibilities for gold in the area.