Auriferous Quartz Veins from Northeastern Brazil: A Fluid-Inclusion Study

Abstract

Auriferous quartz veins are known to exist in more than two dozen prospects, encompassing an area of 500 km 2 northward from Serrita township (state of Pernambuco) in northeastern Brazil. Gold-bearing veins occur either with a strike of 70° to 110°, crosscutting muscovite schists of the Middle Proterozoic Salgueiro Group, or with a strike of 330° in granodiorite intrusions in the same schists. Small amounts of pyrite, galena, arsenopyrite, chalcopyrite, and sphalerite commonly are observed. Sericite, chlorite, and epidote are the most common wallrock alteration products.Fluid inclusions were studied in samples of mineralized quartz veins from the Barra Verde III prospect in a small granodiorite body, and from the Ingá, Saburá, and Riacho do Meio prospects in the Salgueiro schists. Some samples of barren quartz veins also were studied for comparison.Primary and pseudosecondary inclusions in the mineralized veins are triphasic or biphasic aqueous-carbonic at room temperature. The wide range of the CO 2 /H 2 O volume ratio (between 2:1 and 1:3) in a single group or trail suggests the coexistence of two immiscible fluids during the penecontemporaneous processes of quartz crystallization, deformation, mineralization, and recrystallization. Total homogenization of these inclusions beginning at 290° to 310°C and 1.3 to 1.8 kbar provides the trapping conditions of the heterogeneous, effervescent fluid. CO 2 melting temperatures of ~-57° to -59°C indicate low CH 4 or N 2 contents. Clathrate melting close to 6.3°C indicates a low salinity of ~6.9% NaCl equiv. In addition, the low CH 4 content of the fluid in equilibrium with sulfides and alteration minerals suggests an oxygen fugacity between 10~3 0 and 10 27 , a total sulfur activity of 10 -2 to 10°, and a neutral pH of ~5.

Anomalia de ouro em sedimentos de corrente na região das rochas metassedimentares da Unidade Independência, Complexo Ceará, norte da Província Borborema

“…outros depósitos neoproterozoicos (580-530 Ma) encontrados em rochas metassedimentares dos grupos Seridó e Cachoeirinha, nos estados do Rio Grande do Norte e de Pernambuco, respectivamente (COU-TINHO, 1994;LEGRAND et al, 1996;BEURLEN;SIL-VA;SANTOS, 1997;LUIZ-SILVA, 2000;ARAÚJO et al, 2002;2005).…”