doi:10.1016/S0016-7037(00)00462-9 Carbon isotopes and petrography of kerogens in 3.5-Ga hydrothermal silica dikes in the

• Main Authors: North Pole Area, Western Australia, Yuichiro Ueno, Hideyoshi Yoshioka, Shigenori Maruyama, Yukio Isozaki
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2003
• Subjects: North Pole
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.502.2671; http://ea.c.u-tokyo.ac.jp/earth/Members/Isozaki/04Ueno.pdf

Abstract—More than 600 specimens of 3.5 Ga-old hydrothermal silica dikes from the North Pole area, Pilbara craton, Western Australia, have been studied petrographically. The kerogens in 44 samples have been analyzed isotopically (C and N) and chemically (C, N, and H). The silica dikes are composed mainly of fine-grained silica (modal abundance: 97%) and are classified into two types by minor mineral assemblages: B(black)-type and G(gray)-type. The B-type silica dikes contain kerogen (0.37 to 6.72 mgC/g; average 2.44 mgC/g, n 21) and disseminated sulfides, dominantly pyrite and Fe-poor sphalerite. In some cases, carbonate and apatite are also present. Their silica-dominated and sulfide-poor mineral assemblages suggest precipitation from low-temperature reducing hydrothermal fluid (likely 100–200°C). On the other hand, the G-type silica dikes are sulfide-free and concentrations of kerogen are relatively low (0.05 to 0.41 mgC/g; average 0.17 mgC/g, n 13). They typically contain Fe-oxide (mainly hematite) which commonly replaces cubic pyrite and rhombic carbonate. Some G-types occur along secondary quartz veins. These textures indicate that the G-type silica dikes were formed by postdepositional metasomatism (oxidation) of the B-types, and that the B-types probably possess premetasomatic signatures. The 13C values of kerogen in the B-types are 38.1 to 33.1 ‰ (average 35.9‰, n 21), which are 4 ‰ lower than those of the G-types (34.5 to 30.0‰; average 32.2‰, n 19), and 6 ‰ lower than bedded chert (31.2 to 29.4‰; average 30.5‰, n 4).