Abstract
Greenstone rocks, which include Banded Iron Formations (BIFs), tuffs, volcanic flows (basalt, andesite and rhyolite), and clastic sedimentary rocks (shale-mudstone, greywacke-sandstone and conglomerate), crop out around Geita Hills and are flanked by granites and granodiorites. BIFs and tuffs occupy larger area than other lithological units, which crop out as patches. Structural analysis indicates that layers of green-stone rocks are folded and display a regional fold axis with an attitude of 320o/40o. Low-grade metamorphic mineral assemblages (actinolite-epidote-chlorite in basalts and muscovite-epidote-chlorite in granitoids) are common in these rocks; this indicates a regional metamorphism at greenschist facies. However, BIFs and basalts are locally metamorphosed to epidote-amphibolite and amphibolite facies. Basalts belong to the tholeiite series whereas granites, diorites and rhyolites belong to the calcalkaline series. Chondrite normalized rare earth element pattern of basalt is flat and plot slightly below the average N-MORB values suggesting the enrichment of the light rare earth elements, which means that mantle magma source was an E-MORB. Granitoids and rhyolites have strong affinities to the continental arc source magma displaying strong enrichments in the LREEs with (La/Sm)N values ranging between 2.53 and 3.95 in rhyolites and between 4.08 and 5.40 in granitoids. The granitoids are classified as the I-type synorogenic metaluminous granites and granodiorites. Geochemical signatures suggest that the Geita Hills basalts erupted at the enriched mid ocean ridge setting of the back arc setting, and the granites, granodiorite and rhyolite formed in a volcanic arc setting particularly the continental arc.
Highlights
Most of the world Archean provinces (4.0 - 2.5 Ga), which are popular for gold mineralization, are composed of granite-greenstone terranes associated with or without high-grade metamorphic provinces [1]
Sheath folds, and lineations are common in Banded Iron Formations (BIFs) and volcanic sequences at Geita Hills manifesting complex deformational history of which [16] proposed as a result of a single deformation
Simplified stratigraphy of the Archean greenstone sequences of Geita Hills shows from the bottom to the top: volcanic flows, pyroclastic volcanics, chemical sediments (BIF and chert layers), and clastic sediments
Summary
Most of the world Archean provinces (4.0 - 2.5 Ga), which are popular for gold mineralization, are composed of granite-greenstone terranes associated with or without high-grade metamorphic provinces [1]. The terranes are characterized by discontinuous greenstone belts engulfed in a “sea” of granite and gneisses. The major granitegreenstone of the world occur in the Superior and Slave provinces in North America, in the Zimbabwe and Kaapvaal provinces in southern Africa and in the Yilgarn and Pilbara provinces of Western Australia [2]. Higher grades typically occur around the margins of greenstone belts, probably caused by intrusion of plutons [3]. Structural analysis of Archean greenstone belts shows that they are considerably complex; most successions have undergone two or three periods of major deformation and metamorphism [2] and the references therein
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