Annika Dziggel

Growth of zircon and titanite during metamorphism in the granitoid-gneiss terrane south of the Barberton greenstone belt, South Africa

Abstract SHRIMP U-Pb zircon and titanite dating have been used to constrain the timing of mid- to lower-crustal metamorphism (~650-700°C and 8-11 kbar) and syn-kinematic melting in the granitoid gneiss-dominated terrane south of the Barberton greenstone belt, South Africa. This study is concentrated on a clastic metasedimentary unit exposed in one of several greenstone remnants and a late-kinematic trondhjemite intrusive into spatially associated mixed gneisses. Locally, the clastic metasediments show extensive replacement of garnet and plagioclase by epidote and titanite. The titanites yield an upper intercept date of 3229±9 Ma, and provide a minimum age for the peak of metamorphism. Zircons separated from the same unit record a range of concordant and near-concordant 207Pb/206Pb dates between ~3560 and 3230 Ma, the youngest group yielding a weighted mean date of 3227±7 Ma. This range of dates is interpreted to be due to a combination of metamorphic recrystallization and high- temperature Pb-loss in originally detrital zircons during regional metamorphism. A minimum age for the timing of deformation is given by the emplacement age of 3229±5 Ma for the late-kinematic trondhjemite. Thus, geochronological data support the notion of a major metamorphic episode that coincided with the proposed short-lived terrane accretion event in the centre of the Barberton greenstone belt.

Eoarchean within-plate basalts from southwest Greenland: COMMENT

[Jenner et al. (2013)][1] reported the occurrence of, what they interpret as, Earth’s oldest ocean island basalts (OIBs) on the island of Innersuartuut, southwest Greenland. However, this interpretation hinges critically on the incompatible trace element contents of the presented rocks. Compared

Chapter 5.8 Tectono-Metamorphic Controls on Archean Gold Mineralization in the Barberton Greenstone Belt, South Africa: An Example from the New Consort Gold Mine

Publisher Summary This chapter discusses the tectono-metamorphic controls on archean gold mineralization in the Barberton Greenstone belt, South Africa using an example of consort gold mine. The Barberton Greenstone Belt, South Africa, forms part of the oldest nucleus of the Kaapvaal Craton. It contains a well-preserved ca. 3570–3220 Ma volcano-sedimentary sequence, which is surrounded by various generations of tonalite-trondhjemite-granodiorite gneiss domes and sheet-like potassic granites emplaced between ca. 3500–3100 Ma. It is found that although more than 350 gold deposits have been recorded in the Barberton Greenstone Belt, the bulk of the ca. 320 t of gold has been produced from the Sheba–Fairview, New Consort, and Agnes–Princeton mining complexes in the northern part of the belt. The majority of these and associated smaller gold deposits are clustered in a crescent-shaped zone in greenschist facies supracrustal rocks along the eastern margin of the ca. 3230 Ma, tonalitic Kaap Valley Pluton. The New Consort gold mine is situated in the eastern part of the Jamestown schist belt. The systematic variations of ore and alteration assemblages and metamorphic conditions at the New Consort gold mine suggest that the timing of gold mineralization was synchronous with the peak of metamorphism in Fig Tree Group rocks, whereas the main phase of mineralization in rocks of the Onverwacht Group clearly postdates the peak of metamorphism there.

Textural characteristics and trace element distribution in carbonate-hosted Zn-Pb-Ag ores at the Paleoproterozoic Black Angel deposit, central West Greenland

The Black Angel deposit represents the most important base metal deposit in Greenland, having produced 11.2 million tons of Pb-Zn-Ag ore from 1973 to 1990. The deposit is hosted by a greenschist facies calcitic marble of the Mârmorilik Formation of the Paleoproterozoic Karrat Group. The ore consists of sphalerite, pyrite, and galena, with minor amounts of chalcopyrite, arsenopyrite, tetrahedrite, freibergite, tennantite, stannite, briartite, rutile, and graphite. Pyrite occurs as porphyroclasts and as fine-grained euhedral grains and is commonly surrounded by sphalerite, galena, and chalcopyrite. Deformation textures such as pyrite annealing, fracture healing by sulfides as well as “Durchbewegung” textures are common. Electron microprobe analysis and laser ablation-inductively coupled plasma-mass spectrometry trace element analysis show that tetrahedrite-freibergite and galena are enriched in Ag and represent the main Ag carriers. Galena also hosts substantial amounts of Sb, Sn, and Bi. The presence of Ge-rich chalcopyrite and briartite (Cu2(Zn,Fe)GeS4) inclusions in sphalerite indicates that the Black Angel deposit may be host to an hitherto unrecognized Ge endowment. It is suggested that sphalerite and briartite co-precipitated from a hydrothermal fluid having an intermediate sulfidation state. The origin of the deposit remains ambiguous. The calcitic host rock, epigenetic style of mineralization, presence of anhydrite, and occurrence of hydrothermal breccias are consistent with an origin as Mississippi Valley-type deposit. However, much of the mineralization is syn- to late-tectonic and syn-metamorphic, and the sulfide textures are consistent with a metamorphic overprint. An origin as carbonate-hosted polymetallic Kipushi-type deposit is thus more likely, since these deposits constitute the primary deposit type from which briartite has been previously documented.