CALDERITE-RICH GARNET AND FRANKLINITE-RICH SPINEL IN AMPHIBOLITE-FACIES HYDROTHERMAL SEDIMENTS, GAMSBERG Zn-Pb DEPOSIT, NAMAQUA PROVINCE, SOUTH AFRICA

Abstract

Calderite-rich garnet and franklinite-jacobsite-rich spinel occur in amphibolite-facies hydrothermal metasedimentary rocks that stratigraphically overlie the Proterozoic Gamsberg Zn-Pb deposit, South Africa. The minerals are restricted in occurrence to an irregularly developed Mn-Fe-rich unit at the top of the ore-bearing horizon (the Gams Formation), which is interpreted to be genetically related to the underlying sulfide mineralization. The rocks are well laminated on a centimeter to decimeter scale and consist of alternating layers of garnet, pyroxenoid, Mn-rich clinopyroxene and Fe oxide (Zn-Mn-rich ferrite spinel, magnetite, hematite). The garnet contains up to 36% of the calderite component in solid solution with andradite and spessartine, whereas the spinel reaches a maximum franklinite component of 39% in solid solution with magnetite and jacobsite. Associated minor to accessory minerals include manganoan calcite, manganoan and zincian tremolite, Sr-rich epidote, hyalophane, pyrophanite, melanotekite and mimetite. The occurrence and spatial distribution of calderitic garnet, Zn-Mn-rich spinel and hematite constrain oxygen fugacities during metamorphism to conditions close to the HM buffer for the uppermost part of the Gams Formation, and indicate that considerable gradients in f(O 2) and f(S 2) existed within this specific horizon. The preserved assemblages are products of complex reduction and decarbonation reactions of protoliths composed of Mn-Ca-rich carbonate, Fe-Mn oxides and hydroxides, silica and aluminous clay. Under the observed conditions of P, T and f(O 2), the contrasting parageneses are a function of delicate variations in the bulk composition of the precursor sediments. The Mn-Fe-rich and Zn-S-poor nature of these metahydrothermal rocks indicates that they represent a dispersion halo of low-temperature and oxidized fluids that postdated base-metal mineralization. Collectively, the unusual mineralogical and chemical composition of these rocks provides an indication of the spectrum of rock types that may be associated with metamorphosed deposits of base-metal sulfides.

Thermal Condensation of Glycine and Alanine on Metal Ferrite Surface: Primitive Peptide Bond Formation Scenario

“…Transition metal ions present in the primordial oceans as well as in the Earth’s crust undoubtedly became part of the evolutionary chemistry of life and thus became a vital part of living cells [ 59 ]. These spinel type metal ferrite minerals are of special interest because not only are they present in the Earth’s crust [ 60 , 61 ]; they can also be found in meteorites [ 62 , 63 , 64 ]; hydrothermal vent systems [ 65 ]; and ancient sediments [ 66 , 67 ], as well as the sea bed [ 68 , 69 ]. The main objective of this paper is to ascertain the suitability of these specific oxides with respect to the condensation of amino acids and their relevance to the chemical evolution of life.…”

Caracterização do hidrotermalismo “verde” em rochas siliciclásticas do Grupo Pajeú (Supergrupo Oliveira dos Brejinhos), Espinhaço Setentrional, Bahia

“…A andradita foi também descrita por Livingstone (1989) como fase mineral secundária associada à zeólitas em lavas basálticas da Escócia. Stalder & Rozendaal (2005) identificam o mineral calderita, uma solução sólida entre a andradita e espessartita, associada a sedimentos metamorfisados hidrotermalmente na fácies anfibolito, em depósitos de Pb e Zn, na Africa do Sul. Ao contrário do que se observa com relação à maioria da granada de origem hidrotermal citada na literatura, a andradita descrita neste trabalho, preenche o espaço poroso de arenitos arcosianos e não possui qualquer relação com tactitos ou escarnitos.…”

Syn-metamorphic sulfidation of the Gamsberg zinc deposit, South Africa

“…Even though high-grade meta-evaporites are hard to identify, calc-silicate rocks at the base of the Koeris Formation at Gamsberg (Praekelt et al 2006) could well represent former evaporite beds. Be that as it may, depending on the pre-metamorphic oxidation state a more or less pervasive fluid flow would be necessary, which is in conflict with local observations of low fluid/rock ratios (Frimmel 1993;Stalder and Rozendaal 2005a).…”

“…Although the long-term high-temperature history of the ore district would make complete diffusional homogenization of garnet likely, the observed zonation with respect to Fe points at very low fluid/rock ratios, which markedly decrease diffusion rates (Florence and Spear 1995). Such low fluid/rock ratios are supported by the local layer-to-layer variations in oxygen and HF fugacity developed during metamorphism (Frimmel 1993;Stalder and Rozendaal 2005a). Apart from the above-mentioned variations in chemistry, the strong contrast between size and quantity of sulfide inclusions in the two garnet domains speaks for different degrees of dissolution and reprecipitation.…”