A new genetic model for the North African metasomatic siderite deposits

Abstract

Abstract. Based on published accounts of the diagenetic evolution of evaporite diapirs and geological, preliminary geochemical, and fluid inclusion data a new genetic model is presented for the North African metasomatic siderite deposits.Essentially, hot brines from the buried evaporites, gypsum-derived and/or other formation waters are thought to have reacted with their host rocks during early diagenesis becoming acid and reducing, thus allowing solution and transport of Fe and other cations. They were channeled toward the apical parts of diapirs where they deposited their solute by metasomatic processes in fractured roof carbonates. Other depositional modes by pressure and temperature drop as well as by mixing with nearsurface waters are indicated by the occasional vein-type iron ores and very minor barite and fluorite. The model is quite comparable to recently published thoughts on the origin of the Pb-Zn deposits of the same area. Obviously, both groups of deposits share not only temporal and spatial but also causal metallogenic characteristics.R~sum& Bas~e sur les notes publi6es sur l'~volution diag6-n~tique des diapirs 6vaporitiques, et sur les donn6es g6ologiques et les r6sultats pr61iminaires concernant la g6ochimie et les inclusions fluides, un nouveau modble g6n6tique est pr6sent6 pour les minerais sid6ritiques m6tasomatiques du Maghreb.Dans l'ensemble, des saumures chaudes, issues des ~vaporites enfouies, et/ou des eaux deriv6es du gypse et d'autres sediments, semblent avoir r6agi avec leurs roches h6tes durant la diagen~se pr6coce, devenant ainsi acides et r6ductrices. Ceci entraine la mise en solution et le transport du fer et d'autres cations. Ces saumures sont canalis~es vers les parties apicales des diapirs, off elles d6posent leurs solut6s par processus m6tasomatiques dans les fractures du toit carbonat& D'autres modes de d6p6t, aussi bien par chute de temp6rature et de pression que par mdlange avec les eaux de subsurface, sont marqu6s par des fllons occasionels min6ralis6s en fer et tr6s rarement en barytine et fluorine.Le mod61e est tout fi fair comparable /~ ce publi~ r~cemment sur l'origine des min6ralisations ~ Pb-Zn de la m~me r6gion. I1 est 6vident que les deux groupes de min6ralisation partagent des caract6ristiques m~tallog6-niques non seulement spatiotemporeIles mais aussi causales.

Geochemistry of heavily altered Archean volcanic and volcaniclastic rocks of the Warrawoona Group, at Mt. Goldsworthy in the Pilbara Craton, Western Australia: Implications for alteration and origin

“…2d) and of minute carbonate particles in silicified crystal pseudomorphs. The Fe-oxide rhombs are likely to have formed by oxidation of iron-rich carbonates such as ankerite and siderite, neither of which are common as primary igneous minerals but can be observed in altered volcanic rocks (Pohl et al, 1986;Morton and Nebel, 1984;Kitajima et al, 2001;Nakamura and Kato, 2004). The presence of minute carbonate particles in pseudomorphs can be interpreted to be the result of carbonatization followed by silicification of precursory non-carbonate minerals.…”

Improving the routine analysis of siderite for δ13C and δ18O in environmental change research

“…Siderite is a naturally occurring iron-rich carbonate mineral (FeCO 3 ) found in freshwater and marine sediments, [4][5][6][7][8] hydrothermal and metasomatic iron mineralisation, 9,10 and carbonatite magmatism. 11 In addition, siderite may be an important mineral on Mars, where it is implicated in the decarbonisation of the Martian atmosphere.…”

“… 3 Whilst the mineral used in the majority of studies is calcite (CaCO 3 ), many carbonate‐bearing sediments do not contain calcite (endogenic or authigenic) but instead preserve other carbonates, including siderite. Siderite is a naturally occurring iron‐rich carbonate mineral (FeCO 3 ) found in freshwater and marine sediments, 4 , 5 , 6 , 7 , 8 hydrothermal and metasomatic iron mineralisation, 9 , 10 and carbonatite magmatism. 11 In addition, siderite may be an important mineral on Mars, where it is implicated in the decarbonisation of the Martian atmosphere.…”