A mechanism for the formation of the mineralized Mn deposits at Merehead Quarry, Cranmore, Somerset, England

Abstract

Mississippi Valley type galena deposits emplaced into Carboniferous limestones throughout the Mendip Hills during the late Permian or Triassic period were locally exposed to the action of seawater during the Jurassic period following regional uplift and erosion of the intervening strata. Oxidation of galena initiated the deposition of manganate minerals from the seawater, and these adsorbed heavy metals from both the seawater and local environment. A subsequent hydrothermal event heated the leadmanganate deposits causing decomposition of the galena and creating the conditions which led to the formation of the suite of unusual secondary minerals À including a number of rare oxychlorides À now found at Merehead. Heating of the manganate phases converted them to Mn oxides and released the entrained heavy metals which were then incorporated into unusual mineral phases. The impervious Mn oxide coating which enclosed the cooling Pb-rich areas isolated them chemically, leading to closedsystem behaviour. The high-T phases at Merehead are similar to those found in the Pb-bearing silicic skarns at Långban, whilst the suite of secondary minerals which evolved in the closed-system environments bears striking similarities to the 'anomalous sequence' of minerals found at the Mammoth-St. Antony Mine. The complexity of these formation processes probably explains the rarity of Mendip-type Pb-Mn deposits. The collective importance of the disconformity, the hydrothermal event, and subsequent sealing of the deposits are recognized for the first time, and the temperature of the hydrothermal event is shown to have been much greater than has heretofore been realized. Silurian volcanic strata underlying the Carboniferous limestones which have previously been assumed to be the source of heavy metals are shown to have been uninvolved in the process.

Somersetite, Pb8O(OH)4(CO3)5, a new complex hydrocerussite-related mineral from the Mendip Hills, England

“…Across the Mendip Hills, this structural geology has controlled mineralization, including the emplacement of Mississippian Pb ore veins bearing galena, fluorite, calcite and baryte into the Carboniferous limestone, resulting in a large area being irregularly mineralized with sub-parallel ore veins. After their deposition, regional uplift, erosion and subsequent marine transgression allowed the ingress of seawater into many of these Pb veins, where auto-catalysis then caused deposition of manganate minerals (Turner, 2006). Much later, multiple hydrothermal events occurred, and hot hydrothermal fluids followed the same structural features.…”

Yeomanite, Pb2O(OH)Cl, a new chain-structured Pb oxychloride from Merehead Quarry, Somerset, England

“…Finally, the opportunity to closely study a statistically significant number of manganese pods in situ allowed observations to be made that eventually permitted the complex processes behind manganese pod formation to be determined (Turner, 2006).…”

“…FOR some time, the authors have been investigating the mineralogy of the Mendip Hills with particular reference to the assemblages of Pb oxychlorides and other Pb secondary minerals contained within the manganese pods that occur there, as well as similar oxychloride occurrences elsewhere in the world including Långban in Sweden, the Kombat mine in Namibia, the Mammoth St. Antony mine in Tiger, Arizona and the Kunibert mine in Rhine-Westphalia (see e.g. Turner, 2006;Rumsey 2008;Siidra et al 2008;Krivovichev et al 2009;Turner and Rumsey, 2010;Rumsey et al 2011;Turner et al 2012a;Williams et al, 2012, Siidra et al, 2013a.…”

Chloroxiphite Pb3CuO2(OH)2Cl2: structure refinement and description in terms of oxocentred OPb4 tetrahedra

“…Chloroxiphite, Pb 3 CuO 2 (OH) 2 Cl 2 , was first described by Spencer and Mountain (1923) from the Mendip Hills, Somerset, England. It occurs here as olive-green bladed crystals embedded entirely in mendipite (Turner, 2006). Later, chloroxiphite was identified by Vergasova and Filatov (1993) in the products of fumarolic activity of the great Tolbachik fissure eruption (1975À1976, Kamchatka Peninsula, Russia).…”