I've started a thesis(bachelor) on a Mn mine in Poggio S.Vicino(IT), i went for some field researches and to collect minerals in situ(Mn oxides) usually found as Coatings on other minerals such as Quartz and Calcite. Fe is present aswell in the form of Goethite, probably indicating that Mn was not totally separated from Fe during deposition. Stratigraphy of the area is the typical Umbro-Marche succession(Limestones) covering an interval from early Jurassic to early Miocene. The Mn deposit is located on Calcare Massiccio(180Ma). My question is about the causes that led to the deposition of these metals, assumed to be occurred in shallow seawater.
The sedimentary Mn-deposits may be categorized as follows:
(1) Sedimentary manganese deposits
(1) Shallow marine Mn deposits with /without remobilization in subaerial regimes (8a I-8a K)
(1) Clastic-hosted deposits (8a I)
(2) Carbonate-hosted (8a K)
(2) Mn deposits in black shales (8c J)
(3) Mn nodules in deep-marine depositional environments (in parts endogeneous) (8a M)
(4) Supergene manganiferous deposits (8a H)
(1) Bog iron ores on siliciclastic bedrocks
(2) Manganiferous replacement deposits hosted by calcareous rocks
See: DILL, H.G. (2010) The “chessboard” classification scheme of mineral deposits: Mineralogy and geology from aluminum to zirconium.- Earth Science Reviews, 100: 1-420.
Your type may most suitably be described as follows:
• The Fe-(Mn) deposits of the "Hunsrueck-type" were derived from the products of extensive chemical weathering of basalts, greywackes and shales in the Variscan basement complexes of the Rhenish Massif (Waldalgesheim D), (Bottke 1969).
• Oxide hydroxides of Al, Fe-bearing silcretes, goethite, poorly-hydrated Mn oxides and manganomelane are the main components of these supergene deposits
• Ore types: (1) ferricretes sensu stricto, (2) pebble Fe ores, (3) ferruginous conglomerates and breccias (Fe-Mn cement), (4) Fe-Mn replacement ores, and (5) limonitic gossans (Dill 1985). The Fe-Mn enrichments, irrespective of their host rocks, were taken as remnants of hydromorphic soils of non-lateritic origin (Plio-Pleistocene).
• cavity fillings, limonitic bed-load deposits in Tertiary shallow valley, vein-like deposits on calcareous rocks. No sharp mineralogical boundary between supergene Fe-Mn deposits and limonitic gossans underlain by manganiferous siderite deposits (Romanèche-Thorins (France)
• K/Ar and Ar/Ar isotopic dating of ore-forming events (25 to 1 Ma, Miocene and Pliocene) (Hauptmann & Lippolt 2000).
• Vein-like deposits of Ca and Al-sulphate-phosphates (APS) in karst pockets on limestones (e.g. Lahn-Phosphorite (Germany)) and in clay deposits (e.g. Lohrheim Kaolin deposit) on the Devonian rocks of the Rhenish Massif (German et al. 1981, Dill et al. 1995, Dill 2001) with carbonate-fluorapatite (francolite), With their extremely low values for Sr and anomalous high iodine contents (mean value 0,044%), the Lahn phosphorites differ basically from marine phosphorites.
• APS minerals: gorceixite BaAl3(PO4)2(OH)5·H2O, florencite-(Ce) CeAl3(PO4)2(OH)6, goyazite SrAl3(PO4)2(OH)5·H2O.
See also:
DILL, H.G. (1985) Terrestrial ferromanganese ore concentrations from Mid-European Basement Blocks and their implication concerning the environment of formation during Late Cenozoic (N Bavaria / F.R. Germany). - Sedimentary Geology, 45: 77-96.
DILL, H.G., PÖLLMANN, H. and TECHMER, A. (2013) 500 Million years of rift- and unconformity-related Mn mineralization in the Middle East: A geodynamic and sequence stratigraphical approach to the recycling of Mn.- Ore Geology Review 53: 112-133.
DILL, H.G., WEBER, B. and BOTZ, R. (2013) Metalliferous duricrusts (“orecretes”) - markers of weathering: A mineralogical and climatic-geomorphological approach to supergene Pb-Zn- Cu-Sb-P mineralization on different parent materials.- Neues Jahrbuch für Mineralogie Abhandlungen, 190: 123-195.
Literature is available on the RG server
H.G.Dill
Possibly an orogenic style deposit from a sea deposition might agree it could be shallow however conducting orogenic gold exploration have found deposits of manganese within quartz veins that indicate a much deeper deposits some related to chert-sulphide beds and perhaps formed from chemically precipitated exhaletes. Perhaps created in a back arc basin
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