Answers to the question of the origin and the formation mechanism of each ore deposit can only be found after a careful comprehensive mineralogical geological and petrological investigation. It must be clear to the questioner what this means in terms of money, time and manpower.

many thanks for you comment. i was hoping to get directions on phase relationship and diagrams that can pin-point the mineralogical classes and possibilities. Can this be possible?

You can find the interrelationship between the various Fe deposits and their secondary acumulations in the attached file and the text in:

DILL, H.G. (2010) The “chessboard” classification scheme of mineral deposits: Mineralogy and geology  from  aluminum  to zirconium.- Earth Science Reviews, 100: 1-420.

•Source minerals: olivine, pyroxene, amphibole, mica

•Fe oxides contain Fe at a higher quantity thanFe silicates but they are more

resistant to weathering than silicates and in the majority of cases

accommodateFe in thetrivalent state

•An intermediate product during supergene alteration of Fe silicates is

ferrihydrite(Fe4(O3H2)3) which occurs in podsolic soils(Bs horizon).

•Microbial processes involvedin the decomposition ofFe silicates may

impede ferrihydrite to convert into goethite

•Under arid conditions the final product ofsupergene alteration ofFe silicates

is hematite instead of goethite

•Organo-mineralic complexes (tartric acid, citric acid, salycilic acid) are

secreted byplant rootlets or originate from fermentation of organic material

and play an important rolein mobilization or fixation ofiron in the

pedosphere because Fe is bound by more than one ligand.

•Chelat complexes are the only means to prevent Fe from being precipitatedin thefluvial drainage systems and to maintain the transport of Fe in meteoric

waters (black rivers). Complexediron is transported by the rivers to the sea

where thes eFe chelate complex is destroyed as the freshwater getsin

touch with seawater.

•This one key factor for the deposition of Fe in the coastal environment

•Under oxidizing conditions, trivalent Fe is onlysoluble < pH 4, bivalent Fe is only stablein solutions under reducing conditions.

•Fe is mobilized fromsource rocks and may be transported byhydrothermal fluids(hot brines) or by

•meteoric fluidsand drained away by riverine waters.

•Seawater is among the fluids most strongly depletedin iron.

•Only close to the spreading center of diverging lithospheric plates orat the mid-ocean ridge, the content ofFe may substantially increase(e.g. Red Sea). Exchange reactions take place as seawater is deeply circulating into thesubmarine basaltsMg 2+ from the seawater subtituting fro Mn 2+, Fe 2+ and Ca 2+.

•The temperatur of reaction isca. 300°C.

•Due to the low solubility ofFe and Mnin seawater ochre precipitates around the thesubmarine vents. Bivalent Fe is converted intoist trivalent state at lowerEh than Mnand hence concentrated much closer to the vent than Mn.

•As a result ofsubmarine hydrothermal activity, a series of metastable mineral phases(nontronite, jarosite, vivianite, greigite, mackinawite) starts to develop as the hot brines getin contact with the seawaterand subsequently graduallyalter toFe mineralssuch as goethite, hematiteand and pyrite as the more stable mineral phases(„Black Smokers“).

•Bacteria, e.g., hydronicrobium metallgenium havea mediating effectin the process ofore formation.

Eh, pH and soluble diagrams see:

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.

DILL, H.G. (2015) Orecretes linking geosciences and life sciences.- Environmental and Economic Significance of Gossans; Williamson, M.-C. (ed.), Geological Survey of Canada, Open File 7718: 40-47.

DILL, H.G. (2017)  Residual clay deposits on basement rocks: The impact of climate and the geological setting on supergene argillitization in the Bohemian Massif (Central Europe) and across the globe.- Earth Sciences Reviews 165: 1-58.

All papers are available for free download on the RG server

HGD

Quels processus concourent à la mise en place des gneiss à magnétite dans le socle precambrien ?

Merci en avance