Autochtonous refers to sediments that are native to its location (e.g. carbonat precipitation in a lacustrine facies),
· On the other hand,
Allochtonous refers to sediments originating from a place other than where it was found (e.g. glacial dropstones in marine sediments),.
Dear Dr. Harraz,
is an Fe-(Mn) bearing in situ mineralization created by weathering atop of sulfidic ore bodies by leaching S and heavy metals. S and metals are part of the emplacement of the gossan sensu stricto. There is only one way of forming allochthonous Gossan deposits by reworking these duricrusts and eventually leading to some kind of a placer called pebble Fe-(Mn) ore. The internal structure of the pebbles is identical to what we might expect in the gossan, proper, while the overall structure of the pebble Fe ore depends upon the processes of erosion, transport and distance and morphoclimatic conditions.
More specific answers to one or the other question you may find in (available on the RG server) the following studies. Your question lies between Fe-Mn-bearing duricrusts on one side and Fe-bearing placer-deposits on the opposite side:
1. 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
2. DILL, H.G., TECHMER, A., WEBER, B. and FUESSL, M. (2008) Mineralogical and chemical distribution patterns of placers and ferricretes in Quaternary sediments in SE Germany: The impact of nature and man on the unroofing of pegmatites.- Journal of Geochemical Exploration, 96: 1-24.
3. 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.
4. DILL, H. G. and ULRICH, H.-J. (1987) Geoelectric deep sounding - A contribution to constrain the genesis of siliceous ferromanganese duricrusts on deeply weathered Palaeozoic basement rocks (N Bavaria, F.R. Germany).- Zeitschrift für Geomorphologie., N.F., 31: 361-370.
5. DILL, H. G. (1988) Diagenetic and epigenetic U, Ba, and base metal mineralization in the arenaceous Upper Triassic "Burgsandstein" (Southern Germany). With special reference to mineralization in duricrusts. - Mineralogy and Petrology, 39: 93-105.
6. DILL, H.G. (2007) Grain morphology of heavy minerals from marine and continental placer deposits, with special reference to Fe -Ti oxides.- Sedimentary Geology, 198: 1-27.
7.DILL, H.G. and WEMMER, K. (2012) Origin and K/Ar age of cryptomelane-bearing Sn placers on silcretes, SE Germany.- Sedimentary Geology, 275/276: 70-78.
8. DILL, H.G., GOLDMANN S., and CRAVERO, F., (2018) Zr-Ti-Fe placers along the coast of NE Argentina: Provenance analysis and ore guide for the metallogenesis in the South Atlantic Ocean. Ore Geology Reviews 95: 131-160.
With kind regards H.G.Dill
Problems:
1)distinguished between Ironstones and Gossans formed by weathering of iron-rich rocks containing some disseminated sulfides (Blanchard , 1968) .
2)“Ironstone” as a major complication in mineral exploration because of the need to distinguish between gossaneous rocks related to base metal deposits and other ironstones (Andrew, 2000).
3)True gossan can be difficult to distinguish from Ironstone and other Iron Oxide accumulation such as Laterite.
4) Autochthonous (or True Gossan) and Allocthonous (or False Gossan) types are difficult to distinguish due to their similar textural appearance and mineralogy.
Allocthonous includes gossanous material that has been variably transported.
Introducing the term ironstone in this case would put you totally on the wrong scent. Ironstone is a term used for Phanerozoic sedimentary Fe deposits of coastal marine origin. They have nothing to do with the gossan which is the Fe-bearing cap of sulfide deposit undergoing supergene alteration. There may be Fe concretions reworked into these coastal deposits , e.g. Cretaceous Fe deposits near Salzgitter-Peine, Germany, which formed in depressions caused by dissolution of salt in the substrate.
To make it clear:
True gossan: Supergene alteration zone of sulfidic ore deposits and oxidic ones (see Dill et al. 2013 im my recent answer)
False gossan: Fe-bearing duricrusts ferricretes (Dill 1985), e.g., in laterites, bauxites etc. chemical sediments or residues of weathering affecting Fe-bearing country rocks unrelated to primary ore deposits
Ironstones: Phanerozoic sedimentary Fe deposits from the Cambrian through the Quaternary . See also BIF Banded Iron Formation of Precambrian age.
If you carefully investigate the mineral assemblage, the ore texture, the geological setting and the chemical composition you should be able to make a distincton between the three types of Fe-bearing aggregates.
H.G.Dill
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