Metalliferous duricrusts (orecretes) - markers of weathering: A mineralogical and climatic-geomorphological approach to supergene Pb-Zn-Cu-Sb-P mineralization on different parent materials

Orecretes are defined in the current study as a special type of duricrusts (chemical residues) with heavy metals, accumulated near-surface, but excluding deeper parts that developed under reducing conditions. More than 70 study areas across the globe, some of which containing more than one mineralized site, have been selected during this geomorphological study to cover the various morphoclimatic zones from pole to pole. The supergene mineral assemblages were investigated by means of XRD, SEM-EDS, EMPA (electron microprobe analysis), optical methods and oxygen isotope analysis of carbonates. While these methods remain incomplete, morphoclimatic studies supplemented by data from literature were conducted to determine the physical-chemical regime of orecretes in the course of supergene processes during the most recent parts of the Earth history. The current research is focused on the topmost metalliferous chemical residues and near-surface phosphate-bearing encrustations which act as a sink to a wide range of elements in addition to base metals. The orecretes are subdivided into oxicretes (oxide plus hydrate), carbocretes (carbonate), silicacretes (silica), halcretes (halogenides: Cl, J, F, and Br), sulcretes (sulfate plus APS minerals/aluminium phosphate sulfate minerals), phoscretes (phosphates), arsenocretes (arsenates), and vanadocretes (vanadates). Se-, Mo-, and oxalate-bearing orecretes, of less widespread occurrence than the afore-mentioned chemical residues, were named accordingly. The orecretes contain Pb, Cu, Zn, In, Fe, Mn, Ni, Co, W., REE, and Ag, as qualifier, each of which is added to the principal denominator (e.g., silicacrete-(Cu)). The development of orecretes is controlled by their parent material, the landforms, and the climate. The three principal factors render the orecretes under study a first-hand morpho-climatical marker for geomorphologists and climatologists, alike, as illustrated in a flow sheet. Climatologists might learn what was going on in the recent past but also get a tool to predict what we might expect in the near future if the morphoclimatic zones shift across the globe. (1) Orecretes-(Se-Mo) and arsenocretes only developed on parent material strongly enriched in the marker elements As, Se and Mo. Orecretes and carbocretes that occur in some sites as relic forms (see 3) may develop mineral deposits of their own near-surface, e.g., bauxites, ferricretes or uraniferous calcretes. (2) In poorly-reliefed areas, low-lands, plateaus, and highly-eroded mountain belts of Precambrian through Mesozoic age, the full range of orecretes from oxicretes through orecretes may be expected. In rift and graben structures as well as modern mountain belts of Cenozoic age, halcretes, vanadocretes, and phoscretes are less widespread, since the ratio of uplift vs. chemical weathering has a detrimental effect on these effervescence to be preserved when the morphoclimatic zone changed either by altitude or in time. Among the orecretes considerable changes in type and quantity may be observed along with latitudinal climatic zonation from the pole through the equator. These global horizontal changes are equivalent to the mineralogical variation encountered along with the vertical microclimatic zonation in mountainous regions with the (peri) glacial zone developing on top of the mountain chain. However along these vertical transects in highly-reliefed areas, the full spectrum of orecretes does rarely show up due to a more accelerated large-scale uplift and down wrapping relative to flat-lying areas. Nevertheless, orecretes offer a better insight into the weathering phenomena in mountainous areas than common weathering products such as clay minerals for their accommodation of physico-chemically critical elements into their lattice. (3) The latitudinal climatic zonation of orecretes has been discussed for the most recent time slices of the earth history, the Quaternary (<2 Ma) and Neogene (<20 Ma). The majority of orecretes investigated during this study formed during the Quaternary, as revealed by the shift of morphoclimatic zones across the globe and by the vertical zoning in modern mountain ranges. Using the oxygen isotopes of carbocretes to determine the paleotemperatures of orecretes along a transect perpendicular to various morphoclimatic zones, yielded a trend similar to that of the Quaternary climate curve. Irrespective of the precise age of formation, which can only be achieved for some U-, Mn- and C-bearing orecretes, these metalliferous duricrusts may be attributed to pedological and hydrological processes whose physico-chemical regime may be constrained based on thermo-dynamical calculations. The physico-chemical regime is mainly controlled by the chemical composition of the meteoric fluids (pH changes) under oxidizing and near-ambient hydrous conditions. (a) Oxicretes-(Fe-Mn-Cu -In) occur over the full pH range in the humid zone of moderate chemical weathering and likewise in tropical humid zones with pervasive chemical weathering. They often mark the onset of supergene alteration and occupy the position of relic forms marking, in places, a climate, which no longer exists. (b) Carbocretes-(Pb-Cu-Zn-Cd) formed within a pH-range similar to that of the oxicretes down to pH 4. Mountainous and periglacial regions are the most favorable morphoclimatic zones due to the retarded decomposition of organic matter in their soil types. Temperatures of carbocretes-(Cu-Zn-Pb) obtained from O isotope studies along the N-S trend fit the latitudinal morphoclimatic zonation across the globe. The temperature of formation controlled by the organic redox systems is higher in the humid tropical and mid latitude zones than in more arid zones relative to the mean annual temperatures due to the elevated groundwater level and increased rate of precipitation. (c) Silicacretes-(Cu-Zn) form in a pH range that only partially overlaps with those of carbocretes and oxicretes (absent in the strongly alkaline regime) and show up later in the succession of supergene alteration. Silicacretes favorably develop under intermediate morphoclimatic conditions with a balanced ratio between precipitation and evaporation. (d) For halcretes-(Cu-Ag), due to their high solubility, low precipitation plus high evaporation rates are decisive for their preservation. They are exclusive to the tropical semiarid and arid morphoclimatic zones in a low to neutral pH regime. (e) Sulcretes-(Fe-Cu-Pb-APS) similar to halcretes used to precipitate from acidic solutions, including the man-made acid-mine-drainage / AMD. They are indicative of strong evaporation in morphoclimatic zones where the chemical weathering is rather moderate and wind speed is rather high. Temperature is not per se a criterion for the presence or absence of sulfates, but dryness is critical for their precipitation. The boundaries between supergene and hypogene alteration is not sharp in this group of orecretes where APS minerals (aluminium phosphate sulfate) bridge the gap between the supergene and hypogene alteration by their varied solid solution series. Increasing amounts of REE within the APS s.s.s. (= solid solution series) and substitution of phosphate for sulfate in APS s.s.s. takes place as supergene morphoclimatic processes become more dominant in the near-surface alteration process. (f) Phoscretes (Al-Fe-Cu-Pb-REE) are rather heterogeneous with Al-enriched end members prevailing in acidic fluids and Al-free members under neutral to alkaline conditions. In the morphoclimatic humid mid latitude and tropical wet-dry zones phoscretes reveal a relative maximum as a function of the phosphate cycle within the biosphere/pedosphere. (g) Vanadocretes-(Pb-Cu-Zn) take an intermediate position between sulcretes and halcretes, to play a leading role in the sequence of aridity as follows: silicacrete >= sulcrete > vanadocrete > halcrete. They form exclusively under arid conditions as the rate of chemical weathering is low. (Ultra) basic magmatic rocks or (meta)biolites in the hinterland are mandatory to provide the V for the buildup of the orecrete.