Original paper

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

Dill, Harald G.; Weber, Berthold; Botz, Reiner

Abstract

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.

Keywords

duricrustsgeomorphologyclimateore mineralssupergene alterationcenozoic