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Dill H.G.,Federal Institute for Geosciences and Natural Resources | Weber B.,Burgermeister Knorr Str. | Gerdes A.,Frankfurt University
Geomorphology | Year: 2010

Cavernous weathering such as tafoni, alveoles and honeycomb structures have been recorded from a great variety of bedrocks and landforms. In the present study cavernous weathering from late Variscan granites was discussed as to its physical-chemical regime of formation. U/Pb dating yielded a maximum age of 1.52. ±. 0.03. Ma. Supergene U mineralization is accompanied by kaolinite, nontronite and Fe(III) phosphates. Based upon Eh-pH diagrams calculated for U-Fe-P mineralization the physical-chemical conditions may be described as oxidizing with pH values fluctuating around neutral at near-ambient temperatures of 25. °C. Alteration occurs in two stages: dissolution of rock-forming minerals and neoformation of hydrosilicates under mildly acidic conditions, followed by phosphate precipitation under near-neutral conditions. © 2010 Elsevier B.V. Source

Dill H.G.,Federal Institute for Geosciences and Natural Resources | Berner Z.,Fritz Haber Institute | Kaufholt S.,Federal Institute for Geosciences and Natural Resources | Weber B.,Burgermeister Knorr Str. | Metz U.,Cemex
Sedimentary Geology | Year: 2013

Baryte with or without base metal sulfides is quite common in sediments deposited in open marine environments or in continental sedimentary basins. Its precipitation is caused by hydrothermal processes, related to diagenesis, and frequently mediated by biogenic processes. The current study is focused on siliciclastic sandstones of Miocene (Aquitanian) age in an estuarine environment in the Wetterau region of the Rhein Graben, central Germany. In the estuarine environment only the central basin and the landward delta are host to a diagenetic and subsequent hydrothermal mineralization.Diagenesis took place under near-ambient (T. ≈. 25. °C) conditions and resulted in strong pyritization (0.75. <. Eh. <. +. 0.25, pH. >. 5) in the central basin. Diagenesis is more landward represented by a pervasive silicification (pH. <. 12) in deltaic sandstones.Epigenetic mineralization (100°-130. °C) with pyrite in the central basins was succeeded by Cu-Zn-(Sb) minerals (0.75. <. Eh. <. 0/5. <. pH. <. 11), silicification and kaolinization (2. <. pH. <. 9.5) and eventually by the formation of gibbsite (3. <. pH. <. 8). At the transition from the delta to the estuarine funnel, baryte is of very widespread occurrence. Its variegated texture and crystal morphology allow for a precise determination of the hydraulic system as marine phreatic, freshwater phreatic, and freshwater vadose. The narrow size of the rift graben and its sealing against the open sea fostered concentration of Ba and enhanced the redox processes. Hypogene brines along with Miocene volcanic activity provided the metals, and marine ingressions in this transitional environment supplied the sulfur. Sulfides were concentrated in the finer-grained rocks because of their enrichment in organic material, while sulfates accumulated in the more permeable coarser sandstones. © 2013 Elsevier B.V. Source

Dill H.G.,Federal Institute for Geosciences and Natural Resources | Weber B.,Burgermeister Knorr Str. | Melcher F.,Federal Institute for Geosciences and Natural Resources | Wiesner W.,Armanspergstrasse 4 | Muller A.,Geological Survey of Norway
Ore Geology Reviews | Year: 2014

Black heavy mineral (HM) aggregates of metallic luster and composed of ilmenite and rutile were named "nigrine" (amount of rutile. >. ilmenite) and "antinigrine" (amount of ilmenite. >. rutile). They contain inclusions of, e.g., columbite-(Fe), pyrochlore group minerals, wolframite solid solution series (= s.s.s.), monazite, zircon, Fe oxides and sulfides as well as alteration minerals such as pseudorutile and Fe-Ti-Nb-Ta-Al-P compounds, whose precise chemical composition and mineralogical affiliation cannot be determined. These titaniferous HM aggregates are of widespread occurrence in gneisses and shear zones cutting through them as well as alluvial, fluvial and colluvial deposits at different distances from rare-element phosphate pegmatites within the crystalline basement in SE Germany. "Nigrine" and "antinigrine" may be subdivided into three types which formed at different periods relative to the Variscan tectonic disturbances and which show different qualities as markers for the origin and presence of Nb-bearing pegmatites along the western edge of the Bohemian Massif, SE Germany.Type A developed pre- to synkinematically within or near deep-seated shear zones which formed below 730. °C as early as 321 to 329. Ma. These HM aggregates are poor in Nb and impoverished in accessory minerals. The HM aggregates having developed in shear zones mark the "kitchen" where friction and heating contributed to the formation of felsic intrusive bodies, such as pegmatites and aplites. This type of Ti-bearing HM aggregates may be held as distal proximity indicator.Type B is early postkinematic and enriched in niobian rutile, rife with lots of inclusions, especially columbite-(Fe). It precipitated in the crystalline country rocks at temperatures around 600. °C concomitant with the nearby rare-element pegmatites between 302 and 311. Ma. It is the most proximal member of this group of HM aggregates.Type C "nigrine" is enriched in W and late postkinematic relative to the shearing processes in the crystalline basement. It formed around 470. °C during the same period of time as type C. It does, however, not qualify as a marker for rare-element pegmatites, as it is unrelated to these felsic intrusives.During the late Neogene the Variscan basement was strongly uplifted and many pegmatites were stripped off their roof rocks. As a consequence of this, "nigrine" and "antinigrine" were released from their host rocks and became part of the terrigenous sediments laid down in the drainage systems which upstream cut into the crystalline basement and the pegmatites/aplites. Due to their high resistance to chemical weathering these titaniferous HM aggregates acted as "armored relics" for less resistant minerals such as columbite and pyrochlore. When these titaniferous HM aggregates got decomposed by attrition these mineral inclusions of lesser stability appear in the fluvial sediments further afield from their source than expected, considering the low stability to weathering of single crystals of columbite- and pyrochlore group minerals."Antinigrine" and "nigrine" may be spotted in the HM suites of drainage systems around pegmatites at a distance of ≤10km with correlation coefficient of RNb-Ti=+0.42, while in the range 2-5km RNb-Ti increases to +0.77, in the range 1-2km RNb-Ti is +0.85 and around 1km from the rare-element pegmatite RNb-Ti stands at +0.92."Nigrine" and "antinigrine" are no ideal mineral aggregates to form placer-type deposits of their own due to their variegated mineralogy, excluding some Ta-enriched subtypes in Colombia and Sierra Leone. The variable mineralogy detrimental to its use as a source of Ti, is the strong point as an exploration tool for rare-element pegmatites. © 2013 Elsevier B.V. Source

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