Publication details
The new Nb-P aplite at Reinhardsrieth: A keystone in the lateral and depth zonations of the Hagendorf-Pleystein Pegmatite Field, SE Germany
| Authors | |
|---|---|
| Year of publication | 2015 |
| Type | Article in Periodical |
| Magazine / Source | ORE GEOLOGY REVIEWS |
| MU Faculty or unit | |
| Citation | |
| Doi | http://dx.doi.org/10.1016/j.oregeorev.2015.04.015 |
| Field | Geology and mineralogy |
| Keywords | Niobium; Phosphate; Zonation; Pegmatite; Hagendorf-Pleystein; SE Germany |
| Description | The Hagendorf-Pleystein Pegmatite Field (HPPF) is the largest pegmatite concentration in Central-Europe, which has been intensively studied for its mineralogy, mainly at Hagendorf-South pegmatite stock. It was not until recently that the lateral and depth zonations of the HPPF could be clarified when the newly discovered tabular aplite at Reinhardsrieth aplite has been found at transition into the so-called barren zone devoid of rare metals. Thereby the theory of the derivation of the HPPF felsic mobilizates as a result of a successive differentiation of a pegmatitic melt from the Flossenburg Granite has to be discarded. From the structural and lithological point of view, stock-like, well zoned pegmatites, e.g. Hagendorf-South, developed in the hinge zone, whereas tabular aplites, such as Reinhardsrieth, evolved along the limb zone of strongly folded basement rocks. In the latter case, the accommodation space was smaller and the cooling of the felsic mobilizates much faster, giving rise to fine-grained aplitic rather than coarse-grained felsic mobilizates (chilled margin effect). During the incipient stages of the evolution in the HPPF, at Reinhardsrieth, a stronger impact of granitization can be determined than at the neighboring aplites and pegmatites based upon the REE fractionation. A tripartite subdivision may be established for the various pegmatites and aplites in the HPPF: (1) Barren zone (minor amounts of LREE in a system otherwise dominated by feldspar and quartz), (2) marginal zone (mainly Fe-Al phosphates plus columbite-(Fe)), (3) central zone (Fe-Zn-Li phosphates plus columbite-(Fe)). The afore-mentioned zonation shows an E-W trend and a bilateral symmetric arrangement of the various areas. There are significant indications of an oxidizing facies to substitute for a more reducing fades at the transition from the central into the marginal zone of the HPPF. The key mineral for the primary pegmatite mineralization in the marginal and central zones is columbite-(Fe). Its Mn/Fe ratios plotted vs. the Ta/Nb ratios allow for an assessment to what extent metamorphic and magmatic processes have contributed to the built-up of the pegmatites and aplites in the HPPF. Even the most primitive "nigrine"-hosted columbites fit well into this pattern, forming a lithochemical halo in the roof rocks of the pegmatites. The heavy mineral aggregated of rutile-ilmenite ("nigrine") giving host to the columbite as armored relicts got released from the basement rock on top of the pegmatites into the stream sediments where they are operative as an ore guide. The morphology of zircon in the stream sediments has been utilized as an economic ore indicator. The Zr silicate is also indicative of the temperature of formation and closely supports the tripartite zonation established above. Manganiferous apatite is a good depth-of-emplacement indicator for each aplite and pegmatite, respectively. The higher the FeO + MnO content of apatite, the shallower the depth of emplacement of its host pegmatite or aplite. Vivianite, in turn, marks the onset of the hydrothermal alteration of the pegmatites and aplites and its Mn contents relate to the depth where alteration has taken place or emplacement of the secondary phosphates. The higher the Mn content the shallower the depth of alteration. Peculiar element ratios were selected to design some key diagrams so as to corroborate the field-based mineralogical and geological results in terms of zonation of genetic economic geology as well as to cater for applied economic geology as proximity indicators and ore guides when exploring for rare element pegmatite. |