Harald Dill; Wilfried Kantor:

Depositional Environment, Geochemical Facies, and a Tentative Classification System of Selected Types of Phosphate Occurrences

1997. 43 pages, 12 figures, 2 tables, 17x24cm, 150 g
Language: English

(Geologisches Jahrbuch Reihe D, Band D 105)

ISBN 978-3-510-96087-3, paperback, price: 7.00 €

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Abstract top ↑

Geochemical facies analysis (trace element, esp. REE) and mineralogical studies (XRD and microscopy) of phospate rocks were supplemented by a study of the literature on phospate rock sedimentology and depositional environment. Samples were taken from phosphorites, phoscretes, and phosphate bearing ingneous rocks. A tentative classification system for phosphate deposits was set up on the basis of these chemical and mineralogical studies and literature review. We employed the family tree concept ant the catena concept of soil science to shed some light on the interrelationships between various environments. Subdivision of the three basic environments -- --marine -- continental and magmatic -- is based on rock composition, --depositonal structure and the degree of supergene alteration. Marine phosphates may be further subdivided on the basis of (a) the tidal range of their depositional environment, (b) the pyroclastic deposits interbedded with them, (c) the impact of rift-related igneous activity on phosphate deposition, (d) subaerial weathering, (e) the pH-range of meteoric fluids and (f) the input of detrital material into the environment in which the phosphates were formed.

Contents top ↑

1 Introduction 5
2 Sampling Technique and Methodology 6
3 Geology and Mineralogy of Phosphate Mineralizations and their
Depositional Environment 7
3.1 Marine phosphates 7
3.1.1 Siliciclastics 8 High energy, nearshore conditions 8 Outer shelf deposits with volcaniclastic interbeds reworked in
clastic shore zones 8 Clastic shore zone: microtidal zone 8 Clastic shore zone: mesotidal to macrotidal zone 11 Clastic shore zone: supratidal zone 14 Low-energy, offshore conditions (distal estuarine) 14
3.1.2 Carbonates 15 Ramp deposits 15 Insular deposits 16
3.2 Continental phosphates 16
3.2.1 Phoscretes s. s. 16 APS duricrusts 16 Apicretes 17
3.2.Z Layered phosphates 18 Perennial, especially organic lakes 18 Ephemeral lakes 18
3.3 Igneous phosphates 19
3.3.1 Primary phosphates in igneous rocks and associated ore deposits
of magmatic origin 19
3.3.2 Secondary phosphates on top of igneous phosphates 19
4 The Chemical Composition of a Phosphate and its Bearing on the
Physicochemical Conditions of Formation 19
4.1 Major elements 19
4.2 Trace elements 22
4.2.1 Cobalt and nickel 22
4.2.2 Chromium and vanadium 23
4.2.3 Barium and strontium 24
4.2.4 NIolydenum and base metals 2245
4.2.5 Thorium and uranium
4.2.6 Cadmium and arsenic 26
4.3 REE distribution 26
4 3.1 Chondrite-normalized REE patterns 6
4 3.2 Tb/La vs. Tb/Ca plot 27
4.3.3 Ce/Ce* vs. LREE/HREE 31
5 Discussion and Conclusions 33
5.1 Pigeonhole versus family tree-catena classification of
phosphate-bearing environments
5.2 Environmental differences and their implications for element
partitioning 34
6 References 37