The Fe-Mn phosphate aplite ‘Silbergrube’ nearWaidhaus, Germany: epithermal phosphate mineralizationin the Hagendorf-Pleystein pegmatite province

H. G. DILL1,*, B. WEBER

2, A. GERDES3

AND F. MELCHER1

1 Federal Institute for Geosciences and Natural Resources, P.O. Box 510163, D-30631 Hannover, Germany2 Burgermeister-Knorr Str. 8, D-92637 Weiden i.d.OPf., Germany3 Frankfurt University, Institute of Geosciences, Petrology and Geochemistry, Altenhoferallee 1, D-60438 Frankfurt

am Main, Germany

[Received 11 June 2008; Accepted 11 December 2008]

ABSTRACT

The Silbergrube Aplite (SA) in the Hagendorf-Pleystein Pegmatite District, near Waidhaus, Germany,is a mildly peraluminous NW–SE directed leucogranite dyke. It occurs in association with quartz dykesand aplitic metamorphic mobilizates in the NE Bavarian crystalline basement. The SA differs fromother aplitic mobilizates in the region in having a less well developed strain-related mineral orientationand in containing only minor amounts of garnet and tourmaline. The aplitic metamorphic mobilizatesand the SA are chemically and mineralogically almost identical and yield the same age of formation of~302 Ma (stage I). The age of formation of the Hagendorf pegmatites seemingly post-dates theemplacement of the SA. The SA was emplaced at the boundary between fine-grained biotite granitesand metamorphic country rocks within a zone of structural weakness, favouring the formation ofdisseminated late magmatic to hydrothermal mineralization of Li-bearing Fe-Mn phosphates (stages IIand III). Brittle deformation along this zone was conducive to the faultbound Fe-Mn-Ca phosphates.Mineral telescoping is evident from the presence of Fe2+, Fe3+ and Mn2+ phosphates in fissures andvugs in a texturally highly variable host-rock environment (stage IV). This intimate intergrowth ofphosphate minerals reflects contrasting physical and chemical conditions prevailing in a near-surface/shallow epithermal S-deficient phosphate system (stage IV), similar to what is known from Cu-Auepithermal systems. The most recent mineral assemblages that formed under predominantly oxidizingconditions are correlated with the subtropical weathering during the Neogene which resulted in theformation of a peneplain truncating the SA and its country rocks (stage V). The SA is the root zone ofthe felsic aplitic-pegmatitic mobilizates in this region and is overprinted by an epithermal phosphatesystem.

KEYWORDS: aplite, Carboniferous, Li-Fe-Mn phosphate, structure-bound, epithermal, supergene, Neogene.

Introduction

THE Silbergrube (Silbergrube = Silver Mine is a

misnomer used by local people mistaking shiny

flakes of white mica as silvery material) aplite

near Waidhaus is the last operating feldspar mine

in the Hagendorf-Pleystein Pegmatite Mining

District in Bavaria, Germany. The Hagendorf

pegmatite is one of the biggest feldspar-quartz

pegmatite deposits in Europe, and has produced

4.4. Mt of ore (Forster et al., 1967) (Fig. 1). It is

the type locality for the mineral hagendorfite

[NaCaMn(Fe2+Fe3+Mg)2(PO4)3] and contains a

great variety of other mineral species (Scholz,

1925; Forster, 1965, Forster and Kummer, 1974;

Strunz, 1961; Strunz et al., 1975; Uebel, 1975;

Mucke, 1988, 2000; Keller et al., 1994; Mucke et

al., 1990). The Silbergrube Mine (SA), the only* E-mail: dill@bgr.deDOI: 10.1180/minmag.2008.072.5.1119

Mineralogical Magazine, October 2008, Vol. 72(5), pp. 1119–1144

# 2008 The Mineralogical Society

pegmatite-based feldspar mine in Germany,

operated between 1938 and 1958 by underground

mining down to a depth of 60 m. At present, the

existing opencast area measures ~150,000 m2

with an annual production of 40,000 to 50,000 m3,

mainly used by the ceramic industry. The raw

material of 42% Na-feldspar, 38% quartz and

20% K-feldspar is used to produce floor and wall

tiles and sanitary ceramic products.

Pegmatites are important sources of Li and Cs

(Bessemer City, USA, Greenbushes, Australia,

Bikita, Zimbabwe), which are mainly contained in

spodumene, Li mica and pollucite. Gem-quality

amblygonite is exploited at Governador

Valadares, Brazil (Delaney 1996). Between 1960

and 1972, 1000 t of Li ore were also extracted,

mainly from triphylite within the Hagendorf

pegmatite. The phosphates of the Silbergrube

aplite were not, however, mined specifically for

the recovery of lithium (Schmidt, 1955).

There have been few detailed mineralogical or

chemical studies on the evolution of the SA

(Vochten et al., 1995; Breiter and Siebel, 1995;

Novak et al., 1996; Schluter et al., 1999). Indeed,

there is a general paucity of information on

phosphate-rich pegmatites worldwide compared

with the wealth of papers on pegmatites in general

(Cerny et al., 1995; Abella et al., 1995; Anderson

et al., 1998; Wise, 1999; Novak and Cerny, 2001;

Ercit, 2005). Phosphate pegmatites hosting Li-Fe-

Mn phosphates are not so common and have only

previously been reported from France, Finland,

Germany, Namibia, Rwanda, Spain and the Czech

Republic (Cech et al., 1961; Fransolet, 1980;

Lahti, 1981; Mucke, 1981; Fransolet et al., 1983;

Keller, 1991; Cuney and Raimbault, 1991; Roda

Robles et al., 1998). In the aforementioned

papers, emphasis was placed on the primary

Fe-Mn phosphates, their Fe-Mn partitioning and

microchemical studies. Often, investigations

could only be based on specimens taken from

collections for lack of accessibility to the

pegmatites, aplites and aplitic granites such as

the SA themselves. In our paper, we intend to

provide full coverage of the mineralogy and

petrology of the SA from its early phases of

emplacement through to the most recent stages of

alteration of primary phosphates. We will not

revisit minerals already listed in textbooks as to

their crystal structure, but concentrate on the

morphology and outward appearance of phos-

phates as a means of constraining ‘pegmatite

minerostratigraphy’, including their ages of

formation and the physicochemical conditions of

phosphate deposition and alteration. Based upon

the minerals present, we correlate our findings

FIG. 1. (a) The position of the Hagendorf-Pleystein Pegmatite Mining District, Oberpfalz, Germany and (b) the

geological setting (geology modified from Forster, 1965). The granites belong to the Flossenburg Granite Complex.

1120

H. G. DILL ET AL.

with the mineralized rocks in the neighbourhood.

By doing so, a contribution can me made to the

‘telescoping’ (mineral assemblages very different

in their physicochemical conditions of formation

which are found side-by-side with each other or

intimately intergrown) – opposed to zonation – of

mineral assemblages in pegmatites and aplites.

Geological setting

The study area, part of the northeastern Bavarian

basement, is underlain mainly by Moldanubian

paragneisses composed of variable amounts of

biotite, sillimanite, cordierite, quartz, garnet and

feldspar (Forster, 1965) (Fig. 1). Psammopelitic

rocks underwent regional metamorphism under

low-pressure and high-temperature conditions

during the Variscan orogeny. Structural adjust-

ments in the NE Bavarian basement were

constrained to the period 450 to 330 Ma (Weber

and Vollbrecht, 1989). Carboniferous felsic

intrusive rocks are second in abundance, the

most important of which is the Flossenburg

granite (Fig. 1b). This granite has been dated by

the Rb/Sr whole-rock method at 311.9S2.7 Ma

(Wendt et al., 1994). Cooling of the granite based

on isotopic studies on muscovite yielded an age of

300 Ma, and from samples of biotite a

significantly younger age of 292 Ma was

obtained. Petrographically, the granite has been

classified as a monzogranite (Wendt et al., 1994).

During post-Variscan times the NE Bavarian

basement saw a strong uplift which did not

allow the Mesozoic marine incursions to encroach

upon this crystalline basement. It was not until the

Neogene that Miocene-Pliocene weathering and

geomorphological processes left their imprints on

the crystalline rocks of the study area by leaving

behind a thick regolith and U yellow ores

amenable to U/Pb dating (4.55S0.02 Ma) (Carl

and Dill, 1985; Dill, 1985a; Dill et al., 2007a).

Analytical techniques

More than 100 samples were examined using thin

and polished sections. X-ray diffraction (XRD)

analysis was performed using a Phillips PW 3710

with Cu-Ka radiation, a fixed primary slit system,

and a secondary monochromator. X-ray fluores-

cence (XRF) analysis of powdered samples was

carried out using a PANalytical Axios and a PW

2400 spectrometer. Samples have been collected

over several years from various parts of the SA

openpit, the majority derived from the phosphate-

rich area marked in Fig. 2b. Electron microprobe

analyses (EMPA) were carried out using a

CAMECA SX100 equipped with five wave-

length-dispersive spectrometers and a Princeton

Gamma Tech energy-dispersive system. Oxide,

phosphate and silicate phases were analysed at an

acceleration voltage of 20 kV and a sample

current (measured on brass) of 20 nA. The

minerals albite, chromite, kaersutite, almandine,

apatite, magnetite, pentlandite, biotite, rutile,

rhodonite and galena and pure metals were used

as standards.

The scanning electron microscope FEI

QUANTA 600 FEG with an energy-dispersive

system (SEM-EDS), was used to assist in mineral

identification and image analysis for morpholo-

gical studies.

Columbite-group minerals were analysed in

situ in polished thick sections for U, Th and Pb

isotopes by a laser ablation-inductively coupled

plasma-mass spectrometry (LA-ICP-MS) tech-

nique, using a Thermo-Scientific Element II

sector-field ICP-MS coupled to a New Wave

UP213 ultraviolet laser (Gerdes and Zeh, 2008).

Laser spot-sizes varied from 12 to 30 mm for

torbernite and from 20 to 80 mm for columbite.

Data were acquired in peak jumping mode over

800 mass scans during 20 s background measure-

ment followed by a 30 s sample ablation. A

teardrop-shaped, low-volume laser cell was used

to enable sequential sampling of heterogeneous

grains (e.g. growth zones) during time-resolved

data acquisition (cf. Janousek et al., 2006). The

signal was tuned for maximum sensitivity for Pb

and U while keeping oxide production, monitored

by 254UO/238U at <<1%. Raw data were corrected

offline for background signal, common Pb based

on the interference- and background-corrected204Pb signal, laser-induced elemental fractiona-

tion, instrumental mass-discrimination, and time-

dependent elemental fractionation of Pb/U. The

interference of 204Hg (mean = 97S17 cps; counts

per second) on the mass 204 was estimated using

a 204Hg/202Hg of 0.2299 and the measured 202 Hg.

In approximately one-third of the analyses the

interference- and background-corrected 204Pb was

below the estimated limit of detection (~10 cps).

In general, the 206Pb/204Pb was >4000, a level

where the common Pb correction has a negligible

effect on the 206Pb/238U age. Zircon crystals GJ-1

(Jackson et al., 2004) and Plesovice (Slama et al.,

2008) were used for external standardization.

Previous studies have shown the possibility of

using non-matrix matched standardization for LA-

FE-MN PHOSPHATE APLITE, SILBERGRUBE, GERMANY

1121

ICP-MS-U-Pb dating (e.g. Meier et al., 2006;

Horstwood et al., 2003). Late Proterozoic

monazite dated by the same method as above

yielded concordant results where the 207Pb/206Pb

and the 206Pb/238U ages agreed to better than 1%

(Meier et al., 2006). This indicates, in accordance

with our concordant torbernite results, a negli-

gible difference in the U-Pb fraction between

phosphates and zircon after correction of the time-

dependent element fraction. In the present study,

the latter was rather low due to the low energy

density (<0.5 J/cm2) and repetition rate (5 Hz)

used. The 206Pb/238U increases during the ablation

by ~10% in the case of the GJ-1 zircon (60 mm

spot) and <16% in the case of the torbernite. Thus,

the difference between the corrected and uncor-

rected ratio is relatively small. Reported uncer-

tainties (2s) were propagated by quadratic

addition of the external reproducibility (2 s.d.)

obtained from the reference zircon (n = 12) during

the analytical session and the within-run precision

of each analysis (2 s.e.). Concordia diagrams (2s

error ellipses) and concordia ages with 2suncertainty were produced using Isoplot/Ex rev.

2.49 (Ludwig, 2001). For further details on the

analytical protocol and data processing, see

Gerdes and Zeh (2008).

Results

Petrology of the SA from f|eld observationsThe SA strikes NW–SE and dips at an angle of

~45º beneath a fine-grained biotite granite.

Biotites within the granite are aligned with a

NW strike, showing the same NE dip orientation

as the enclosing country rocks (Forster, 1965).

The contact between the SA proper, and its

hanging-wall biotite granite, is marked by a

pegmatitic zone called ‘stockscheider’ displaying

a faint alignment of feldspar (Fig. 2a). The SA

contains aplitic, more Fe-rich and Fe-poor parts

with an irregular tectonic zone rich in phosphates

(Fig. 2b). The major minerals in the SA are

quartz, K-feldspar (more or less kaolinized),

FIG. 2. The Silbergrube aplite (SA) (fine-grained muscovite-albite granite). (a) Close-up view of the SA geology in

cross section (modified from Foster, 1965). (b) Image of the Silbergrube openpit exhibiting the various types of

aplite and the core zone where most phosphates were concentrated. This part of the SA is not mined.

1122

H. G. DILL ET AL.

albite and muscovite (Table 1, Fig. 3). At the

margin of the SA, in its hanging wall, biotite is

converted into Fe-Al chlorite (Table 1). The rock-

forming minerals within the SA are randomly

oriented (Fig. 4a), whereas some aplites sampled

for reference near Pleystein show a preferred

orientation of tourmaline (dravite–schorl) in their

marginal garnet-tourmaline zones (Fig. 4b,c).

Tourmaline solid solution series (s.s.s.) as well

as garnets are also sporadically found in the SA.

Garnet mineralogy

Garnets in the aplites show strong zonation, with a

core enriched in the spessartite component.

Garnet grains belong to the pyralspite (pyrope–

almandine–spessartite) s.s.s. (Matthes, 1961). For

the pure spessartite composition, the lower

reaction limit at pressures between about 200

and 1500 atm. is at 410ºC. According to Mathes

(1961), in spessartite–almandine s.s.s., the limit

rises with increasing almandine content from

410ºC (spessartite90 almandine10) to 500ºC

(spessartite50 almandine10).

Sulphide and non-sulphide ore minerals

Mineralogy

The SA is poor in sulphides, such as pyrite,

marcasite and sphalerite, whilst oxide minerals are

more widespread in the Fe-rich zone. Whereas the

Fe-Mn ‘limonite’ appeared very late in the

evolution of the SA, the columbite-group minerals

formed early, contemporaneously with the felsic

rock-forming minerals, during the emplacement of

the SA. Columbite-(Fe) shows no differentiation as

to its Mn/Fe and Nb/Ta ratios (Fe = 10.3 wt.%, Mn

= 3.8 wt.%, Nb = 44.3 wt.% and Ta = 21.3 wt.%).

The aplitic columbite-group minerals being studied

in this area belong to the ferrocolumbite (niobite)

group (Fig. 5a,b). The ferrocolumbite sampled in

stream sediments of creeks and rivulets of the

drainage system covering the Hagendorf-Pleystein

District is close to the end-member ferrocolumbite,

whereas ferrocolumbite sampled from aplites at the

outcrop is slightly enriched in Ta (Dill et al.,

2007b). This orthorhombic oxide mineral occurs

mostly in crystals flattened to the prism faces

{010} and intergrown with quartz.

Age datingColumbite-(Fe) as shown in Fig. 5c was used

for U/Pb dating. Ferrocolumbite was analysed for

U, Th and Pb isotopes by LA-ICP-MS techniques.

A cluster of data on the concordia plot yielded an

age of formation of 302.8S1.9 Ma (Fig. 5d,

Table 2). The ages of ferricolumbites from SA,

Hagendorf (299.6S1.9 Ma) and Pleystein

(302.1S3.3 Ma), are quite similar (Dill et al.,

2007b).

Phosphate minerals

All phosphate minerals recorded in the SA are

listed, irrespective of the level of certainty, in

Table 3, together with the the rock-forming and

ore minerals also present. Many of the phosphates

are otherwise known only from lapidary and

mineral collectors’ journals.

Apatite seriesApatite is either commonly disseminated in the

SA or forms dark green anhedral masses abundant

in Mn and, hence, classified as manganapatite

(Table 1, Figs 3, 5a). Manganiferous apatite has

been described from zoned lithium-rich pegma-

tites and the highest MnO concentration has been

recorded from granitic pegmatites in the range

3.0�10.3 wt.% MnO (Cruft, 1966; Falster et al.,

1988).

Triploidite seriesThis series of Fe-Mn phosphates is represented

in the SA by triploidite, zwieselite and wolfeite.

Like apatite, these minerals occur as dissemina-

tion and anhedral masses in the SA (Fig. 6b).

Triphylite seriesThree members of this series – triphylite,

ferrisicklerite and heterosite-purpurite – are

found in the SA. Based upon the data listed in

Table 1, a Mn-bearing heterosite [(Fe0.7Mn0.3)

(PO4)] replaces Mn vivianite in the interstices of

the aplitic groundmass (Fig. 6c).

Vivianite-ludlamite-landesiteSamples of vivianite [(Fe2.4Mn0.5Mg0.1)

(PO4)2·8(H2O)] taken at the SA are significantly

enriched in Mn and chemically not at variance

with vivianite described from Hagendorf (Strunz

et al., 1975, table 1). It is developed along grain

boundaries of the aplitic groundmass and is found

lining walls in small fissures (Figs 3, 5b,c).

Vivianite is stable over a wide range and also

occurs late in the mineralization, forming earthy

encrustations (German: ‘Blaueisenerde’ = blue

coloured clays). Ludlamite was recorded by

Novak et al. (1996) as filling cracks and replacing

FE-MN PHOSPHATE APLITE, SILBERGRUBE, GERMANY

1123

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1.8

74

0.0

08

0.0

03

0.0

09

0.0

04

0.0

32

0.0

02

S0.0

06

Cl

0.0

19

Tota

l5.0

25

4.9

79

4.9

73

14.0

04

7.9

88

8.0

03

4.9

82

4.9

59

7.8

98

7.8

55

7.9

66

1.9

93

1.9

99

2.0

74

2.0

66

n.a

.,notan

alyse

dTiO

2(<

0.0

4%

),Sc 2

O3(<

0.0

4%

)an

dA

s 2O

3(<

0.0

6%

)an

alyse

d,butnotdet

ecte

d.M

usc

.:m

usc

ovite;

allFe

calc

ula

ted

asFe 2

O3fo

rfe

ldsp

ar,fr

ondel

ite,

mitridat

ite,

var

sici

tean

dhet

erosi

te,an

das

FeO

form

usc

ovite,

apat

ite

and

viv

ianite.

FIG

.3.The

evolu

tion

ofth

eSA

intim

e(s

ubdiv

ided

into

five

stag

esco

rres

pondin

gto

five

min

eral

izin

gpro

cess

es)an

dsp

ace

(show

nby

the

Fe-

pooran

dFe-

rich

central

and

mar

gin

alfa

cies

and

fourse

quen

ces

instag

eIV

(Ato

D)co

ntrolled

by

var

ious

par

entm

ater

ials

and

by

diffe

rentre

action

pat

hw

ays)

.The

mar

gin

alfa

cies

reflec

ts

the

reac

tion

of

phosp

hat

e-bea

ring

solu

tions

with

Al-rich

rock

sfrom

the

surroundin

gco

untry

rock

s,th

ece

ntral

faci

esdes

crib

esth

ein

tra-

aplitic

pro

cess

es.

The

physico

chem

ical

conditio

ns

for

each

min

eral

izat

ion

may

be

ded

uce

dfrom

the

colo

urco

mposite

.

FE-MN PHOSPHATE APLITE, SILBERGRUBE, GERMANY

1125

white mica interstitially. Landesite is one of the

minerals listed by the Mindat.org database for that

locality but it was not identified in the present

study.

Strunzite seriesWhile strunzite is easily identified by its Mn

content using SEM-EDX methods and macro-

scopically by its acicular stellate aggregates on

rockbridgeite and minerals of the jahnsite-

whiteite series, distinction between ferrostrunzite

and ferristrunzite is fraught with difficulties in

these micromounts, even by microchemical

analyses (Fig. 6d). Vochten et al. (1995) used

Mossbauer spectroscopic studies of some strun-

zite varieties to address this question. As the

current study has a different focus from theirs, our

attribution to ferristrunzite or ferrostrunzite is

based on the phosphate minerals with which

ferrostrunzite and ferristrunzite are associated in

time and space. In Fig. 6e, an intimate inter-

growth of needle-shaped ferrostrunzite and cruci-

form rockbridgeite is representative of an

Fe2+/Fe3+ regime. Beraunite and oxiberaunite

either form acicular crystal aggregates or coat

other Fe-bearing minerals, mainly rockbridgeite

(Fig. 6f). One of the latest Fe phosphates to be

identified in the phosphate mineralization of the

SA is cacoxenite (Fig. 6g,h). It is found covering

Fe phosphates as well as non-ferrous minerals.

FIG. 4. Silbergrube aplite compared with an aplitic mobilizate sampled for reference near Pleystein. (a) Silbergrube

aplite with albite, K feldspar and muscovite in random distribution. This specimen is bounded on the right-hand side

by a veinlet hosting laueite, keckite, strunzite, rockbridgeite and strengite. The dark green zone proximal to the vein-

type mineralization is enriched in manganapatite and triphylite. Black spots disseminated in the aplite are vivianite.

(b) Pleystein aplitic mobilizate showing a preferred orientation of dravite–schorl s.s.s. within the marginal

tourmaline-garnet zone of the aplite. (c) Tourmaline scattered in a matrix of alkaline feldspar and quartz. Thin

section, crossed polars. For chemical composition of both aplites see Table 2.

1126

H. G. DILL ET AL.

FIG. 5. Columbite-group minerals from aplites of the NE Bavarian Basement and on a worldwide scale.

(a) Subdivision of columbite group minerals. The aplitic columbite-group minerals under study belong to the

ferrocolumbite (niobite) group. (b) Ferrocolumbites from the study area in NE Bavaria compared to a large number

of ‘coltan’ (columbite-tantalites) analysed world-wide (Melcher et al., 2008). Ferrocolumbite from placer deposits in

the Hagendorf-Pleystein District are the most primitive of all columbite group minerals analysed worldwide.

Ferrocolumbite sampled from aplites at outcrop are slightly enriched in Ta relative to ferrocolumbite from placer

deposits in the study area. (c) Ferrocolumbite from the Silbergrube Aplite in quartz (SEM-EDX). (d) LA-ICP-MS U/

Pb age dating of ferrocolumbite from Silbergrube Aplite (Fig. 5c).

FE-MN PHOSPHATE APLITE, SILBERGRUBE, GERMANY

1127

TA

BLE

2.LA

-ICP-M

SU

,Pb

and

Th

isoto

pe

dat

aof

ferroco

lum

bite

from

the

Silber

gru

be

Aplite

.

207Pb

aU

bPb

bTh

b206Pb/

206Pb/

2s

207Pb/

2s

207Pb/

2s

rho

d206Pb/

2s

207Pb/

2s

207Pb/

2s

Num

ber

(cps)

(ppm

)(p

pm

)U

204Pb

238U

c(%

)235U

c(%

)206Pb

c(%

)238U

(Ma)

235U

(Ma)

206Pb

(Ma)

A1

1596

28

1.3

0.0

02

886

0.0

4946

4.0

0.3

581

7.1

0.0

5251

5.9

0.5

6311

12

311

19

308

134

A2

2835

56

2.6

0.0

01

4523

0.0

4854

2.5

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535

3.7

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5282

2.7

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9306

8307

10

321

61

A4

1563

20

0.9

0.0

04

1409

0.0

4913

3.0

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503

5.1

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5171

4.1

0.5

8309

9305

13

273

94

A5

4200

44

2.1

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01

5610

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4851

2.3

0.3

558

3.9

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5320

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8305

7309

11

337

72

A6

5161

45

2.2

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14

1156

0.0

4862

2.8

0.3

609

9.5

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9.1

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0306

8313

26

364

204

A8

2853

39

2.0

0.0

02

3066

0.0

5379

2.4

0.3

935

3.6

0.0

5305

2.7

0.6

6338

8337

10

331

61

A10

1905

30

1.6

0.0

01

3518

0.0

5645

2.6

0.4

230

4.6

0.0

5434

3.8

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6354

9358

14

385

85

A14

3070

75

3.3

0.0

01

5839

0.0

4807

2.3

0.3

497

3.9

0.0

5276

3.2

0.5

9303

7304

10

318

72

A15

3576

85

3.8

0.0

01

6269

0.0

4842

2.2

0.3

553

3.8

0.0

5322

3.1

0.5

9305

7309

10

338

70

A16

6434

84

4.2

0.0

01

3136

0.0

4749

2.5

0.3

388

5.4

0.0

5174

4.8

0.4

6299

7296

14

274

109

A17

3446

75

3.4

0.0

01

1309

0.0

4748

3.4

0.3

383

7.0

0.0

5167

6.1

0.4

9299

10

296

18

271

141

A19

2184

71

3.1

0.0

01

4061

0.0

4760

2.8

0.3

544

4.3

0.0

5400

3.2

0.6

5300

8308

11

371

73

A20

1898

68

3.0

0.0

01

3621

0.0

4853

2.3

0.3

528

4.9

0.0

5272

4.3

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7305

7307

13

317

99

A21

1212

47

2.1

0.0

02

2321

0.0

4809

3.2

0.3

474

5.0

0.0

5239

3.8

0.6

4303

9303

13

303

87

B1

3881

48

2.3

0.0

01

5517

0.0

4868

2.2

0.3

550

4.9

0.0

5288

4.4

0.4

4306

6308

13

324

99

B2

8408

61

3.1

0.0

03

6751

0.0

4753

2.4

0.3

434

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0.0

5240

3.3

0.5

8299

7300

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303

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B3

2873

33

1.5

0.0

01

1199

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4692

2.3

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418

4.8

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7296

7299

13

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96

B4

4474

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984

4.8

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0267

6265

11

246

95

B6

8347

77

3.4

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3316

0.0

4828

2.1

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474

2.6

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1.6

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6303

7294

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B7

7883

76

3.2

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4259

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2.1

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321

2.8

0.0

5207

1.8

0.7

6292

6291

7289

41

B8

4516

26

1.2

0.0

02

5360

0.0

4832

2.1

0.3

591

4.9

0.0

5390

4.4

0.4

3304

6312

13

367

100

B9

6150

56

2.5

0.0

01

10719

0.0

4873

2.1

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564

2.9

0.0

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2.0

0.7

3307

6310

8331

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B10

4427

37

1.6

0.0

01

7398

0.0

4733

2.3

0.3

423

3.5

0.0

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2.7

0.6

4298

7299

9305

62

B12

6243

57

2.5

0.0

01

10380

0.0

4743

2.1

0.3

493

3.1

0.0

5342

2.2

0.6

9299

6304

8347

50

B13

5197

51

2.3

0.0

01

9562

0.0

4967

2.2

0.3

681

3.6

0.0

5375

2.9

0.6

0312

7318

10

360

65

B14

3536

34

1.6

0.0

02

6372

0.0

5118

2.0

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664

4.0

0.0

5192

3.5

0.5

0322

6317

11

282

80

B15

7415

71

3.1

0.0

01

13569

0.0

4788

2.1

0.3

389

3.1

0.0

5133

2.2

0.7

0301

6296

8256

51

B16

7551

56

2.6

0.0

02

10878

0.0

4870

2.6

0.3

502

4.7

0.0

5216

3.9

0.5

5307

8305

12

292

89

Dia

met

erof

lase

rsp

ots

wer

e60

(A1–A

21)

and

80mm

(B1–B16),

resp

ectivel

y;dep

thofab

lation

crat

er15–20mm

.a

Within

-run

bac

kgro

und-c

orrec

ted

mea

n207Pb

signal

.b

Uan

dPb

conte

ntan

dTh/U

ratio

wer

eca

lcula

ted

rela

tive

toG

J-1

refe

rence

(LA

-SF-ICP-M

Sval

ues

,G

erdes

,unpublish

ed).

cco

rrec

ted

for

bac

kgro

und,w

ithin

-run

Pb/U

frac

tionat

ion

and

com

mon

Pb

and

subse

quen

tly

norm

aliz

edto

GJ-

1(ID

-TIM

Sval

ue/

mea

sure

dval

ue)

;207Pb/2

35U

calc

ula

ted

using

207Pb/2

06Pb/(

238U

/206Pb6

1/1

37.8

8).

See

Ger

des

and

Zeh

(2006,2008)fo

rer

ror

pro

pag

atio

nan

ddet

ails

on

dat

apro

cess

ing.

dRho

isth

eer

ror

correl

atio

ndefi

ned

aser

r206Pb/2

38U

/err

207Pb/2

35U

.

1128

H. G. DILL ET AL.

TABLE 3. Rock-forming, ore and phosphate minerals recorded from the Silbergrube Aplite.

Mineral Formula

Albite NaAlSi3O8

Al-Strunzite ?Apatite-(CaF) Ca5(PO4)3FArsenopyrite FeAsSAutunite Ca(UO2)2(PO4)2·10�12(H2O)Beraunite Fe2+Fe3+

5 (PO4)4(OH)5·4(H2O)Biotite K(Mg,Fe2+)3AlSi3O10(OH,F)2Cacoxenite (Fe3+,Al)25(PO4)17O6(OH)12·75(H2O)Chalcopyrite CuFeS2

Childrenite Fe2+Al(PO4)(OH)2·(H2O)Chlorite Group’ (Fe,Mg,Al)6(Si,Al)4O10(OH)8Columbite Fe2+Nb2O6

Cryptomelane KMn4+6 Mn2+2O16

Earlshannonite (Mn,Fe2+)Fe3+2 (PO4)2(OH)2·4(H2O)

Eosphorite Mn2+Al(PO4)(OH)2·(H2O)Fairfieldite Ca2(Mn,Fe2+)(PO4)2·2(H2O)Ferrisicklerite Li(Fe3+,Mn2+)PO4

Ferristrunzite Fe3+Fe3+2 (PO4)2(OH)3·5(H2O)

Ferrostrunzite Fe2+Fe3+2 (PO4)2(OH)2·6(H2O)

Garnet Group’ pyrope-almandine-spessartiteHeterosite Fe3+(PO4)Hureaulite Mn5(PO3OH)2(PO4)2·4(H2O)Jahnsite CaMn2+Mg2Fe3+

2 (PO4)4(OH)2·8(H2O)Kastningite (Mn2+,Fe2+,Mg)Al2(PO4 )2(OH)2·8(H2O)Keckite Ca(Mn,Zn)2Fe3+

3 (PO4)4(OH)3·2(H2O)Kingsmountite (Ca,Mn2+)4(Fe2+,Mn2+)Al4(PO4)6(OH)4·12(H2O)Landesite (Mn,Mg)9Fe3+

3 (PO4)8(OH)3·9(H2O)Laueite Mn2+Fe3+

2 (PO4)2(OH)2·8(H2O)Leucophosphite KFe3+

2 (PO4)2(OH)·2(H2O)Limonite Fe3+O(OH)Ludlamite (Fe2+,Mg,Mn)3(PO4)2·4(H2O)Mangangordonite (Mn2+,Fe2+,Mg)Al2(PO4)2(OH)2·8(H2O)Mantienneite KMg2Al2Ti(PO4)4(OH)3·15(H2O)Marcasite FeS2

Meurigite KFe3+7 (PO4)5(OH)7·8(H2O)

Mitridatite Ca2Fe3+3 (PO4)3O2·3(H2O)

Muscovite KAl2(Si3Al)O10(OH,F)2Orthoclase K AlSi3O8

Paravauxite Fe2+Al2(PO4)2(OH)2·8(H2O)Phosphophyllite Zn2(Fe2+,Mn)(PO4)2·4(H2O)Phosphosiderite Fe3+PO4·2(H2O)Plagioclase (Na,Ca)(Si,Al)4O8

Pseudolaueite Mn2+Fe3+2 (PO4)2(OH)2·7�8(H2O)

Pyrite FeS2

Quartz SiO2

Rittmannite Mn2+Mn2+Fe2+Al2(OH)2(PO4)4·8(H2O)Robertsite Ca6Mn2+

9 (PO4)9O6·3(H2O)Rockbridgeite (Fe2+,Mn)Fe3+4(PO4)3(OH)5Rutile TiO2

Scorzalite (Fe2+,Mg)Al2(PO4)2(OH)2Sillimanite Al2SiO5

Sphalerite ZnSStewartite Mn2+Fe3+

2 (PO4)2(OH)2·8(H2O)Strengite Fe3+PO4·2(H2O)Strunzite Mn2+Fe3+

2 (PO4)2(OH)2·6(H2O)Torbernite Cu(UO2)2(PO4)2·8�12(H2O)Tourmaline Group dravite-schorlTriphylite LiFe2+PO4

Triploidite (Mn,Fe2+)2(PO4)(OH)Variscite AlPO4·2(H2O)Vivianite Fe2+

3 (PO4)2·8(H2O)Wavellite Al3(PO4)2(OH,F)3·5(H2O)Whitmoreite Fe2+Fe3+

2 (PO4)2(OH)2·4(H2O)Wolfeite Fe1.2Mn0.8(PO4)OHZwieselite (Fe2+,Mn)2(PO4)F

1130

H. G. DILL ET AL.

Childrenite seriesSlender prismatic crystals of white to yellowish

childrenite-dominated members of the chil-

drenite–eosphorite s.s.s., displaying the faces

{110}, grow into vugs and solution cavities

(Fig. 7a).

Paravauxite seriesKastningite (Silbergrube is the type locality for

kastningite), stewartite and paravauxite form

colourless crystals growing into vugs and

developing crystals flattened parallel {001}

(kastningite and stewartite), or elongated prisms

FIG. 6 (facing page and above). Phosphate minerals on a microscopic and macroscopic scale. (a) Anhedral

manganapatite within a matrix of albite. The white spot at the edge of apatite is Pb which was smeared into cracks on

polishing of the rock section (back scatter electron (BSE) image). (b) Anhedral masses of wolfeite (wo) which are

replaced from the rim and along fissures by Mn vivianite (vi) (thin section, plane polarized light). (c) Mn heterosite

(he) replaces Mn vivianite (vi) along the grain boundaries and in the interstices of the muscovite-quartz-albite

groundmass. Muscovite is replaced by variscite (va) and by vivianite (vi) forming some sort of slip fibres

(sub)parallel to the fissure wall. (d) Aggregates of acicular strunzite (st) growing into vugs alongside keckite (ke).

(e) Cruciform rockbridgeite (ro) associated with ferrostrunzite (fs) in stage IV reflecting partial oxidation of Fe2+ and

separation of Mn from Fe. (f) Beraunite (be) sprays developing together with phosphosiderite (ps) in solution cavities

of rockbridgeite (ro). (g) Meshing slender prisms of cacoxenite under the SEM-EDX. (h) Stellate aggregates of

cacoxenite scattered on Fe phosphate. (i) K feldspar (kf) bounded by a massive mineralization of frondelite-

dominated rockbridgeite–frondelite s.s.s. (fr) with relics of Mn apatite (ap) (BSE).

FE-MN PHOSPHATE APLITE, SILBERGRUBE, GERMANY

1131

FIG. 7. Phosphate minerals on a macroscopic scale and their crystal morphology using the common notation by

means of Miller indices. (a) Sprays of white crystals of an Fe-enriched member of the childrenite–eosphorite s.s.s. -

inset shows crystal morphology. (b) Flattened crystals of kastningite � inset shows crystal morphology. (c) Flattened

crystals of stewartite � inset shows crystal morphology. (d) Elongated crystals of paravauxite with the terminating

faces {011} and {001} corroded � inset shows crystal morphology. (e) Yellow stubby laueite crystals in vugs �inset shows crystal morphology. (f) Encrustations made up of botryoidal variscite.

1132

H. G. DILL ET AL.

with faces {100} and {110} (paravauxite) (Fig. 7

b,c,d). The terminating faces {011} and {001} are

strongly corroded and not well represented

(Fig. 7d). Laueite forms stubby crystals (Fig. 7e)

and also contributes together with strengite,

strunzite and keckite to the group of phosphates

infilling fissures and cracks (Fig. 8).

Phosphosiderite-strengite-variscite seriesPoorly hydrated Fe and Al phosphates occur in

three different textural types. They form

botryoidal nodular masses covering silicates and

Fe phosphates such as variscite (Figs 6c, 7d), grow

into vugs such as phosphosiderite (Fig. 9a) or

infill fissures such as strengite in the form of

botryoidal encrustation and stellate aggregates

(Fig. 8). Wavellite is also associated with variscite

in the most recent mineral assemblages of the SA.

Rockbridgeite-frondelite seriesThese Fe-Mn phosphates are the most common

phosphates in the SA (Table 1, Figs 6e,f, 8),

occurring as massive sub-economic phosphate

ore. They are seldom well-shaped, the crystals

closely intergrowing with each other. They do not

grow into cavities but are corroded by late-stage

solutions. Whitmoreite, although structurally

different from the rockbridgeite–frondelite

series, is frequently associated with these Fe2+-

Fe3+ phosphates.

HureauliteThe monoclinic Mn phosphate hureaulite was

observed only in vugs where it displays two

different crystal morphologies (Fig. 9b,c). Type I

has the faces {111} and {512} terminating the

prism while type II shows a simpler arrangement

of faces, with faces {512} reduced to almost zero

such that {001} becomes dominant. These

different types formed side-by-side on rock-

bridgeite. Notably, this mineral shows a colour

zonation from white, through pink to deep red

(oxi-hureaulite).

Calcium-bearing Fe phosphatesThis group forms a heterogeneous assortment

of phosphate minerals encompassing minerals of

the jahnsite-whiteite group and minerals such as

kingsmountite and mitridatite, the latter forming

parts of different structural groups but treated

together for their Ca contents. Excluding mitri-

datite, these Ca phosphates are commonly found

in open-space fills and fissure-wall linings. The

phosphate jahnsite, in this case with the suffix

(CaMnFe), contains a variation of different

elements, but is listed among the phosphate

minerals by Nriagu and Moore (1984). Keckite

was only determined by morphology and SEM-

EDX. The minute grain size did not yield reliable

XRD data. As none of the Ca-bearing phosphates

investigated from the SA contained Zn at any

FIG. 8. Apatite and aplitic groundmass (stage II/late magmatic) intersected by a vein filled with rockbridgeite I ,

strengite (stage IV/late hydrothermal breakdown of Fe phosphates accompanied by partial oxidation of Fe2+>> Fe3+).

The coating is made up of keckite, strunzite and crystals of rockbridgeite II.

FE-MN PHOSPHATE APLITE, SILBERGRUBE, GERMANY

1133

FIG. 9. Phosphate minerals on a microscopic and macroscopic scale. (a) Plates of phosphosiderite on rockbridgeite.

(b) Hureaulite type I on rockbridgeite with stubby crystals terminated by the faces {111} and {512}. (c) Hureaulite

type II with terminating face {001} becoming the dominant one. (d) Frondelite (fr) overgrown by leucophosphite

crystals (SEM-EDX). (e) Leucophosphite beside beraunite; inset shows crystal morphology. (f) Platy autunite

crystals in a quartz druse.

1134

H. G. DILL ET AL.

significant level, the grain is likely to be keckite

sensu stricto. This is not an unexpected result in

view of the chemical composition of the SA and

the small amounts of sphalerite present in the SA

relative to the neighbouring Hagendorf pegmatite

(Table 4). If Zn-enriched end-members such as

keckite are present, they certainly have not had a

great impact on the evolution of phosphates in the

SA.

LeucophosphiteThis is one of the late phosphate minerals

overgrowing minerals of the rockbridgeite–

frondelite s.s.s. in druses (Fig. 9d). It stands

out from the fibrous phosphates beraunite and

ferristrunzite with which it shares the solution

cavities in older Fe-Mn phosphates (Fig. 9e).

This phosphate warrants mention due to the

presence of the unusual cation K+, accommo-

dated in the structure of phosphates containing

Fe3+ as a major component. The conspicuous

crystal structure eases its identification

(Fig. 9e).

Aluminium and phosphate-bearing ‘leucoxene’Mantienneite was determined from the neigh-

bouring pegmatite Hagendorf South by Birch et

al. (1996) and is also listed on Mindat.org for the

SA. In stream sediments of the drainage system

intersecting the Hagendorf-Pleystein pegmatite

province, we found some grains rich in Ti, Al, Fe

and P. Electron probe analyses yielded a chemical

composition that pointed to a Ti-rich phase

(76 wt.% TiO2) with appreciable concentrations

of impuri t ies , e .g. 2.0�2.5 wt.% FeO,

6.1�8.5 wt.% Al2O3, 3.5�4.1 wt.% P2O5, and

subordinate amounts of V, Si and Ca. Totals in the

range 90�95 wt.% suggest considerable incor-

poration of H2O or the (OH)� complex. The phase

was considered a special type of ‘leucoxene’, i.e.

submicroscopic intergrowths of TiO2, Al-rich

phosphates and silicates (Dill et al., 2007b).

This Al-P ‘leucoxene’ is presumed to have

formed very late in the sequence.

Uranyl phosphate seriesYellow U ore minerals (‘gummites’, hydro-

silicates and phosphates) are ubiquitous in

mineral deposits of the Oberpfalzer Wald and

also outside the U mining districts (Dill, 1985a).

In the SA, autunite plates were identified in cracks

filled with quartz similar to what has been

described by Locock and Burns (2003) (Fig. 9f).

Another U yellow ore mineral is torbernite.

Discussion

The emplacement of the aplite: (Stage I)

According to the chemical analyses by Novak et

al. (1996), the SA is a mildly peraluminous

leucogranite with a slight dominance of Na over

K (Table 4). It is not strikingly different, in

respect of the chemical compositions, from aplitic

mobilizates found in the neighbourhood near

Pleystein (Table 4), but it is at variance with

respect to the structure and petrography. Rock-

forming minerals in the SA are randomly

oriented, whereas equivalent aplites from

Pleystein show an alignment of biotite and

tourmaline parallel to the foliation of the

enclosing country rocks (Fig. 4b). Lineation of

elongated mineral aggregates such as tourmaline

and quartz discs in this aplitic granite were not

produced by tectonic transport and deformation

because of the post-kinematic nature. Preferred

orientation of some minerals is interpreted as a

product of strain-related post-kinematic growth of

boron minerals near the contact with the

metamorphic country rocks. Where the strain

rate was lower, as in the SA , a random orientation

of minerals is the rule. The feldspar-quartz

assemblage in the SA and the Pleystein aplites

is a leucosome invading the metamorphic country

rocks in the waning stages of metamorphic and

structural overprinting of the psammopelitic

country rocks of the Moldanubian zone.

Not surprisingly, the ages of aplite-hosted

ferricolumbites from the SA (302.8S1.9 Ma) and

Pleystein (302.1S3.3 Ma) are almost identical

(Dill et al., 2007a). The Hagendorf ferricolum-

bites, albeit within the range of error, tend to be

younger (299.6S1.9 Ma).

Breiter and Siebel (1995) distinguished five

mappable units during their cross-border studies

of the Rozvadov Pluton, an igneous complex of

mainly Variscan granitoid rocks. The afore-

described aplitic rocks at Pleystein and SA were

described as slightly deformed leucocratic meta-

aplite/metapegmatite dykes containing garnet and

tourmaline. According to the authors, rare earth

element data suggest that meta-aplite/pegmatites

are the result of a batch partial melting process.

The primitive nature and non-fractionation in the

aplites under consideration is also indicated by the

composition of the ferricolumbites which are

rather homogenous.

The aplitic mobilizates of SA and near

Pleystein are typically single-stage fillings,

whereas the neighbouring Hagendorf stockso

FE-MN PHOSPHATE APLITE, SILBERGRUBE, GERMANY

1135

resulted from a multistage-emplacement (Uebel,

1980; Rose, 1981).

As indicated by the idiomorphic shape, garnet

formed in these aplitic mobilizates under static

conditions. Quartz, apatite and zircon, rarely

associated with garnet, are randomly oriented

and interpreted as a post-kinematic growth to

garnet in the felsic mobilizate. No direct contact

exists between garnet and other Fe-bearing

minerals available for any Fe and Mg partitioning.

Sirbescu et al. (2008) evaluated the crystallization

regime of a zoned pegmatite dyke and the degree

of magma undercooling. Based on their heating

experiments, the liquidus temperature of the

TABLE 4. Chemical composition of the Silbergrube Aplite compared with an aplitic mobilizate near Pleysteinshowing a preferred orientation of mafic constituents (Fig. 4b, c).

Location Pleystein Pleystein Silbergrube Aplite Silbergrube ApliteRock type Aplite with tourmaline

and garnetMarginal zoneof the aplite

(muscovite-albite granite) (muscovite-albite granite)

Reference this study this study Novak et al. (1996) Breiter and Siebel (1995)

SiO2 77.81 74.24 76.31 72.99TiO2 0.03 0.04 0.02 0.01Al2O3 13.01 13.85 12.57 14.92Fe2O3 0.76 1.04 0.55 0.50MnO 0.03 0.25 0.00 0.07MgO 0.17 0.11 0.12 0.01CaO 0.36 0.97 0.41 0.09Na2O 3.38 1.60 4.14 5.02K2O 2.90 4.40 3.71 3.71P2O5 0.60 1.52 0.20 0.43

Ba 28 31 <50 2Ce 23 30 n.d n.d.Co 3 3 10 n.d.Cr 4 4 <5 n.d.Cs <3 <3 n.d. 33Hf <6 <6 n.d. 2Mo <3 <3 <10 n.d.Nb 20 18 27 31Pb 20 21 17 <7Rb 84 157 982 1011Sc 6 87 n.d. n.d.Sn <4 <4 23 27Sr 93 163 47 2Ta 5 5 n.d. 25Th 3 3 n.d. 1U 3 23 n.d. 14Y <3 <3 <36 0V <5 <5 <20 n.d.Zn 44 19 28 26Zr 31 35 23 <7B 1348 90 n.d. n.d.Li 3 14 14 180Nb/Ta 4.29 3.60 n.d. 1.24

Data from SiO2 to P2O5 are given in wt.%Data from Ba to Li are given in ppmn.d. = not determined

1136

H. G. DILL ET AL.

pegmatite magma was ~720ºC. The magma

crystallized sequentially, starting with a thin

border zone which formed in <1 day at an

average temperature of ~480ºC, indicating

240ºC undercooling.

Where does all the P required to build up the

great variety of phosphates in the aplite come

from? Judging from the chemical compositions of

Table 1, K-feldspar is a more likely source of P

than albite during alteration. This agrees well with

the petrographic studies of Breiter and Siebel

(1995) and Fryda and Breiter (1995), who reported

elevated amounts of P in K-feldspar from nearby

Czech granites at Podlesi (0.83 wt.% P2O5) and at

Homolka (0.77 wt.% P2O5). There is a continuous

succession of P-bearing minerals from the igneous

stage through apatite into the variegated mineral

assemblage comprising Fe-Mn-Ca phosphates in

the aplites and pegmatites of the Hagendorf-

Pleystein province. Feldspars are deprived of P

during the younger hydrothermal alteration. The

variation in P in granitic rocks has been studied by

Broska et al. (2004) and in experiments by Tollari

et al. (2006). The influences of Fe content and

oxidation state on the saturation of phosphate

minerals in magmatic systems have been studied

by the latter authors in the temperature range from

1030�1070ºC and oxygen fugacity from 1.5 log

units below to 1.5 log units above the fayalite-

magnetite-quartz buffer.

The preponderance of P-enriched granites and

aplitic and pegmatitic derivates on both sides of

the Czech-German border in this part of the NE

Bavarian Crystalline Basement may be geneti-

cally related to phosphate-bearing metasediments

(metabiolites, metaphosphorites, phosphate-

bearing paragneisses) (Dill, 1985b). There may

be an increase in P on differentiation as shown by

geochemically specialized albite-zinnwaldite-

topaz granites (Krızovy kamen/Kreuzstein

granite) with indications of Sn-Nb-Ta mineraliza-

tion and associated phosphorus-rich pegmatite

cupolas (Breiter and Siebel, 1995). However, the

abnormally high values of phosphate in the aplitic

mobilizates and in the pegmatite cupolas are

convincing evidence that P specialization of the

geodynamic setting of this part of the

Moldanubian zone was crucial for the overall

presence of phosphate in these igneous rocks.

The SA is part of the root zone of the

Hagendorf pegmatite province, revealing mainly

post-kinematic growth of its major components

including P-rich phases and emplaced within a

stress-free to stress-poor geodynamic environ-

ment. The Hagendorf pegmatite fades out into an

aplitic root zone underneath the main pegmatite

body and aplitic bodies form the earliest felsic

mobilizates in the Hagendorf-Pleystein pegmatite

province (Dill et al., 2007b). This aplitic body

marks the transition of lenses and dykes of

synkinematic primitive tourmaline- and garnet-

bearing aplitic bodies, previously mapped and

denominated as metaaplites or orthogneisses by

Forster (1965), into post-kinematic fractionated

granites whose root zones are deduced so far only

from gravity maps (Casten et al., 1997). The

youngest and most fractionated felsic igneous

rock type is the Kreuzstein albite-zinnwaldite-

topaz granite which is abundant in Sn, Ta and Nb.

The marginal facies of the SA reflects the

reaction of phosphate-bearing solutions with Al-

rich rocks mainly from the surrounding country

rocks, the central facies describes the intra-aplitic

processes (Fig. 3).

The magmatic phosphate stages: II

There is little doubt that the mineral assemblage

with Mn-apatite and triphylite can be attributed to

the early magmatic processes accompanying the

emplacement of the SA (Fig. 3). Stage II of

phosphatization in the SA may be compared with

the initial stages recorded from phosphate

pegmatites elsewhere (Lahti, 1981; Fransolet et

al., 1983; Keller, 1991; Roda Robles et al., 1998)

and may also be correlated with the initial stages

described by Forster et al. (1967) from the

pegmatites exposed next to the SA in the

Hagendorf-Pegmatite Province. From a structural

point of view, stage II phosphate mineralization in

SA contrasts with Li phosphate concentrations at

Hagendorf South, where ~1000 t of Li were

mined from 1960 to 1972 (Walther and Dill,

1995). In Hagendorf South, Li phosphates were

concentrated in a stocklike body on top of the

quartz core, whereas the SA phosphate minerals

of stage II are disseminated along zones of

structural weakness with stage II phosphates

disappearing from these structures without any

Li concentrations of commercial interest (Figs 2,

3). The onset of sulphide mineralization correlates

with this stage. While the redox conditions for

these stages may easily be constrained by the

presence of divalent Fe and Mn accommodated in

the structure of the phosphates of the triphylite

and triploidite series, geoacidometer or mineral

associations, applicable as geobarometers or

geothermometers, are missing and only circum-

FE-MN PHOSPHATE APLITE, SILBERGRUBE, GERMANY

1137

stantial evidence can be provided to describe the

physicochemical conditions, particularly in terms

of the temperature of formation. The temperature

estimation follows the discussion in the preceding

section and is assumed to be <400ºC.

The early hydrothermal stage: III

Apart from the sulphide minerals mentioned

earlier, there are two mineralization events

which warrant individual treatment. Mn-vivianite

and ludlamite resulted from hydration of stage II

phosphates while reducing conditions were still

preserved. This Mn-bearing Fe phosphate miner-

alization opens up two different pathways of

mineralization (Fig. 3). One type of mineraliza-

tion is disseminated, as is the previous event, the

other is related to mineralized fault zones,

attesting to a change from ore impregnation

with little or no brittle deformation to open-

space filling along tectonic lines (Fig. 6c). To

account for the mineral transformation of

secondary phosphate, the Eh-pH stability fields

were calculated by means of the interactive

software programme ‘The Geochemist’s

Workbench’ from Rock Ware Inc. The thermo-

dynamic parameters were taken from the library

of the program. Basic chemical data to constrain

the physico-chemical regime are provided by

Nriagu (1972) and Wagman et al. (1971). The

system Fe2+�HPO42� is not controlled by temp-

erature changes under subcritical conditions.

Vivianite is stable above pH 4 and Eh <0.25 V.

The ore texture definitely points to a hypogene

rather than supergene mineralization. However,

based on available data, it is not possible to give a

more precise temperature of formation than

<300ºC. This temperature may also be applied

to the sulphide mineralization and that character-

ized by minerals of the childrenite–eosphorite

series which resulted from phosphate-bearing

solutions reacting with aluminium from the

surrounding metamorphic host rocks (Fig. 3 –

the marginal facies). The fluid system was

dominated by hydrogen phosphate compounds

and carbon dioxide or sulphur compounds have

neither played a significant part during stage III

nor in the wake of it. Had this been so, carbonate

minerals would have come into existence or the

stability fields of aluminum-phosphate-sulphate

compounds, e.g. crandallite-woodhouseite series

minerals, might have been entered (Stoffregen,

1993; Stoffregen and Alpers, 1987; Stoffregen et

al., 1994).

As a rare constituent, secondary brazilianite

was observed among the primary phosphates in

the Hagendorf pegmatite. Elsewhere in the NE-

Bavarian pegmatite province, crandallite was

identified, in places, in fault fissures (Strunz,

1974). It is believed to have been washed into

these cavities from above when these granites

were subject to supergene alteration � see later.

Alunite s.s.s. or woodhouseite s.s.s. are absent in

the zones of hydrothermal aplitic alteration under

study because this alteration took place under

conditions of strong S-limitation and reducing

conditions.

The late hydrothermal to early (epithermal) weathering: IV

Stage IV mineralization is transitional from

fissurebound to vuggy as far as the structural

inventory and morphology of mineralized sites

are concerned. It is composed of minerals

accommodating Fe2+, Fe3+ and Mn2+ in their

structure, thereby demonstrating small-scale fluc-

tuating oxygen fugacities in time and in space.

There is an overall tendancy of oxygen fugacity to

increase in the course of mineralization but with

many episodes of more reducing conditions as

demonstrated by the alteration of rockbridgeite

and strengite in various generations (Figs 3, 8).

There are four different sequences, (Fig. 3a�d)

the derivation of which can be explained by

different parent phosphates and/or reaction path-

ways. Ferrosicklerite in sequence A is derived

from the decomposition of triphylite and ends up

with precipitation of heterosite in the classical

sequence of alteration (‘Mason-Quensel

sequence’). Keller (1991) described heterosite–

purpurite s.s.s. and ferrosicklerite as a topotactic

alteration product of triphylite. Due to the paucity

in lithium, this sequence is less well represented

in the SA than in the neighbouring Hagendorf

pegmatite where Li reached ore grade. This is also

true for keckite, as far as its Zn analogue is

concerned (sequence B). While Hagendorf

displays a sequence of Zn phosphates evolving

from the massive Fe-enriched sphalerite, the poor

sphalerite mineralization in the SA (stage II-III)

was unable to create anything but a ‘starved’

jahnsite–whiteite sequence. Both jahnsite–

whiteite series minerals and mitridatite originated

from the decomposition of apatite and from the

small amounts of CaO released during decom-

position of alkali feldspar. Kingsmountite and

kastningite are representatives of the open-space

fillings of the Al-bearing phosphates already

1138

H. G. DILL ET AL.

discussed under stage III (sequence D). Iron

phosphates, with or without Mn form the bulk

of phosphate minerals in the SA and may be

related directly to Fe-Mn phosphates of stages II

and III with which they locally form grain-to-

grain contacts (sequence C).

The statement by Fransolet (2007) that, as

temperature decreases, the behaviour of Fe and

Mn tends to become more and more independent,

in that Mn enters into the structure of oxides or

hydroxides, whereas Fe, mainly Fe3+, is still

engaged in phosphate minerals, cannot fully be

supported by our observations and may simply be

explained by supergene processes. In stage IV, Fe

and Mn are partitioned into different minerals.

Soluble Mn has a larger stability field than Fe

under moderately reducing conditions. Mn2+ is

mobilized into the pore water, while Fe2+ remains

fixed in its compounds. Oxidation of Mn2+ to

Mn3+, which is only achieved under the condi-

tions of stage V, trails behind the conversion of

Fe2+ into Fe3+ due to different stability fields of

Mn and Fe compounds under different Eh-pH

regimes. In the pH range 5 to 8, a considerable

quantity of phosphates may be removed from

natural aquatic systems by amorphous Fe oxide

(Williams et al., 1970). Experimental results by

Warry and Kramer (1976) showed the

Fe oxyhydroxide-phosphate system produced

strengite and phosphosiderite under oxidizing

conditions and temperatures ~100ºC.

Based on thermochemical calculations, at log

aH2PO4� = �4, logaFe2+ = �4, and log aMn2+ = �4

and temperatures of >200ºC, the stability field of

Mn2+ phosphate is diminished so that the

formation of rockbridgeite is unlikely.

Therefore, temperature values of <200ºC are

more realistic in stage IV (sequence C). Based

upon this temperature interval and the above-

mentioned chemical composition, the physico-

chemical conditions can be modelled for stage IV.

The pH is assumed to be >6 for the Fe2+-Fe3+-

Mn2+ phosphate compounds with an Eh of

~0.25 V. An increase in the Mn2+ activity, a

slight increase in the Eh value to >0.25 V and a

slightly increasing alkalinity to pH>7 accompa-

nied by a decrease in the temperature of formation

expand the stability field of Mn compounds and

may account for the presence of Fe3+-Mn2+

phosphates such as strunzite and or laueite.

Iron phosphate, deprived of Fe, reflects a

reduction in the alkalinity to a pH of <7 while

other parameters may be kept constant. The

structural relationship between leucophosphite

[KFe3+2 (PO4)2(OH)·2(H2O)] and tinsleyite

[KAl2(PO4)2(OH)·2(H2O)] resembles that

between jarosite-(K) [KFe3+3 (SO4)2(OH)6] and

alunite [KAl3(SO4)2(OH)6], both typical of the

so-called high-sulphidation type or alunite-type

epithermal (shallow) ore deposits (Albinson et al.,

2001; Hedenquist et al., 2000). Epithermal means

low temperature and refers to a heat system or

hydrothermal system emplaced at depths of

<1 km in comparison to the deep-seated

mesothermal or orogenic vein-type deposits of

higher temperature.

A slight acidification of the mineralizing fluids

may cause the breakdown of K- and Al-bearing

silicates such as muscovite and decomposition of

pre-existing phosphates of stages I�III as it is the

case with sulphides and hydrogen sulphide

complexes within epithermal systems.

Oxi-hureaulite reflects the variability of redox

states expressed by its variation in colour. It has a

much wider stability field than the afore-described

Fe3+ phosphates and is stable under oxidizing to

reducing conditions of Eh> �0.25 V at temp-

erature <100ºC and a pH >6 (sequence D).

Phosphates, such as kastningite, accommo-

dating divalent Mn and Fe in their structure, can

exist only where the couple of Mn2+ and Fe2+ is

not dissociated under less oxidizing conditions

(Eh <0.25 V, pH >6). Those vuggy mineraliza-

tions with Fe2+ phosphates standing alone can

only plausibly be explained by reducing condi-

tions and a moderate reduction in alkalinity in the

range 5<pH<7. For a more intensive drop below

pH 5, no indications are found in this S-limited

environment.

In conclusion, stage IV mineralization shows

vuggy and stockwork-like textural features

formed from low-temperature mineralizing fluids

fluctuating within a rather narrow range of pH and

Eh. Iron and Mn phosphates, related to the stage

IV mineralization and the source of contained

elements, have to be sought in decomposed, early

phosphates and co-existing silicates. Such a

mineralogical and textural scenario can only

plausibly be accounted for by a shallow/near

surface hydrothermal system with a ‘telescoping’

of minerals similar to shallow epithermal ore

mineralization and prohibiting any depth-related

mineral zonation. Questions as to whether this

phosphate mineralization is of hydrothermal

origin or caused by supergene processes become

more or less academic in this S-deficient

epithermal Fe-Mn-phosphate mineralization

which occurs within a phreatic through vadose

FE-MN PHOSPHATE APLITE, SILBERGRUBE, GERMANY

1139

aquatic system. The time of formation cannot yet

precisely be defined. Mineralization began with

the uplift of the NE Bavarian basement at the end

of the Palaeozoic.

The crystal morphology of the most widespread

phosphates of stage IV is displayed in Figs 7 and

9, and addresses the issue of crystal habit and the

environment of deposition or, in other words,

variation in the growth rate of faces as a function

of changes in the chemical composition and

varying physical conditions.

The late weathering stage: V

Minerals of stage V, including phosphates,

limonite and kaolinite, are not exclusive to the

SA, with the exception of heterosite. Age dating

of U yellow ore was carried out in the

neighbouring Hagendorf Pegmatite (Dill et al.,

2007a). Torbernite, which is present in the SA,

albeit in minor quantities, was analysed for U, Th

and Pb isotopes by LA-ICP-MS techniques. Data

yielded an age of formation of 4.55S0.02 Ma,

which corresponds to Miocene-Pliocene weath-

ering and geomorphological processes in the

study area. Textural characteristics (encrustation

vugs) combined with data gathered during

regional geological studies rule out any high-

temperature alteration.

The physicochemical data published by

Vieillard et al. (1979) for kaolinite and Al

phosphates attest to the presence of acidic

meteoric fluids during alteration of stage V.

The size and shape of the stability field of

kaolinite is independent of changing redox

conditions and only controlled by the availability

of the Al(OH)2+ complex in the meteoric waters.

Its increase in the meteoric waters may shift the

position of the stability field of kaolinite down to

~pH 4. Around this pH, Al phosphate can coexist

with kaolinite only if log aHPO42� exceeds �3.

Raising the Eh to >0.4 mV causes Fe3+

phosphates to precipitate together with the

afore-mentioned phyllosilicate and Al phosphate.

Oxidizing conditions only exist in the upper part

of the weathering zone. Redox conditions in the

lower part, existing along fissures in the SA,

allowed for the development of Fe2+ phosphates

(‘blue iron earth’ = earthy vivianite). Vivianite,

stable <0.25 mV in this more recent alteration

zone, is not very common because Fe2+ phosphate

can come into existence only when the mobility of

Fe is very high or a rather unrealistic log aAl3+ =

�7 and a high log aHPO42� = �2 are assumed.

Synopsis and conclusions

The mildly peraluminous leucogranite of the SA

is an intrusive dyke stretching in a NW–SE

direction along with quartz dykes and aplitic

metamorphic mobilizates from which the SA

differs only in respect of a less intensive strain-

related mineral orientation. The aplitic meta-

morphic mobilizates and the SA are chemically

and mineralogically almost identical and yield the

same age of formation of ~302 Ma (stage I),

representing the root zone of the pegmatites and

aplites in this region. Formation of the Hagendorf

pegmatite (299 Ma), although within the range of

error, appears to post-date the emplacement of the

SA. We are in need of more precise age data to

confirm these assumptions.

The NW–SE striking boundary between the

fine-grained biotite granite and its metamorphic

country rocks was a zone of structural weakness

which favoured the formation of disseminated Li-

bearing Fe-Mn phosphates evolving from late

magmatic to high-T hydrothermal solutions

(stages II and III) and the brittle deformation

along steeply dipping tectonized zones conducive

to the faultbound Fe-Mn-Ca phosphates. Mineral

telescoping during stage IV, evident from Fe2+,

Fe3+ and Mn2+ phosphates in a texturally highly

variable host rock environment with fissures and

vugs, can best be accounted for by a near-surface/

shallow epithermal S-deficient phosphate system.

Epithermal alteration affected the entire SA and is

considered as linear, parallel to the principal

strike of the aplite.

Stage V mineral assemblages, which are not

exclusive to the SA, occur on a regional scale and

exclude some minerals such as heterosite whose

genetic position cannot yet be defined. This

mineral assemblage which formed under predo-

minantly oxidizing conditions is correlated with

the subtropical weathering during the Neogene on

an old peneplain truncating the SA and its country

rocks.

The very complex, late-stage phosphate miner-

alization in the SA is made up of Fe-Mn-Zn-Ca

phosphates intimately intergrown with each other

and indicative of contrasting physicochemical

conditions. The existing spider diagrams do not

plausibly explain why these various mineral

reactions take place. A common model applied

to shallow sulphide ore deposits (epithermal ore

deposits) is used to describe this phosphate-

bearing mineralization, transitional from hypogene

into supergene mineralizing processes at shallow

1140

H. G. DILL ET AL.

depth. Further investigations of this late-stage

mineralization in other phosphate-bearing pegma-

tites world-wide need to be carried out to test the

idea of epithermal phosphate mineralization.

Acknowledgements

We are indebted to J. Lodziak who conducted the

electron microprobe analyses. Chemical analyses

were carried out in the laboratory of BGR by F.

Korte. The preparation of samples and SEM

analyses were performed by I. Bitz and D. Klosa,

D. Weck carried out the XRD analyses. We also

thank W. Baeumier for providing some samples

from his collection for our investigations. We are

grateful to Karel Breiter and an anonymous

reviewer who have reviewed our paper for the

Mineralogical Magazine. We extend our gratitude

also to Mark D. Welch and Peter W. Scott for

their editorial handling of the manuscript.

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