B Lugovic - Mn-crust todorokite mineralization on sw backshore cretaceous limestones from the island of dugi otok - страница 1

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ISSN: 0001-5113

ACTA ADRIAT.,

UDC:552.54 (210.5)(210.7

AADRAY

49(1): 53 - 63, 2008

Dugi Otok)

Mn-crust todorokite mineralization on SW backshore Cretaceous limestones from the island of Dugi Otok (Central

Adriatic, Croatia)

Bosko LUGOVIC 1, Branimir SEGVIC 1 * and Tanja SEGVIC 2

1Faculty of Mining, Geology and Petrol Engineering, University of Zagreb,

Pierottijeva 6, HR-10000 Zagreb, Croatia

2Institute of Oceanography and Fisheries, P.O.Box 500, HR-21000 Split, Croatia

* Corresponding author, e-mail: bsegvic@rgn.hr

The southwest shore of the island of Dugi Otok is characterized by the unusual sample occurrence of Mn-hydrated oxide mineralization in the form of botryoidally and globular, fine-laminated concentric aggregates, crusting the Upper Cretaceous backshore limestones. The mineralization consists mainly of Fe impoverished 10-A tecto-manganate todorokite (enriched in Mg and transition metals) accompanied by accessory MnOOH. Todorokite chemical composition is typical of a hydrogenous origin in an oxic shallow marine environment. According to our model, Mn has been recurrently leached and mobilized from the Late Pleistocene sea floor sediments located around 50 nautical miles south of Dugi Otok. We propose a link between the Mn-geochemical anomaly of these sediments and the alteration of distinct, spatial close pre-Holocene tephra, dispersed over the entire recent Adriatic area.

Key words: 10-A manganates, todorokite, hydrogenous origin, pre-Holocene tephra, Dugi Otok island, Adriatic

INTRODUCTION

The island of Dugi Otok is a part of the Adriatic carbonate platform and belongs to the Dinaridic orogenic mountain chain. The coastal outline of the orogenic topography was ultimately reshaped by extensive sea level rise during Holocene deglaciation (Fig. 1). The island consists entirely of gastropods fosiliferous Upper Cretaceous limestone and minor dolomite (MAMUZIC et al., 1967) showing various karstification morphology phenomena (DZAJA, 2003). The Mn-mineralization presented by the predominant todorokite and accessory

MnOOH occurs as fine-laminated films, crusts or small Mn-cretes onto the bedding planes of the host carbonate rocks whereby spatially adjacent goethite mineralization (SEGVIC et al., 2008) may be also coated by Mn-crusts (Fig. 2). Lithological and geochemical characteristics of Dugi Otok surface rocks preclude them as a potential source of Mn for mineralization.

Mn-(Fe) hydrated oxides are widespread and common marine precipitates. They occur in the oceans as nodules, micro-concretions, coatings and crusts (CRERAR & BARNES, 1974) representing the key to understanding marine geochemical cycles, submarine hydrothermal processes, location of oxygen minimum zones, etc. (e.g. DYMOND et al., 1984; HALBACH, 1986; ABOUCHAMI & GOLDSTEIN, 1995). They may be either hydrogenous precipitates from hydrothermal solutions and ambient sea-water or formed by diagenetic processes showing different respective chemical compositions and distinctive mineralogy (HEIN et al., 1992). In the Adriatic the presence of Mn is ubiquitous, especially in the north Adriatic sea-bottom mainly due to the inputs of the river Po (TANKERE et al., 2000). Ferromanganese coated structures are reported in the Jabuka pit, central Adriatic (DOLENEC, 2003) and Mn-anomalies were detected in the central Adriatic depression (DOLENEC et al., 1998).

15 16 17

Longitude E

Fig. 1. Map of the Adriatic Sea and the surrounding mainland showing the location of Mn-mineralization on the island of Dugi Otok and the position of the related Mn-anomaly in the middle Adriatic Depression. The relevant sites of Neapolitan Yellow Tuff dispersed from the Campi Flegrei caldera near Naples are indicated as PALIKLAS (CALANCHI et al. 1998), KET (PATERNE et al. 1988), KORCULA and GLJEV

Fig. 2. Photography of Mn-crete onto the bedding plane of host limestone. Note the sharp crevice infilled by genetically unrelated goethite partly coated by Mn-crete (pen bottom)

In this work we present, for the first time, evidence of relatively abundant 10-A Mn min­eralization predominantly confined to the SW backshore zone of the island of Dugi Otok (Fig. 1). The aim of this research is to characterize mineralogical and geochemical characteristics of the Mn-mineralization and determine the ori­gin and source of Mn.

MATERIAL AND METHODS

Manganese crusts and films were manually separated from the host carbonate rocks. The remaining carbonates were dissolved with pH 4.5 buffer solution of CH3COONa (NaAc/HAc). Powdered samples were analyzed by X-ray diffraction (XRD) at the Faculty of Science, University of Zagreb using a Philips diffractom-eter 1820 and CuKa radiation with a graphitic monochromator (V=40 kV, I=35 mA). The samples were scanned at a rate 2 °/min over the range of 4-63° 20. The diffraction patterns were interpreted using the files from the International Centre for Diffraction Data (JCPDS-1996). The phases were identified as follows: todorokite-JCPDF card number 00-013-0164; MnO-OH-JCPDF card numbers 01-089-2354, 01-088­0648; quartz-JCPDF card number 01-086-1630;

illite-JCPDF card number 00-002-0056.

The chemical composition of todorokite was measured by SEM EDS and EPMA at the Mineralogical Institute of Ruprecht - Karls Uni­versity of Heidelberg using the CAMECA SX51 electron microprobe equipped with five wave­length-dispersive spectrometers. The operating parameters were 15 kV accelerating voltage, 20 nA beam current, ~1 [am beam size and 10 s counting time for all elements. Natural minerals, oxides (corundum, spinel, hematite and rutile) and silicates (albite, orthoclase, anorthite and wollastonite) were used for calibration. Raw data for all analyses were corrected for matrix

effects with the PAP algorithm (POUCHOU &

PICHOIR, 1984; POUCHOU & PICHOIR, 1985) imple­mented by CAMECA. Formula calculations were done using a software package designed by Hans-Peter Meyer (Mineralogical Institute of Ruprecht-Karls University of Heidelberg, Germany).

The SEM imagery of the Mn crust was performed at the Geology Department of the Faculty of Science, University of Zagreb using TESCAN instrument equipped with Oxford Instruments INCA analyzing system.

RESULTS

Shape and size

The black to earthy colored Mn crust is 5 jim to 300 (jm thick, forming aggregates of mostly tecto-manganates. The aggregates show botryoidal, dendritic and globular concentric textures and comprise predominantly of todorokite, accompanied by MnO-OH and very minor quartz and illite. Scanning electron microscope (SEM) imagery depicts manifold surface morphology comprising of aggregates of spherulites, plates and disks as may be clearly observed under different magnification (Fig. 3a, 3b, 3c, 3d).

XRD mineralogy

Todorokite is identified as the principal mineral phase by the strongest diagnostic peaks of 9.8150 A and 4,8350 A corresponding to the reflection pattern of 10-A manganates. A complete data pattern along with the reference data on the todorokite from the Todoroki mine in Japan is reported in Table 1. Other determined phases are MnO-OH, identified by peaks of

Fig. 3. SEM imagery of Mn-mineralization showing manifold todorokite morphologies at four different magnifications. The highest magnification depicts mostly aggregated spherulites and minor plates and disks (3d)

4.2136/4.1996 A, 2.6972/2.6875 A and 2.8080/ 2.8175 A, minor quartz and illite.

EPMA mineralogy and chemistry

Quantitative EPMA data for todorokite chemistry and calculated structural formulae are presented in Table 2. Mn is arbitrarily assumed to be Mn4+. Besides Mn, the most abundant is Al (0.147-1.067 c.p.f.u.). A significant abundance of Mg (0.440-0.881 c.p.f.u.) suggests isomorphic substitution in the M2 position at the edges of the triple MnO6 octahedra chain framework (BURNS & BURNS, 1978). Alkali elements (Na, K), alkali-earth elements (Ca, Ba), Ce, and transition metals (Cu, Co, Ni and Zn) enter the cell to balance the charge and stabilize the manganate structure (e.g. BURNS & BURNS, 1978; LECLAIRE &

PERSEIL, 1978). The abundances of large cations K and Ba are highly positively correlated with Mn content (Figs. 4a and 4b) or show a weak

0.5 -

0.3

0.1

и-1-1-r

©

Table 1. d-spacing (d/A) and intensity       for the reference todorokite and todorokite from the Mn-crusts of Dugi

Otok

40

42 44 Mn

46

Todoroki/Japan

Sample BZ-3

(00-13-164)

todorokite

d

I

dobs Iobs

9.68

100

9.82 100

7.15

2

7.38 42

4.80

80

4.84 65

3.22

15

3.23 28

2.46

20

2.44 56

2.22

20

2.21 7

1.75

10

1.72 26

 

In parentheses JCPDF card number.

correlation trend for Ca and Na (not shown)

whilst the abundance of small cations Al and Mg

(Figs 4c and 4d) as well as

Si (not shown) are

negatively correlated. ->

 

 

• •

1          1 1 •

2

Я

m

 

 

1

•     • •

-

 

і               і               і і

(S)

1                       1 1

и

40 42

Mn

44 46

~i-1-1-1-1-1-г

_l_I_I_1_

_l_I_l_

40

42 44 Мл

46

Fig. 4. Plot of K, Ba, Al and Mg versus Mn (all elements in wt%) for todorokite from Dugi Otok Mn-mineralization

Table 2. Selected microprobe chemical analyses (in wt%) of todorokite from the Dugi Otok mineralization and calculated number of cations per formular unit (c.p.f.u.)

Analysis No. bo-3

bo-7

bo-8

bo-11

bo-12

bo-18

bo-21

bo-24

bo-25

Na2O

0.75

0.95

1.05

0.95

1.06

0.97

1.36

1.32

1.14

Ce2O3

0.41

0.98

0.72

0.73

1.01

0.17

0.67

0.66

0.71

BaO

1.18

3.03

2.30

2.30

3.02

0.50

2.07

1.80

2.13

CoO

0.12

0.05

0.07

0.09

0.08

2.99

1.97

1.52

0.07

NiO

1.02

0.20

0.50

0.49

0.27

2.76

1.24

1.82

0.62

CuO

0.21

0.25

0.24

0.23

0.23

0.34

0.17

0.23

0.32

MgO

5.40

2.86

4.37

3.30

2.83

6.18

3.91

4.72

3.60

Al2O3

6.37

1.19

2.69

3.25

2.15

8.58

1.92

4.64

2.82

SiO2

0.18

0.04

0.19

0.08

0.05

0.14

0.03

0.08

0.08

K2O

0.27

0.58

0.59

0.47

0.55

0.19

0.65

0.38

0.49

CaO

6.12

5.01

3.29

7.33

3.77

2.23

2.24

2.72

7.82

MnO2

65.53

72.76

73.29

67.06

72.55

58.20

70.07

66.73

64.13

FeO

0.07

0.00

0.00

0.17

0.00

0.01

0.00

0.00

0.00

TiO2

0.00

0.01

0.00

0.04

0.00

0.01

0.00

0.02

0.00

ZnO

0.15

0.01

0.07

0.10

0.07

0.25

0.09

0.12

0.14

Totala (wt%)

87.97

89.46

89.60

86.71

87.83

83.72

86.55

86.93

84.16

Number of c.p.f.u. on the basis of 12 oxygens and all Mn

as Mn4+ and all Fe

as divalentb

Na+

0.147

0.193

0.206

0.194

0.214

0.198

0.279

0.266

0.241

Ce3+

0.015

0.038

0.027

0.028

0.039

0.006

0.026

0.025

0.028

Ba2+

0.005

0.012

0.009

0.010

0.012

0.002

0.009

0.007

0.009

Co2+

0.010

0.004

0.006

0.007

0.007

0.253

0.167

0.127

0.006

Ni2+

0.083

0.017

0.040

0.041

0.023

0.234

0.106

0.152

0.054

Cu2+

0.017

0.020

0.018

0.018

0.018

0.027

0.013

0.018

0.026

Mg2+

0.811

0.445

0.657

0.518

0.440

0.972

0.617

0.732

0.584

Al3+

0.756

0.147

0.319

0.404

0.264

1.067

0.240

0.569

0.361

Si4+

0.018

0.004

0.019

0.008

0.005

0.014

0.003

0.005

0.009

K+

0.017

0.038

0.038

0.031

0.037

0.013

0.044

0.025

0.030

Ca2+

0.661

0.561

0.355

0.827

0.422

0.252

0.254

0.302

0.911

Mn4+

4.560

5.258

5.105

4.884

5.232

4.243

5.120

4.793

4.823

Fe2+

0.006

0.000

0.000

0.015

0.000

0.001

0.000

0.000

0.000

Ti4+

0.002

0.001

0.000

0.003

0.000

0.001

0.000

0.002

0.000

Zn2+

0.011

0.000

0.005

0.008

0.006

0.019

0.007

0.009

0.011

Totalc

7.115

6.739

6.805

6.996

6.718

7.304

6.884

7.033

7.094

a Chlorine (0.14-0.23 wt%) is included in the totals. The deficiency from 100 wt% corresponds to 3.4-4.0 H2O p.f.u (approx. 9 wt% H2O).

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B Lugovic - Mn-crust todorokite mineralization on sw backshore cretaceous limestones from the island of dugi otok