3 Publications

7/31/2019 3 Publications

1/23

Publications

Franco Bonavia

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Minniti, M., & F. ,1984. Copper-ore grade hydrothermal mineralization

discovered in a seamount in the Tyrrhenian sea (Mediterranean): is the

mineralization related to porphyry-copper or to base metal lodes? Marine

Geology, 59, 271-282.

Minniti, M., F. , C. Dacquino, & G. Raspa, 1984. Distribution of Mn, Fe,

Ni, Co, Cu in young sediments on the Palinuro seamount in the southeast

Tyrrhenian sea, Marine Mining, 5(3), 277-305.

, F., & W.H. MacLean, 1986. Geology and ore enrichment factors at

Radiore mine, Quebec, Mineralium Deposita, 21(2), 137-146.

Beraki, W.H., F. , T. Getachew, R. Schmerold, & T. Tarekegn, 1989. The

Adola fold and thrust belt in Ethiopia: a re-examination with implications for

Pan-African evolution, Geological Magazine, 126(6), 647-657.

Boccaletti, M., A. Getaneh, & F. , 1991. The Marda fault: a remnant of

an incipient aborted rift in the Paleo-African Arabian Plate, J.Petroleum

Geology, 14(1), 79-92.

Tolessa, S., F. , M. Solomon, A. Haile-Meskel & E. Teferra, 1991.Structural pattern of the Pan-African around Moyale, southern Ethiopia,

Precambrian Research, 52, 179-186.

Balemwal, A., & F. , 1991. Precambrian rock structure and late

Pleistocene strikeslip tectonics around Mega (south Ethiopia), Journal of

African Earth Sciences, 13, (3/4), 169-172.

Morten, L., A.M. De Francesco, F. , G. Haileselassie, G.M. Bargossi, & M.

Bondi, 1992. A mantle xenolith locality from Sidamo region, southernEthiopia, Mineralogical Magazine, 56, 422-425.

, F., & J. Chorowicz, 1992. Northward expulsion of the PanAfrican of

NE Africa guided by a re-entrant zone in the Tanzania craton, Geology, 20,

1023-1026.

, F., V. Diella, & A. Ferrario, 1993. Precambrian podiform chromitites

from Kenticha Hill, southern Ethiopia, Economic Geology, 88, 198-202.


2/23

Publications

Franco Bonavia

, F., & J. Chorowicz, 1993. Neoproterozoic structures in the

Mozambique Orogenic belt of southern Ethiopia, Precambrian Research, 62,

307-322.

J. Chorowicz, B. Collet, F. , & T. Korme, 1994. NW to NNWextensiondirection in the Ethiopian rift deduced from the orientation of large

extensional structures and fault-slip analysis, Geological Society American

Bulletin, 106, 1560-1570.

, F., J. Chorowicz, & B. Collet, 1995. Have wet and dry Precambrian

crust controlled Cenozoic Trap volcanism in Arabia and East Africa?

Geophysical Research Letters, 22, 2337-2340.

MacLean, F. , & G. Sanna, 1997. Argillite debris converted to bauxitein Sardinia, Italy, Mineralium Deposita, 32, 607-616.

T. Korme, J. Chorowicz, B. Collet, & F. ,1997.Volcanic vents rooted on

extension fractures and their geodynamic implications in the Ethiopian Rift,J.

of Volc. & Geoth.l Res., 79, 205-222.

J. Chorowicz, B. Collet, F. , P. Mohr, J.F. Parrot, & T. Korme, 1998. The

Tana Basin, Ethiopia: intra-plateau uplift, rifting, subsidence, Tectonophys.,

295, 351-367.

J. Chorowicz, B. Collet, F. , & T. Korme, 1999. Left-lateral strike-slip

tectonics and gravity induced individualisation of continental blocks in

western Afar, Eclogae Geologicae Helveticae, 92, 149-158.

B. Collet, H. Taud, J.F. Parrot, F. , & J. Chorowicz, 2000. A new

kinematic approach of the Danakil block by means of DEM representation,

Tectonophys., 316, 343-357.


3/23

Publications

Franco Bonavia

[1]

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Massive sulfides have been discovered

dredging the Palinuro seamount in the

Tyrrhenian sea, off the Calabrian coast,

in Italy (map, Google Earth).

Mineralization was discovered made of

indurated sulfide crust, mixed with dark

ooze of organic and volcanic origin, and

reddish layers produced by oxidation. A

dust-like precipitate was observed close

to the mineralization by a submersible.No edifice resembling the black-smokers

were identified.

Microprobe analyses have revealed the

presence of bismuthinite, stibnite, rutile,

pyrite, barite and tennantite-tetrahedrite

crystals. Well-formed quartz, twig-shaped

native copper & sphalerite crystals were

identified by other means. Fe-smectite,

aragonite, calcite, feldspar and quartz

minerals were found in ooze.

Tennantite-tetrahedrite crystal mounting

bisbuthinite crystal, both surrounded by

needle-shaped stibnite crystals (photo

Sentimenti & Tangerini , CERIVE).


4/23

Publications

Franco Bonavia

[2]

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An offshore exploration campaign with the aim of identifying and evaluating the Fe-Mn bearing

encrustations and micronodules discovered on the Palinuro seamount provided the means for

constructing Mn, Fe, Ni, Co, Cu geochemical maps and interpret a complex metal history. Zoning

supports the idea that encrustations and micronodules are of hydrothermal origin. The presence

of ferromanganese minerals associated with low redox facies formed during the remobilization of

previously deposited Mn mineral phases, thus diagenetic processes also played a significant role in

the final deposition.


5/23

Publications

Franco Bonavia

[3]

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A cut section of the ore deposit was mapped in detail (left figure) before being mined out in1980.

The stratiform ore is interpreted a distal-type VMS lens, having a well-developed sedimentary

structures, a chlorite-biotite alteration zone, and a significant tuff component in the rhyolite unit.

Assuming the seawater-derived brines leached metals from the underlying rocks, precipitating

them onto the seafloor, the metal enrichment factor was calculated. Enrichment factors (not to be

confused with partitioning coefficients) are by-and-large controlled by (1) the efficiency of

circulating brines in dissolving metals, and (2) efficiency of the precipitating mechanism at exit

point. The geothermal system was mostly efficient in concentrating Cu, Cd, Zn, Au and Ag. There

were difficulties in both solubility and precipitation of Bi, Sn, As, Co, Mo, whereas W, Cr, V, Mn,

and Ni did not concentrate in the ore (right figure)


6/23

Publications

Franco Bonavia

[4]

A north-south trending Neoproterozoic belt (AFTB), made of volcanic-sedimentary and ophiolite-like rocks, overly gneiss and granitic gneiss, was found to be gold-bearing at Lega Dembi, Sakaro,

Digati, and that tectonic structures controlled the distribution of Au-quartz veins.

Detailed structural (Fig. A, left) and microfabrics (Fig. A, right) analysis show two thrusting events

(D1, D5) and three folding phases (F2,F3,F4). The degree of metamorphism during deformational

cycles reached the highest greenschist to medium amphibolites facies, and retrograded minerals

formed near thrusts post-dating metamorphic peaks (Fig.B). Accepting an age of 103040Ma of

the AFTB, the belt is postulated to be an immature island arc .

:: regional structural trends and sample localities. : quartz c-axis of Fe-Mn quartzites from

different localities, labeled 1 to 5. Stippled: pole-free areas. Synoptic diagram for fabric skeletons is

labeled 6: principles axes of finite strain x>y>z; stretching lineation lstr and foliation Sm; sense of

vorticity indicated by curved arrow.


7/23

Publications

Franco Bonavia

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Figure ( ): P-T history deduced from

sample collected in five different areas,

labeled a to d. Stability field of staurolite(broken line).

Mineral reactions (solid lines):

: chlorite+muscovite+qtz = cordierite+

biotite+Al2SiO5+ H2O;

: muscovite+qtz = k feldspar+ Al2SiO5 ;

: partial anatexis of granitic material.

andalusite (And), kyanite

(Ky) and sillimanite (Sill): solid line.

represent P-T conditions

of dominant assemblages (inferred).


8/23

Publications

Franco Bonavia

[5]

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The MFZ is the main structural element

in the Horn of Africa. Gravity anomalies

suggest it to be a fracture extending

from the Red Sea through Ethiopia and

Somalia (Ogaden region) in the Indian

Ocean. The MFZ is marked by the Wadi

Sheebele river [see Landsat TM (Google

Earth from 3016 km altitude) + SRTM +

JAXA composite image, left figure].

Although evidence of its continuationbeyond the Afar Depression remains

speculative, the pre-drift restoration of

Africa and Arabia shows a remarkable

alignment with the Red Sea, supporting

the interpretation that the MFZ is a

deep-rooted structure.


9/23

Publications

Franco Bonavia

[6]

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The regional structure shows

north-trending, upright, doubly

plunging, tight to isoclinals folds.

Later events produced large scalefolds with NE-SW axial trace in

the central-western regions and

N-S to NW-SE shear orientations

( ).

NW-SE shears activated thrusting,

and the structure of the region is

interpreted in plan and in section

as a flower structure ( ).


10/23

Publications

Franco Bonavia

[7]

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Distinctive morphotectonic features, assumed to have been the result of strike-slip motion, are

observed all along the escarpment which in southern Ethiopia delimits the Kenyan plain. The

fault pattern is discussed.


11/23


12/23

Publications

Franco Bonavia

[9]

[PUBLICATION 10]

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In southern Ethiopia and northern Kenya, higher graderocks of the Mozambique Orogenic Belt (MOB) and lower

grade Arabian-Nubian Shield (ANS) are juxtaposed.

The presence of steeply dipping asymmetric ductile shear

zones continue vertically at depth or flatten into low-lying

thrusts as overriding or detachment zones, and the large

volume of crust, deformed at 15-20 km depth, in the core

zone, are interpreted to represent roots of the northward

expulsion of the ANS from the MOB (note todays structure

at erosion level).


13/23

Publications

Franco Bonavia

[10]

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Contrasting interpretations are given by different authors on the nature of ultramafic bodies

throughout the Mozambique Orogetic Belt and the Arabian Nubian Schield. In the Adola belt,

at Kenticha Hill, in southern Ethiopia, serpentinatized peridotite host chromitites. A few were

analyzed by fire-assay neutron activation for PGE. Those formed by fractionation and cumulus

process differ from podiform chromitites. Chondrite-normalized platinum-group element ratios

for Kenticha Hills indicate they share features with the podiform clan.


14/23

Publications

Franco Bonavia

[11]

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The paper describes the arrangement

of the ophiolites mapped in the field

or interpreted from Landsat MSS .

In Fig. a, the hook-shaped ophiolite

(2), bounded south by gneissic (4) or

schistose rocks (1), contrast with the

northward linearity of the belt (Fig.b).

The internal fold-style (close, upright,

slightly overturned, doubly plunging

folds conform with regional structure.

At the southernmost end of the belt,

fold orientation changes from NE to

NW. This change in fold orientation is

marked by curved faults.

No evidence of large (hundreds of

kilometer) displacement along shear

zones was observed. Shear zones

appear to have developed as large-

scale thrusts having the geometry offront and lateral ramps.


15/23


16/23

Publications

Franco Bonavia

[13]

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Cenozoic volcanism from the Arabian peninsula as far as East Africa does not fit the model of a

unique plume impinged beneath the Afar. It is postulated that the alkali Afro-Arabian province

formed because controlled by compositional differences of the lihospheric crust passing over a

plume. Thermal perturbations beneath juvenile crust, wet and fusible, caused melting leading

to foundering of dense material and the injections of lighter material solidified before reaching

the surface both in Arabia and Nubia, (C: wet crust ); whereas the same process beneath crust

dominated by amphibolite-granulites did not produce immediate melting and lavas extruded

well before magma underplating commenced in East Africa (C: dry crust).


17/23

Publications

Franco Bonavia

[14]

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The Olmedo bauxite deposit is located in NW Sardinia, in Italy. The bauxite profile constitutes a

basal limestone overlying desiccated marl and argillite, and passing upwards through bauxite

argillite, argillaceous bauxite and compact red and white bauxite (left figure). The bauxite

argillite is an identifiable unit where quartz has disappeared, illite is largely depleted, kaolinite

enhanced, and bohemite (rare gibbsite) mineral makes its first appearance. This unit grades into

red (hematite-rich) and then to white bauxite as kaolinite, hematite decreases and bohemite

becomes the dominant mineral. Oolites and pisolites are common. Oolites are small in the

white bauxite, and form a micro-oolitic texture which grades to homogeneous compact bauxite

as hematite is leached and the oolitic texture is masked.

Plot of TiO2 (wt%)vs Zr (ppm) shows for each sampled unit that many elements are residually

concentrated at constant TiO2/Zr, proof that they were relatively immobile during the bauxite-forming process and derived from an homogeneous source.


18/23

Publications

Franco Bonavia

[15]

Most extension fractures in the Ethiopian rift are related to rift dynamics and have consistently

the same orientation for hundreds of kilometers. Clear relationships between tectonics and

volcanism were obtained from SPOT images by mapping extension structures (fissures, open

fractures or cracks), elongated vents and linear volcanic clusters.

NW- to NNW-oriented local extension directions (hidden lines) obtained from extension

fractures in (a) the central segment of the Main Ethiopian Rift ( ) and in the Awassa area ( ).

( ) Local extension directions deduced from directly observed extension fractures

have a star at the center of hidden lines, the inferred ones instead have a small dot at thecenter.


19/23

Publications

Franco Bonavia

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20/23

Publications

Franco Bonavia

[16]


21/23

Publications

Franco Bonavia

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In the Ethiopian plateau, the Tana lake

is perched on topographic high. The

lake, located at the center of three

converging grabens (Gondar, Debre

Tabor, Dengel Ber) is impressed into

mid-Tertiary flood basalt pile.

Satellite image observation, DEM, and

fault-slip analyses of paleostress tensor

1, 2, , calculated using the right-

dihedral method (Angelier & Mechler),

and the orientation of slikensides and

grooving indicate the lake is a region

of subsidence.

[Landsat-TM (Google Earth from

242,58 km altitude) + SRTM + JAXA

composite image].


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3 Publications

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