3 publications
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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.
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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.
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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).
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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.
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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)
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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.
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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).
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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.
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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 ( ).
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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.
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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).
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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.
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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.
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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).
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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.
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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.
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Franco Bonavia
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Franco Bonavia
[16]
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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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