©Geol. Bundesanstalt, Wien; download unter www.geologie.ac.at

Verh. Geol. B-A.

Jahrgang 1978

Heft3S. 279-283

Wien, Dezember 1979

Proceed. 3 rd ISMIDA (Leoben, Oct. 7-10, 1977)

S. 105-109

Wien, Dezember 1979

Structural Control of Mineralization in the Apuan Alps
(Tuscany, Italy)*)
By L. CARMIGNANI, G. DESSAU andG.

DUCHI**)

With 1 figure

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Apuaner Alpen
Erzmineralisation
Tektonik
BariteDeckenbau
Geochronologie

Abstract
The strongly uplifted and deeply eroded massiv of the Apuan Alps allows deep insight into theoverall nappe structure of the Northern Apennines. Its present setting is the result of several tectonic
phases. The lowest unit, the so-called "Autochthon", contains several polymetallic and barite deposits,
which originated during the first tectonic phase and were later metamorphosed.

1. The geological backgroundT h e " A p u a n A l p s " , a mountain massif roughly ellipsoidal in shape, stretches 30 k m in a
N N W - S S E direction along the Tyrrhenian Sea. T h e y constitute a tectonic w i n d o w where
the most complete succession of the tectonic units of the Apennines is exposed. Besides
one metamorphic rock-type of outstanding economic value, - the " C a r r a r a m a r b l e " - theyalso contain small deposits of various ores and barite.
W e k n o w from classical literature that the marble quarries were flourishing at least since
R o m a n times. M o s t probably, the mines of iron, lead and silver go back to the same, if n o t
to greater depths of antiquity, but historical records are available only since the Middle
Ages. T h e exploitation of the ores terminated a few years ago, and only barite continues tobe mined today (apart from the quarrying of marble).
The structurally lowest unit is a sequence characterized b y syntectonic greenschist facies
metamorphism. It ranges in age from at least Silurian to Oligocene. We maintain for it thetraditional name of " A u t o c h t h o n " , and w e distinguish a basement of Paleozoic phyllites,
porphyroids presently considered to be Permian, and quartz pebble conglomerates andquartzites ("Verrucano") of Lower Triassic age. T h e basement is overlain by a cover be­
ginning with U p p e r Triassic dolomitic limestones ( " G r e z z o n i " ) and b y the Liassic marble.
This is followed b y a mainly carbonatic sequence and b y an Oligocene flysch("Pseudomacigno").
*) Research carried out with grants from the Italian Council of Scientific Research.
**) Author's addresses: Institute of Geology and Paleontology, University of Pisa, 56100 Pisa, Via
S. Maria 53.
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279

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The "Autochthon" is structurally overlain by the "Tuscan Nappe", a sequence similar tothe Mesozoic and Tertiary portion of the "Autochthon", but it is not metamorphosed andit begins with Triassic evaporites. Interposed between the two units is the "Massa Unit", aset of tectonic slices, which might have been the original basement of the Tuscan Nappe.
The Tuscan Nappe is in turn overlain by eugeosynclinal sequences, the "Ligurian Nappes", which as a result of erosion are presently exposed only on the periphery of the massif.
The actual setting of the Apuan Alps, as schematically shown in its tectonic evolution asin Fig. 1 (showing also the emplacement of the mineral deposits), is the result of three
Oligo-Miocene compressive phases, followed by a Plio-Quaternary distensive phase.
During the first compressive phase - by far the most important one - the "Autochthon"
was metamorphosed, overthrusted by the Tuscan and Ligurian Nappes, and subsequently
altered into large-scale isoclinal recumbent folds. Most of the crustal shortening across themassif occurred during this phase, which according to radiometric investigations by KLINGFIELD, HUNZIKER & SHAMEL (1977) is dated at 26 m. y. The second phase generated folds
with axes at right angles to the prevailing NNW-SSE trend of the massif, whereas the thirdphase affected the whole sequence with its large folds trending again in the NNW-SSE direction, - having westward vergences on its western and eastward ones on its eastern flank.
The result was that the whole massif assumed the shape of a large conjugate box fold. The
radiometric age of the third phase is 11-14 m. y.
The uplift of the fault-bounded massiv is related to the late, Plio-quaternary distensive
phase.
The mineralizations are related to the first tectonic phase, which had generated isoclinal
recumbent folds. These were accompanied by a penetrative axial plane schistosity (Si),which often completely transposed the original bedding, resulting in L-S tectonites.
The greatest first-phase folds sometimes have lower limbs extending 2-3 km. Thesestructures are often accompanied by ductile faults parallel to the axial plane of the folds,
which at a late stage can develop into true overthrusts. As we shall see, these late tectonic
surfaces are the structural guides for the most important mineralizations.
On the Si-surfaces an extension lineation is well developed, which trends NE-SW in thewhole massif. On the other hand the hinge lines of the first major and minor folds do not
maintain a constant attitude, but form variable angles with the extension lineation.
On the western slope of the metamorphosed Apuan Alps the first phase axes trend
NW-SE, and therefore form an angle of 90° with the extension lineations. Advancing
further to the East these axes gradually rotate, until they grow parallel to the extensionlineation (CARMIGNANI & GIGLIA, 1977). The progressive change towards parallelism between the direction of maximum extension and the first phase fold axes is well known inthe case of considerable stretching due to simple shear (SANDERSON, 1973; ESCHER & WATTERSON, 1974).
Summing up, the structural investigations of CARMIGNANI, GIGLIA & KLIGFIELD (1977)

show that the Apuan metamorphic complex exhibits flat-lying schistosities, strongly developed mineral extension lineations, rotated fold axes, L-S tectonites, which, according toESCHER & WATTERSON (1974) and MATTAUER (1975), are all characteristic of mobile belts.
The latter are gently dipping, deep-rooted shear zones, found in orogenic belts associatedwith great overthrusts.
The Apuan mobile belt originated on the western margin of the Italo-Adriatic continental crust, as a result of the collision with the Sardinian-Corsican continental fragment. The
accompanying shears might have supplied the channelways for the mineralizing fluids,
perhaps coming from a syntectonic acid magma. These fluids deposited the epigenetic mineral deposits, all of which belong to the first tectonic phase.
280

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©Geol. Bundesanstalt, Wien; download unter www.geologie.ac.at

2. The Mineral Deposits
We shall only briefly mention the insignificant ores of sedimentary origin within the"Autochthon" of the northern part of the massif:
near Vagli; quartz stringers with some chalcopyrite, probably originated through thegeochemical concentration of Cu contents of the Cretaceous-Eocene sediments (nowmetamorphosed into sericitic slates), and near Scörtico; a seam of manganese silicates andcarbonates of a metamorphic paragenesis, interstratified within the same formation.
The epigenetic, mineral paragenesis, of any present or past importance include:
1. Polymetallic veins of galena, sphalerite and lead-antimony sulphosalts in chiefly quartzose gangue, situated in the basement of the "Autochthon".
2. Deposits of barite, pyrite and iron oxides, with traces of sulphides and sulphosalts,
located mainly in metasomatic bodies in the dolomitic limestones of the Upper Trias, butalso in veins in the basement of the "Autochthon" and in the phyllites of the "Massa Unit".
3. Small quartz veins with cinnabar in the phyllites of the "Autochthon" and of the"Massa Unit".
These deposits are all clustered in the southern part of the massif, around Monte Lietoand Monte Ornato, where epigenetic tourmalinization is widespread near the contact zone
between the "Autochthon" and "Massa Unit". For their setting within the stratigraphic
and tectonic framework, we refer again to Fig. 1.
Not fitting within this scheme is the paragenesis:
4. The frigido vein farther north, a rather undisturbed fissure vein cutting through thephyllites of the "Massa Unit". Its main ore is spatic siderite, - with some calcopyrite containing cubanite and sphalerite starlets, some pyrite with pentlandite, and traces of several
other sulphosalts, such as fahlerz, ullmannite, vaesite, etc.
The only important example of paragenesis (1) is the vein known as the "Bottino vein",which strikes NW-SE for several km through the Paleozoic phyllites, more or less conformably with their schistosity (Si). It is folded and tectonized by the later compressive
phases. Its main ores are galena, extremely Fe-rich sphalerite, chalcopyrite, pyrrhotite, andin addition to tetrahedrite, a whole array of silver-rich lead-antimony-sulphosalts (as wellknown from metamorphic deposits). Quartz is the dominant gangue-type, often studded
with microscopic tourmaline needles, and Sn is present in traces. The brittle gangues arestrongly cataclastic and kneaded with the ores, which also show evidence of the enduredstresses. In vugs, however, the iron-rich sphalerite and all other ores and gangues arefound in excellent, totally undisturbed condition, therefore testifying to post-tectonic crystallization. The vein also shows zoning vertically and along its strike. In the vicinity, unimportant outcrops within the marble are more copper-rich, containing Hg-bearing tetrahedrite and much fluorite as well.
We have grouped as "Paragenesis (2)" the deposits of barite, pyrite and iron oxides,
which appear, however, in two completely different settings, - namely in fissure veins
within the phyllites, and in replacement lodes in the dolomitic limestones (and sometimes
in marbles). They occur along tectonic or stratigraphic contacts between the carbonatic andthe phyllitic rocks, always within the "Autochthon". At Valdicastello barite veins cut ore
veins of Paragenesis (1), and therefore are somewhat younger, although belonging to thesame tectonic phase.
The most important deposit, belonging to the second type and presently under exploitation, is the one of Monte Arsiccio. It has already produced around one million tons of barite, as well as minor quantities of pyrite and iron oxides. It shows vertical zoning, withpyrite towards the phyllites at the bottom, and iron oxides (we presume derived from sid282(108)©Geol. Bundesanstalt, Wien; download unter www.geologie.ac.aterite) towards the upper contact with the carbonatic rocks. F o r a tentative physico-chemical explanation of these characteristics w e refer to o u r forthcoming paper (CARMIGNANI, D E SSAU & D U C H I , 1976). Notwithstanding t h e different settings, there are similarities between
the barite veins and t h e lodes, - for instance traces of the same sulphides and sulphosalts,
and the fairly uniform strontium contents (see later); the sulphosalts point also towards
Paragenesis (1). T h e veins might have been the feeders of the lodes. Evidence against asedimentary origin of t h e lodes is their emplacement frequently along tectonic contacts,
and replacing rocks of different ages. A further example of the M o n t e Arsiccio-type deposit is the smaller one of Buca della Vena near Stazzema.
According t o BARBIERI, MASI & TOLOMEO (in print), and t o the kind verbal information ofBARBIERI, t h e strontium content (7 analyses for the three deposits) is fairly uniform, near
5000 p p m , and might have been extracted b y a convective circulation from the generally
Sr-bearing Tuscan evaporitic rocks. Although the same explanantion is tentatively advanced
also for the barium contents, t h e latter w o u l d rather point towards t h e Ba-contents of thefeldspars of a magmatic rock.
Paragenesis (3) is represented by t w o practically monomineralic vein deposits; of cinnabar in quartz gangue, at Levigliani within the Paleozoic of the " A u t o c h t h o n " , and at
Ripa within phyllites of t h e "Massa U n i t " , n o w attributed t o the Trias. T h e Ripa veinshows the same significant feature of the Bottino vein - vugs with undisturbed, perfect ore
crytals within a surely tectonized vein.
As schematically s h o w n in Fig. 1, the more important mineral deposits are within the" A u t o c h t h o n " , in the proximity of the overthrust of the "Massa U n i t " . Smaller deposits
are instead contained in slices ("Parautochthonous slices of Stazzema") torn from the A u tochthon and transported t o t h e north-east b y t h e Tuscan N a p p e . T h e latter deposits are
therefore u p r o o t e d , w i t h o u t continuity in depth. O n t h e other hand the " A u t o c h t h o n "might deserve further investigation where it is hidden below the "Massa U n i t " .
T h e age of the A p u a n deposits is of the order of 26 m . y . , whereas the far more important ore deposits of Southern Tuscany are connected with post-tectonic granitic intrusions
aged 7 m. y . or less. T h e t w o groups of deposits belong therefore to different metallogenic
cycles.
References
The present "abridged version" has space for a few principal references only, quoted below. In
these, and in the "full version", detailed bibliographies may be found.
BARBIERI, M., MASI, U. & TOLOMEO, L.: Significance of strontium for the origin of the epithermal
barite deposits from the Apuan Alps (north-western Tuscany, Central Italy) — In press, 1978.
CARMIGNANI, L., DESSAU, G. & DUCHI, G.: I giacimenti minerari delle Alpi Apuane e loro correlazioni con l'evoluzione del gruppo montuoso. - M e m . Soc. Geol. It., 11, 417-431, Roma 1972.
CARMIGNANI, L., DESSAU, G. & DUCHI, G.: Una mineralizzazione sintettonica: il giacimento di Val-dicastello (Alpi Apuane). - Boll. Soc. Geol. It., 95, Roma 1976.
CARMIGNANI, L. & GIGLIA, G.: Analisi mesostrutturale della zona occidentale delle Apuane metamorfiche. - Boll. Soc. Geol. It., 96. Roma.
ESCHER, A. & WATTERSON, J.: Stretching fabrics, folds, and crustal shortening. - Tectonophysics. 22,
2 2 3 - 2 3 1 , 1974.
KLIGFIELD, R., HUNZIKER, J. C. & SCHRAMEL S.: K/Ar ages of multiply deformed metasedimentaryrocks from the Alpi Apuane, Northern Apennines, and their tectonic implications (Abstract). Paper read at the " E C O G V - Fifth European Colloquium of Geochronology, Cosmochronology
and Isotope Geology", Pisa. 5-10, Sept. 1977.
MATTAUER, M.: Sur le mecanisme de formation de la schistosite dans l'Himalaia. - Earth and planetary science letters, 28, 144-154, 1975.
SANDERSON, D. J.: The development of fold axes oblique to the regional trend. - Tectonophysics, 16,55-70, 1973.
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Zeitschrift/Journal: Verhandlungen der Geologischen Bundesanstalt
Jahr/Year: 1978
Band/Volume: 1978
Autor(en)/Author(s): Carmignani L., Dessau Gabor, Duchi G.
Artikel/Article: Structural Control of Mineralization in the Apuan Alps (Tuscany,Italy) 279-283