Geological and Geochemical Characteristics of Permian Tourmalinization at Val Trompia (Southern Alps, Northern Italy) and Relationship with the Orobic Tourmalinites
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Per. Mineral. (1999), 68, 2, 185-212 An InternationalJournal of �·� PERIOD! CO di MINERALOGIA MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, established in 1930 ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, ArchaeometJ)' and Cultural Heritage Geological and geochemical characteristics of Permian tourmalinization at Val Trompia (southern Alps, northern Italy) and relationship with the Orobic tourmalinites * LUISA DE CAPITANI1, MARILENA MORONI1 and FRANCO RODEGHIER02 1 Dipartimen to diSc ienza della Terra, Un iversita diMil an o, Via Botticelli 23, I- 201 33 Milan o, Italy 2 Dipartimen to diGeorisorse e Territorio, Politecn ico diTori n o, Corso Duca degli Ab ruzzi 24 , I- 10129 Torin o, Italy Submitted December I998- Accepted May I999 ABSTRACT. - Tourmaline breccias in the pre as having formed by magmatic-hydrothermal fluids Alpine basement in Val Trompia (Southern Alps, that produced metasomatic tourmalines with Northern Italy) are related to late Palaeozoic granites different compositions at different distances from and are associated with Sn-W -bearing ore deposits. the fluid source (magma). Compositional analogies The breccias are rich in acicular tourmaline with between tourmalinites of the two areas may variable textural and compositional characteristics. therefore reflect similarities between the At least four stages of tourmaline crystallization are tourmalinization processes that developed in relation distinguished, mainly by Mg/Fe ratios. All tourmali to large-scale Permian magmatic-tectonic activity. nes can be classified as «alkali group» varieties The Val Trompia tourmalinites thus may represent using recent nomenclature schemes. Compositions deeper metasomatic products, closer to the magma vary from schorl-dravite for earlier, main-stage type chamber, whereas the Orobic tourmalinites would I and II coarse crystals, to highly Fe-enriched tour have formed from hydrothermal fluids injected malines, probably approaching the povondraite along shallower faults during coeval volcanic endmember (typical of oxidized hydrothermal envi activity. ronments), for type Ill and IV fine-grained tourmali nes within late veinlets. Whole-rock REE geochemi RIASSUNTO. - Brecce a tormalina affioranti nel cal data show variable, locally remarkable enrich basamento sudalpino in Val Trompia, in relazione ments in LREE related to the action of hydrothermal con lineamenti e granitoidi tardo-paleozoici ed asso fluids. Coarse tourmalines occurring in the crypto ciate a mineralizzazioni a Sn e W, sono caratterizzate crystalline groundmass of the Orobic tourmalinites, da tormaline con caratteri tessiturali e composizionali which are hosted in cataclastic zones along the tecto variabili. Tormaline appartenenti ad almeno quattro nic contact between basement and Permian volcano stadi genetici sono riconoscibili principalmente dalla sedimentary cover rocks in the Lake Diavolo area variazione del rapporto Mg/Fe. Tutte le tormaline (southern Alps), have schorl-dravite compositions sono definibili come «alkali group tourmalines», in similar to type I and II, main-stage Val Trompia accordo con la piu recente classificazione. La compo tourmalines. The Orobic tourmalinites were conside sizione varia da termini della serie schorlite-dravite, red of metasomatic origin by previous studies. per i cristalli riferibili agli stadi di cristallizzazione The Val Trompia tourmalinites can be interpreted piu antichi (tipo I e Il), fino a quelli estremamente arricchiti in Fe affini al termine povondraitico (tipico * Correspon dingauthor, di ambiente idrotermale ossidato), per tormaline E- ma il: ma rilena@r 10 . terra. un imi.it entro vene tardive (tipo Ill e IV). Dati relativi alle 186 L. DE CAPITANI, M. MORON! an d F.RoDEGHIERO terre rare su roccia totale mostrano arricchimenti geochemical characteristics of tourmaline-rich variabili, talora notevoli di terre rare leggere, proba breccia rocks recently found in Val Trompia bilmente legati all' azione di fluidi idrotermali. Sono (central southern Alps, northern Italy). There, state analizzate anche le tormaline di maggiori Alpine compressive tectonics only partly dimensioni nelle tormaliniti criptocristalline affioran ti entro fasce cataclasitiche al contatto tra basamento concealed late Palaeozoic geological and e copertura vulcano-sedimentaria permiana nella structural elements belonging to the post zona del Lago del Diavolo (Alpi Orobie), gia prece Variscan rifting, which greatly influenced the dentemente definite di origine metasomatica. Queste geology and metallogeny of the entire southern tormaline ricadono nel campo composizionale della Alpine domain in northern Italy. V al Trompia serie schorlite-dravite e sono analoghe a queUe trum tourmalinites show textural complexities and pline di tipo I e II, la cui formazione e attribuita spatial relationships to late Palaeozoic all' evento principale di tormalinizzazione, fortemente influenzato dal chimismo della roccia originaria. Le lineaments, magmatic rocks, and ore deposits. tormaliniti dell a V al Trompia so no interpretabili For these reasons they have been studied in come originate da fluidi magmatico-idrotermali che, detail in order to evaluate their compositional a distanze differenti dalla sorgente, per reazione variability and gain insights into their genesis. metasomatica hanno prodotto tormaline di differente These tourmalinites are further compared with composizione. Analogie composizionali tra le torma tourmaline-rich rocks of metasomatic origin liniti delle due aree sembrano quindi riflettere una hosted by Permian palaeo-faults (Zhang et al., somiglianza tra i processi di tormalinizzazione avve nuti su larga scala e legati all' attivita magmato-tetto 1994; Slack et al., 1996) and those geologically nica permiana. In questo ambito le tormaliniti della related to the Permian boron-bearing U Val Trompia potrebbero rappresentare i prodotti mineralization at N ovazza and V al V edello in metasomatici di maggior profondita, in prossimita the central Orobic Alps (Fuchs and Maury, della camera magmatica, mentre quelle orobiche 1995). An attempt is also made to construct a rappresenterebbero il prodotto legato ai fluidi, convo model relating the Val Trompia and Orobic gliati lungo sistemi di fratture, che alimentavano il tourmalinizations to the late Palaeozoic vulcanismo superficiale. geologic and metallogenic framework of the KEY WORDS: Southern Alps, Permian, tourmaline central southern Alps. chemistry, tounnalinite, boron metasomatism. REGIONAL GEOLOGY INTRODUCTION The areas of Val Trompia and of Lake Tourmalines and tourmaline-rich rocks occur Diavolo-Lake Publino (indicated below as in direct and indirect association with a vmiety Lake Diavolo in short) are located in Lombardy of geologic environments and ore deposits of (Northern Italy) and belong to the central sector diverse age, as summarized extensively in the of the South Alpine domain (fig.l). recent volume on boron edited by the The Val Trompia area (labelled 1 in fig. 1; Mineralogical Society of America (Grew and fig. 2) includes the southernmost exposure of Anovitz, 1996). the pre-Alpine crystalline basement: it consists Due to its complex chemical nature, of a small crystalline massif 3-4 km wide that tourmaline is known to acquire a specific crops out for about 15 km in a ENE-WSW compositional signature according to different direction. The Lake Diavolo area (labelled 2 in conditions of crystallization. Therefore fig. 1; fig. 4) is at the southern margin of the tourmaline represents a valuable tool for central part of the wide Orobic Massif (Orobic evaluating the geochemistry of a mineralizing Alps), exposed with continuity over a distance environment and may be an important of more than 100 km in a E-W direction from pathfinder for ore deposits. This work the Insubric Line southwards. investigates for the first time the geological and The main structure of the region reflects the Geological and geochemical characteristics ofPermian tourmalinization 187 0 10 20 30km ---=====-- 0 Bergamo South-AlpineDomain Alpine Domain CD crystallinebasement Va/Trompia AdamelloPluton cover sequence ® (late Alpine) Lake Diavolo Fig. 1 - Simplified regional geological sketch map of the cen tral sector of the South Alpin e domain showin g relative distribution of Va riscan ba semen t an d post-Variscan cover, main tecton ic lin eamen ts, an d the two stud y areas of Val Trompia (a rea 1) an d La ke Diavolo-La ke Publin o (a rea 2) . Labels: P: La ke Pescegallo; VV: Val Ved ello; N: Novazza; M: Man iva Pass-Val di Va ia . complex tectonic relationships between the pre post-Variscan cover sequence occurred during Alpine, Variscan basement and post-Variscan, the Alpine orogeny. However, during upper upper Palaeozoic-Lower Triassic volcano Palaeozoic time several of these lineaments sedimentary cover sequence. were already active as transtensional faults. Major tectonic lineaments show two main They controlled development of the so-called structural trends: E-W ( «Orobic» trend) and «Collio Basin» s.l., a major E-W -trending NNE-SSW («Giudicarie» trend). The Insubric, volcano-tectonic strike-slip and pull-apart basin the Orobic, and the V al Trompia lineaments system, consisting of a series of E-W -trending belong to the first trend. The second trend is asymmetric graben basins (Orobic Basin, named after the Giudicarie Lineament, which Boario Basin, Collio Basin s.s., etc.) separated connects the «Orobic»-type Val Trompia by basement horsts ( Cassinis, 1985; Cassinis