Hirtopanu, and three co-authors, reported in 2013 on the Ditrau alkaline intrusive complex (DAIC) situated within the crystalline rocks of the inner part of the East Carpathians. It has a distinct ring structure and a succession of magmatic events from gabbroic and dioritic magma to syenitic and various post-magmatic events as well, developed between a Triassic extensional stage and a Jurassic rifting stage. The DAIC has a high petrographic complexity and a great diversity of mineral occurrences. More than 200 new minerals were recently identified, many of them being first occurrences in Romania.
The rare element minerals which were determined in DAIC belong to the following classes, in the predominant order:
I. LREE(Y)-carbonates;
II. Nb, Ta, REE(Y),Ti, Zr, Th, U-oxides
III. REE(Y)-Phosphates;
IV. REE-(Y), Th, Zr, Pb-Silicates.
Among silicates, the following minerals occur in DAIC, specially in Jolotca area: allanite-(Ce), thorite, thorogummite, cerite-(Ce), chevkinite-(Ce), törnebohmite-(Ce), stillwellite-(Ce), tritomite-(Ce), britholite-(Ce) and Th-zircon.
Allanite-(Ce) occurs as a constituent mineral in mineralizing Jolotca area and as an accessory one in the rest of the massif. The chemical composition of allanite-(Ce) has high Fe2O3 content, showing oxidizing conditions for its genesis. The effect of changing redox conditions could be one possible cause of the neoformation of allanite-(Ce) on the other REE silicates.
The allanite-(Ce) is quite commonly attacked by fluids and transformed into REE-fluorcarbonates minerals, most often into bastnasite-(Ce). The opposite allanitization process of REEcarbonates is widely spread in DAIC and it is illustrated in many photos. Also, the allanite-(Ce) can form on monazite-(Ce) and this transformation process is reversible. It presents many generations, having specific the continuous decreasing of REE contents from the old to the new members, shown by their microprobe analyses. Oscillatory zoning of allanite-(Ce) seen only in backscattered electron images, would result from changes in temperature, pressure and REE-concentrations in fluid, during its crystallization. The textural relations of the REE-silicates, mineralogical and chemical evidence demonstrate that hydrothermal processes were responsible for the REE mineralization at DAIC.
The authors concluded that the rare elements minerals determined in DAIC, belong to the following 4 mineral classes:
REE- Carbonates, Nb,Ta,
REE–Oxides,
REE Phosphates and
REE-Silicates.
The allanite-(Ce) belongs to silicates class among other, like chevkinite-(Ce), törnebohmite-(Ce), cerite-(Ce), britholite-(Ce), stillwellite-(Ce) (?), tritomite-(Ce), thorite, thorogummite and zircon. It occurs as mineral constituent of the ore, showing big prismatic crystals and many generations.
The chemical composition shows high Fe2O3 content and a diminution of REE content from old to new allanite-(Ce) generations. The high Fe2O3 content indicates an oxidizing conditions of allanite-(Ce) formation. The allanite-(Ce) is associated closely with REE-carbonates, REE-phosphates and with other REE-silicates present in the DAIC. Also, the intergrowth of allanite-(Ce) with sulphides, specially with sphalerite and pyrite, is very common, being linked by a metallogenetical process. The textural relations between diverse REE-minerals, showed that the-allanite-(Ce) has many generations, being substituted/ neoformed by/on many REE minerals. Replacement of earlier REE minerals by later REE minerals is common in DAIC such as the replacement of monazite by bastnasite+apatite and reaction of bastnasite and apatite to form monazite and fluorite. Also, the neoformation of allanite-(Ce) on monazite and bastnasite-(Ce) occurs frequently.
The substitutions of REE minerals belonging to the same class took place in DAIC. The bastnasite-parisite transformation is very likely to result from variations of either Ca2+ and CO2 2- activities in the fluids. The substitutions among the REE-silicates were described: allanite-(Ce) substitutes törnebohmite-(Ce), chevkinite-(Ce) and cerite- (Ce), by decreased REE-content, because some fluctuations in the availability of SiO2 and in REE-bearing fluids. Allanite-(Ce) is formed under conditions with higher SiO2 activity and higher concentrations of Al, Ca and Fe than that the conditions favorable for the crystallization of the other REE-silicates shown above. Some changes in pH, oxidizing conditions and Temperature have probably occurred, and in the REE concentrations, as well.
The DAIC display extensive evidence of hydrothermal alteration, (natrolitisation, albitisation etc.), which is widespread in those parts of the massiff that is most enriched in HFSE-, high fieldstrength elements (Zr, Nb, U, Y, REE) and in halogens, especially fluorine. The REE are transported mainly as fluoride complexes in hydrothermal fluids derived from alkaline/ peralkaline magmas. This is supported by the fact that the principal REE ore mineral is bastnasite- (Ce), and the fluorite is an important gangue mineral in DAIC. The sulphate and carbonate ligands may play an important role in REE transport and their deposition, as carbonate and sulphate REE complexations.
For many years, the REE had been considered to be immobile in hydrothermal fluids. However, now is has become increasingly apparent that the REE can be concentrated to ore grades via hydrothermal processes. It is known, that the alkaline igneous rocks arguably contain the highest concentrations of HFSE of any igneous rock type, and intrusions of such rocks commonly host exploitable resources of these elements.Now, the DAIC hosts Ti, Nb, Ta, Th, U, Y, Zr, and REE mineralization, being a site of a currently subeconomic, albeit geologically interesting types of mineralization.
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