Maffeis, A.
1*, Ferrando, S.
1, Castelli, D.
1& Frezzotti, M.L.
21Department of Earth Sciences, Università degli Studi di Torino, Italy; 2Department of Earth and Environmental Sciences, Università degli Studi di Milano-Bicocca, Italy; *andrea.maffeis@unito.it
The direct study of the fluids that circulated during active subduction of continental crust at sub-arc depth is fundamental to understand the Deep Carbon Cycle (Kelemen and Manning, 2015). The investigation of metamorphic and fluid evolution of UHP impure marbles is promising to follow the fate of carbonates during deep subduction.
In the UHP Brossasco-Isasca Unit (BIU) of the Dora-Maira Massif, there are some hectometre-scale lenses of impure marbles that experienced peak metamorphic conditions at ~4.0 GPa and
~730 °C (Castelli et al., 2007). Some marbles preserve evidence for multiple events of dissolution-precipitation of Dol (all mineral abbreviations after Whitney and Evans, 2010) occurred during UHP prograde to early-retrograde evolution in presence of a complex, solute-rich aqueous fluid (Ferrando et al., 2017).
An impure marble, consisting in the simple peak mineral assemblage Cc (ex Arg) + Dol + Di + Ol, has been selected to asses both the composition and evolution of the fluid phase during exhumation.
A detailed petrographic investigation allowed distinguishing five main groups of synchronous inclusions within zoned Di:
• Group I: large (5-25 µm) primary bi-phase multisolid aqueous inclusions (liquid + different kinds of solids) within the UHP core of porphyroblastic Di; (Fig. 1);
• Group II: small (<1-8 µm) primary, within the mantle of porphyroblastic Di, bi-phase aqueous inclusions containing liquid and vapour (L+V) [L/(L+V)~80-90%];
• Group III: small (<1-6 µm) secondary intragranular (grain internal), bi-phase aqueous inclusions containing liquid and vapour (L+V) [L/(L+V)~60-80%]
• Group IV: small (<1-8 µm) secondary (intragranular, grain boundary → grain boundary) tri-phase aqueous inclusions containing a liquid, a vapour bubble and a cubic solid (probably a salt; L+V+S);
• Group V: variable sized (<1-24 µm) secondary (intragranular: grain boundary → interior in Di bigger than 200-240 µm; grain boundary → grain boundary in Di smaller than 200-240 µm) bi-phase aqueous inclusions containing liquid and vapour (L+V) and characterised by a dominant vapour bubble [L/(L+V) ~30-40%].
Regarding the multiphase aqueous inclusions, micro-Raman analyses allowed to recognise the presence of Mg-Cc + Tlc +Tr ± Dol ± H2Oliq ± a non-Raman active phase. Based on results from optical
microscopy, the latter phase can be a cubic daughter mineral (i.e. a salt). SEM-EDS analysis on open inclusions, revealed appreciable Cl content in Tr and Tlc (Ferrando et al., 2017). This finding points to a solute-rich aqueous COH fluid, containing Ca, Mg, Si, CO32- and Cl. This solute-rich fluid may be responsible for the UHP dissolution-precipitation of carbonates during subduction. The subsequent evolution during exhumation occurred in presence of more diluted aqueous fluids characterised by distinct salinities and by the lack of dissolved Si and CO32-.
References
Ferrando S. et al. (2017) Am. Mineral. 102:42-60.
Castelli D. et al. (2007): J. Metamorph. Geol. 25:587-603.
Kelemen P.B. and Manning C.E. (2015) P. Natl. Acad.
Sci. USA 112:E3997-E4006.
Whitney D.L. and Evans B.W. (2010) Am. Mineral.
95:185-187.
Fig. 1. Typical microstructural relationships between diopside-hosted, primary Group I bi-phase multisolid aqueous and primary Group II and secondary Group III aqueous inclusions.
Stacked microphotographs (50X), from 0 to 6 µm of depth in the crystal.
Fluid constraints in the Miguel Vacas Cu deposit (Ossa-Morena Zone, Portugal) Maia, M.
1,2*, Moreira, N.
1,2, Mirão, J.
1,3, Noronha, F.
2,4& Nogueira, P.
1,21University of Évora, Faculty of Sciences and Technology, Portugal; 2Institute of Earth Sciences, Portugal;
3HERCULES Laboratory, University of Évora, Portugal; 4University of Porto, Faculty of Sciences, Portugal;
*mcmaiageo@gmail.com
The ancient Miguel Vacas copper mine belongs to a cluster of ancient Cu mines and occurrences in the Ossa-Morena Zone, specifically in the Sousel-Barrancos metallogenic belt (SBMB), such as Mociços, Ferrarias and Bugalho. This deposit is characterised by an anastomosing vein structure mainly composed of quartz + carbonate + sulphides that extends over 2 km in length with an NNW-SSE direction. Chalcopyrite is the main primary Cu mineralisation; strong supergene enrichment is evidenced by the mineral assemblage of libethenite, malachite, bornite and digenite. Prospects in the area also reveal the presence of arsenopyrite. An epigenetic epithermal model for the Cu mineralisation is proposed for Miguel Vacas, as well as for other SBMB deposits (Mateus et al., 2003). This study aims to the characterisation of the fluids related with the mineralising processes. Sampling was done in a borehole at different depths (eight samples from 53 to 188 m). This study allowed identifying three different FI types regarding their composition.
LVS1-3 where L and V is H2O, S1 corresponds to halite, S2 is a carbonate and S3 an opaque phase;
most of the inclusions contain a halite crystal. LV consists of two-phase inclusions where L and V are H2O. L1-2V where L1 is H2O; L2 and V are CO2. Miguel Vacas fluids studied in mineralised veins show a wide range of homogenisation temperatures (Th= [62.8 – 350 ºC]; 𝑥 = 181 ºC) and salinities ([0.35 – 30.9]; 𝑥 = 17.89 wt% NaCl equiv.). First ice melting temperatures (Te= [68 – -14 ºC]; 𝑥 = -46 ºC) suggest the presence of solutes such as CaCl2 and MgCl2.
LV microthermometry data allowed to differentiate between two major groups (LV-a and LV-b), enhancing that even though these inclusions are gathered in the same FI assemblage they belong to a different fluid pulse. LVS1-3 and LV-a inclusions were studied in quartz + calcite ore-bearing veins, with comb-zoned textures, where these fluids coexist. Figure 1 suggests mixing between high salinity (LVS1-3) and lower salinity fluids (LV-a) (arrow trend), which may indicate different fluid sources present in the system. It should be emphasised that halite-bearing FI´s show Th(L+V→L) at lower temperatures than halite melting (TmHal= [83.1 – 181.4 ºC]; 𝑥 = 144.7 ºC). LV-b FI´s are found in shallower milky quartz samples (53 and 65 m), close to a dolerite intrusion and are distributed in cluster assemblages. LV-b thermometric data advocate for a much different fluid source, perhaps related with the dolerite intrusion. Petrographic evidence
suggests that this quartz crystallised earlier than the quartz + calcite + sulphides paragenetic sequence.
The fluid inclusions from the Miguel Vacas, as well as from the Mociços Cu deposit, suggest that Cu ore-bearing fluids in SBMB ancient mines share a similar source with an important contribution of magmatic fluids.
Acknowledgement
To HERCULES Laboratory of the University of Évora for Raman Spectroscopy equipment usage and to the Institute of Earth Sciences. This work is a contribution to the project "ZOM-3D Metallogenic Modelling of Ossa-Morena Zone: Valorisation of the Alentejo Mineral Resources" (ALT20-03-0145-FEDER-000028), funded by Alentejo 2020 (Regional Operational Program of Alentejo) through the FEDER / FSE / FEEI. M.
References
Mateus A. et al. (2003) in: VI Congresso Nacional de Geologia, Lisboa (Portugal), Ciências da Terra (UNL), Lisboa, nº esp. V, CD-ROM, F90-F93.
Fig. 1. Salinity (wt% NaCl equiv.) versus Th from the obtained data from the different Types of fluid inclusions.
Fluid constraints in the Mociços Cu deposit (Ossa-Morena Zone, Portugal) Maia, M.
1,2*, Moreira, N.
1,2, Vicente, S.
1,2, Mirão, J.
1,3, Noronha, F.
2,4& Nogueira, P.
1,21University of Évora, Faculty of Sciences and Technology, Portugal; 2Institute of Earth Sciences, Portugal;
3HERCULES Laboratory, University of Évora, Portugal; 4University of Porto, Faculty of Sciences, Portugal;
*mcmaiageo@gmail.com
The Mociços copper deposit is located in the Ossa-Morena Zone, at the Sousel-Barrancos metallogenic belt (SBMB) and belongs to a cluster of ancient Cu mines and occurrences, such as Miguel Vacas, Ferrarias and Bugalho. This deposit is characterised by a main sulphide assemblage of chalcopyrite + pyrite in a quartz + carbonate decametric lode, with an NNW-SSE general trend, which cross-cuts metasedimentary Paleozoic successions with circa 1 km length.
The model proposal for SBMB deposits (Mateus et al., 2003) states that the Cu contents were remobilised from metasedimentary host rocks by low to moderate salinity and temperature fluids.
The study of more than 200 fluid inclusions (FI´s) allowed identifying four main FI types. LVS1-3
where L and V are H2O, S1 is halite, S2 is a carbonate and S3 is an opaque. Most of these inclusions contain a halite crystal. L1-2VS1-2 each phase representing L1 is H2O, L2 and V are CO2, S1
is halite and S2 is a carbonate or opaque. L1-2V where L1 is H2O, L2 and V are CO2. LV consists of two-phase inclusions where L and V are H2O. In addition, CO2-rich biphasic FI´s (L and V are CO2) were studied in milky deformed quartz. These inclusions suggest a metamorphic fluid circulation with possible CO2 devolatilisation from the metasedimentary host rocks.
Figure 1 plots Th versus salinity for all the studied FI assemblages. A two-stage fluid circulation model is suggested by the presence of a high-salinity and a relatively lower high-salinity fluid (NaCl range= [0.18 – 44.3 wt% NaCl equiv.]; 𝑥 = 21.85 wt% NaCl equiv.) (see arrow). The high salinity FI´s are almost exclusively halite-bearing, aligning in the halite saturation curve. These high salinity fluids may be related to brines of magmatic origin, contributing with hot ore-bearing fluids which are progressively replaced with meteoric fluids (Th
range= [68 – 420 ºC]; 𝑥=182.8 ºC).
The majority of the halite-bearing FI´s (TmHal
range [138.3 – 370 ºC]; 𝑥=242 ºC) show a behaviour where Th(L+V→L) < TmHal, corresponding to a pressure of ≈ 50-100 MPa. This inclusion behaviour has been reported for several magmatic-hydrothermal ore deposits (Becker et al., 2008).
Several carbonates and daughter minerals were identified by Raman spectroscopy. First ice melting temperatures ([-5 – -70 ºC]; 𝑥= -48.4 ºC) indicate the presence of solutes other than NaCl, like CaCl2
and MgCl2 as suggested by the presence of the carbonates. Fluid inclusion data from the Mociços deposit suggests that a more complex and deeper
fluid circulating system must be present and further studies will be carried out.
Acknowledgement
To HERCULES Laboratory of the University of Évora for usage of Raman Spectroscopy equipment and to the Institute of Earth Sciences. This work is a contribution to the project "ZOM-3D Metallogenic Modelling of Ossa-Morena Zone: Valorisation of the Alentejo Mineral Resources" (ALT20-03-0145-FEDER-000028), funded by Alentejo 2020 (Regional Operational Program of Alentejo) through the FEDER / FSE / FEEI. M.
References
Becker S.P. et al. (2008) Econ. Geol. 103:539-554.
Mateus A. et al. (2003) In: VI Congresso Nacional de Geologia, Lisboa (Portugal), Ciências da Terra (UNL), Lisboa, nº esp. V, CD-ROM, F90-F93.
Fig. 1. Salinity (wt% NaCl equiv.) versus Th from the obtained data for the different fluid inclusions Types.