『Abstract
This contribution examines two types of Mn deposits, which were
exploited in the past in Sardinia. Both types are Tertiary in
age, but not strictly coeval and they are both linked to a continental
arc setting characterized by the presence of calcalkaline volcanics
and shallow marine sediments. Mn-deposits formed in terrestrial
or shallow marine environments are unusual compared to the most
common deep oceanic ones, and therefore poorly studied. The Mn-ores
of Sardinia occur either as stratabound deposits with stratiform
geometry associated to rudites with littoral facies or as vein-type
deposits hosted within volcanic rocks. In the stratiform deposits
the Mn ores form the matrix or cement of a transgressive conglomeratic
bed overlain by shallow water limestones and underlain by calc-alkaline
volcanics. The vein-type deposits consist of Mn oxides hosted
by latite lava domes, comenditic lava flows and rhyolitic ignimbrites.
Although these geological settings are markedly different, the
main Mn-bearing phases are the same, namely pyrolusite and hollandite.
The deposits differ in the presence of accessory minerals, such
as todorokite, manganite, coronadite, cryptomelane and gangue
minerals.
Rare earth element geochemistry is the most effective tool in
discriminating the two types of Mn deposits. These analyses indicate
that the vein-type deposits were formed from geofluid mixtures
linked to the Oligocene-Miocene calcalkaline volcanic cycle. Mineralogical
and geochemical data indicate that the crystallization of the
Mn oxides in the stratabound deposits was diagenetic, related
to sudden changes in Eh and pH in coastal, meteoric waters that
came in touch with the marine coastal water after having flushed
the calcalkaline volcanics.
Keywords: Manganese: Geochemistry; Ores; Hydrothermal; Marine
sediments; Sardinia』
1. Introduction
2. Geological setting
2.1. Geology of the Mn deposits
2.1.1. Sediment-hosted Mn deposits (SH group)
2.1.2. Volcanic- hosted Mn deposits (VH group)
3. Sampling and analytical methods
4. Results
4.1. Mineralogy and petrography
4.1.1. SH Mn ores
4.1.2. VH Mn ores
4.2. Geochemistry
4.2.1. Major elements
4.2.2. Trace elements
4.2.3. Rare earth elements
5. Discussion
5.1. Sediment-hosted Mn ores
5.2. Volcanic-hosted Mn ores
6. Summary and conclusions
Acknowledgements
References