Opals from Pedro II and Buriti dos Montes, in the Piauí State, are the most important
occurrences of this gemstone in Brazil, both in terms of volume and -gemological quality that
is comparable to the famous Australian opals. However, informality in the extraction and
marketing of these opals, as well as the lack of information about the genesis of these deposits
do not permit prospecting for new deposits, and the establishment of a certificate of origin for
Piauí opals, would allow their formal participation in the international gemological market.
Some authors have been studied these opals, revealing strong evidences of their hydrothermal
origin, but until now, no work discussed the physico-chemical characteristics of the fluids that
would have originated these opals deposits. Thus, the main objective of this work was to
understand the hydrothermal system responsible for the genesis of opals from Piauí, i.e., to
characterize the fluids that originated the mineralization and show its relationship with the
geological context of this region. Pedro II and Buriti dos Montes counties are located in the
northeastern portion of the Piauí State, at approximately 230 km east of the capital Teresina.
The opal occurrences are on the basis of the Parnaíba Basin, constituting veins and veinlets in
the sandstones of the Serra Grande (Buriti dos Montes) and Canindé (Pedro II) groups, which
are cut by diabase sills and dikes of the Sardinha Formation. They also occur in cementing
breccias and as colluvial and paleochannel deposits. Associated to opals, locally there are
veins of quartz, chalcedony, barite and hematite (or goethite). In general, opals from Pedro II
present play-of-color, are mostly white or bluish with a milky appearance, semitranslucent to
opaque, and have solid inclusions little bit apparent. In contrast, orange opals from Buriti dos
Montes do not show play-of-color, their color ranges from light yellow to brownish red, they
are semitransparent to translucent, and contain a large variety of solid inclusions. The
obtained data reveal that the opals from Pedro II are typically of amorphous type (opal-A),
while the opals from Buriti dos Montes range between amorphous and cristobalite-tridymite
(opal-CT). In the precious opals, the typical play-of-color is caused by the regular
arrangement of the silica spheres that constitute them. The absence of opaline cement among
the spheres reinforces the beauty of this effect. On the other hand, the orange opals do not
show play-of-color, but they have greater transparency due to the small size of the spheres.
The solid inclusions also produce beautiful effects in the studied opals, mainly in the orange
variety that is more transparent. Besides this, the solid inclusions set reveals intrinsic
characteristics to hydrothermal processes that originated the studied opals. Botryoidal,
dendritic and nodular aggregates are examples of inclusions formed by fragments of the host sandstones, which were carried by the hydrothermal fluids that generated the opals. In the
opals from Buriti dos Montes, the red, orange and yellow hues are produced by the partial
dissolution of the Fe oxy-hydroxides inclusions. Similarly, the green color in the precious
opals is related to Co-pentlandite microcrystals included in them. The set of minerals
associated to opals leads to a mineralogical-geochemical signature marked by high contents of
Fe and Al in opals with hematite/goethite and kaolinite inclusions, such as the considerable
increase in the rare earth elements contents, in the opals that have kaolinite and apatite
inclusions. Among the trace elements, Ba is the most abundant, and it probably was
incorporated to hydrothermal fluid, considering that veins of barite are frequently found in
this region of Parnaíba Basin. Some features such as flow structures in the opals, corrosion
and partial dissolution in the hyaline quartz crystals and mineralogical inclusions,
hydrothermal quartz veinlets that overgrew to detrital grains, and zoning in the quartz crystals,
confirm that these opals have hydrothermal origin. The Gondwana rupture would have caused
a wide fissural basic magmatism, which was the responsible for the heat supply that generated
the first convective cells of hot fluids. The water contained in the sandstones certainly filled
the system and enriched in silica through partial or total dissolution of the quartz grains of
sandstones. This hydrothermal fluid was subsequently hosted and cooled in the fracture
systems, precipitating the opal and associated minerals.
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