The Project Evaluation of Deposits of Opals of Pedro II

The Project Evaluation of Deposits of Opals of Pedro II, an action from the Geology of Brazil Program (PGB) supported by CPRM/Geological Survey of Brazil, is inserted in the Growth Acceleration Program (PAC) of the Federal Government. Started in 2011, was executed by Residence of Teresina. Aims to provide geological subsidies to Local Productive Arrangement (APL) of Opal of Pedro II, through the geological characterization of deposits, aiming at an orderly and proper exploration, besides elaborating registration and description of mineral occurrences. The work of geological mapping associated with bibliographic information allowed to individualize on project six lithostratigraphic units of Paleozoic and Mesozoic rocks, and Cenozoic sediments. The Paleozoic rocks are represented by formations: Jaicós (Serra Grande Group) - sandstones and occasionally interbedded mudstones, ferruginous sandstones and conglomeratic sandstones beds (Silurian); Pimenteira (Canindé Group) - fine clastic predominantly pelitic facies (siltstones and shales) with layers intercalated of fine sandstones (Devonian); Cabeças (Canindé Group) - quartz sandstone, locally with ferruginous levels, lateritizated, levels of laterite and lateritic conglomerates, and in Pedro II and neighborhoods, overlapping sandstones to diabases, are highly silicified and fractured, and may contain noble opal filling fractures (Devonian). All these formations are part of the Parnaíba Basin. Mesozoic igneous rocks of basic character, constituting large lenticular bodies or thick sills (dykes rarely), intrude the Paleozoic rocks, in particular, the sandstones of Cabeças Formation. Dated held in diabase collected in Roça opal-digging presented age between 194 and 209 Ma (206Pb/238U) and Concordia Age of 203 ± 2 Ma, associated with an MSWD 0.44 and probability of agreement of 0.51 - the result representing the crystallization age of the body (Upper Triassic - Lower Jurassic), thus compounding the Mosquito Formation. Colluvium-eluvial and Alluvial deposits represent the Cenozoic sediments. Treatment of collected data from fractures indicate fractures with average dips greater than 85o and predominance of the directions E-W and NE-SW. In the area, fractures form intersecting families, usually with two directions almost orthogonal to each other. Based on conjugate pairs σ1 inferences were made which resulted in two main directions: SW-NE and SE-NW. In the western portion of the area, prevails σ1 according to SW-NE direction, where there are no records of occurrence of opal. In the region adjacent to Pedro II despite still being observed σ1 according to SW-NE direction, near Boi Morto mine and Mamoeiro opal-digging, main occurrences of opal mines, was observed σ1 with NW-SE direction. This direction is consistent with fotoalignments, suggesting this as the main direction of mineralization in opals. As for metallogenesis, primary opal deposits are hosted in sediments of Cabeças Formation, notably in the areas of contact between them and the basic rocks that intrude them. Opal occurs filling fractures, fissures and veinlets in silicified sandstones, more rarely, in siltstones, and on the capa of diabase sills, where this was altered forming clay level that containing smectite. The genesis of opals of Pedro II is closely related to a hydrothermal environment, arisen with the intrusion of basic rocks in siliciclastic rocks of Cabeças Formation (Gomes e Costa, 1994). Essential data to corroborate this genetic model were presented by Marques et al. (2013), based on analyzes of fluid inclusions, mineralogical and chemical composition of these opals and their solid inclusions, and in large part of the solid inclusions partial dissolution features were observed, also referring to the hydrothermal environment, intense migration of fluids, added to own mineralogy of the same. However, some authors associate the genesis of the ore to lateritic weathering. Thus, we have as main factors associated with generation of primary deposits of opals in the region of Pedro II, the hydrothermal system and the structural pattern of the area. Opal also occurs in secondary deposits derived from the weathering, erosion and transport of primary mineralization, forming alluvial deposits and talus deposits, resulting from the breakdown of Cabeças Formation sedimentary rocks. The productive chain of Opal of Pedro II presents as bottlenecks: lack of daily specialized technical monitoring, what prevents
 a mining streamlined shape, leading to visible environmental liabilities; lack of financial resources to enable equipment to facilitate the mining; lack of continuous improvements in facilities that support the miners in the various opal-digging in the region; manpower drain; evasion of hand labor, because local young people, children and grandchildren of miners, are shunning the opal-digging, and leaving the branch in search of better professional horizons in large urban centers.