The mineral deposit of Sapucaia, located in Bonito county, northeast of Pará state, is part of a
set of occurrences of lateritic aluminum phosphates located predominantly along the coastal
area of Pará and Maranhão states. These deposits were subjetc for study since the beginning of
the last century, when the first descriptions of "phosphorous bauxites" were mentioned in the
NW region in Maranhão. In the last decades, with the increase demand for fertilized products
for agricultural world market, various mineral exploration projects were started or expanded
their resources in Brazilian territory, among them there is the economic viability of deposits of
aluminous phosphates, as Sapucaia, which will be the first project of economic mineral
production and marketing of thermophosphates of Brazil. This study aimed to characterize the
geology, mineral composition and geochemistry of lateritic aluminum phosphates profile of the
Sapucaia hill. The macro-region inclued lands mostly dominated by Precambrian to Paleozoic
rocks land, locally defined by Pirabas Formation, Barreiras Formation, latosoils and recent
sediments. The morphology of the deposit is characterized by a discrete elongated hillock that
offers smooth and continuous slopes at its edges, which become rare natural exposures of the
laterite profile horizons. So, the methodology used to characterize the deposit was based on the
program of geological research performed by “Fosfatar Mining”, hitherto owns their mineral
rights, has provide two trenches and samples of 8 drill cores. Sampling was limited to the extent
of lithological lateritic profile, with the selection of 44 samples at intervals of 1m, and
underwent a course of preparation and laboratory analysis. According with the other
occurrences of the Gurupi region, the phosphates of Sapucaia contains an individualized
horizon, called simply aluminum phosphates or aluminophosphatic crust, which varies
texturally to massive horizon, cavernous, porous microporous, which to the top transitions to
phosphate ferroalumino crust, “leopard skin”, cavernous, composed of nodules of hematite and
/ or goethite cemented by aluminum phosphates, with similar characteristics to the underlying
phosphate horizon. The aluminophosphatic crust to the base of the profile transition to a thick
horizon kaolinitic clay with sand layers, which rests on weathered heterolithic fine-grained
sediments, sericite clay aspect, sometimes interspersed with sandy horizons, and have no
apparent correlation with other rocks outcrop in the region. Approximately 40% of the surface
of the hill is covered by colluvium composed of mineralized fragments of the crust and sandy
sediment from Barreira Formation. In the crust, the aluminum phosphates are predominantly
represented by the subgroup of crandallite: i) Crandallite-goyazite (average 57.3%); ii)
woodhouseite-svanbergite (average 15.8%); and iii) wardite-millisite (average 5.1%). Associated with phosphates are: hematite, goethite, quartz, kaolinite, muscovite and anatase,
with volumes that vary according to the corresponding laterite horizon. As the heavy minerals
in the accessory level are zircon, staurolite, tourmaline, anatase, andalusite and sillimanite. The
horizon of phosphates and iron phosphate crusts shows be clearly rich in P2O5, and Fe2O3, CaO,
Na2O, SrO, SO3, Th, Ta and LREE such as La and Ce relative to the horizon saprolite. The
contents of SiO2 are pretty high, but much lower than those identified in lower clay horizon. In
profile as a whole, there is an inverse correlation between SiO2 and Al2O3, between Al2O3 and
Fe2O3, and positive between SiO2 and Fe2O3, which confirms the nature of the lateritic profile.
Different from what is expected by bauxitic laterites, the content of P2O5, CaO, Na2O, SrO and
SO3 have a elevated concentration, considered typical of phosphate deposits of aluminum rich
in crandallite-goyazite and woodhouseite-svanbergite. The succession of horizons, the
mineralogical composition and geochemical patterns allow correlating the deposit with other
aluminum phosphates presents in the region, more specifically Jandiá (Pará) and Trauíra
(Maranhão), as well as others located beyond the Brazilian territory, thus indicating that the
aluminum phosphates of Sapucaia are the genesis of a lateritic mature and a complete profile,
products whose source rock may be related to mineralized rocks in phosphorus, such as those
observed in Pimenteiras Formation, partially outcropping on the edge of the Parnaíba Basin.
Possibly, the current ore body was integrated of Pirabas paleocost sea, since drill holes on the
side of the body, made clear the relationship of lateral contact between these units.
|
