The Ni-Cu-PGE sulfide ores of the komatiite-hosted Fortaleza de Minas deposit, Brazil: Evidence of hydrothermal remobilization

The Fortaleza de Minas Ni–Cu–PGE sulfide deposit is hosted by Archean komatiitic rocks of the Morro do Ferro greenstone belt, near the southwestern margin of the Sao Francisco craton, Minas Gerais state, Brazil. The deposit contains 6 million tonnes of ore with an average grade of 2.2 wt% Ni, 0.4% Cu, 0.05% Co and 1.2 ppm PGE+Au, and comprises (i) a main orebody, which is metamorphosed, deformed and transposed along a regional shear zone, consisting mainly of disseminated, brecciated and stringer sulfide ores that are interpreted to be of early magmatic origin, and (ii) PGE-rich discordant veins that are hosted in N–S- and NE–SW-trending late faults that cross-cut the main orebody. The discordant PGE-rich ore (up to 4 ppm total PGE) is characterized by thin, discontinuous and irregular veins and lenses of massive sulfides hosted by serpentinite and talc schist, and is relatively undeformed if compared with the early types of ore. It is composed mainly of pyrrhotite, pentlandite, chalcopyrite, magnetite, carbonates, and amphiboles, with minor cobaltite–gersdorffite, sphalerite, ilmenite, and quartz, and rarely maucherite (Ni11As8), tellurides and platinum-group minerals (PGM). Omeeite, irarsite, sperrylite, and Ni-bearing merenskyite are the main PGM, followed by minor amounts of testibiopalladite and an unknown phase containing Ru, Te, and As. The PGM occur either included in, or at the margins of, sulfides, sulfarsenides, silicates and oxides, or filling fractures in pyrrhotite, pentlandite, and chalcopyrite, suggesting that they started to precipitate with these minerals and continued to precipitate after the sulfides were formed. The mantle-normalized metal distribution of the two samples of discordant veins shows distinct patterns: one richer in Ni–Pd–Ir–Rh–Ru–Os and another with higher amounts of Cu–Pt–Bi. Both are strongly depleted in Cr if compared with the metamorphosed magmatic ore of this deposit, which follows the general Kambalda-type magmatic trend. On the basis of structural, mineralogical and geochemical evidence, we propose that the PGE-rich discordant ore may have formed by remobilization of metals from the deformed, metamorphosed magmatic orebody (which shows a depleted pattern in these elements) by reduced (pyrrhotite – pentlandite – pyrite are stable), neutral to alkaline and carbonic fluids (carbonate-stable). The PGE may have been transported as bisulfide complexes, and precipitated as tellurides (mainly Pd) and arsenides (Pt, Rh, Ru, Os, Ir) in the late N–S and NE–SW-trending faults owing to a decrease in the activity of S caused by the precipitation of sulfides in the veins.

[1]  A. J. Naldrett.  A history of our understanding of magmatic Ni-Cu sulfide deposits , 2005 .

[2]  Conny Bockrath,et al.  Fractionation of the Platinum-Group Elements During Mantle Melting , 2004, Science.

[3]  Robert E. Jones,et al.  PETROGRAPHIC AND GEOCHEMICAL CHARACTERISTICS OF POSTMAGMATIC HYDROTHERMAL ALTERATION AND MINERALIZATION IN THE J–M REEF, STILLWATER COMPLEX, MONTANA , 2004 .

[4]  W. Maier,et al.  REPLACEMENT OF BASE METAL SULFIDES BY ACTINOLITE, EPIDOTE, CALCITE, AND MAGNETITE IN THE UG2 AND MERENSKY REEF OF THE BUSHVELD COMPLEX, SOUTH AFRICA , 2004 .

[5]  S. G. Carvalho,et al.  Grau metamórfico das formaçóes ferríferas do Greenstone Belt Morro do Ferro: abordagem utilizando texturas de exsoluçáo em anfibólios , 2003 .

[6]  M. Pimentel,et al.  Idade SM-ND de komatiitos do Greenstone Belt do Morro do Ferro, Fortaleza de Minas (MG) , 2002 .

[7]  Y. Lahaye,et al.  ORIGIN OF VARIATIONS IN NICKEL TENOR ALONG THE STRIKE OF THE EDWARDS LODE NICKEL SULFIDE OREBODY, KAMBALDA, WESTERN AUSTRALIA , 2001 .

[8]  M. Pimentel,et al.  Granites and the geodynamic history of the neoproterozoic Brası́lia belt, Central Brazil: a review , 1999 .

[9]  F. Gervilla,et al.  Platinum-, palladium- and gold-rich arsenide ores from the Kylmäkoski Ni-Cu deposit , 1998 .

[10]  F. Gervilla,et al.  Platinum-Group Element Sulpharsenides and Pd Bismuthotellurides in the Metamorphosed Ni-Cu Deposit at Las Aguilas (Province of San Luis, Argentina) , 1997, Mineralogical Magazine.

[11]  A. Choudhuri,et al.  Sulfur Isotopes in Komatiite-Hosted Ni-Cu Sulfide Deposits from the Morro do Ferro Greenstone Belt, Southeastern Brazil , 1997 .

[12]  José M. Torres-Ruiz,et al.  Immiscibility between arsenide and sulfide melts; a mechanism for concentration of noble metals , 1996 .

[13]  D. H. Watkinson,et al.  Geochemical Evolution of the Epidote Zone, Fraser Mine, Sudbury, Ontario: Ni-Cu-PGE Remobilization by Saline Fluids , 1996 .

[14]  Y. Hasui,et al.  Gravimetria e compartimentaçao crustal do sul de Minas Gerais , 1996 .

[15]  W. McDonough,et al.  The composition of the Earth , 1995 .

[16]  M. Fleet,et al.  Platinum-group minerals and gold in Arsenic-rich ore at the Thompson mine, Thompson Nickel Belt, Manitoba, Canada , 1993 .

[17]  N. Machado,et al.  U-Pb evidence of late Archean tectono-thermal activity in the southern São Francisco shield, Brazil , 1992 .

[18]  Campos Neto,et al.  A porção ocidental da faixa Alto Rio Grande: ensaio de evolução tectônica , 1991 .

[19]  D. C. Harris,et al.  Partitioning of the platinum-group and other trace elements in sulfides from the Bushveld Complex and Canadian occurrences of nickel-copper sulfides , 1990 .

[20]  C. Marchetto.  Platinum-group minerals in the O'Toole (Ni-Cu-Co) deposit, Brazil , 1990 .

[21]  J. Thompson,et al.  The O'Toole nickel deposit, Morro do Ferro greenstone belt, Brazil , 1990 .

[22]  Gergely Andres Julio Szabó.  Contexto geológico e petrologia das rochas metaultramáficas de Alpinopolis, MG , 1989 .

[23]  A. J. Naldrett,et al.  Magmatic Sulfide Deposits , 1989 .

[24]  D. Gee,et al.  Platinum group elements in mafic-ultramafic rocks of the Western Gneiss Terrain, Western Australia , 1987 .

[25]  H. C. H. Dillon-Leitch,et al.  Distribution of platinum-group elements in the Donaldson West Deposit, Cape Smith Belt, Quebec , 1986 .

[26]  Y. Hasui,et al.  The gravimetric information and the Archean-Proterozoic structural framework of Eastern Brazil , 1982 .

[27]  D. Groves,et al.  Nickel sulfide deposits in Western Australia; a review , 1981 .

[28]  R. Keays,et al.  Precious metals in volcanic peridotite-associated nickel sulfide deposits in Western Australia; II, Distribution within the ores and host rocks at Kambalda , 1981 .

[29]  F. Almeida,et al.  Informacoes geofisicas sobre o oeste mineiro e seu significado geotectonico , 1980 .

[30]  A. J. Naldrett.  Partitioning of Fe, Co, Ni, and Cu between sulfide liquid and basaltic melts and the composition of Ni-Cu sulfide deposits; reply and further discussion , 1979 .

[31]  R. Keays,et al.  Precious metals in volcanic-type nickel sulfide deposits in Western Australia; I, Relationship with the composition of the ores and their host rocks , 1979 .

[32]  J. Danni,et al.  Geologia da raiz de um greenstone belt na regiao de Fortaleza de Minas. Minas Gerais , 1979 .

[33]  A. J. Naldrett,et al.  Partitioning of Fe, Co, Ni, and Cu between sulfide liquid and basaltic melts and the composition of Ni-Cu sulfide deposits , 1978 .