The Au-Pd mineralization at the Conceicao iron mine, Itabira district, southern Sao Francisco craton, Brazil: An example of a Jacutinga-type deposit

Citation
Gr. Olivo et al., The Au-Pd mineralization at the Conceicao iron mine, Itabira district, southern Sao Francisco craton, Brazil: An example of a Jacutinga-type deposit, ECON GEOL B, 96(1), 2001, pp. 61-74
Citations number
59
Categorie Soggetti
Earth Sciences
Journal title
ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS
ISSN journal
03610128 → ACNP
Volume
96
Issue
1
Year of publication
2001
Pages
61 - 74
Database
ISI
SICI code
0361-0128(200101/02)96:1<61:TAMATC>2.0.ZU;2-I
Abstract
The Cunceicao mine, which is located in the Itabira district, southern Sao Francisco craton, is a rare example of gold and palladium mineralization ho sted by jacutinga, an altered Lake Superior-type iron-formation. The mine s equence consists of three metamorphosed lithostratigraphic units, which are from the oldest to the youngest: (1) an Arehean volcano-sedimentary sequen ce, (2) a Paleoproternzoic iron-formation, and (3) a Paleoprotcrozoic quart zite. These units are cut by mafic intrusive rocks and, with the exception of intrusive contacts, all contacts between the various units are structura l and interpreted as thrust faults. The rocks were subjected to three phase s of folding: (1) D-1, characterized by tight asymmetric and isoclinal fold s, which develop locally into sheath folds (the D-1 fabrics are the S-1 fol iation and elongation lineation); (2) D-2, represented by tight, large-scal e folds and associated parasitic folds; and (3) D-3, characterized by open folds with an associated crenulation cleavage. The D-1 and D-2 deformation are interpreted to be products of simple shear with a. transport direction consistently east over west. These rocks were metamorphosed to typical mine ralogy of amphibolite facies conditions during the D-1 phase of folding. The Conceicao orebody consists of a single quartz vein surrounded by jacuti nga. The vein is parallel to the S-1 foliation, which is locally oriented w est-southwest and dips 55 degrees to 75 degrees NW, and was folded by De de formation . It extends 100 m downdip in the direction of the L-e elongation lineation and 400 m along strike. The jacutinga consists of centimeter-sca le layers of muscovite, specular hematite (hematite I), and octahedral hema tite grains (hematite II) with remnants of magnetite, and a finely laminate d envelope around the quartz vein, which is composed mainly of Fe hydroxide s, hematite (hematite III), kaolinite; muscovite, and tourmaline. The quart z vein is dismembered and quartz grains are partially corroded and filled b y Fe hydroxides and kaolinite. The ore minerals, interpreted to have formed during the main hydrothermal event, are palladian gold (up to 3.2 wt % Pd) and an unknown Pd-Sb-As mineral (tentatively identified as mertierite II), which are typically included in hematite II octahedra. A few grains of pal ladian gold (up to 0.9 wt % Pd) were found mantling hematite II grains. Nat ive gold is commonly associated with Fe hydroxide in the finely laminated a lteration envelope around the quartz vein and is interpreted to have precip itated during weathering. The similarities between the Au-Pd orebodies at die Conceicao and Caue depo sits (referred to as "jacutinga-type" deposits) suggest that they were form ed by the same processes and their characteristics favor an epigenetic mode l for gold and palladium concentration in the iron-formation. Since the Cre taceous, these Archean and Proterozoic locks have been weathered, and the h ydrothermal silicates and oxides have been replaced by kaolinite and Fe(Mn) hydroxides; gold and palladium have been remobilized and reprecipitated As pure metals or Pd oxides.