Paleoproterozoic Iron Formations Research Articles

The Au-Pd Mineralization at the Conceicao Iron Mine, ItabiraDistrict, Southern Sao Francisco Craton, Brazil: An Example of aJacutinga-Type Deposit

The Conceicao mine, which is located in the Itabira district, southern Sao Francisco craton, is a rare example of gold and palladium mineralization hosted by jacutinga, an altered Lake Superior-type iron-formation. The mine sequence consists of three metamorphosed lithostratigraphic units, which are from the oldest to the youngest: (1) an Archean volcano-sedimentary sequence, (2) a Paleoproterozoic iron-formation, and (3) a Paleoproterozoic quartzite. These units are cut by mafic intrusive rocks and, with the exception of intrusive contacts, all contacts between the various units are structural and interpreted as thrust faults. The rocks were subjected to three phases of folding: (1) D1, characterized by tight asymmetric and isoclinal folds, which develop locally into sheath folds (the D1 fabrics are the S1 foliation and elongation lineation); (2) D2, represented by tight, large-scale folds and associated parasitic folds; and (3) D3, characterized by open folds with an associated crenulation cleavage. The D1 and D2 deformation are interpreted to be products of simple shear with a transport direction consistently east over west. These rocks were metamorphosed to typical mineralogy of amphibolite facies conditions during the D1 phase of folding. The Conceicao orebody consists of a single quartz vein surrounded by jacutinga. The vein is parallel to the S1 foliation, which is locally oriented west-southwest and dips 55° to 75° NW, and was folded by D2 deformation. It extends 100 m downdip in the direction of the Le elongation lineation and 400 m along strike. The jacutinga consists of centimeter-scale layers of muscovite, specular hematite (hematite I), and octahedral hematite grains (hematite II) with remnants of magnetite, and a finely laminated envelope around the quartz vein, which is composed mainly of Fe hydroxides, hematite (hematite III), kaolinite, muscovite, and tourmaline. The quartz vein is dismembered and quartz grains are partially corroded and filled by 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 palladian gold (up to 0.9 wt % Pd) were found mantling hematite II grains. Native gold is commonly associated with Fe hydroxide in the finely laminated alteration envelope around the quartz vein and is interpreted to have precipitated during weathering. The similarities between the Au-Pd orebodies at the Conceicao and Caue deposits (referred to as jacutinga-type deposits) suggest that they were formed by the same processes and their characteristics favor an epigenetic model for gold and palladium concentration in the iron-formation. Since the Cretaceous, these Archean and Proterozoic rocks have been weathered, and the hydrothermal 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.

Was the Deposition of Large Precambrian Iron Formations Linked to Major Marine Transgressions?

Geochemical evidence suggests the iron- and silica-rich precipitates that compose large Neoarchean and Paleoproterozoic iron formations formed along a chemocline between iron-rich deep water and iron-poor surface water. Sedimento-logical evidence indicates many large iron formations accumulated in marine shelf environments during major transgressions. We suggest this occurred because the chemocline could only impinge on continental shelves during times of sea-level rise or highstand. On the other hand, the stratigraphic record indicates that deposition of iron formations ceased during regressions in some basins and transgressions in others. To explain this, we propose that iron concentrations in the Precambrian ocean were greatest at mid-water depths in a zone on the order of 100 m thick. Radiometric age dates from the Hamersley Basin of Western Australia suggest major iron-formation deposition had a periodicity of ca. 20 to 125 m.y., close to second-order cycles in Phanerozoic sea level driven by long-te...