Chalcophile Element Concentrations in the Pechenga Sulphides; Comparison with the Cape Smith Sulphides.

Barnes* Sarah-Jane, Victor A. Melezhik+ and Stanislav V. Sokolov&

*CERM, Universite du Quebec, Chicoutimi, G7H 2B1, Canada.,

+Geological Survey of Norway, Trondheim, Norway.

&Pechenga Nickel Combinate, 185415 Zapolyarny, Russia.

 

In the Pechenga area of northeastern Russia, Ni-Cu sulphide ores are hosted by ferropicritic sills and flows, which were emplaced into pyrite-rich black shales. The sulphides from these Ni-deposits are normally classified into 4 types; disseminated, massive, brecciated and vein. We have determined the composition of different sulphides and reported them below.

Median Values for the Chalcophile Elements in Pechenga Sulphides and Ultramafic Tuff.

 type  n  Fe  S  Cu  Ni  Ag  As  Co  Sb  Se  Zn  
     %        ppm            
 disseminated  21  48.68  37.20  4369  8.38  38  30  2549  4.1  93  714  
 massive  33  51.60  37.63  1.82  8.35  10  20  2020  0.77  75  232  
 brecciated d*  11  54.17  38.24  1.60  6.49  21  92  1855  1.55  70  469  
 brecciated m*  8  52.22  37.78  1.95  8.30  14  146  1994  0.66  87  252  
 sedimentary  7  46.49  53.42  0.06  0.04  1  387  69  10  29  589  
 type  Os  Ir  Ru  Rh  Pt  Pd  Au  U/Th  Pd/Ir  Ni/Cu  Cu/PdN  Ni/IrM
   ppb                      
 disseminated  28  29  69  33  797  980  161  0.18  46.1  2.07  6.59  8.8
 massive  82  44  121  63  353  283  84  <0.1  6.4  5.77  5.26  3.82
 brecciated d*  30  26  52  35  576  714  85  0.93  21.5  3.32  3.43  4.88
 brecciated m*  89  46  89  45  273  264  125  0.50  4.8  5.23  5.70  3.10
 sedimentary  <2  0.34  <10  3  <20  <20  44  2.7    0.7    2

*Both disseminated and massive ore occurs in the brecciated form, these have different compositions so they are reported separately.

 

The disseminated, brecciated and massive sulphides all have similar Ni, Co and Se contents. All of the sulphides are depleted in PGE and Au relative to Ni and Cu with Ni/IrN and Cu/PdN of 3 to 9. This may indicate that these sulphides are evolved and that more primitive sulphides rich in PGE are present at depth. The massive sulphides, which occur in both normal and brecciated form, are depleted in Cu, Ag, Au, Pt, Pd, Sb and Zn and enriched in Os, Ir, Rh, Rh relative to disseminated sulphides, which also occur in both normal and brecciated form. The enrichment of Os, Ir, Ru and Rh in the massive sulphides could be because they contain monosulphide solid solution as a cumulate phase. The brecciated sulphides are enriched in As and have higher U/Th ratios than unbrecciated sulphides. This may reflect a contribution from the sediments which are depleted in most metals, but have high As concentrations and U/Th ratios.

The tectonic setting, host rocks and age of the Pechenga sulphides are very similar to the Ni-sulphide deposits of the Cape Smith Fold Belt of northern Quebec. The Ni and Co concentrations of the Cape Smith sulphides are slightly higher, but the PGE and Au concentrations are an order of magnitude higher. Possibly this is because the Cape Smith sulphides represent primitive sulphides.