MODELING OF VARIATIONS ACCOMPANYING MINERAL REACTIONS IN PELITIC XENOLITHS AT THE VOISEY'S BAY NI-CU-CO DEPOSIT, LABRADOR, CANADA

Ni-Cu-Co sulfide mineralization at Voisey’s Bay is in part attributed to reaction between country rocks and host magma, as evidenced by the close association of sulfide mineral concentrations and xenolith laden horizons.  Potential country rock contaminants include the Tasiuyak paragneiss and a variety of mafic to quartzofeldspathic gneisses.  Typical samples of the Tasiuyak gneiss consist of garnet, plagioclase, pyroxene, quartz and biotite.  Xenoliths derived from the Tasiuyak gneiss primarily consist of hercynite, magnetite, corundum and plagioclase.  d¹⁸O values of protolith mineral assemblages indicate equilibration temperatures in excess of 800°C.

We have modeled the closed system destruction of garnet via a reaction that produces cordierite, orthopyroxene and K-feldspar:

K₂Fe₆Al₂Si₆O₂₀(OH)₄ + 2Fe₃Al₂Si₃O₁₂ + 9SiO₂=Fe₂Al₄Si₅O₁₈(2H₂O) + 10FeSiO₃ + 2KAlSi₃O₈

Computed d¹⁸O values (‰) for a 11.3‰ protolith are: cordierite - 11.5, orthopyroxene - 10.3, and K-feldspar - 12.7.

Progressive dehydration of cordierite results in expulsion of ¹⁸O-enriched water and the production of hercynite and quartz (Fe₂Al₄Si₅O₁₈(2H₂O)=2FeAl₂O₄ + 5SiO₂ + 2H₂O), with final values of 8.7 and 16.3‰, respectively.  Xenoliths at Voisey’s Bay are quartz deficient, and characterized by bulk d¹⁸O values near 7‰.  Final open system interaction between xenoliths and magma resulted in the loss of a siliceous, ¹⁸O-enriched partial melt, and the transfer of Fe²⁺ from the magma to yield hercynite that is pseudomorphic after corundum.