2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 10
Presentation Time: 3:15 PM

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


MARIGA, Jeffrey1, RIPLEY, Edward M.2 and LI, Chusi2, (1)Department of Geological Sciences, Indiana Univ, Bloomington, IN 47405, (2)Department of Geological Sciences, Indiana Univ, 1001 E. 10th St, Bloomington, IN 47405, jmariga@indiana.edu

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.