EVIDENCE FOR SULPHUR LOSS IN THE MARATHON PGE-CU DEPOSIT, ONTARIO

The Marathon PGM-Cu deposit is hosted by the Coldwell alkaline complex, which was emplaced at 1108 Ma as part of the Mid-Continent Rift System. Mineralization at the Marathon PGM-Cu deposit is hosted by the Two Duck Lake gabbro, a fresh olivine-bearing gabbro. The economic mineralization is largely contained within two zones. The W Horizon is a high-grade PGE horizon characterized by low S content, low Cu/Pd and high Cu/Ni. The dominant sulphide minerals are chalcopyrite and bornite. The Main Zone has higher S, higher Cu/Pd and the dominant sulphide minerals are chalcopyrite and pyrrhotite.

Detailed petrographic study has shown that large scale low temperature hydrothermal alteration is not present in the W Horizon. From whole rock geochemistry and microprobe analyses through the W Horizon there is no evidence to support repetitive intrusions that underwent fractional crystallization that is typical of mineralization in layered intrusions. However, the sulphide metal tenors are too high to be explained by R Factor models. Sulphur dissolution upgrading in a magmatic conduit system was proposed by Good (2010), and is the preferred model for forming the W Horizon. In this model sulphur undersaturated basaltic magma interacts with an immiscible sulphide liquid in a magma conduit, resulting in dissolution of the sulphide liquid and PGE enrichment in the residual sulphide liquid.

The low Cu/Pd and high metal tenors in the W Horizon are modeled by sulphur dissolution upgrading. Bornite exsolution lamellae are common in chalcopyrite grains in the W Horizon. Chalcopyrite grains within the W Horizon are commonly mantled by magnetite, which differs from primary magmatic magnetite in that it lacks ilmenite exsolution lamella and has nearly a pure end-member composition. These two textures are interpreted as evidence of S loss. Whole rock S and Se contents were determined to further investigate S loss. During crystallization of a silicate melt or the formation of a sulphide liquid it is expected that the S/Se ratio will remain constant. However the removal of S through desulphidization to a gas or liquid is expected to decrease S/Se. Average S/Se values are 800 for the W Horizon, 1980 for the Main Zone and 1700 in unmineralized samples. The low S/Se found within the W horizon is additional evidence that desulphidization has occurred.