APPLICATION OF LA-ICP-MS TO SULFIDE MELT INCLUSION ANALYSIS: EXAMPLE FROM BROKEN HILL, AUSTRALIA

It has recently been suggested that during peak metamorphic conditions at Broken Hill, the orebody became partially molten. New research has revealed the existence of polyphase sulfide melt inclusions (SMINCs) hosted in garnetites associated with “remobilised” ore and are believed to represent the only direct evidence of a trapped sulfide melt. SMINCs (up to 40µm in diameter) are composed of a combination of sulfide daughter minerals including galena, tetrahedrite, chalcopyrite, bornite, arsenopyrite and sphalerite amongst others. Due to the presence of daughter phases, SMINCs are inhomogenous resulting in analytical difficulties for traditional area-based analysis techniques (WDS, EDS) which cannot provide a representative bulk composition from exposed SMINCs.

LA-ICP-MS is a technique well suited for multi-element in situ analysis of silicate hosted SMINCs as it permits analysis of fully submerged inclusions, for major, minor and trace elements. Ablation of individual SMINCs hosted within garnets produce a fairly complex ablation signal, and within the signal, the relative intensities of different elements indicate which daughter phases are present.

A series of homogenisation experiments were undertaken; partial melting was observed at 620°C, 5kbar and complete homogenisation at 720-840°C, 5kbar. EDS analysis of homogenized SMINCs was undertaken to determine an internal standard which was used to calculate bulk SMINC compositions from the LA-ICP-MS data.

SMINC bulk compositions are dominated by Pb and Cu with minor Sb and As, and a ~17 fold increase in the Ag/Pb ratio relative to the average lead lodes.

Results from homogenisation experiments and bulk compositions from the LA-ICP-MS combined with estimated peak metamorphic conditions for Broken Hill (810°C, 5kbar) strongly suggest that polyphase sulfide inclusions represent trapped sulfide melts, in support of the suggestion that the Broken Hill orebody partially melted during peak metamorphic conditions.