Melt inclusion chemistry and associated Cu-Mo-Au mineralization in 2.7-Ga porphyry intrusions and volcanics, Timmins, Ontario

Abstract

Melt inclusions are tiny samples of melt that are trapped by crystals growing in a magma. In this study, melt inclusions hosted in quartz phenocrysts from the Paymaster Porphyry, Crown Prophyry and Krist fragmental unit, Timmins, Ontario, were analyzed to provide a better understanding of the processes involved in compositional evolution of Archean porphyry magmas. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of the melt inclusions as well as Back Scattered Electron – Scanning Electron Microscopy (BSE-SEM) analyses of mineral surfaces from the Pearl Lake porphyry were used to further investigate the behaviour of Cu, Mo and Au. The mineralogy of the porphyry rocks from four localities comprises mostly quartz (~25-35%), seriticized plagioclase (~20-40%), seriticized K-feldspar (~15-20%), chloritized biotite (<5%), calcite (~5%) and trace amounts of chalcopyrite. SEM data shows that Cu and Au coprecipitated in the porphyry ores and this implies that Cu and Au are genetically related; therefore, any processes that influenced Cu enrichment might have also influenced Au. The melts from the Paymaster and Crown porphyry along with the Krist fragmental fall in the adakitic field (high Sr/Y, low Y and high La/Yb(N), low YbN). The adakitic evolutionary trend that is shown by these melts cannot be portrayed by bulk rock analyses as they are limited in compositional range and implies fractionation of metals at depth. Paymaster and Crown porphyry melt inclusion compositions along with those from the Krist fragmental unit demonstrate that the melts in these intrusive systems were sourced from the same parental liquid, but show some differences in Cu-Mo-W-As-Sb-Ag-Bi and Sn. The adakitic trend is further supported and illustrated by other geochemical discriminations, such as the Yb/Ta discrimination diagram which places the melts in the VAG field, confirming that the three localities are most consistent with orogenic volcanic arc granite setting. A TAS classification diagram places the bulk composition of the melts mainly in the dacite field. The melt inclusion data shows that the magmatic history of the system is protracted since the porphyry melts are compositionally similar over a ~10 Ma window of intrusion + fragmentation (Krist) and intrusion (Paymaster + Crown). (Yb)N vs (La/Yb)N, Y and Cu vs Sr/Y and Nb/Ta vs Zr/Hf plots illustrate a relationship showing the fractionation of elements and the evolution of metals, as the Nb, Ta and Cu concentrations seem to be following trends that are not necessarily explained by the crystallization of minerals in a chamber at depth, but instead possibly inform us about magma degassing in the Archean. This study confirms that melt inclusion analyses conducted on Archean age rocks can quantify metal contents.