The Strathcona Deep Copper zone consists of a complex suite of massive sulfide veins that occupy a series of fractures in the footwall gneisses and Sudbury breccia. It is characterized by the occurrence of branching quartz veinlets and hydrothermal alteration selvages along the margins of massive sulfide veins. Hornblende (or actinolite), epidote, calcite, and ferropyrosmalite [(Fe,Mn) 8 Si 6 O 15 (OH,C1) 10 ] occur in the inner zones of the alteration selvages adjacent to the massive sulfide veins. Annite, chlorite, and albite are developed in the outer zones. Fe-rich silicates of the alteration assemblages contain significant amounts of chlorine. Ferropyrosmalite contains 5.79 to 6.54 wt percent Cl; hornblende contains up to 2.99 wt percent Cl; annite contains up to 2.13 wt percent Cl.Primary fluid inclusions in the quartz from the branching veinlets are mutiphase consisting of vapor, liquid, and more than four solids. The solid crystals of opened inclusions identified by scanning electron microscope are +Na, Ca, K, K-Pb, K-Fe, and Fe-Mn chlorides. The sequence of melting events and the nature of the solid phases suggest that the trapped fluids are NaCl-CaCl 2 -H 2 O solutions with minor K, Fe, Mn and Pb. These fluid inclusions are highly saline (avg 60 wt %: NaCl + CaCl 2 ), calcium-rich (Ca/Na >1, by weight), and homogenize to liquid at temperatures 260 degrees to 420 degrees C (342 degrees + or - 35 degrees C, mean + or - std. dev.).Two options are proposed for the origin of the fluids: melt-fluid phase separation during sulfide liquid crystallization, and convection of ambient fluids across the contact of the sulfide veins. The Cl-rich fluids may have participated in the precipitation of Pd and probably other platinum group elements (PGE) at a later stage, but they did not transport the PGE very far from the magmatic sulfide veins.