HIGH CHLORINE ALTERATION MINERALS AND CALCIUM-RICH BRINES IN FLUID INCLUSIONS FROM THE STRATHCONA DEEP COPPER ZONE, SUDBURY, ONTARIO

The Stratheona Deep Copper zone consists of a complex suite of massive sulfide veins that occupy a series of fractures in the footwall gneis ses and Sudbury breccia. It is characterized by the occurrence of bran ching quartz veinlets and hydrothermal alteration selvages along the m argins of massive sulfide veins. Hornblende (or actinolite), epidote, calcite, and ferropyrosmalite [(Fe,Mn)8Si6O15(OH,Cl)10] occur in the i nner 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 am ounts of chlorine. Ferropyrosmalite contains 5.79 to 6.54 wt percent C l; 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 b ranching veinlets are mutiphase consisting of vapor, liquid, and more than four solids. The solid crystals of opened inclusions identified b y scanning electron microscope are Na, Ca, K, K-Pb, K-Fe, and Fe-Mn ch lorides. The sequence of melting events and the nature of the solid ph ases suggest that the trapped fluids are NaCl-CaCl2-H2O solutions with minor K, Fe, Mn and Pb. These fluid inclusions are highly saline (avg 60 wt %: NaCl + CaCl2), calcium-rich (Ca/Na > 1, by weight), and homo genize to liquid at temperatures 260-degrees to 420-degrees-C (342-deg rees +/- 35-degrees-C, mean +/- std. dev.). Two options are proposed f or the origin of the fluids: melt-fluid phase separation during sulfid e 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 f rom the magmatic sulfide veins.