- GeoRef, Copyright 2004, American Geological Institute.
The Las Canas stratabound Pb-Zn deposit, in northern Chile, is hosted by the Lower Cretaceous back-arc marine carbonate and subordinate andesitic volcanic and volcaniclastic rocks of the Chanarcillo Group, one hundred meters above a thick pile of magmatic arc-related basaltic to andesitic lavas of the Bandurrias Formation. The deposit is not obviously related to local intrusive rocks, which include an Upper Cretaceous granodiorite batholith and dacite dikes emplaced along normal faults. Stratabound Pb-Zn-Ba mineralization is constrained within several porous stratigraphic units, including a 50 cm thick lapilli-tuff and less well mineralized algal-laminated limestone. Veins that locally carry some Pb-Zn occur along normal faults that displace the stratabound orebodies, and this study shows that they represent a different type of mineralization. Fluid inclusion microthermometric data for ore-stage quartz and barite indicate saline fluids (6 to 30 wt% NaCl eq ) and a formation temperature in the range of 80 degrees C to 160 degrees C for the stratabound mineralization. Two types of fluid inclusions occur in the veins. The type I vein fluid inclusions indicate less saline (average 7 wt% NaCl eq ) and slightly higher temperature fluids, in the range of 110 degrees C to 220 degrees C. The type II vein fluid inclusions indicate highly saline fluids (25 to 40 wt% NaCl eq ) which might be related to minor copper mineralization. 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios for galenas from the stratabound mineralization, with mean values of 18.42, 15.59, and 38.25, respectively, fall within the range of stratabound polymetallic deposits of northern and central Chile, and indicate a Lower Cretaceous magmatic rock source. Galenas from veins have slightly more radiogenic 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios, with mean values of 18.47, 15.61, and 38.34, suggesting two distinct episodes of mineralization. delta 34 S values for galena and barite range from negative values (-31.2 per mil to -12.5 per mil and -7.7 per mil to +12 per mil, respectively) in the mineralized tuff to higher values in the algal-laminated limestone (-2.5 per mil to +1.2 per mil and +13.2 per mil to +17.4 per mil, respectively), close to Lower Cretaceous marine sulfate values for barite, and homogeneous values for the galena from veins (-12.0 per mil+ or -1 per mil). Changing redox conditions can partly account for delta 34 S variations in the tuff. Sulfur sources include magmatic rock sulfur (-1.5 per mil to +9.7 per mil) as well as sedimentary sulfides and marine sulfate.