Use of intrusive and calc-silicate compositional data to distinguish contrasting skarn types in the Darwin polymetallic skarn district, California, USA
The polymetallic Pb-Zn-Ag-W-Cu-Au skarn and vein ores of the Darwin area, California, represent a metal association not common in skarn deposits. Detailed and regional geologic mapping combined with modal and chemical analyses of igneous rocks and electron microprobe and assay studies of calc-silicate rocks shows that there are three separate mineralizing systems which have been superimposed by structural and intrusive events. Scheelite-bearing skarns are associated with the alkaline Darwin stock; small Cu±Au skarns are adjacent to sheared (thrust-emplaced) granite-granodiorite masses; and Pb-Zn-Ag vein skarns are zoned around small granite porphyry dikes and plugs. Calc-silicate mineral compositions are also indicative of three different mineralization systems: W-skarns have high Fe pyroxenes and moderate Fe garnets; Cu-skarns have low Fe-pyroxenes and high Fe-garnets; and Pb-Zn skarns have high-Mn pyroxenes and garnets (patterns seen in other skarn deposits). Vein ores contain metals compatible with the subjacent skarns and apparently represent lower-temperature remobilization of skarn ores.