Nearly all biotites in a 2 km section across the Providencia stock, Zacatecas, Mexico contain between 0.25 and 0.50% chlorine. Their average chlorine content is twice to three times the mean chlorine content of biotites in granitic rocks; the bulk chlorine content of the stock exceeds the average chlorine content of granitic rocks by approximately the same amount. Quartz throughout the stock contains highly saline postmagmatic fluid inclusions. Since ore metals are strongly concentrated from silicate melts into highly saline aqueous fluids, we suggest that the development of the lead-zinc-silver ore deposits of the Providencia area was due in part to the abnormally large chlorine content of the stock. A relatively large chlorine content in intrusives may well be a necessary, though hardly a sufficient, prerequisite for the formation of associated magmatic-hydrothermal ore deposits. The chlorine content of intrusive rocks may therefore develop into a useful diagnostic prospecting tool.