Lead-zinc ore deposition in the light of fluid inclusion studies, Providence Mine, Zacatecas, Mexico

Abstract

In the Providencia area elongate pipe-like bodies of Pb-Zn sulfide ore occur in steeply dipping Upper Jurassic and Cretaceous limestones. Most of the ore bodies have a close spatial relationship to the Providencia granodiorite stock which is continuous at depth with the Concepci6n del Oro stock that has been dated at 40 n 2 million years. The mineralogy of the ore bodies is comparatively simple; sphalerite, galena and pyrite are the major sulfide minerals, and calcite and quartz are the gangue minerals. The bulk of the sulfide ore was formed by replacement of carbonate wall rocks, and the remainder by deposition of minerals in vugs. Ore deposition appears to have been structurally controlled by both fractures and bedding. Sulfur isotope studies on minerals from the ore bodies and veinlets within the stock gave 8 S a4 values within the magmatic hydrothermal range. Fluid inclusion studies were made on sphalerite, quartz, calcite and fluorite, the bulk of the data being obtained from sphalerite crystals. The results are consistent and indicate that the massive ore was emplaced at temperatures above 350 o C, whereas zoned sphalerite crystals were emplaced over the temperature interval from 350-200 o C. Calcite and quartz were generally formed at temperatures below 350 o C. 425oC is considered to be a reasonable upper limit for hydrothermal ore deposition at Providencia. The fluid inclusion data suggest that vertical temperature gradients within the conduits during mineralization were less than 50 o C/Km. Considerations of the cooling history of the Providencia stock and the rocks in its vicinity indicate that at least 10 o years must have elapsed after intrusion of the granodiorite prior to the deposition of sphalerite at temperatures as low as 200 o C. Salinity studies on fluid inclusions demonstrate that the salinity of the ore-forming solutions varied between 5 and 40 equiv. wt. % NaC1 and that the pattern of these variations was complex. The possible mechanisms by which such strong and repeated salinity variations could have taken place during ore deposition are discussed. Simple calculations show that the volume of hydrothermal solution needed to deposit the Providencia ore bodies was probably of the order of 10 km a. The source of these solutions is concluded to be a large magmatic body at some depth.