Chemical composition of the hydrothermal fluids responsible for the lead-zinc deposits at Providencia, Zacatecas, Mexico

The chemical composition of 22 samples of primary fluid inclusions in quartz, calcite, and sphalerite from Providencia, Mexico, has been determined. Samples were prepared, crushed, and leached as described by Roedder et al. (1963). Cation analyses of leachates were made by atomic absorption spectroscopy, and the anions were analyzed by a special X-ray fluorescence technique.

The Na, K, Ca, and C1 concentrations show a considerable range that reflects the widespread salinity variations in the hydrothermal fluids observed by Sawkins (1964). The K/Na atomic ratios of most samples range from 0.18 to 0.43 and the ratios tend to increase with the salinity of the inclusions. The temperatures indicated by the K/Na ratios from the published curves of K/Na ratios vs. temperature are mostly at least 100 ø C greater than corresponding filling temperatures. The Ca/Na atomic ratios of inclusions in sphalerite range from 0.12 to 0.61. The K/Na and Ca/Na data agree well with previous δO¹⁸ data (Rye, 1966; Rye and O'Neil. 1968) which indicate that the hydrothermal fluids were derived from the magma related to the Providencia granodiorite stock and that the fluids reacted only slightly with the crystalline stock during the late phases of ore deposition. The data also indicate that the salinity variations in the hydrothermal fluids probably occurred at the source of the fluids and may have resulted from boiling in the magma chamber.

Mg concentrations are generally no more than a few hundred parts per million. Most of the Mg in the hydrothermal fluids was evidently removed during the formation of calcium-magnesium silicates in the lower levels of the ore pipes. Mg/Ca atomic ratios are generally less than 0.08 and are consistent with the paucity of dolomite in the area.

Zn and Cu concentrations in water leaches of inclusions in calcite are generally less than 50 ppm. Maximum base-metal concentrations from water leaches of two samples of inclusions in quartz are 890 ppm Zn and 530 ppm Cu.

Chloride is the major anion in the fluids, and C1 concentrations vary with corresponding cation concentrations. With two exceptions, sulfur concentrations of the inclusions in calcite and quartz, analyzed as SO₄, are below the level of detection