HYDROTHERMAL GEOCHEMISTRY OF SILVER-GOLD VEIN FORMATION IN THE TAYOLTITA MINE AND SAN DIMAS MINING DISTRICT, DURANGO AND SINALOA, MEXICO (SIERRA MADRE, FLUID INCLUSIONS).

Abstract

The San Dimas mining district, including the Tayoltita mine, is a Tertiary silver-gold epithermal vein system deposited in a calcalkaline volcanic pile. Hydrothermal alteration and vein formation is temporally related to a granite batholith intruded into the volcanics. Alteration mineralogy in andesites is compatible with a hydrothermal flow model in which heated water rises through the batholith, cools to 260°C, and flows out into the volcanics. In the process, a(Na)⁺/a(H)⁺, a(K)⁺/a(H)⁺, a(Ca)⁺⁺/a²(H)⁺, a(SO₄)⁼.a²(H)⁺, and a(H₂S) increase; a(A1)⁺⁺⁺/a³(H)⁺ decreases; and a(Fe)⁺⁺/a² (H)⁺ remains constant, all relative to original fluid conditions in the andesites. Lateral elongation of Ag:Au ratio zoning plotted on vertical projections of veins is interpreted to reflect hydrothermal fluid flow principally in a horizontal direction during ore deposition. Quartz vein-filling, accompanied by chlorite, calcite, rhodonite, and adularia, is widest in a vertical interval approximately 500 to 1,000 meters below the original surface. Pyrite is widely distributed, but silver minerals, electrum, and base-metal sulfides are restricted to the upper portion of the vertical interval of veining in a zone termed the ore horizon. Paragenetic relationships among vein minerals of the Cinco Senores vein in the Tayoltita mine indicate that a(H₂S) decreased; a(Cu)⁺/a(H)⁺, a(Ag)⁺/a(H)⁺, and a(Au)⁺/a(H)⁺ increased; and a(Fe)⁺⁺/a²(H)⁺ and a(SO₄)⁼.a²(H)⁺ remained nearly constant during the initial stage of ore deposition. Fluid inclusion studies of quartz from the Cinco Senores vein indicate that ore deposited at an average temperature of 260°C from boiling fluids of apparent salinities ranging from 0.15 to 0.3 m(NaCl) equivalent. The greater apparent salinities probably reflect dissolved gases as well as chloride salts. Correlation of Ag:Au ratios in deposited vein with ice-melting temperatures in fluid inclusions suggests that evolution of ore fluids in space was accompanied by both increase in deposited Ag:Au ratios and decline in fluid solute concentration. Correlation of ice-melting temperatures with paragenetic age of associated quartz suggests that vein-depositing hydrothermal fluids evolved in both space and time from relatively concentrated to dilute conditions. Both boiling and mixing could have caused this decline in solute concentration.