HYDROTHERMAL GEOCHEMISTRY OF SILVER-GOLD VEIN FORMATION IN THE TAYOLTITA MINE AND SAN DIMAS MINING DISTRICT, DURANGO AND SINALOA, MEXICO (SIERRA MADRE, FLUID INCLUSIONS).
KeywordsMines and mineral resources -- Mexico -- Sinaloa (State)
Geochemistry -- Mexico -- Sinaloa (State)
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PublisherThe University of Arizona.
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AbstractThe 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.