Abstract

The Topia district lies on the western flank of the Sierra Madre Occidental structural arch and Tertiary volcanic plateau. Beneath unmineralized mid-Tertiary silicic ignimbrites lies a kilometer-thick sequence of early Tertiary andesitic lavas and pyroclastics resting on Mesozoic limestones and marine clastic sedimentary rocks. Laramide geanticlinal uplift along the Sierra produced regional tilting and major fracturing of the volcanic pile. Following an interval of major erosion, the sequence was intruded by granodiorite stocks, and a northeast-trending set of faults was mineralized as Ag-Zn-Pb-Au-Cu-rich fissure veins. Mineralization was followed by an interval of tectonic quiescence and then by eruption of more than a kilometer of Oligocene silicic ignimbrites.K-Ar dating of igneous rocks and mineralization yields an age of 46.1 m.y. for one of the granodiorite stocks, a date of 43.8 m.y. for vein adularia, 43.5 m.y. for hydrothermal celadonite, and 44.0 m.y. for an andesite lava the K-Ar age of which was reset by the mineralization thermal event. A date of 37.9 m.y. on the lowermost rhyolite welded tuff corresponds to initiation of widespread silicic ignimbrite eruptions in the Sierra. Fissure-filling mineralization was accompanied by development of hydrothermal alteration comprising (1) a narrow (<1 m) quartz-sericite-pyrite alteration envelope (+ or - adularia + or - chamosite + or - kaolinite + or - hematite + or - celadonite) that grades outward into (2) a variably silicified oxidative alteration zone that occupies the entire interval between veins at elevations in and above the ore horizon. The distal oxidative facies envelopes consist of an inner intense celadonite + quartz + hematite + calcite + illite + clinochlore subfacies and an outer subfacies of hematite + calcite veinlets in nearly fresh rock. At elevations below the ore horizon, the local oxidative alteration facies is absent and the inner silicification zone passes outward into conventional propylitic facies envelopes. Chemical gains and losses and porosity changes have been monitored quantitatively across the succession of alteration envelopes.Oblique tensional slip along faults having gentle convex and concave irregularities and steps in the slip surface produced a succession of alternating dilatant (fissure) and convergent (shear zone) intervals along each fault. Open fissure intervals were filled by more or less symmetric, crustified precipitation on the walls, whereas wall-rock-replacement mineralization characterizes narrow veins in shear zone intervals. In each vein, ore occupies a restricted vertical interval of 100 to 200 m (the "ore horizon"), which grades downward into barren coarse-grained quartz-rich filling and grades upward into barren, cherty quartz-calcite-barite-rich vein filling. The ore horizon commonly contains subequal amounts of sphalerite, pyrite, galena, and quartz, with minor arsenopyrite, marcasite, chalcopyrite, Ag-rich sulfosalts, electrum, chamosite, muscovite, and carbonates. Locally, massive sphalerite as millimeter- to centimeter- thick crustification bands dominates the paragenetic sequence. Crustification bands dominated by quartz, calcite, and barite are predominantly fine grained; they precipitated from low-salinity, boiling fluids. Intercalated bands dominated by sulfides are characteristically very coarse grained (to 10 cm) and have high-salinity fluid inclusions (7-23 wt %); no primary vapor-dominant inclusions have been recognized in sphalerite or pyrargyrite. Precipitation of crustification bands dominated by coarse-grained sulfides was punctuated by recurrent effervescence of ore fluids within the ore horizon, accompanied by precipitation of electrum + quartz + or - calcite + or - barite and dissolution of sphalerite. At currently exposed levels above the ore horizon, diluted fluids were boiling more or less continuously.Voluminous bulk ore assay data, electron microprobe analyses of spatial variations in compositions of mineral solid solutions (Zn/Fe in sphalerite and Sb/As of polybasite), and analysis of trends in modal abundance of vein chlorite, barite, and fluorite reveal a systematic gradient of northeastward increase in the proportions of Ba, F, and Pb in the ore, the Zn/Fe ratio of sphalerite, and the Sb/As ratio of sulfosalts. The lateral zoning trend is definitely not due to lateral flow of mineralizing solutions but whether it is due to lateral variations in the lithologic constitution of the rock columns with which ascending solutions reacted, or to other causes, is still to be determined.Fluid inclusion studies indicate ore deposition mainly in the 280 degrees to 200 degrees C interval, with a reasonably systematic vertical decrease in temperature of about 25 degrees C/100 m. Direct analyses of fluid inclusions that trapped the liquid phase of vapor-saturated solutions yield CO 2 contents of 0.30 to 0.57 molal, increasing with depth. Fluid inclusion geobarometry is used (1) to show that regional tilting of the host andesites is preore, (2) to show that the 100- to 200-m-thick, districtwide ore shell formed at paleodepths ranging from about 500 to 1,250 m, and (3) to constrain the amount of postore erosion preceding silicic ignimbrite eruptions at about 115 m.

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