THE VOLCANO-SEDIMENTARY SETTING OF THE KUROKO TYPE VHMS DISTRICT OF CUALE, JALISCO, MEXICO

The Cuale mining district, situated in the Cordillera Madre del Sur, ca. 30 km SE of Puerto Vallarta, comprises a number of small, high-grade Pb, Zn, Cu, Au and Ag rich massive sulfide deposits. The mineralization is hosted by a rhyolitic volcano-sedimentary complex that is cut by subordinate late andesitic dikes. Regionally, the Cuale volcanic rocks are part of the Guerrero Terrane and are interpreted as a roof-pendant within the Sierra Madre del Sur Batholith. They exhibit generally intense chloritic, sericitic and silicic hydrothermal alteration, but have not undergone significant regional metamorphism and are only weakly deformed. U -Pb geochronology on zircons from two rhyolite samples yielded Middle to Late Jurassic dates of 162.4 +/- 0.7 and 155.9 +/- 1.6 Ma, respectively. To date, three sub-units have been defined within the rhyolite package on the basis of petrographic, geochemical and field relationships: 1) a footwall sequence of quartz and plagioclase phyric rhyolite flows and tuffs exhibiting low Zr/Y (1 - 1.9) and Zr/Nb ratios (6.5-14); 2) an aphyric to minor quartz phyric rhyolite with Zr/Y: 1.6 - 4.9 and Zr/Nb 9.5-26; and 3) a quartz and plagioclase phyric rhyolite intruding and unconformably overlying the former units with Zr/Y: 2.2 - 7.6 and Nb/Zr: 8 - 26. The overall trace element signature indicates subalkaline tholeiitic compositions with a weak trend to a calc-alkalic affinity. Ore bodies that exhibit characteristics of a proximal setting (high Cu and Au, content, stockwork mineralization) are spatially and, by inference, genetically related to aphyric rhyolite flows and domes. Distal ore bodies, rich in Pb, Zn and Ag, are spatially associated with black shale and locally exhibit sedimentary textures. They are interpreted to have been deposited in anoxic basins adjacent to the rhyolite domes.