Geology, tectonic environment, structural controls in the San Martin de Bolanos District, Jalisco

Exxon Co. U.S.A., Midland, TX, United States

The San Martin de Bolanos mining district is located in the southernmost portion of the Barranca section of the Sierra Madre Occidental province and is approximately 190 km by road from Guadalajara, Jalisco, Mexico. The volcanic rocks, consisting of intercalated ash and lava flows, are early Miocene in age, and mineralization shows a close affinity to those of andesitic composition. Other lithologies present include extensive postmineralization capping ash flows and a bimodal suite of basalts and rhyolites which appear to be restricted to the floor of a structural depression. These basalts are subalkaline to weakly alkaline in composition, but a rhyolitic volcanic neck of late Miocene to Pliocene age is strongly alkaline. Situated within the valley is a large accumulation of stratified volcaniclastics and conglomerates. Along the western escarpment are laharic flows which unconformably overlie the stratified sediments.Structurally, the district is situated in a large north-northeasterly trending graben that formed in Miocene time with later readjustments of Pliocene age. This graben is a downdropped block 35 to 40 km in width and 150 km in length. The five stages of graben evolution consist of: (I) Oligocene-Miocene "ignimbrite flare-up," (II) arching, fracturing, and fissure vein mineralization, (III) postmineralization ash-flow tuff emplacement, (IV) partial subsidence of the arched structure due to volcano-tectonic collapse and partial overlap with later collapse due to extension that began in middle to late Miocene time, and (V) continued minor periodic subsidence during the final stages of valley development. Apparent back-arc extension or incipient spreading ridge jumps following convergent plate motion are most likely responsible for the structural grain.Ore mineralization consists primarily of stromeyerite, argentitc, silver oxides, and small amounts of gold in an epithermal environment. Quartz is the primary gangue mineral and there are lesser amounts of specularitc, sphalerite, fluorite, chalcopyrite, galena, pyrite, and barite. Electron microprobe analyses show that the average value of silver in galena is only 0.08 wt percent Ag and the iron content of sphalerite is 0.95 wt percent Fe. The veins are steeply dipping and the fractures pinch and swell, localizing the metals in the dilations. The late-mineralization fluids at San Martin de Bolafios were evidently of low temperature (140 degrees C) and salinity (0.5 equiv wt % NaCl), as interpreted from the fluid inclusion study of fluorite. Although the source of the mineralizing fluids is unknown, an adjacent rhyolitic intrusion emplaced in the western escarpment of the graben may be genetically related to mineralization.The total vertical extent of mineralization in the area is not known, but near-vertical mineralized intervals are known to exceed 250 m. Dating of pre- and postmineralization volcanic flows indicates that the mineralization occurred within the time span of 23.7 to 21.3 m.y. It does not appear from a comparison of the attitudes of the fault and fracture patterns of the graben structure to those containing mineralization that the mineralization was genetically related to the development of the graben structures since metallization predates the main taphrogenic episode.

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