Abstract

The Batopilas mining district is one of the few mining districts where the major ore mineral is native silver. The native silver occurs as fissure vein deposits in granodiorite, fine-grained diorite, and dacite. Orebodies are irregularly shaped pods of massive native silver in calcite and may contain up to 75 wt percent silver. The veins formed along north- and northeast-trending fractures and faults.The geologic history of the district is summarized as follows: intrusion of the Pastrana dacite, Dolores microquartz diorite, and Tahonas granodiorite, sequentially, into preexisting Mesozoic sediments and/or igneous rocks. Some of these intrusive events may have vented to the surface. After the intrusions were eroded, the San Jose, Arenal, and Casas Coloradas flow breccias were deposited. Between each episode of flow breccia deposition, there occurred periods of tectonism and erosion. After a fourth major episode of tectonism and erosion, the massive ignimbrites of the upper volcanic series represented by the Yerbanis Formation were deposited. Contemporaneous with Yerbanis deposition was the intrusion of rhyolite dikes. Numerous andesitc dikes of uncertain age are also present.The native silver orebodies are generally less than 30 m in length with the width varying from a few centimeters to 4.6 m and averaging 0.8 m. The ore-grade mineralization terminates abruptly in both vertical and horizontal directions. The orebodies are connected vertically and along strike by up to 90 m of unmineralized fractures or vein stringers of calcite, sometimes less than 3 mm wide.Flexures in vein orientation and/or cross fractures are associated with some orebodies, whereas other orebodies have no apparent structural control. Most orebodies are hosted by Pastrana dacite and Tahonas granodiorite.Three types of wall-rock alteration are found with the silver mineralization: (1) chlorite-actinolite alteration, (2) silicification, and (3) pyritization. Chlorite-actinolite alteration is often associated with gray and green argillization. The host rocks of many, but not all, orebodies have experienced chlorite-actinolite alteration and pyritization. Silicification occurs in dikelike masses within the Dolores and Pastrana units. These silicified zones are generally less than 9 m wide. Pyritization is found in the intrusions. Minor quartz-pyrite veins are found throughout.Gangue in the Batopiias ores is calcite and is associated with small amounts of sphalerite, galena, and laumontite. Cobalt-nickel arsenides and ruby silvers have been reported, but their presence is not confirmed by this investigation. Pyrite is abundant in the host rocks but is absent from the ores.Two kilometers southwest of the village of Batopilas is an area of approximately 5 km 2 devoid of vegetation, the hillsides and tops have a thick gossan (Corralitos). The underlying Tahonas granodiorite exhibits widespread sericitic alteration, and extensive to almost complete quartz veining and replacement are present. Chalcanthite stalagtites form on many overhangs along the arroyos. This area is interpreted as the Tahonas porphyry copper deposit.A well-developed pattern of mineralogic zonation exists in the district. This zonation is concentric around the Tahonas porphyry copper deposit. Preliminary fluid inclusion studies document homogenization temperatures (T h ) between 217 degrees and 434 degrees C, and salinities over 20 equiv wt percent NaCl for quartz from the Tahonas porphyry copper deposit. Studies on quartz from outlying regions give homogenization temperature ranges of 224 degrees to 351 degrees C, and salinities of from 9 to 18 equiv wt percent NaCl. Studies of the later generation of calcite, silver, and minor sulfides give (T h ) values between 137 degrees and 257 degrees C for calcite and 170 degrees to 196 degrees C for sphalerite. Salinities in calcite are concentrated between 7.6 and 13.2 equiv wt percent NaCl. The estimated 300 million oz (9.3 million kg) of silver produced to date probably was deposited from circulating connate waters adjacent to the Tahonas porphyry copper mineralizing system.

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