Transactions . t and pitches southeast. Two main sets of faults are responsi-ble for the shape of the orebody, mark the zones of shattering that per-mitted the entrance of the mineral-bearing solutions, and indicate thedirections of movement that followed the period of dike intrusion. The VOL. 386 GEOLOGY AND MINING METHODS AT PILARES MINE first, or easterly set of faults, has an approximate N 70° E trend; thesecond, or northwesterly set, has an approximate N 40° W trend. Varia-tions of 10° to 20° from these directions occur. The easterly set offaults, with the exception of the Esperan
Transactions . t and pitches southeast. Two main sets of faults are responsi-ble for the shape of the orebody, mark the zones of shattering that per-mitted the entrance of the mineral-bearing solutions, and indicate thedirections of movement that followed the period of dike intrusion. The VOL. 386 GEOLOGY AND MINING METHODS AT PILARES MINE first, or easterly set of faults, has an approximate N 70° E trend; thesecond, or northwesterly set, has an approximate N 40° W trend. Varia-tions of 10° to 20° from these directions occur. The easterly set offaults, with the exception of the Esperanza fault zone and a fewminor slips, dip steeply to the south; the northwesterly faults dip, as arule, steeply to the southwest. Tangential movement, causing a fracturing of the corners of the paral-lelogram formed by the intersection of the two sets of faults, would ac-count for the oval outline of the mine. The corners of the oval, otherthings being equal, containing the greatest number of fractures, would. Fig. 3. -Vertical section through mine oval from Esperanza to Pilares SHAFTS. also become the most favorable places for ore deposition. Post-oremovement appears to have been along two directions: the earlier, whichthe diabase followed, was northwest; the later, which cuts the dike andoffsets the orebodies, was easterly. The combination of a soft dike andpronounced fault zones has produced a real difficulty in the method ofmining. Ore-bearing Rocks.—The only volcanic rocks that have been cutby the mine workings are the latite and andesite breccias. The latitebreccia has a field habit that, in places, is anything but that suggestiveof a breccia. On Paulina Mountain, the breccia habit is clearly defined,but masses of rock resembling parts of a flow are frequently encountered W. ROGERS WADE AND ALFRED WANDTKE 387 und(n-ground. Some of these masses, where they arc cut by mine work-ings, are fully 40 ft. (12 m.) across, and at least 10 ft. (3 m.) in theother directions. In
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Keywords: ., bo, bookcentury1800, bookdecade1870, booksubjectmineralindustries
