Page 102 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 101
section and decreasing value along the intersections of the two series of fractures. Along the strike under the shale, the ore may be continuous from one deposit to the next for as much as 400 feet. In depth, however, the ore generally pinches out in less than 100 feet below the main ore horizon. The ore is not gener- ally in direct contact with the Percha shale, but is separated from it by a thin layer of limestone, through which veinlets of sulfide ore extend. The rocks adjacent to the contact between the Percha shale and the underlying Fusselman limestone have been silicified in a manner very similar to that at Hillsboro and Lake Valley. In some places it is the underlying limestone which has been extensively altered along a zone of very irregular thick- ness to a white to pink flint breccia, often with crystals of quartz lining the irregular spaces between the breccia fragments. In other places the limestone is apparently unaffected, all the re- placement having occurred in the shale, which has been converted into a dark gray to black flint breccia, often with areas of red jasper-like material along the immediate contact. Specks and nodules of pyrite are irregularly distributed within the shale, but none were detected within the limestone. As at Lake Valley and Hillsboro, the silicification and brecciation extend downward along fractures and faults, and where faulting is extensive, the silicification has extended out on both sides of numerous parallel fractures until the whole mass of the rock throughout a wide zone of shearing has been changed into flint breccia, and much of the local structure is thereby concealed. It is reported that this silicified material usually contains small amounts of gold. The rock is locally called "quartzite" in the various districts where it is found. The writer interprets this silicification as the last episode of the intrusion of stocks in the region preceding the introduction of the ore solutions. It is believed that silica-bear- ing waters rose up along fault planes and pervaded the rocks along an extensive bedding fault that had developed at the con- tact of the Percha shale and the Fusselman limestone, due to the differential stretching of these beds at the time of their arching. Brecciation along both the fault zone and the bedding fault would provide a favorable zone in which the siliceous solutions could completely replace the limestone and shale fragments and the finer interstitial material, and thus form an impervious rock which the later ore-bearing solutions could not penetrate.
The primary minerals of the district consist of silver-bear- ing galena and chalcopyrite, sphalerite, pyrite, and the gangue minerals, rhodochrosite, manganiferous calcite and quartz. Ala- bandite, the sulfide of manganese, has been recognized in speci- mens from the district, and it is apparently fairly common, although none was seen by the writer. Some argentite (silver sulfide) occurs associated with the other sulfides, and gold in small quantities is generally present in the quartz. The mineral-