Geology and Ore Genesis of Silver–Barite Mineralization in the ...

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volcanism was occurring at the southeast end of the Calico Mountains as recently as 16. 5 Ma. This is difficult to reconcile with a transpressive environment. Such a setting would result in compressional stress inhibiting the rise of silicic magmas to the surface. It seems more likely that dacitic volcanism was related to extension during the waning stages of detachment. Furthermore, the Waterloo and Langtry deposits are cleanly offset 2–3 kilometers by the Calico fault (Fletcher, 1986). This must have occurred after the ore was emplaced at 17–17.5 Ma. Had dextral shear been contemporaneous with ore emplacement it is expected the ore bodies would be elongated or “strung out” parallel to that fault, or that at least pods of mineralization would be found scattered between the two ore bodies. Neither is the case; field evidence supports the emplacement of mineralization before the Calico fault underwent right-slip motion. Figure 8. Cartoon model for silver-barite mineralization in the central Mojave. Figure 8 is a suggested model for silver-barite mineralization in the central Mojave. Northeast transport of the detached block from 25 to 19 Ma created a linear northwest–southeast-trending basin. Extension resulted in volcanism (Pickhandle Formation) followed by erosion and deposition of the Barstow Formation. Extension resumed around 17.5 Ma and continued until 16.5 Ma (Singleton and Gans, 2008. Volcanism occurred in the eastern Calico Mountains and heat from the pluton(s) drove a convective geothermal system to the west. Meteoric water moved downward along the detachment fault surface, was heated, and moved convectively upward along listric faults in the overlying plate. As the fluid cooled or was diluted, ore deposition occurred within the Pickhandle Formation. Evidence suggests ore deposition in the overlying Barstow Formation was contemporaneous. This requires that fluids rose locally along faults into the Barstow Formation. Impermeable horizons (shales?) would have prevented fluid migration unless they were breached by faulting. The nature of faulting in the Barstow Formation is uncertain. Fletcher (1986) states that 1% of all Barstow mineralization is vein controlled, but that the veins have a random orientation, unlike the northwest strike of all Pickhandle veins. Perhaps extension occurred when the lower Barstow was only poorly indurated and hosts for mineralization were porous and brecciated sediments of the Barstow Formation. Right oblique-slip on the Calico fault then offset mineral zones developed in the Barstow Formation 2–3 kilometers. In addition, transpression caused reverse faulting of the block north of the Calico fault resulting in uplift and anticlinal arching of the Calico Mountains (Dibblee, 1980; Singleton and Gans, 2008). This model agrees with Rosso et al. (1992) and Fletcher (1986) who argue that Pickhandle mineralization was emplaced at depths of one–1.5 kilometers while mineralization in the Barstow Formation was deposited at depths of less than a few hundred meters. The present topographic relationship requires at least a kilometer of uplift to account for Pickhandle silver–barite veins at the head of Wall St Canyon situated 300 meters above disseminated mineralization of the Waterloo deposit in the Barstow Formation.
While the above model fits much of the analytical data and field observation, some questions nonetheless remain. Barite mineralization in the Mitchel Range occurs in northwest-trending veins within Waterman Gneiss (lower plate of the Waterman Hills detachment fault). How can this be reconciled with the model? Furthermore, Dokka (1986) states that the Pickhandle Formation has undergone less than 5% extension, a conclusion that led him to exclude rocks north of the Calico fault from the extended terrain of the central Mojave Desert. If the Calico Mountains have undergone minimal extension, how was the fault-controlled open space for the Pickhandle mineralization created? The paragenetic sequence requires that ore fluids were initially oxidizing (barite/hematite/magnetite), becoming more reduced (pyrite, other sulfides) over time. What caused the Eh drop and why did it occur late in the ore fluid evolution? References Cited Bartley, J.M., Glazner, A.F., and Schermer, E.R., 1990, North-south contraction of the Mojave block and strike-slip tectonics in southern California: Science, v. 248, p. 1398-1401. Bowen, O.E., 1954, Geology and mineral deposits of the Barstow Quadrangle, San Bernardino County, CA: California Division of Mines and Geology Bulletin 165, p. 185. Burke, D.B., Hillhouse, J.W., McKee, E.H., Miller, S.T., and Morton, J.L., 1982, Cenozoic rocks in the Barstow basin area of southern California—Stratigraphic relations, radiometric ages, and paleomagnetism: U.S. Geological Survey Bulletin 1529-E, p. 1-16. Byers, F.M., 1960, Geology of the Alvord Mountain Quadrangle, San Bernardino County, California: U.S. Geological Survey Bulletin 1089A, 71 p. Cox, B.F., and Wilshire, H.G., 1993, Geologic map of the area around the Nebo Annex, Marine Corps Logistics Base, Barstow, California: U.S. Geological Survey Open-File Report 93-568, 36 p. DeLeen, J., 1950, Geology and mineral deposits of the Calico Mining District: Unpublished Master's Thesis, University of California-Berkeley, 86 p. Dibblee, T. W., Jr., 1967, Areal geology of the western Mojave Desert, California: U. S. Geological Survey Prof. Paper 522, 153 p. Dibblee, T. W., Jr., 1970, Geologic map of the Daggett Quadrangle, San Bernardino County, California: U. S. Geological Survey Misc. Geological Inv. Map I-592. Dibblee, T. W., Jr., 1980a, Cenozoic rock units of the Mojave Desert, in Fife, D.L., and Brown, A.R., eds., Geology and mineral wealth of the California desert: Dibblee Volume: Santa Ana, California, South Coast Geological Society, p. 41-68. Dibblee, T. W., Jr., 1980b , Geologic structure of the Mojave Desert, in Fife, D.L., and Brown, A.R., eds., Geology and mineral wealth of the California desert: Dibblee Volume: Santa Ana, California, South Coast Geological Society, p. 69-100. Dokka. R.K., 1979, Styles and timing of late Cenozoic faulting, central Mojave Desert, California: Geological Society of America Abstracts with Programs, Annual Meeting, p. 414.
Page 1 and 2: Geology and Ore Genesis of Silver-
Page 3 and 4: Stratigraphy General Geology Portio
Page 5 and 6: Widespread gravels in the Yermo Hil
Page 7 and 8: Veins with comb-textured barite lin
Page 9 and 10: concordant to bedding of the volcan
Page 11: veins are hosted by upper plate roc
Page 15 and 16: Jessey, D. R., and Tarman, Donald W

Geology and Ore Genesis of Silver–Barite Mineralization in the ...

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