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

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The overall similarity of paragenesis for the Pickhandle veins and Barstow disseminated ores is supported by the atypical nature of the paragenetic sequence (oxides early—sulfides late). Most epithermal deposits are characterized by early sulfide mineralization with later deposition of oxides and carbonates. As both the Calico veins and Waterloo/Langtry deposits are paragenetically atypical, it seems more likely there was a single evolving fluid responsible for ore deposition and not two unrelated events. Figure 6. Detailed paragenesis for the vein mineralization of the Calico District and disseminated ores of the Barstow Formation (Waterloo/Langtry paragenesis after Fletcher, 1986). Fluid Inclusion Geothermometry Fluid inclusion data are summarized in Figure 7. Homogenization temperatures for barite range from 160° to 310° C. Calico District barite fluid inclusions are often large (>50 microns) and are restricted to Type I, indicating the fluids were not boiling at the time of barite crystallization. Homogenization temperatures range from 240° to 310° C and salinities from 2–4 wt% NaCl. Utilizing paragenesis, barite geothermometry and salinity, and Eh-pH relationships, Rosso (1992) calculated depth of barite emplacement at one to 1.5 kilometers. Limited analysis of quartz (not shown) has yielded homogenization temperatures similar to but slightly higher than those for barite (average 290°C). Fletcher (1986) analyzed both barite and quartz from the Waterloo and Langtry deposits within the Barstow Formation. He reported homogenization temperatures from 140° to 230°C with salinities of 4–6 wt% NaCl. Barite inclusions were generally smaller than those from the Pickhandle veins and Type II inclusions, indicative of boiling, were common. Utilizing similar data and logic to that of Rosso, Fletcher (1986) calculated depth of emplacement of Barstow mineralization at no greater than a few hundred meters. Figure 7 combines data for the Waterman Hills and Mitchel Range. While homogenization temperatures show considerable overlap (Waterman Hills 180° to 220° C and Mitchel Range 180-270°C) there are significant differences between the two localities (Jessey and Tarman, 1994). Waterman Hills barite
veins are hosted by upper plate rocks correlative to the Miocene Pickhandle Formation (upper plate WHDF). Inclusions were the smallest observed in the central Mojave Desert (average diameter 10-15 microns) and evidence for boiling the most widespread. Although salinity could not be determined, field observation suggests barite was emplaced at shallow depths. Mitchel Range barite was deposited in veins cutting basement rocks (Waterman Gneiss) in the lower plate of the WHDF. Barite occurs as coarse-grained rosette-shaped aggregates, often heavily iron stained. Crystals are water-clear and contain large inclusions (>100 microns). There is no evidence of fluid boiling. Figure 7. Fluid inclusion homogenization temperatures for central Mojave Desert barite occurrences (data for Waterloo/Langtry from Fletcher, 1986). Mt. General barite (not shown in Figure 7) is dark, cloudy, and yields few usable inclusions. Furthermore, upon heating the inclusions often fracture making homogenization temperature difficult to measure. Data from a small number of observations (12) suggests the average homogenization temperature is approximately 210°C. Discussion Any model for the genesis of silver–barite mineralization must take into account structural and stratigraphic relationships, as well as mineral paragenesis and fluid inclusion geothermometry and geobarometry. The first question that must be addressed is the age of mineralization. Fletcher (1986) states that two samples of potassically altered Barstow Formation yielded K/Ar dates of 17.1 and 17.5 Ma respectively. This is consistent with timing of deposition of the Barstow Formation. The relationship of the mineralization to structural evolution is more problematic. Singleton and Gans (2008) argue that unroofing of the Waterman Hills core complex occurred between 25 Ma and 19 Ma. Bartley et al. (1990) place the onset of dextral shear in the central Mojave at 19 Ma, although it is unclear if this includes the Calico fault. Singleton and Gans (2008) state, however, that active dacitic
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Geology and Ore Genesis of Silver–Barite Mineralization in the ...

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