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

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