The Waterman Mine

History

The Waterman Mine is located four miles north of Barstow just west of the Old Fort Irwin Road. The mined produced approximately 50,000 tons of ore from 1880 to 1887. Barite-silver mineralization occurs in two northwest-striking veins covered by two patented mining claims. Three shafts were present although one has been recently bulldozed and a second is caved and inaccessible. The remains of the third shaft on the "Principle Vein" provide access to the underground workings. The mine was briefly reactivated in the 1930's when silver was extracted from the mine dumps and a series a shallow open cuts along the strike of the veins.

Regional Stratigraphy

The Waterman Gneiss is generally regarded as the oldest rock in the Waterman Hills. It has been reported to be of Precambrian, Paleozoic or Mesozoic age by various authors. Like many of the Mojave/Sonora metamorphic core complexes its age remains controversial Mesozoic granodiorites and diorites intrude the Waterman Gneiss along the north edge of the Waterman Hills. Both the gneiss and intrusives have been overprinted by pervasive chloritization, cataclastized and mylonitized.

Overlying the basement rocks are a series of volcanic, volcaniclastic and epiclastic rocks of Miocene age. These rocks have been loosely grouped into the Jackhammer, Pickhandle and Barstow Formations to the north in the Mud Hills and Calico Mountains. Similar Miocene age rocks age present within the Waterman Hills (see discussion below), but significant lithologic differences make correlations with the type sections difficult. Present practice is to simply map the Miocene rocks of the Waterman Hills as undifferentiated.


Regional Structure

The main structural feature in the Waterman Hills is the Waterman Hills Detachment Fault (WHDF), a low angle normal (detachment) fault formed during Miocene extension of the central Mojave. The WHDF juxtaposes lower plate Waterman Gneiss and Mesozoic intrusive and upper plate Tertiary volcanics and sedimentary rocks. Kinematic indicators suggest top to the northeast shear. Subsequent rotation must have occurred since northeast transport would necessitate a northeast dip for the master detachment surface. Field mapping reveals the existing fault planes dips 2-25° to the southwest.

High angle normal faults cut both the basement complex and Tertiary rocks. The relationship of these faults to the structural evolution of the central Mojave is enigmatic. In the Calico Mountains to the northeast, these faults appear to be listric faults related to upper plate transport during Miocene extension. However, in the Mitchell Range to the east and to a lesser extent in the Waterman Hills these faults can be seen to cut the metamorphic basement complex and hence would appear to be post-detachment.

The Calico fault, a right lateral strike-slip fault probably formed during very late Miocene or Pliocene. Although the Calico fault has a similar strike to the high angle normal faults, it appears to be unrelated to this episode of faulting. Curiously, the Calico fault cannot be shown conclusively to cut any high angle normal faults. Evidence in the Calico Mountains, however, does suggest the Calico fault is younger than the normal faults and unmineralized.

Mine Geology

Stratigraphy

The basement complex at the mine site consists of the Waterman Gneiss and Mesozoic intrusives ranging in composition from diorite to quartz monzonite. All basement rocks have been overprinted by mylonitic fabric, cataclastized and moderately chloritized. This overprinting probably occurred during Tertiary detachment. All basement rocks appear to lie within the lower plate of the WHDF.

The upper plate of the WHDF is comprised of massive volcanics of rhyolitic to dacitic composition and associated tuff and tuff breccias. These are overlain by sedimentary rocks described by Glazner et al (1988). The basal portion of the sedimentary sequence are water-lain tuffs and conglomerates comprised of volcanic clasts. Above these are a series of sandstones, shales and siltstones. Near the south edge of the mine site is a limestone of enigmatic origin. It unconformably overlies a rhyolite breccia. It cannot be correlated to the sedimentary rocks to the north described by Walker and its origin is uncertain. None of the sedimentary units of the Waterman Hills can be correlated with Barstow Formation to the north in the Rainbow Basin.

Structure

All sedimentary units at the mine site dip to the southwest at moderate to steep angles. Limited mapping to the southwest suggests the sedimentary rocks are part of a northwest-striking synclinal trough. Similar northwest striking folds in the Barstow Formation to the north have been attributed to right-slip along the Calico Fault. However, the Waterman mine lies several kilometers south of the Calico fault making it difficult to relate these folds to movement along the Calico fault. Furthermore barite mineralization is locally coincident with bedding suggesting folding prior to introduction of the barite. Movement along the Calico fault is believed to post-date barite mineralization.

Since the WHDF daylights both to the north and south of the mine it was possible to make a projection of the depth to the fault surface based on dip angles and topography. These projections suggest the fault should lie no more than 200 meters beneath the present erosional surface. Since the workings reportedly reached only depths of 80 meters the detachment fault was never encountered.

The main ore control for the Principle Vein is a northwest striking normal fault. The fault strikes approximately north 10 west near the south edge of the property becoming more northwesterly in strike to the north. Dips are 70-85 degrees to the southwest. Slickensides are common along strike of the vein. The change in strike of the fault coincides with a change in fault geometry, from a clearly crosscutting relationship with rock units to the south to a bedding plane slip to the north. A second subparallel fault possibly lies to the west hosting a second vein, but mining activity has obliterated much of the surface outcrop of the second vein.

Barite Mineralization

Much of the barite-silver mineralization in the Waterman Hills consists of open space filling within the Principle Vein. Disseminated grains of barite are common in the hanging wall of the vein. The host rock for the Principle Vein varies from tuffaceous sandstone to tuff breccia. The vein mineralization strikes northwest and dips steeply to the southwest. Slickensides are common along vein walls suggesting fault control of the mineralization.

Reported ore minerals include native silver, cerargyrite, and argentite. Gangue includes pyrite, barite, iron and manganese oxides, and chalcedony. Barite, the most common mineral present, occurs as milky white, tabular crystals up to 2 cm in length. The barite generally forms crustiform bands that have been extensively brecciated and replaced by later oxides, quartz and sulfides.

Alteration consists of silification, minor propylitization and supergene oxidation of primary sulfides and oxides. Fluid inclusion homogenization temperatures for Waterman mine barite range from 180 to 220 degrees C. The small size of inclusions made salinity determination difficult. The few inclusions measured suggest salinities of approximately 3wt% NaCl.

REFERENCES CITED

Glazner, A.F., Bartley, J.M., and Walker, J.D., 1988, Geology of the Waterman Hills detachment fault, central Mojave Desert, California: This Extended Land-Geological Journeys in the Southern Basin and Range, Special Publication 2, D.L. Weide and M.L. Faber eds., pp. 257-237.