To/
date (May 1996) the authors have examined five (5) hydrothermal
barite-silver occurrences (sub-districts) in the central Mojave
desert (Calico Mountains, Barstow Formation in Wall Street Canyon,
Waterman Hills, Mitchell Range and Mt. General). All are located
in San Bernardino County near the town of Barstow, CA. While each
occurrence is characterized by a remarkably similar mineralogy,
the host rocks, style of the mineralization and temperatures of
barite emplacement differ significantly. Our research utilizes
fluid inclusion geothermometry, paragenetic studies and field
relationships to relate barite mineralization to the Cenozoic
structural evolution of the central Mojave and constrain timing
of major tectonic events.
Our thanks go out to Kevin Rosso, Mary Ireland and Ian Fletcher
who provided some of the inclusion data and initial interpretation
of ore fluid geochemistry. The conclusions stated herein are those
of the authors (DRJ and DWT) who welcome any feedback.
The basement in the central Mojave consists of crystalline metamorphic rocks, termed the Waterman Gneiss, and granitic to dioritic intrusives of Cretaceous to Jurassic age. Age of the Waterman Gneiss is enigmatic, but most be pre-Jurassic. These rocks are unconformably overlain by Miocene volcanics and sediments of the Jackhammer, Pickhandle and Barstow Formations. Within the Waterman Hills and Mt. General sub-districts similar lithologies are present, but the stratigraphic position of these units is uncertain. Younger Plio-Pliestocene rocks are present locally, but none are mineralized.
The most prominent structural feature in the central Mojave is
the Waterman Hills-Mitchell Range (WHMR) detachment fault. Top
to the northeast shear has unroofed the crystalline basement in
the Waterman Hills and Mitchell Range resulting in an upper plate
of Tertiary rocks and a lower plate of Waterman Gneiss/intrusives.
Rocks of the Calico Mountains appear to have moved northeast several
kilometers to their present position. Barite mineralization is
closely related to detachment. Complicating the structural picture
are a series of Late Tertiary-Quaternary northwest-striking strike-slip
faults. The Calico fault has moved the Calico Mountain block northwestward
to its present position. The amount of offset along the Calico
fault remains the subject of debate with some geologists suggesting
as much as 20 km of motion. Fletcher, however, suggests the offset
of the Waterloo-Langtree silver deposit by the Calico fault is
only 2-3 km. There is no evidence to suggest that any barite mineralization
is related to strike-slip faulting.
Detailed
paragenetic studies of the Calico vein mineralization by Jessey
and the Barstow disseminated ores by Fletcher have shown only
minor differences in the sequence of ore mineral deposition. Additional,
less detailed studies of the Waterman Hills, Mitchell Range and
Mt. General indicate a similar, albeit less complex, paragenesis.
To generalize, barite and chalcedony/quartz were deposited early,
followed by hematite, magnetite and manganese oxides. These were
in turn followed by various sulfides. The final stage of mineralization
included secondary iron oxides, silver chlorides and calcite deposited
by supergene fluids. Studies by Rosso suggest the sequence of
ore mineral deposition can best be explained by changes in fluid
pH. Despite the appearance of sulfides late in the paragenetic
sequence Eh changes were minimal. The accompanying paragenetic
diagram is for the Barstow barite deposits, but the diagram for
the Calico mineralization is quite similar.
Bluish-green, propylitic alteration (chlorite+calcite±epidote)
is the most pervasive and readily recognizable form of alteration
in the central Mojave. Unfortunately, the alteration is so widespread
it is often a poor ore guide. Silicification, consisting of fine-grained
chalcedony and jasperoid is more closely associated with the barite-silver
mineralization. Secondary iron oxides have formed at the apex
of many veins in response to circulating supergene fluids. However,
the formation of secondary iron oxides is by no means ubiquitous
and appears to be controlled by the postore faulting in many of
the veins. Local kaolinization is present in host rocks containing
phenocrysts of plagioclase.
In
the Calico Mountains, epigenetic barite-silver veins occur within
the Early Miocene Pickhandle Formation (upper plate WHMR detachment
fault). The Pickhandle is comprised of flows of dacite to rhyodacite
composition, associated tuffs, and volcanic breccias. All barite
mineralization is localized within northwest trending (N25-75°W)
faults (over 100 veins have been mapped) which crosscut the volcanics
at steep angles. Mineralization has been deposited as open space
filling along dilatant zones and as a breccia-filling. Barite
crystals are euhedral, and often one centimeter or more in length.
In many veins the barite has been brecciated and the interstices
filled by later iron oxides and sulfides.
Barite fluid inclusions are often large (>50 microns) and show
little evidence of boiling. Homogenization temperatures range
from 240° to 300° C and salinity from 2-4 wt% NaCl.
Rosso calculated depth of barite emplacement at 1000 to 1500 meters.
Limited analysis of quartz has yielded homogenization temperatures
similar to those for barite.
Ian Fletcher studied the barite-silver mineralization of the Middle Miocene Barstow Formation in the vicinity of Wall Street Canyon. The Barstow which conformably overlies the Pickhandle, is comprised of lucustrine sandstones, siltstones and mudstones with minor interbedded volcaniclastics. Fletcher reports that barite usually occurs as disseminated grains and cementing material within siltstone (99%) and more rarely as veinlets (1%). The veinlets have a random orientation in contrast to the northwest strike of those in the underlying Pickhandle volcanics.
Fluid inclusion studies of barite and quartz suggest homogenization temperatures from 140° to 230°C with salinity from 4-6 wt% NaCl. Barite inclusions were generally smaller than those from the Pickhandle and evidence for boiling was more common. Fletcher calculated depth of emplacement for the mineralization at a few hundred meters.
Barite in the Waterman Hills occurs in two northwest striking veins. The veins crosscut Tertiary sedimentary and volcaniclastic rocks of uncertain age (Geologic Map from Ireland). The volcanic units are generally more silicic than those of the Pickhandle Formation of the Calico Mountains, while the sedimentary strata are both coarser grained and contain more volcanic debris than Barstow Formation exposed to the north in Rainbow Basin. The more evolved (silicic) volcanics suggest Tertiary rocks of the Waterman Hills are younger (Middle Miocene) than those of the Calico Mountains. The Waterman Hills detachment fault outcrops less than one kilometer to the north and south of the barite veins. Projections from surface outcrops indicate the fault contact should lie less than 400 meters below the present erosional surface.
Barite occurs as coarsely crystalline aggregates, often displaying extensive brecciation, The barite veins strike to the northwest parallel to the enclosing sedimentary hosts. Elevated inclusion temperatures and field mapping suggest the barite is clearly epigenetic and lies along a northwest trending fault. Locally, the barite follows a sedimentary contact, but in general the veins dip more steeply to the southwest crosscutting the sedimentary layers. Furthermore, the vein footwall is slickensided.
Fluid inclusion homogenization temperatures for the Waterman Hills ranged from 180° to 220° C. Inclusions were the smallest observed in the central Mojave (average diameter 10-15 microns) and evidence for boiling the most widespread. Due to the small size of the inclusions accurate freezing stage measurements were impossible and salinity could not be determined. Field observations suggest barite was emplaced at very shallow depths.
Pre-Cenozoic Waterman Gneiss (lower plate WHMR detachment fault) hosts a group of northwest trending barite veins in the Mitchell Range. The veins crosscut the Waterman Gneiss at steep angles and postdate a prominent mylonitic fabric. Vein contacts are slickensided suggesting emplacement within fault zones. The latter relationship requires significant unroofing of the Waterman Gneiss and transformation from ductile mylonitic fabric to brittle deformation and high angle faulting.
Barite occurs as rosette-shaped aggregates, often heavily iron stained. Crystals lack the cloudiness of other sub-districts and contain some of the largest inclusions in the central Mojave (>100 microns). There is no evidence of fluid boiling. Homogenization temperatures range from 180° to 270° C; inclusion salinity has not yet been measured. Lack of evidence for boiling and the large size of the inclusions possibly indicates barite emplacement at moderate depths.
The barite mineralization of the Mt. General area remains the most enigmatic. Limited mapping suggests host rocks similar to those in the Waterman Hills. Stratigraphic position of these rocks is uncertain. Perhaps the most unusual aspect of the Mt. General occurrence is the barite itself. The barite occurs within indistinct northwest trending zones which lack continuity along strike. Mineralization has been extensively sheared and brecciated. Rounding of the barite clasts indicates subsequent transport.
Barite is dark, cloudy and yields few usable inclusions. Further, upon heating inclusions often crack and valid temperature measurements are difficult at best. To date, the average of a dozen inclusion measurements is 208°C.
Fluid inclusion data are summarized on the accompanying figure (Fluid Inclusion Data). Homogenization temperatures range from 140° to 300° C, however, each sub-district appears to be unique. When plotted on a simple topographic map no trends are apparent. Attempts to relate homogenization temperature to stratigraphic position are more revealing. Stratigraphically lower rock have higher homogenization temperatures, but the correlation is only fair as temperatures for barite mineralization in the Waterman Gneiss are less than those for the overlying Pickhandle Formation..
To
more fully explain the observed temperature trends it is necessary
to prepare a post-detachment reconstruction of the central Mojave.
This results in barite mineralization in the Waterman Gneiss being
emplaced at stratigraphically lower, but structurally higher levels
than that in the overlying Pickhandle. The latter has moved down
dip during northeast transport. This further implies mineralization
was emplaced after much of the detachment had occurred and that
homogenization temperatures are a function of depth of emplacement.
(Note the question marks in the cross section which denote the
structural uncertainty across the Barstow-Daggett trough.)
Fletcher, Darby Ian, 1986 Geology and genesis of the Waterloo-Langtree silver-barite deposits, CA: Unpublished Ph. D. dissertation, Stanford University. 158 p.
Ireland, Mary C., 1993, Barite mineralization, alteration and geochemistry of the Waterman Mine, San Bernardino County, CA: Unpublished Senior Thesis, California Polytechnic University-Pomona, 40 p.
Jessey, David R., and Tarman, Donald W., 1988, Geology of the Calico Mountains in Geologic Excursions in the Eastern Mojave Desert, Lori Gaskin ed, National Association of Geology Teachers Far Western Section, Spring Conference, p 1-80.
Rosso, Kevin M., 1992, Fluid inclusion microthermometry and geochemistry of the silver-barite mineralization in the Calico mining district, San Bernardino County, CA: Unpublished Senior Thesis, California Polytechnic University-Pomona, 32 p.
