Geology and Mineralization
The Calico district lies within the central Mojave Desert region of southern California. This portion of the Mojave Desert is known as the Western Mojave Block (Fletcher, 1986). It is bounded on the north and northwest by the Garlock left-lateral strike-slip fault zone and on the south and southwest by the San Andreas right-lateral strike-slip fault zone (Figure 7- 1).
Figure 71 Location of the Calico mining district within the northwest-trending belt of silver and gold mineralization, southern California
Basement rocks in the central Mojave consist of crystalline metamorphic rocks (Waterman gneiss) and granitic to dioritic intrusives of Cretaceous to Jurassic age. These rocks are unconformably overlain by Miocene volcanics and sediments of the Jackhammer, Pickhandle, and Barstow formations. Younger rocks of Plio-Pleistocene age are present locally but none is mineralized. The general geology of the region is shown on Figure 7-3. A generalized stratigraphic column is shown on Figure 7-3. Figures 7-2 and 7-3 and the formation descriptions that follow are from Jessey (2010).
The Waterman gneiss varies from dioritic gneiss, to impure marble and quartzite, to various types of mylonite. When not affected by later retrograde events, Waterman gneiss falls within the amphibolite facies of regional metamorphism. Subsequent retrograde metamorphism associated with regional detachment has locally superimposed a chlorite-grade greenschist facies.
The Coyote group consists of marble, schist, metamorphosed andesite and minor metamorphosed basalt. These rocks lie along the north flank of the Calico Mountains. Paleozoic fossils were found near the middle of the sequence but some investigators believe the upper half of the group is of Mesozoic age.
Figure 72 Generalized geologic map of the Calico Mountains area
Figure 73 Generalized stratigraphic section for the central Mojave region
The basal unit above the Waterman Hills Detachment Fault (WHDF) is the Jackhammer formation. At its type section in Jackhammer Gap it consists of 700 feet of tuff, tuff breccia, volcanogenic sedimentary rock, arkosic conglomerate, and basalt. Dokka and Woodburne (1986) report an age date of 25.6 Ma (Oligocene) for a basalt near the top of the Jackhammer.
The overlying Pickhandle formation is one of two major ore-bearing units in the Calico district, hosting much of the vein-type barite–silver mineralization. The Pickhandle is, in general, a series of intercalated pyroclastic rocks and volcanic flows, the latter predominantly of rhyodacitic to dacitic composition. Minor volcaniclastic sedimentary units occur throughout the sequence, but are more common near the contact with the overlying Barstow Formation. Age dates by various authors suggest the bulk of the Pickhandle Formation was deposited by 19 Ma, although a series of dacite domes near the southeast end of the Calico Mountains have yielded ages as young as 16.8 Ma suggesting the waning stages of Pickhandle volcanism may be coeval with deposition of the overlying Barstow Formation. The Pickhandle is named for exposures in Pickhandle Pass of the northern Calico Mountains.
The Middle Miocene Barstow Formation overlies the Pickhandle volcanics. The basal contact marks the transition from volcanics to sedimentary rocks. Field relationships suggest there is overlap between Pickhandle volcanism and Barstow deposition, the latter beginning, perhaps, as early as 18. The Barstow Formation was deposited in the east-trending, fault-controlled basin. Thickness ranges from 2,400 feet in the northwest part of the basin to 1,200 feet in the Alvord Mountains. Lithologically, the Barstow consists of a wide array of sediment types reflecting deposition in a shallow lake, tributary stream systems, and alluvial fans. In the Calico Mountains, a gradual upward coarsening can be observed with rocks grading from calcareous mudstones and sandstones to conglomerates. Impure limestones with thicknesses ranging from fractions of inches to a few feet are locally present at the base of the Barstow Formation in the Calico Mountains. The Barstow is the second major ore host in the region. The ore occurs as disseminated grains and randomly oriented stockwork veinlets in permeable siltstones and porous sandstones.
The pre-Tertiary metamorphic rocks are exposed mainly in the southwestern Mojave region. The plutonic rocks are of Mesozoic age; they are the most widely and abundantly exposed pre-Tertiary rocks in the Western Mojave Block. Sedimentary rocks of Mesozoic age are exposed in only thin, isolated blocks within the western Mojave region. Following the period of Cretaceous and early Tertiary erosion, several middle Tertiary volcanic centers developed in the Mojave region. Dibblee (1970) mapped the volcanic centers along a northwest-trending mineralized belt (Figure 7-1) as dominantly intermediate in composition and of middle to late Tertiary in age. Volcaniclastic rocks derived from these centers are commonly intercalated with the basal strata of the sedimentary rocks that accumulated in the basins. A few scattered eruptions of basaltic flows and cinder cones occurred in late Pliocene and Quaternary time along northwest-trending fault zones.
Rocks within the western Mojave block were disrupted and deformed in Cenozoic time by differential movement and stress associated with the San Andreas and Garlock fault zones (Fletcher, 1986).
The main Cenozoic extensional structure in the central Mojave Desert is the Waterman Hills detachment fault, which places brittlely deformed Miocene rocks (Tertiary volcanics and sedimentary rocks ) on ductilely and cataclastically deformed footwall rocks (Waterman gneiss and intrusives). The upper plate strata were apparently deposited during extensional faulting. These strata were tilted, folded, and intruded by rhyolite plugs that are cut off at the detachment fault.
The WHDF is a low angle normal (detachment) fault formed during Miocene extension of the central Mojave. Evidence suggests slip on the Waterman Hills detachment fault to be about 40–50 km. This is also consistent with other offset markers, such as the western edge of a Mesozoic dike swarm.
The Calico fault is a right lateral strike-slip fault that probably formed during very late Miocene or Pliocene. The Calico Mountains appear to have moved to their present position by several kilometers (km) of northeast movement along the WHDF detachment surface and up to 20 km of northwest of displacement along the northwest- trending Calico fault.
The eastern edge of the Western Mojave Block is defined by a belt of silver and gold mineralization that comprises a northwest-trending, generally down-faulted block, in which hydrothermal mineralization commonly is associated with volcanic rocks of Tertiary age that are intruded into granitic and metamorphic rocks of Cretaceous age and older. This mineralized belt, which is about 100 miles long and 10 to 30 miles wide, extends southwesterly across the desert from the Randsburg- Johannesburg gold-silver district on the northwest through the Calico silver-barite district to the Lava Beds silver district and the Stedman gold-copper-silver district to the southeast. The Calico district lies near the middle of the mineralized belt (Figure 7-1).