Calico Mining District

The colorful history of the Calico Mining District has been described by many authors; Weber (1966) provides an excellent summary. The district flourished from the 1880's to the close of the nineteenth century. The manpower shortage and downturn in mining during World War I and the subsequent Great Depression marked the end of significant activity, but not before the district had established itself as the largest silver producer in California. Total production is thought to have exceeded $20,000,000 by 1940. However, put in the perspective of a true giant like the Comstock Lode (total production of $396,000,000) (Smith, 1943), the Calico District must be considered quite small. During the 1950's an economic boom and a renewed interest in silver resulted in the reopening of several of the district's mines, but production was small. Accelerated petroleum exploration in California, however, made barite an economically attractive commodity. From 1957 to 1961 the Leviathan Mine was the largest barite producer on the west coast. Substantial barite reserves remain, but discovery of the much larger Battle Mountain, Nevada deposits has reduced the economic viability of the Calico District. In the early 1960's, ASARCO Inc. began the exploration and limited development of a disseminated silver deposit along the southwest flank of the Calico Mountains. The Waterloo Project, as it was named, is reported to contain approximately 45MT of ore grading 3-4 troy ounces per ton silver and 10% barite. Subsequent development has been hampered by environmental concerns and the low silver grade of the ore. The property has been inactive for nearly a two decades

GENERAL GEOLOGY

Stratigraphy and Host Rocks

Portions of the Calico Mining District have been mapped by McCulloh (1952,1965), Weber (1965), Dibblee (1967), DeLeen (1950), Mero (1972), Fletcher (1986) and Payne and Glass (1987). The Waterman Gneiss to the south and west of the Calico Mountains is generally regarded as the oldest rock in the region. It has been reported to be of Precambrian, Paleozoic or Lower Mesozoic age by various authors. Like many of the Mojave/Sonora metamorphic core complexes its age remains controversial. No Waterman Gneiss is known to outcrop within the Calico Mining District, however metamorphic clasts are common within tuffaceous horizons of the Pickhandle Formation. Mesozoic granitic to dioritic intrusives lie to the east of the Calico Mining District. While no major outcrops of intrusive occur within the district itself, small exotic blocks of quartz diorite have been noted at many localities, particularly to the northwest of Wall Street Canyon.

The early Miocene Pickhandle Formation is one of two major ore hosts in the district. Much of the vein-type barite-silver mineralization occurs within this formation. In general the Pickhandle consists of a series of intercalated pyroclastics and volcanic flows, the latter of predominantly dacitic composition. Minor volcanoclastic sedimentary units occur throughout the sequence, but are more common near the contact with the overlying Barstow Formation. The mid-late Miocene Barstow Formation unconformably (?) overlies the Pickhandle volcanics, the basal contact marked by transition from volcanics to sedimentary rocks. The Barstow consists of interbedded shales, siltstones and sandstones with minor lacustrine limestone. The lowermost sedimentary units are of distinctly volcanoclastic origin. The Barstow Formation is the second major ore host within the district. The ore occurs as disseminated grains and randomly oriented stockwork veins of barite and various silver minerals.

Dibblee (1970) identified younger (Pliocene) andesitic and dacitic volcanics locally capping the Pickhandle Formation. These younger flows are generally confined to the core of the Calico Mountains. No evidence of mineralization has been found in any of the younger volcanics. McCulloh (1952) mapped local outcrops of unconsolidated gravels in major drainages along the flanks of the Calico Mountains. The term Yermo Formation has been loosely applied to the gravels and a Pliestocene age has been assigned.

Structure

The Calico Fault Zone is the major structural feature within the district. It lies along the southwest flank of the Calico Mountains, trending approximately N 60° W. It is described by Dibblee (1970) as a right lateral strike-slip fault. The Calico Mountains are part of a northwest plunging anticlinorium, itself folded into a series of synclines and anticlines with northwest plunges. Barite veins within the Pickhandle are localized, for the most part, in a series of northwest striking, subparallel, normal faults (See Figure). A few veins have been observed with indications of reactivation and subhorizontal motion. All trend N 20-80° W, with the majority approximating the N 60° W trend of the Calico Fault.

BARITE-SILVER MINERALIZATION

Two distinct types of barite-silver mineralization occur within the district. Vein-type deposits are extensive within the Pickhandle Volcanics and have accounted for much of the past production, particularly from deeply oxidized, near-surface outcrops in the vicinity of Wall Street Canyon. Embolite constituted the main ore mineral with lesser cerargyrite and native silver. Disseminated to stockwork vein deposits of low grade silver-barite mineralization also occur within the sedimentary units of the Barstow Formation. These have been described by Fletcher (1986) and subsequent discussions of this mineralization is taken largely from that reference.

Pickhandle Barite-Silver Veins

Five recognizable, end-member, vein types can be mapped (Jessey, 1986). Of these, only two are common. Gradations exist between all vein types. The accompanying figure is a generalized paragenetic sequence for the Pickhandle barite-silver veins. The most common vein type, termed "black-matrix", consists of brecciated barite fragments in a matrix of iron and manganese oxides and minor sulfides. Rare calcite is paragenetically much later than barite and often quite dark due to the presence of included iron and manganese oxides. Interstices between barite fragments are most commonly filled with a mixture of iron and manganese oxides. Magnetite occurs locally, with partial to total alteration to hematite. A variety of manganese oxides are present, but they cannot be differentiated by simple microscopic analysis.

Sulfides are quite rare. The most common are pyrite and galena with trace chalcopyrite and tennantite. Silver assays as high as 1100 ounces per ton have been reported, but 3-5 oz./ton is closer to the norm. The silver-bearing species are uncertain. Samples of high grade silver ore were examined and found to contain a high proportion of galena suggesting argentiferous galena. However, assays as high as those reported above (1100 ounces) dictate the presence of a primary silver mineral such as argentite or native silver.

Alteration consists of minor to extensive silicification. Envelopes of weak propylitic alteration (calcite+chlorite+epidote) have been recognized adjacent to some veins, but the relationship is not ubiquitous and often can be seen only in thin section. Payne and Glass (1987) report hydrothermal alteration of amphibole to celadonite.

Oxidized barite veins are thought to represent the supergene equivalent of the black matrix barite veins. They are easily recognized by the brick red alteration adjacent to and intimately associated with the barite. The alteration consists of jasperoid and fine-grained secondary hematite. Most primary sulfides have been replaced, with only occasional, heavily corroded, pyrite remaining. Galena has altered to cerrusite. Magnetite has been replaced and pseudomorphed by hematite. Microscopically, the secondary hematite occurs as fine-grained to colloidal veins cutting the altered primary sulfides and oxides. Jasperoid consists of banded aggregates of iron-poor chalcedony and iron-rich jasperoid filling the interstices between barite fragments. Secondary silver minerals, particularly embolite and cerargyrite are present in some veins but absent in others. Silver grades in the oxidized veins of Wall Street Canyon were quite high, exceeding 10 oz./ton, but the norm for most oxidized veins is generally 1-2 oz/ton. The heavily altered and oxidized veins appear to cluster in distinct groups adjacent to kinks or bends in the Calico fault.

A few unusual veins were noted during field studies. In all cases these vein types are of restricted occurrence. Banded veins of jasperoid and barite occur near the St. Louis Mine. They are unusual in the fine development of comb structure, often described as a signature of a true epithermal precious metal district. Iron and manganese oxides are minor and sulfides absent. Late calcite veinlets sometimes crosscut the earlier barite and jasperoid. Northeast of the Leviathan Mine monomineralic veins of white barite can be observed. Alteration is absent. Inclusions of brecciated host rock can be seen at many localities. Southwest of the Leviathan Mine, thin but laterally persistent veins of jasperoid with no attendant barite are present. The marked differences in mineral suites of these two vein types which lie within 300 meters of one another may be explained by their age relationships. Monomineralic barite veins represent an early stage of mineralization while jasperoid veins represent a much later stage of mineralization

Barstow Barite-Silver Mineralization

Mineralization within the Barstow formation is largely disseminated. Fletcher (1986) reports that less than 1% of the primary barite-silver mineralization occurs in recognizable veins. Fletcher recognizes three stages of mineralization which correlate well with the paragenesis of the underlying Pickhandle barite-silver veins. The earliest stage, termed "Early Barite Veins", consists of barite and quartz with minor hematite and trace anhydrite and sulfides. The second stage, "Silver-Silicification Veins" are responsible for the bulk of the silver mineralization. These veins are composed primarily of quartz, lesser barite, minor hematite and calcite and trace sulfides and native silver. Acanthite and native silver have been identified as the chief ore minerals. The final mineralizing stage, "Late Calcite Veins" are predominantly coarse calcite and minor quartz.

Alteration consists of early bleaching which most commonly manifests itself as K-feldspar replacement of detrital grains. Although not termed as such by Fletcher, this alteration in many respects appears similar to the pervasive potassic alteration associated with many base and precious metal mining districts. The alteration is correlative with the early stage of barite-quartz mineralization. Intense silicification accompanied the silver-silicification stage veins. It occurs most commonly as colloform bands of jasperoidal chalcedony although minor recrystallized quartz is also present.

Controls for the Barstow mineralization are rather enigmatic. Studies of the vein systems (Fletcher, 1986) indicate a near random orientation typical of a stockwork vein system overlying a deep seated intrusive. However, these vein orientations are inconsistent with those of the Pickhandle volcanics which strike northwest. Further Fletcher (1986) feels there is no genetic relationship with the Calico Fault since the Waterloo deposit has been offset by the fault. Finally, mineralization is pervasive throughout the lower portion of the Barstow Formation, apparently favoring no particular sedimentary horizon. Fletcher favors mineralization related to the emplacement of a small stock in the vicinity of Wall Street Canyon.

Ore Genesis

Many researchers have considered the Calico District to be a classic example of the epithermal precious metal deposit. Certainly the district has many characteristic features: association with volcanic rocks, Tertiary age, normal faulting suggestive of crustal extension, low temperature mineralization and potassic and propylitic alteration.

The most common epithermal model relates ore deposition to periods of extensional volcanism associated with plate subduction. The timing of mineralization in the Calico District (15-20 MY) is inconsistent with a subduction-related extensional model. Can extension occur by other mechanisms? If so, can any of these be related to the Calico District? Recent attention has focused on the detachment model to account for extension in many of the southern California precious metal districts. Detachment faults were originally described along the flanks of the Calico Mountains by Weber (1967). However, these detachment faults bear little similarity to classic detachment faults described by Davis and others (1980) in the eastern Mojave and Sonora Deserts. The Calico detachments are best termed gravity slides, and are not related to deep crustal extension as are those in other districts. Moreover, even the gravity slide model has been challenged (Payne and Glass, 1987). In addition, the Barstow mineralization can be demonstrated to predate deformation associated with the "gravity slide" (Fletcher, 1986). More recently, Glazner et. al.(1988) have documented detachment faulting in the Waterman Hills a few kilometers to the southwest of the Calico Mountains. Assuming the Calico Mountains represent the upper plate of a detachment block, the high angle northwest-trending barite-silver veins could represent mineralized listric faults described in most detachment terranes.

The following model best explains the district's ore deposits. Extensional stresses during the early Miocene, related to detachment faulting, created a series of normal faults in the upper plate Pickhandle Volcanics. A small stock was emplaced in the vicinity of Wall Street Canyon which drove a hydrothermal convective system mineralizing the normal faults as well as the flat-lying sediments of the lower Barstow Formation. During the late Miocene, strike-slip movement began along the Calico Fault reactivating the dip-slip faults. The reactivated faults underwent additional extension in areas adjacent to bends in the main Calico fault causing further dilation and permitting the circulation of meteoric waters which oxidized the existing mineralization and deposited secondary oxides and silver chlorides.

REFERENCES CITED

Davis, G. A., Anderson, J. L., Frost, E. G., and Shackelford, T. J., 1980, Mylonization and detachment faulting in the Whipple-Buckskin-Rawhide terrane, southeastern California and western Arizona: Geological Society of America Memoir 153, pp. 79-129.

DeLeen, J., 1950, Geology and mineral deposits of the Calico Mining District: Unpublished Master's Thesis, University of California-Berkeley, 86 p.

Dibblee, T. W., Jr., 1967, Areal geology of the western Mojave Desert, California: U. S. Geological Survey Prof. Paper 522, 153 p.

__________, 1970, Geologic map of the Daggett Quadrangle, San Bernardino County, California: U. S. Geological Survey Misc. Geological Inv. Map I-592.

Fletcher, D. I., 1986, Geology and genesis of the Waterloo and Langtry silver-barite deposits, California: Unpublished Ph.D. Dissertation, Stanford University, 158 p.

Glazner, A. F., Bartley, J. M., and Walker, J. D., 1988, Geology of the Waterman Hills detachment fault, central Mojave Desert, California: Guidebook for the 1988 Cordilleran Sectional Meeting Geological Society of America, in press.

Jessey, D. R., 1986, A geologic investigation of the Leviathan-Silver Bow property, Calico mining district, San Bernardino County California: Geological Society of America Abstracts with Programs, Cordilleran Section, p. 121.

McCulloh, T. H., 1952, Geology of the southern half of the Lane Mountain Quadrangle, California: Unpublished Ph.D. Dissertation, University of California-Los Angeles, 182 p.

___________, 1965, Geologic map of the Nebo and Yermo Quadrangles, San Bernardino County, California: U. S. Geological Survey Open-File Map OFR-65-107.

Mero, A. L., 1972, Geology and ore deposits of the south-central Calico Mountains: Unpublished Master's Thesis, Calif. State University-San Diego, 74 p.

Payne, J. G., and Glass, J. R., 1987, Geology and silver deposits of the Calico district, San Bernardino County, California: Guidebook for field trips to bulk mineable precious metal deposits, Geological Society of Nevada, pp. 31-44.

Smith, G. H., 1970, The history of the Comstock Lode 1850-1920: Nevada Bureau of Mines Bulletin 37, 305 p.

Weber, F. H., 1965, Reconnaissance of silver-barite deposits of the Calico Mountains and vicinity: California Division of Mines and Geology Open File Map, Los Angeles.

_____________, 1966, Silver mining in Old Calico: Calif. Division of Mines and Geology Mineral Info. Service, v. 19, pp. 71-80.