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

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Propylitic alteration (chlorite+calcite±epidote) is the most pervasive and readily recognizable form of alteration in the district. The alteration is widespread and commonly not proximal to barite veins. Silicification, consisting of fine-grained chalcedony and jasperoid is more closely associated with the silver-barite mineralization. Payne and Glass (1987) also report hydrothermal alteration of amphibole to celadonite. Local kaolinization is present in host rocks containing phenocrysts of plagioclase. 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 secondary, fine-grained hematite. Most primary sulfides have been replaced, with only occasional, heavily corroded, pyrite remaining. Galena has altered to cerrusite. Magnetite has been replaced by hematite, leaving pseudomorphs of the original magnetite crystals. Secondary silver minerals, particularly embolite and cerargyrite, are present in some veins but absent in others. Silver grades for the oxidized veins of Wall Street Canyon are quite high, exceeding 10 oz/ton. Barstow Formation Fletcher (1986) studied the silver–barite mineralization of the Middle Miocene Barstow Formation. He identified two separate tabular ore bodies: the Waterloo deposit, north of the Calico fault, and the Langtry, 2.5 kilometers to the west and on the south side of the Calico fault. He argued that the deposits were once part of a single ore body that was subsequently offset by right slip along the Calico fault. The Barstow Formation in this area is composed of lacustrine sandstones, siltstones and mudstones with minor interbedded volcaniclastic sandstones. Fletcher reports that barite occurs as disseminated grains and cementing material within siltstone (99%) (Fig. 5) 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. Alteration consists of bleaching that most commonly manifests itself as K-feldspar replacement of detrital feldspar grains. Although not termed as such by Fletcher, this alteration appears similar to the pervasive potassic alteration associated with many epithermal precious metal deposits. The alteration is correlative with an early stage of barite-quartz mineralization. Intense silicification, most commonly as colloform bands of jasperoidal chalcedony with minor recrystallized quartz is also present. For a more detailed discussion of the deposits see Fletcher (1986). Waterman Hills Figure 5. Photomicrograph of disseminated barite (white) in siltstone (from Fletcher, 1986). Silver ore in the Waterman Hills occurs in two northwest-striking veins that can be traced for a distance of over 500 meters along strike (Ireland, 1993). The veins lie within a sequence of volcaniclastic sedimentary rocks and volcanics of uncertain age. Dibblee (1952) correlated the rocks with the Miocene Pickhandle Formation. However, the more silicic nature of the volcanics (rhyolites) and the distinct northwest strike of the sedimentary units suggest that correlation is tentative. The veins, while
concordant to bedding of the volcaniclastic sedimentary rocks at the surface, crosscut the strata in underground workings. The Waterman Hills detachment fault crops out less than one kilometer north and south of the barite veins. Projections from surface outcrops indicate the fault contact should lie less than 150 meters below the present erosional surface. Mineralization resembles that of the Calico District wherein coarse-grained barite has been brecciated and replaced by a matrix of iron oxides. Little is known about the silver occurrence and no sulfides were observed either on the dumps or in underground workings. Mitchel Range The Waterman Gneiss (lower plate of the Waterman Hills detachment fault) hosts a group of northwest trending barite veins at the southeast end of the Mitchel Range. The veins crosscut the Waterman Gneiss at steep angles and postdate a prominent mylonitic fabric. Vein margins are slickensided, suggesting emplacement within fault zones. Barite occurs as coarse-grained rosette-shaped aggregates, often heavily iron stained. Minor iron oxides are present, as are trace amounts of galena. No silver mineralization was observed and the limited extent of the workings suggests ore grades were subeconomic. Mt. General The silver–barite mineralization of the Mt. General area remains the most enigmatic feature of the Calico District. Mapping suggests host rocks are 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. The barite occurs within indistinct northwest-trending shear zones, which lack continuity along strike. Mineralization has been extensively sheared and brecciated. Barite is dark and cloudy, commonly with inclusions of particulate matter. Microscopic examination indicates the barite and minor associated calcite have undergone extensive recrystallization. Iron staining is common in underground workings, but iron oxides occur in only trace amounts. The very limited underground workings and scattered prospect pits suggest low silver grades. Paragenesis Detailed paragenetic studies of the Calico vein mineralization (Jessey, 1992) and disseminated ores in the Barstow Formation (Fletcher, 1986) have shown only minor differences in the sequence of ore mineral deposition (Fig. 6). Less detailed studies of mineralization in the Waterman Hills, Mitchell Range and Mt. General indicate a similar, albeit less complex, paragenesis. To generalize, barite and chalcedony/quartz were deposited early (Early Barite stage). This was followed by an episode of pervasive silicification (initial stage of Silver–Silicification) and subsequently by hematite, magnetite and manganese oxides. A variety of sulfides followed the oxides (late stage Silver– Silicification). The Late Stage Alteration included secondary iron oxides, silver chlorides, and calcite deposited by supergene fluids. Fletcher (1986) and Rosso (1992) attribute the observed paragenetic sequence to changes in fluid pH. However, the elevated salinity of ore fluids (see Fluid Inclusion Geothermometry) makes this difficult to rationalize, as saline fluids typically buffer reactions and resist pH change. Another explanation would be a change in Eh. Early oxidizing ore fluids would deposit sulfates (barite) and oxides (magnetite/hematite). As conditions became more reducing, pyrite, galena and acanthite would form.
Page 1 and 2: Geology and Ore Genesis of Silver-
Page 3 and 4: Stratigraphy General Geology Portio
Page 5 and 6: Widespread gravels in the Yermo Hil
Page 7: Veins with comb-textured barite lin
Page 11 and 12: veins are hosted by upper plate roc
Page 13 and 14: While the above model fits much of
Page 15 and 16: Jessey, D. R., and Tarman, Donald W

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

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