Ferroaxinite from New Melones Lake, Calaveras County, California
Last Updated: 17th Aug 2012
FERROAXINITE FROM NEW MELONES LAKE, CALAVERAS COUNTY, CALIFORNIA, A REMARKABLE NEW LOCALITY
Partial Facsimile of the excellent Mineralogical Record article by:
Demetrius Pohl, Renald Guillemette, James Shigley and Gail Dunning
from the spillway adjacent to New Melones Lake near Copperopolis in
Calaveras county. Since its discovery, this locality has produced some
of the finest ferroaxinite specimens ever found in North America.
At the new Melones Lake spillway area, ferroaxinite was first noticed in the 1970's by engineers during dam construction, when it was observed during overburden removal. The locality has remained dormant since the completion of the dam and access to the site is restricted because the area comprises part of the New Melones Dam installations which are under the jurisdiction of the U.S. Bureau of Reclamation.
The spillway of New Melones Lake has been excavated in rocks of the western Sierra Nevada metamorphic belt. This belt forms part of the western limb of a north west-trending faulted synclinorium (a broad regional syncline on which are superimposed minor folds), the axial part of which is occupied by the granitic rocks of the Sierra Nevada batholith. Rocks in this belt consist of alternating metamorphosed volcanic and sedimentary units which are considered to be part of an early Mesozoic island-arc system and the underlying oceanic crust. In the New Melones Lake area the rocks have been divided into three major units: a basement of melange and serpentinite matrix melanges, an overlying island-arc volcanic unit, and finally an upper unit of slate and graywacke designated as the Mariposa formation. The melange and lower parts of the volcanic succession have been faulted into the overlying Upper Jurassic Mariposa formations.
Although the regional geologic fabric trends northwest, the rocks exposed in the spillway show contacts oriented from west to north-west. The main lithologic units are massive grey-green gabbros with abundant diabase dikes, fine grained and in some places vesicular basalts and volcanics including both tuff and pillow lavas, all of which have been altered to greenstone. These are considered to be part of the Peno Blanco volcanics. Meta-argillites, melange rocks and serpentinites comprise a minor part of the stratigraphic section. All of these rocks have undergone greenshist facies metamorphism and locally show pervasive alteration to fine grained albite-epidote-quartz assemblages. In spite of the extensive alteration, deformation and faulting of the rocks, many original igneous and sedimentary features have been preserved. A shear foliation or shistosity is developed in the rocks only locally in the vicinity of the faults.
Rocks in the spillway are cut by many north-west trending faults and shear zones which dip either steeply to the northeast or gently to the southwest. Extension fractures or tension gashes are abundant in the metagabbro and metabasalt units in the spillway walls, but are absent in the less competent metasediments and serpentinite. These gash veins generally occur as sub-horizontal, en echelon, left-lateral groups dipping gently to the west. Some of these veins form ladder-like sets; these being confined to to basic dikes which intrude the metagabbro and metavolcanics. The length of the gash veins varies from 10 centimeters to 10 meters with a thickness of 2-30 cm. In plan view they are lens shaped. In some cases individuals are stacked so closely as to coalesce, as in areas 1 and 2 (fig. 1), with resultant thicknesses up to one meter. Most veins are undeformed and not offset by later events, and consequently record one of the last tectonic episodes in the area. It is in these gash veins that the ferroaxinite and other minerals of interest occur in the spillway. Ferroaxinite is confined to the veins, in contrast to the other minerals which also occur disseminated in the wall-rock. In general, most of the veins are completely filled with massive ferroaxinite and only rarely will a cavity be encountered. To date only five major veins which contain central cavities with free growing ferroaxinite and associated minerals have been found. These locations - numbered as areas 1 to 5, are shown on figure 1. Mineralogical data presented in this paper were collected from an examination of specimens collected at these localities.
In summary, the rocks found in the New Melones lake area consist of a structurally complex assortment of ancient oceanic crust, melanges and sheets of ultramafic rocks that evidently were deformed prior to the formation of the island arc. This entire assemblage has undergone additional tectonic shuffling during the late Jurassic, both prior to and during the Nevadan Orogeny, when the entire terrane was strongly folded and cleaved. The last event, around 150 million years ago, was most probably responsible for the generation of the gash veins and their spectacular ferroaxinite mineralization.
Although a large number of tension veins exposed in the spillway contain massive, coarsely crystalline ferroaxinite, to date only a few of them have produced well formed euhedral crystals. Locations of the major veins with crystal-lined pockets are shown on figure 1. Ferroaxinite displays slightly differing color, morphology and paragenesis, depending on the location of the vein. The morphology of the crystals is typical of axinite, the crystals being thin and tabular with a wedge-like habit. Both single crystals and complex groups have been found. While its crystal habit is not unusual, the size, color, luster and general quality of this ferroaxinite are exceptional. Individual crystals can range up to 8 cm in longest dimension and parallel growth aggregated up to 12 cm. While much of the ferroaxinite is cloudy or opaque, thin fragments, small crystals and even many of the large ones are transparent and gemmy, particularly at their tips. All ferroaxinite from the spillway is violet or clove brown. Crystals from areas 1, 2, and 4 exhibit strong selective absorption as they are rotated in front of a light source; their color changing from pale violet-brown to deep reddish-violet. Veins from area 1 have produced both the most spectacular and largest number of ferroaxinite to date. This ferroaxinite generally occurs as fans and rosettes of crystals, box-works of bladed crystals, or single, parallel growth crystals implanted on either massive, granular ferroaxinite or a matrix of small epidote, actinolite and albite crystals. While many of the crystals have razor-sharp edges, some show small serrations which upon close examination are found to be multiple terminations or possibly growth hillocks.
Actinolite commonly occurs as white to very pale green flexible fibers in areas 1 and 3 as well as well as many of the smaller veins. In area 1, fibrous crystals to 1 cm cover areas up to 60 X 30 cm and occur with albite to form attractive plates. It has also been found in one vein of area 3 as flexible, long-fibered (up to 15 cm) parallel aggregates.
Albite is found in all major open cavities to date. It occurs as translucent white to water-clear, euhedral, striated, twinned crystals ranging from microscopic to almost 2 cm in diameter. Chlorite phantoms are sometimes seen within specimens from areas 3 and 5. Microprobe analysis show that the albites contain less than one percent anorthite and orthoclase components.
Calcite most commonly occurs as colorless, translucent to transparent very coarsly-crystalline anhedral vein fillings. In areas 2 and 5 it is also found as large, thin, platy crystals forming a box-work among other minerals in the cavities. Several thin, hexagonal plates up to 3 cm across have been found as floaters in granular chlorite. Small, well-defined, blocky crystals are sometimes found growing on platy calcite and directly on ferroaxinite from areas 1 and 2.
Chalcopyrite has been found only at a small, unreferenced vein area across the spillway from area 1. The crystals occur as crude polyhedra up to 6 mm in diameter within chlorite, and as thin veinlets in the vein core and in the surrounding rock.
Chlorite occurs as thick, loose, clay-like fillings within the open vein cavities of areas 3 and 5, as well as in many minor veins throughout the spillway. It typically forms attractive phantoms within quartz and albite. Chlorite inclusions within, and coatings of chlorite on quartz and ferroaxinite can give these crystals a dark grey-green, corroded appearance.
Epidote has been found as small euhedral crystals in areas 1, 3 and 4 and in many of the smaller open cavities, and as granular and fibrous masses in the veins. The crystals rarely exceed 10 mm in length, form parallel groups and crusts, and display the typical olive-green color.
This fibrous, clay-like mineral has only been found in area 1. It occurs as pale brown to orange gel-like or fibrous cavity fillings which completely envelope the central crystals in this area.
Small (less than 1 cm) cubes of pyrite have been found only in area 2, as an early-formed vein and wall-rock phase. It also occurs as disseminated crystals in the wall rocks of the spillway.
Quartz has been found in all major veins except those of area 1. It generally occurs as colorless, translucent to transparent crystals up to 10 cm or more in length, though the average is 2 to 5 cm. Some of the crystals have patches and phantom inclusions of chlorite and more rarely ferroaxinite, albite, epidote and actinolite.
A pale greyish yellow smectite-group mineral - most probably montmorillonite - fills interstices between ferroaxinite and albite crystals in area 2. It also occurs as a coating between calcite plates and possibly as inclusions in calcite.
Each of the major vein pockets from areas 1 to 5 have somewhat different mineral assemblages and the paragenesis of each pocket is shown in figure 18. The absence of particular minerals from pocket to pocket is especially striking. There are also small but significant chemical differences between the pockets as shown by variations in ferroaxinite color and composition, differences in clay mineralogy, and the color of epidote.
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