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Mimetite and wulfenite from the Kintore Opencut, Broken Hill, New South Wales.

During the seven years that the Kintore Opencut was worked, the list of species from Australia's famous Broken Hill deposit and the knowledge of the Broken Hill orebody were greatly expanded. Two of the many oxidized minerals found, mimetite and wulfenite, occurred in a variety of colors and crystal habits that may be unsurpassed at any other locality.

INTRODUCTION

Mimetite and wulfenite have long been considered rare in the oxidized zone at Broken Hill, New South Wales, and specimens are highly prized by collectors. Mining operations at the Kintore Opencut between 1984 and 1991 exposed a large suite of minerals, currently numbering about 120 species, and have resulted in many mimetite specimens and a lesser number of wulfenite specimens being recovered (Birch and van der Heyden, 1988; Birch 1990). A wide variety of colors and crystal habits is exhibited by the two species.

MIMETITE

Although mining at the Kintore Opencut has shown mimetite to be abundant in some areas of the Broken Hill oxidized zone, Broken Hill mimetite specimens have always been quite rare in collections. The scarcity of mimetite specimens recovered during early mining operations is probably not surprising, considering the often inconspicuous appearance of the species compared to the more spectacular specimens that were recovered of many other minerals for which Broken Hill is famous, such as pyromorphite, smithsonite, cerussite and anglesite. A number of mimetites of different color and habit have previously been described by Birch et al. (1982) and Birch and van der Heyden (1988).

Occurrence

Of the arsenates identified from the Kintore Opencut, mimetite is the most abundant. It has been found in direct association with at least 20 other species. The most common associates are minerals related to beudantite and plumbogummite, with compositions occurring in several solid solution series. The "end members" of series thus far identified are beudantite, corkite, hinsdalite, hidalgoite, plumbogummite, philipsbornite and two new species for which the Kintore Opencut is the type locality, segnitite and kintoreite (Birch et al., 1992; Pring et al., 1995). A number of these species are frequently found replacing mimetite, ranging from an overgrowth on the faces of still complete mimetite crystals, through partial replacement to complete replacement. These pseudomorphs are generally cavernous shells, retaining the form of the original mimetite crystal. Coronadite is occasionally also seen replacing mimetite crystals. Some of the other more common associates of mimetite are bayldonite crystals, olivenite crystals in a variety of habits, white to gray smithsonite, and cerussite.

Color

Mimetite from the Kintore Opencut shows a wide variety of colors. Crystals colored pale orange to deep orange and orange-red provide the most spectacular specimens, especially when associated with yellow and green segnitite-beudantite and plumbogummite-like minerals. The most impressive specimens have orange to orange-red crystals to 5 mm richly scattered on coronadite and quartz-rich rocks. Matrix specimens exceeding 30 cm were recovered. Orange crystals commonly show color zoning, with pyramid and pinacoid faces, and sometimes also the second-order prism faces, being yellow or white. Attractive specimens of yellow mimetite were also found; the crystals do not reach the same size and are not as common as the orange crystals, but they still provide excellent specimens. Again there is a variety of shades from a very pale, pastel yellow to more v*vivid colors. Some of the best specimens have gemmy, transparent crystals scattered on goethite matrix, and resemble a smaller version of those from Tsumeb. Some crystals are a brilliant "electric" yellow, and are exceptionally transparent and lustrous. Green is perhaps the most uncommon color for Kintore Opencut mimetites. Transparent crystals to 3 mm ranging in color from yellow through yellow-green to a bright apple-green occur very rarely on garnet sandstone associated with beudantite.

Colorless crystals are also common, and many are water-clear and very lustrous. Opaque, milky white crystals, some of which are skeletal and consist of only a very thin shell, occur on goethite matrix. Mimetite is also found in varying shades of cream, gray, brown and tan, some of which are also bicolored, with white pyramid and pinacoid faces.

Morphologies

Mimetite crystals are usually found as scattered individuals or as random intergrowths of two or more crystals, but sprays of radiating crystals also occur, as do crusts of drusy crystals. The most common habit is short to long prismatic, and although crystals to 8 mm in length were found, they are typically 3 mm or less in size. The majority of the Kintore Opencut mimetite crystals are elongated prisms with the first-order prism {1010} as the predominant form, occasionally accompanied by the second-order prism {1120}. Prismatic crystals are generally terminated by the first-order pyramid {1011} or second-order pyramid {1121}, with both frequently being present on a single crystal, often modified by the pinacoidal base {0001}. More rarely seen are the first-order pyramid {2021} and second-order pyramid {1122}. Many prismatic crystals show multiple terminations with numerous pyramid faces or with what appears to be a single prism branching out at both ends to become a group of radiating individuals, resulting in sheaf or dumbbell shaped groups (see Wilson and Miller, 1971, for discussion of this same habit from the Rowley mine, Arizona). Tabular, yellow and bright orange to orange-red crystals occur scattered on coronadite matrix. The prism {1010} and the base {0001} are the predominant forms with small {1011} faces also sometimes present. The {0001} faces often appear frosted due to the development of many {1011} faces across the surface. Crystals on which a prism is not a dominant face are very rare. Orange bipyramidal crystals which have small {1010} faces accompanying the pyramid {1011} occur on quartz and kaolin-rich rocks associated with smithsonite. Orange bipyramids with the second-order pyramid {1121} modified by small {1011} faces are associated with pale green hinsdalite.

Compositions

Microprobe analysis of several mimetites from the Kintore Opencut were published by Birch and van der Heyden (1988). Most of those analyzed were near end-member composition but a few had intermediate compositions, being phosphatian mimetite.

WULFENITE

Wulfenite specimens from Broken Hill have always been highly prized by collectors. There have previously been only two finds outside of the Kintore Opencut, both from one area of the 45-meter level of the Junction mine. The first of these was in 1890, when very few specimens were saved, and the second was in 1924 when more specimens were collected. The crystals are orange bipyramids modified by prism and pinacoid faces (Birch et al., 1982).

Wulfenite from the Kintore Opencut does not show the same wide variation in color as mimetite, but it does occur in a variety of crystal habits. The finest examples were found in mid-1986 when specimens up to 20 cm across with orange to orange-red crystals on garnet-rich sandstone were collected. The crystals are typically less than 4 mm, with some reaching 8 mm. They are associated with drusy yellow to orange mimetite and white cerussite crystals. The crystals are generally transparent, and vary from paper thin and tabular with dominant {001} faces modified by small pyramid faces, to more equant habits showing similar development of {001} and two or more pyramids.

Apart from these very rich specimens, wulfenite crystals have tended to occur as small isolated individuals and were thus easily overlooked. Tabular orange crystals to 1.5 mm, with a more complex habit, occur on iron-stained and manganese-stained quartz associated with pale yellow mimetite crystals. The dominant faces are the pyramids {114} and {015} with smaller {011}, {013}, {112} and {001} faces. Yellow to orange cuboid crystals to 2 mm occur on quartz, associated with malachite and mimetite. The crystals show dominant second-order prism faces {010} modified by the pyramids {011} and {112} and sometimes also {013} and {114}. The prism is sometimes elongated to form a short prismatic habit. Yellow 1-mm crystals which show the rarely seen hemimorphic character of wulfenite occur on "limonite" gossan; the {013} faces are absent from one half of the crystal. Yellow crystals with a bipyramidal habit occur on quartz and kaolin-rich rocks associated with chlorargyrite. In addition to the dominant second-order pyramid {011} and lesser {013} and {112} faces, a prism, probably {120} is also present. The crystals also sometimes show small {001} faces. The color of these crystals is a bright lemon-yellow or, in rare instances, gray.

PARAGENESIS

Birch and van der Heyden (1988) and Birch (1990) have suggested four broad associations as occurring at the Kintore Opencut, with the arsenate-rich series being the most complex and diverse. All three associations have their origin in ground water dissolution of the primary sulfides galena, sphalerite and chalcopyrite. Local concentrations of arsenopyrite would yield arsenic-rich solutions and small amounts of molybdenite may be the source of molybdenum for wulfenite.

Mimetite and pyromorphite, which form under acidic conditions, were possibly the most abundant minerals during the early stages of oxidation. Finely crystalline corkite-beudantite and plumbogummite may also be among the earliest precipitated minerals. forming directly onto the garnet sandstone or alternately, rich solutions may have attacked and dissolved mimetite and pyromorphite forming pseudomorphs and replacement shells.

ACKNOWLEDGMENTS

The author wishes to thank Brian Beyer, Jack Leach, John Toma and Robert Wehr, who provided specimens, and Bill Birch for some useful suggestions.

[Figures 1-17 ILLUSTRATION OMITTED]

REFERENCES

BIRCH, W. D. (1990) Minerals from Kintore and Block 14 Opencuts, Broken Hill, New South Wales; A review of recent discoveries including tsumebite, kipushite and otavite. Australian Mineralogist, 5(4), 125-141.

BIRCH, W. D., CHAPMAN, A., and PEACOVER, S. R. (1982) The minerals, in: H. K. WORNER and R. MITCHELL, editors, Minerals of Broken Hill, Australian Mining and Smelting Limited, Melbourne, 68-185.

BIRCH, W. D., PRING, A., and GATEHOUSE, B. M. (1992) Segnitite, Pb [Fe.sub.3] H [(As [O.sub.4]).sub.2] [(0H).sub.6], a new mineral in the lusungite group, from Broken Hill, New South Wales, Australia. American Mineralogist, 77, 656-659.

BIRCH, W. D., and VAN DER HEYDEN, A. (1988) Minerals from the Kintore Opencut, Broken Hill, New South Wales. Mineralogical Record, 19, 425-436.

PRING, A., BIRCH, W. D., DAWE, J., TAYLOR, M., DELIENS, M., and WALENTA, K. (1995) Kintoreite, Pb [Fe.sub.3] [([PO.sub.4]).sub.2] - [(OH, [H.sub.2] O).sub.6, a new mineral of the jarosite-alunite family and lusungite discredited. Mineralogical Magazine, 59, 143-148.

WILSON, W. E., and MILLER, D. K. (1971) Minerals of the Rowley mine. Mineralogical Record, 5, 10-30.
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Author:Elliott, Peter
Publication:The Mineralogical Record
Date:Sep 1, 1996
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