"Rock of the Month # 45, posted March 2005" --- Sample GCW 770. Sample collected and donated by A. James Macdonald, 1985.. Chrysocolla is an hydrated, orthorhombic copper silicate, known since ancient times. Like many other oxidized, secondary Cu minerals, it is brightly coloured in hand specimen, although the body colour may be quite pale in thin section. This view was taken in transmitted, crossed polarized light, 125X magnification, long-axis field-of-view 1.0 mm.

Chrysocolla is a green or blue copper mineral, and on detailed study would probably, like goethite and related iron oxides, show a range of compositional and structural properties. Unlike the carbonate malachite, with which it may occur, it does not effervesce in hydrochloric acid.

The hand specimen is an altered metagabbro which preserves a medium-grained igneous texture, but is extensively fractured and invaded by vividly coloured copper minerals. In thin section roughly half the sample is revealed as granular epidote, with abundant chlorite and chrysocolla, some plagioclase feldspar of uncertain composition and origin, and some chloritized relict ferromagnesian mineral, probably an amphibole which was itself formed by alteration of primary magmatic pyroxene. Minor and accessory minerals include quartz, white mica, oxides and pyrrhotite. The chrysocolla is late, occupying veinlets cutting the altered ferromagnesian mineral and granular epidote alike. The coarser material displays strong birefringence, seen in the photomicrograph.

The New Rambler mine lies in the Medicine Bow Mountains of southern Wyoming, and is noted for unusual Cu-Au-Ag-Pt-Pd mineralization. Reported minerals include common sulphides, oxidized species such as chrysocolla and malachite, as well as rarities such as Pd tellurides. Commonly cited as an example of a hydrothermal PGE-enriched deposit (e.g., McCallum et al., 1976), it has also been interpreted as a magmatic sulphide mineralization modified by later deformation and alteration (Loucks, 1989). See also Nyman et al. (1990) for evidence of remobilization of PGE from altered metagabbros and redeposition of the rare metals in shear zones. PGE were first noted in the district in 1901, and the shear zone association was reported by Theobald and Thompson (1968). See Watkinson et al. (2002) for a recent overview of gabbroic-hosted Cu-PGE mineralization in North America.

References

Loucks,RR (1989) Magmatic PGE-Cu-Ni sulfide mineralization, petrology, and structure of the early Proterozoic Mullen Creek layered mafic- ultramafic intrusion, Medicine Bow Mountains, Wyoming, USA. Abs. 5th Internat. Platinum Symposium, in Bull.Geol.Soc.Finland 61 part 1, 64pp., 12-13.

McCallum,ME, Loucks,RR, Carlson,RR, Cooley,EF and Doerge,TA (1976) Platinum metals associated with hydrothermal copper ores of the New Rambler Mine, Medicene Bow Mountains, Wyoming. Econ.Geol. 71, 1429-1450.

Nyman,MW, Sheets,RW and Bodnar,RJ (1990) Fluid-inclusion evidence for the physical and chemical conditions associated with intermediate-temperature PGE mineralization at the New Rambler deposit, southeastern Wyoming. In `Advances in the Study of Platinum-Group Elements' (Barnes,S-J and Duke,JM editors), Can.Mineral. 28 part 3, 629-638.

Theobald,PK and Thompson,CE (1968) Platinum and associated elements at the New Rambler mine and vicinity, Albany and Carbon counties, Wyoming. USGS Circular 607, 14pp.

Watkinson,DH, Lavigne,MJ and Fox,PE (2002) Magmatic-hydrothermal Cu- and Pd-rich deposits in gabbroic rocks from North America. In `The Geology, Geochemistry, Mineralogy and Mineral Beneficiation of Platinum-Group Elements' (Cabri,LJ editor), CIM Spec.Vol. 54, 852pp., 299-319.