PROVENANCE STUDIES OF CU-BEARING FELDSPAR USING CU ISOTOPES ANALYSIS BY SIMS

Red-colored, Cu-rich andesine (labradorite) was discovered in 1980 in Oregon, USA. In 2002, diffusion-treated red feldspar appeared in markets and sold as untreated gem-quality material. The starting material for the diffusion process was pale-yellow andesine from Inner Mongolia, China. Researchers have sought various techniques to distinguish between diffusion-treated and natural red andesine. Several researchers have used Ar-Ar, and trace- and major-element analysis to distinguish between red andesine from Oregon, Mexico, the Democratic Republic of Congo (DRC), Inner Mongolia and Tibet. Ar-isotope studies showed that diffusion-treated andesine is significantly depleted in ⁴⁰Ar but were not able to determine the sources of Cu-rich andesine. Andesine from Oregon, Mexico and Asia can be distinguished by their major-element contents. However, trace- and major-elements could not distinguish between DRC, Tibetan or Inner Mongolian, diffusion-treated minerals. A LA-ICP-MS study analyzed Cu isotopes in red andesine and reported up to 3‰ variation in diffusion-treated feldspar and high intra-sample variability of Cu-isotope ratios in samples from Tibet, and concluded that Cu-isotopes can be used, in conjunction with trace elements, to source Cu-rich andesine and distinguish between diffusion-treated and natural red feldspar.

We measured Cu-isotope ratios using secondary ion mass spectrometry (SIMS) in four uncut pieces and three faceted stones. Our uncut pieces range in color from pale green to deep blue or purple. Although one of our faceted stones consists of alternating bands of light and dark red, the other two faceted stones are a deep-red color. This work can also be considered a blind study as we have no provenance information on these minerals. Cu isotopes varied within a single grain by up to 3.3‰. Cu isotopes in one uncut sample varied by 12.8‰, indicating that naturally occurring Cu-rich feldspar may be isotopically zoned. Regardless, Cu isotopes can be used to distinguish between sources of Cu-rich andesine. In addition, SIMS is relatively non-destructive with exceptional spatial resolution and is ideally suited for provenance studies of gem minerals. Faceted stones can be analyzed (and returned to the owners) with minimal visible damage.