The following brief appears courtesy of Gems & Gemology‘s G&G eBrief
Image courtesy of Gems & Gemology. Photo by L. W. Snee.
Author: Lawrence W. Snee (Global Gems and Geology, Denver, Colorado) and Tommy Wu (Shire Trading Ltd., Hong Kong)
In April 2010, we visited the Montepuez ruby deposit in Cabo Delgado Province, northern Mozambique. We were hosted and guided by Carlos Asghar of Mwiriti Mining (Pemba), which has been granted the mining concession. Mwiriti has an active exploration and mining program under way, but the deposit has been overrun by illegal miners. In fact, we saw several shafts (up to 20 m deep) they had sunk. As many as 4,000 illegal miners have been evicted in recent months, with several arrested by security guards and local police while we were at the deposit.
The rubies are hosted by eluvial material and the underlying weathered bedrock. The latter consists of the Montepuez Complex, a suite of metamorphosed sedimentary rocks (amphibolite-grade schists and gneisses) that were intruded by granite, granodiorite, and tonalite. In the deeply weathered area we examined, the eluvium appeared to lie directly on Montepuez gneisses, which were cross-cut by light-colored veins mostly weathered to clay. These veins ranged up to 20 cm thick and probably originally consisted of syenitic (silica-deficient) pegmatites and aplites. We saw ruby in these veins and in the overlying boulder-rich eluvium. The miners dig pits in the lateritic soil to search for the light-colored, sand-rich layers indicative of underlying boulder beds.
The Montepuez deposits appear to have considerable promise and extend over a large region. Additional ruby finds have been reported nearby, but outside the concession. Reliable local sources told us that rubies of similar color and character were being recovered 10-20 km from the site we visited.
Lawrence W. Snee
Global Gems and Geology, Denver, Colorado
Shire Trading Ltd., Hong Kong