Tag Archives: olivine

PERIDOT

In 1997, I decided to search the Leucite Hills in western Wyoming for diamonds (see photo at left). I didn’t find diamonds, but mark my words – “DIAMONDS WILL BE FOUND HERE”. This is an easy prediction to make based on geology.

The Leucite Hills consist of several lamproite volcanoes & flows that erupted 900,000 to 3.1 million years ago. They started their journey under one of the thickest parts of the Wyoming Craton. Cratons are very old, cool, continental cores necessary to have melting deep within the earth’s upper mantle (where diamonds are formed). While exploring this region, I collected grab samples from lamproite in the northeastern part of the field & two yielded chromites with similar geochemistry to chromites found as mineral inclusions in diamond – thus this tells me some of the volcanoes began their journey at 90 to 120 miles beneath the surface where diamonds are common.

Diamondiferous lamproites are found at Murfreesburo, Arkansas; Argyle & Ellendale, Australia; Majhgwan-Chelima, India; Kapamba, Zambia; Aldan, Russia & Bobi, Ivory Coast. The richest deposits are typically found in olivine lamproites which often alter to serpentinite (a very soft material) that erodes quickly, thus such deposits are usually hidden within a field of leucite lamproites (which are much harder rock). Thus, this all suggests a few diamonds are likely to occur in the lamproites with diamond-stability chromites, but the locations for rich diamond deposits are hidden. Along the northern edge of this field, a large sand dune field that marks the location of a major continental shear – a favorable structure for hidden lamproites.¬†One can almost guarantee there are diamondiferous olivine lamproites in this area hidden under a few feet of sand. Lamproites are also well known for colored stones – brown, yellow & the beautiful, extremely rare Argyle ‘Pinks’. Some have sold for >$US1 million/carat!

While searching for diamonds, I started looking for olivine. In 1997, I came across two green anthills at Black Rock – the ants had collected all of the available olivine in the immediate area & decorated their hills. So I collected the hills & processed them for diamonds – but all we found were 13,000 carats of flawless peridot (gem-quality olivine). Some were 12 millimeters in length. I also found peridot in place that were nearly 0.5 inch across.
Even though olivine had been recognized in this area >100 years ago, no one had ever looked at the quality of the olivine. Later, I mapped the Leucite Hills and identified all of the olivine bearing volcanoes.