Thursday, January 30, 2014

Geo 730: Jan. 30, Day 395: Riddle Me This...

We're at the base of Nickle Mountain, west of Riddle, Oregon in this photo. The lens cap (52 mm) is resting on a block of fresh, unaltered peridotite, and the lighter rock to the upper left is probably mostly silica/quartz.

The Hanna Nickle Mine, closed in 1987, is an oddball. The short and sweet description that will be easy to understand for geologists is that it's a nickle-rich laterite. However, I imagine that's completely opaque to most people, so a longer-winded explanation is in order. The bedrock here is Josephine Ophiolite peridotite, composed mostly of olivine and pyroxene. The general formula for olivine is (Fe,Mg)2SiO4. That means that any proportion of Fe to Mg can exist along that continuum, from 100% of one to 100% of the other. Now, what that general formula doesn't tell you is that other metals can substitute in for either the Fe or Mg, generally in small amounts, but up to maybe 5 or 6 percent of the total, which is exactly what happened with nickle here.

Millions of years ago (I don't know if a date was ever assigned with any confidence), a deep bedrock slump took place on this mountain, creating a depression near its summit, and a slump plane (technically, a shallow fault) that allowed internal drainage. So weathering took place as usual, but "erosion" occurred only by complete solution; no clastic material was transported off the area.

We tend to think of silica/quartz as insoluble at ambient surface conditions, but of course, everything is soluble to some degree. And there are a number of metal oxides that are even less soluble than that of silicon. Among them are aluminum, iron and... nickle! (As an aside, cobalt is another one these highly insoluble oxides; I was involved for a period of time in assessing the feasibility of a cobalt laterite prospect.) As I've mentioned before There's precious little alumina in peridotite, which is why weathering it does not produce clay minerals. However, there's plenty of iron oxides, and after dissolution of silica and magnesium oxides, a commercially attractive concentration of nickle oxides as well. Deposits such as this, where the bulk of the initial rock composition has been removed by solution, leaving behind only the most insoluble components, are referred to as laterites. The most commonly mined laterites are, in fact, bauxites, or aluminum ore. In this case, as I recall, the smelting process resulted in a pair of mutually immiscible mattes, one iron rich, the other nickle rich. The two were separated, and sent elsewhere for further refining.

For many years, the nickle mine at Riddle was the only one operating in the lower 48. I've never been certain whether there are or were any operating in Alaska. In the years prior to its closure, I know the company was looking at importing a similar ore, I think from New Caledonia. At the time, I thought the idea of transporting bulk ore across the Pacific, and inland across the Klamaths, for the initial smelting and refinement, didn't make much sense. Apparently I was correct in that assessment.

By the way: the greenish splotches on the lighter rock? That's the nickle ore: garnierite.

Photo unmodified. May 9, 2013. FlashEarth Location. Indexed.

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