Gossan

Iron oxides and hydroxides

Today I’m attending the 49^th annual field conference of the Tobacco Root Geological Society, a regional Northern Rockies organization that I and my friends Bob and Marian Lankston co-founded 49 (!) years ago. So here’s a rock from the Tobacco Root Mountains of Montana USA.

The brown part of the rock is basically iron oxides and hydroxides, goethite and the mixture limonite (and maybe some hematite in the more purplish zones), reasonably called gossan. Gossan is a result of intense weathering of primary sulfides like pyrite (iron sulfide) or more diverse ore minerals. All that remains is the iron oxides/hydroxides and quartz, and often the quartz is in thin bands between the original sulfide materials that have altered and weathered away, leaving a boxwork pattern of resistant quartz with holes.

This piece doesn’t really show that boxwork appearance. Rather, the iron oxide/hydroxide appears to follow a pre-existing pattern of angular shapes, probably the original crystalline material in the rock, so that this is a pseudomorph (“false form”) of the iron oxide/hydroxide after those original crystals.

The angles generally suggest either amphiboles (crystal angles typically at 56 and 124 degrees), or carbonate rhombohedrons (often around 73° and 107°). I realize that the rock does not necessarily offer nice perpendicular sections through those crystals, but I went ahead and measured some of the most well-defined angles. They're pretty consistent averaging about 68 and 112 degrees, not too far off from amphiboles. But the range is 67-75 and 102-115, which is a little more consistent with carbonate angles, such as might be found in siderite, iron carbonate, a reasonable source for some of the iron.

Sailor Lake, Montana USA. The rock was found a short distance up the trail toward Brannan Lakes. Photo from 2016.

It's almost impossible to get a sense of the third dimension in this rock, but such as it is, the replaced crystals don't seem to be sections through long prismatic crystals like amphiboles, but more through something stubby, a further suggestion of something like a carbonate. And there are a few small patches of some vitreous beige stuff, with cleavage and not quartz, within those mostly limonitic rhomb shapes, that appear to be siderite or perhaps ferroan dolomite – they give a bit of fizz in acid when pulverized, and at least a few bits became magnetic on heating, a test for siderite.

Some of the angular, iron oxide-replaced crystals show breaks suggesting some level of fracturing or brecciation (though not pervasive) before, during, and/or after the iron oxide replacement, which also shows a degree of zonation that might indicate a relatively slow replacement, layer by layer, rather than the crystals dissolving and the remaining hole then filled with iron oxide.

The quartz crystals seem to have grown later, as they cut across the banding/layering in the iron oxide. But they could have grown in association with the original siderite (?) and remained when the limonite replaced only the siderite.

Complex folding and faulting in the quartzo-feldspathic gneiss near Sailor Lake. Photo with Dan Capps and Cody from 2001.

The rock is from the Archaean/Paleoproterozoic quartzo-feldspathic gneiss of the Tobacco Roots, in the talus between Sailor Lake and the Brannan Lakes above Sailor. I collected it in either 1969 or 1970. Those rocks are mostly pretty standard gneiss, probably formed during the Big Sky Orogeny around 1,800 million years ago (Ronemus and others, 2022, Orogens of Big Sky Country: Reconstructing the Deep-Time Tectonothermal History of the Beartooth Mountains, Montana and Wyoming, USA: Tectonics 42:1, 2023).

Old equipment at the Sailor Lake Mine

There has certainly been some later mineralization, probably a result of the intrusion of the granitic Tobacco Root Batholith about 76 million years ago, coeval with the Boulder Batholith. The Sailor Lake Mine, about a half mile or so (a kilometer) from where I picked this rock up, was exploited for gold but apparently only produced a little copper.

Sailor Lake in 1969

One of the Brannan Lakes during an August snowstorm in 1977

Sailor Lake is a cirque lake at an elevation of 8,967 feet (2,733 m) beneath peaks rising to near 10,500 feet (3,200 m). The Brannan Lakes are shallow paternoster lakes in a cirque about 500 feet above Sailor.

The word gossan probably comes from Cornish gossen, derived from gōs, meaning "blood," Cornish miners’ slang in allusion to the sometimes blood-red color of the iron oxides.