Friday, November 14, 2008

Friday Field Photo

Wandering around out in the hills a while ago, I came upon some old mine dumps. (I was in Nevada, after all.) The actual sources for the lower, larger dumps seen in the above photo are, as far as I could tell, up the hill: two smaller dumps you can barely see above and between the lower two dumps (click to enlarge). That is to say, the lower dumps are composed of waste material moved downhill from the upper workings by the oldtimer miners. Usually, that kind of transportation is done underground, but if any lower haulage adits (tunnels) exist near these lower dumps to connect to the upper workings, they weren't obvious, or were long-since caved in.

On one of the lower dumps, I found this interesting and colorful rock, an iron-oxide-rich rock with colors of brown, reddish brown, and yellow brown.

The rock has some large relict pyrite cubes in the upper left, and below that and cutting through the rock are some irregular veinlets of silica, which mark spaces where former rock fragments and former sulfides have been leached and weathered out, leaving holes and soft, punky, yellow and brown Fe-oxides. The host rocks to this oxide material is - limestone!

P.S. Any ideas about the diamond-shaped hole in the upper left below the relict pyrite cubes?


Lost Geologist said...

The diamond-shaped reminds me of either haematite or magnetite, I saw haematite replaced by pyrite sometime, or dolomite - perhaps because right now my head if full of carbonate of my mapping project.

Anonymous said...

How about pyrite? It also forms octahedrons (check your D, H & Z). I once found an outcrop that was full of nice, black octahedral crystals that I initially identified as magnetite, but later smashed a couple and discovered the truth that they were pyrite octahedrons with a thin, black, oxide (perhaps magnetite?) crust.


Calgary, Alberta

Lockwood said...

Looks like boxwork to me. I generally think of this as less-soluable material that remains after most of the ground mass has weathered and dissolved out. In the case of Nickle Mountain, Oregon, quartz had dissolved from surface material during weathering, then reprecipitated in the fractured peridotite below the water table. As weathering and erosion proceded, that rock itself weathered out, leaving the quartz boxwork. Nickle oxides/hydroxides are nearly as insoluable as their aluminum counterparts, so this nickel deposit was lateritic in nature. An older USGS Publication on the deposit is here. This quote also uses the term in context: "Soil, saprolite and silica boxwork nickeliferous ores were formed over the Nickel Mountain peridotite during the development and later destruction of the Klamath peneplane." I'm not finding any good pictures, and most references seem to be cave-oriented rather that economic geology-oriented. But a decent econ geology text should have discussion, and the sample and your description match very well to what I remember.

Lockwood said...

So I guess after all that, I didn't really answer your question- I'm guessing it's just a couple of sets of parallel jointing planes that filled in with silica before the sulfides and carbonates weathered out.

Silver Fox said...

These are all good ideas, and sorry it's taken me so long to get back here.

The diamond-like shape could reflect the erosion of a rock-fragment or the oxidation or leaching of a mineral. It *is* similar to the shape of dolomite.

I think that octahedral shapes (cubic) are usually a bit stubbier, though not quite square - but possibly the shapes vary. I was thinking it could be the shape of a large chalcopyrite crystal (tetragonal). It would be a fairly large crystal, though, and I'm not really convinced! And for some reason, I can't open Mindat right now, so can't link to a chalcopyrite photo.

Boxwork can be used as a pretty generalized term - I usually reserve it for textures that result from weathering or leaching of minerals. It's often used in exploration just for textures that originated from weathering or leaching of sulfide minerals: sulfide boxwork (with Fe-oxide and silica forming the boxes).

So... I don't have a for sure answer - my best guess is an eroded rock fragment, but I'd want to look around on that dump some more, and around the area, and maybe I'd learn something more definitive!