Monday, August 11, 2014

Rhyolite Porphyry from Majuba Hill I

I'm just now getting back to my Majuba Hill series, wherein I'm posting some photos from the spring GSN field trip and some related rock photos. And for those just catching up, the series begins loosely with this first unplanned stop. It then continues with a Keep Out sign, some views of the Middle adit, and some views from the dump at the Middle adit. It then goes on to our first look at the rhyolite porphyry.

What we have today are a few rock specimens — four to start with.
Tourmalinized rhyolite porphyry from Majuba Hill: two fairly "fresh" or unmineralized samples on the left, and two mineralized versions on the right.
The first sample: a weathered surface of tourmalinized rhyolite porphyry. Tourmaline has replaced the feldspar phenocrysts, or filled voids left by the earlier leaching of feldspars, and left only the quartz eyes.
The second sample.
In this second sample, some of the tourmaline in the feldspar sites can be seen to be intergrown with sericite — a very fine-grained, white mica (sericite is usually compositionally the same as muscovite), which in this case formed by hydrothermal alteration.
A second look at the second sample.
The tourmaline mass in the center of this sample contains some reddish brown iron oxides formed after now oxidized and partly leached sulfides. The tourmaline mass below and a little to the right of center shows an example of tourmaline with sericite.

We'll take a look at the next two samples some other day.

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