Wednesday, August 20, 2014

Rhyolite Porphyry from Majuba Hill II

GSN field trip Day 2: we're still at Majuba Hill.
Continuing with our progression from left to right of rhyolite porphyry hand samples, we move on to Rock Number 3, a weakly mineralized version of the tourmalinized porphyry. (Rock 1 and 2 were seen here.)
The feldspar sites in this rock sample contain needles of tourmaline, and powdery white sericite with a soft, blue green copper mineral.
An enlargement of the left part of the same photo.
I'm unclear as to the timing of mineral deposition in the altered, mineralized feldspar sites. Which came first, the tourmaline, the sericite, or the copper minerals (all after the original feldspar phenocrysts)?

The blue, green, and turquoise-colored copper minerals include at least include one spot of azurite, a few spots of possible malachite, and widespread possible chrysocolla — but at Majuba Hill, there are several bluish to greenish copper minerals that might be present (see this mineral list for Majuba Hill), including parnauite and goudeyite, both of which were first identified at Majuba Hill.
Another view of the same rock - this time the right part (turned sideways).
Which came first? I'm still wondering. Maybe we could use some thin sections (not that I have any to offer, sorry).

We'll move on to the highly altered rock on the far right (second photo) in a third episode.

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