Rhyolite Porphyry from Majuba Hill III

GSN field trip Day 2: We've taken a few Majuba Hill samples out of the bag.

We'll now take a look at the farthest right sample, having already seen
Rocks #1 & #2 and Rock #3.

One side shows a weathered surface, with dark brown iron oxides and the copper carbonates malachite and azurite.

I suspect that the reason I picked up this rock was because of the mineralized appearance of the outer surfaces. I probably had no idea this rock would turn out to be yet another sample of rhyolite porphyry when I grabbed it off the dump. Scratching the iron oxide reveals it to be goethite mixed with jarosite — according to the old Bear Creek porphyry copper exploration method matching oxide color to a complex triangle of powdered goethite, hematite, and jarosite with mixes in between. The only partial example I can find of this method online can be seen here.

A fresh surface of the same rock.

Zooming in on the fresh surface, we can see that we indeed have yet another sample of rhyolite porphyry: 

Although the former feldspar phenocrysts are nearly completely gone to black tourmaline needles with a scattering of brown iron oxides (goethite + jarosite) and greenish to blue-green copper minerals, the gray, fresh-looking quartz eyes are clearly recognizable.

What's really interesting about this rock is the white to gray matrix.

The matrix of this current hand sample is considerably more altered than that of our previous sample, Rock #3: The matrix is shot through with tiny tourmaline needles and finely disseminated sericite. Additionally, the tourmalinized feldspar sites within this rock contain only minor copper minerals when compared to the feldspar sites of Rock #3. I was a bit surprised at this lesser amount of copper after spying the obvious blues and greens of copper minerals on the weathered surfaces.

And here's another surface of the rock, one showing a moderate amount of reddish brown iron-oxide (hematite + jarosite), a pale bluish green copper mineral (or combo of minerals), and black tourmaline.

The iron oxides are very fine-grained and powdery, and from a few boxworky textures, I'd say that they at least in part replace former sulfides (probably copper sulfides judging by the color of the iron oxides). Quartz phenocrysts are visible in the light olive gray right hand side of the sample (these are easier to see in person with a hand lens than in this photo because of the shadows in the photo).

And that's the last of our rhyolite porphyry samples, at least of those that haven't been brecciated!

Rhyolite Porphyry from Majuba Hill I
Rhyolite Porphyry from Majuba Hill II
Rhyolite Porphyry from Majuba Hill III (this post)

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.

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.

Reaching One Thousand

So, while I wasn't thinking about it, my 1000th post posted, and this is my 1005th! I'm not sure, really, what to say about this — although I'm also sure I'll think of something — other than Yay!!

I didn't have a particular number of posts or length of time in mind as a goal when I started blogging in earnest back in early 2008 (earlier posts and some early 2008 posts were taken from a defunct site that I had kept offline before I fully transferred to Blogger on February 29th, 2008 — leap day). Read more about my early blogging here on my 5th "Blogger-versary" post, which has links for visiting (or revisiting) more categories than reviewed below.

My original idea for blogging was to convey the life of a mineral exploration geologist and also, ultimately, to post some of my exploration stories, stories which are kept mostly on bits of paper and in old computer files under the bland and misleading title of Mojave Exploration. I have, however, found through my blogging years that I can't really reveal much of my life as an exploration geologist (my current life), because most of what I do is confidential. Additionally, the professional and geographic world in which I live and work is small, and while intending to remain largely anonymous (and certainly pseudonymous), many stories and specifics lend themselves to fairly easy recognition of who I am. In fact, I have been picked out by three people that I know of (two are long-standing friends/colleagues; a third was someone I didn't know at the time but who nevertheless managed to identify me). So I write mostly about things I see on various road trips: those near and far, often those current (recently current) or recent, sometimes those from long ago.

As for the Mojave Exploration stories and files, some of these have been posted here, first beginning with the Highway 8A series — a loose series, not really a continguous batch of posts as linked to with the 8A tag, because the tag includes field trips in and around Big Smoky Valley and other parts of old 8A, including part of the Winnemucca-to-the-Sea Highway — but do see my Highway 8A page for a list of at least some of these posts (I think it's current). You can also find some of the old stories under the tag old times. One of these includes an introductory post about being left out overnight by a helicopter (not the final word, which hasn't been written or at least hasn't been posted here); another is the post about the Mt. St. Helens field trip. A few of the old stories can also be found in one particular set of series, wherein I started running through the 1978 field season, getting as far as mid-June, perhaps.

I've also posted a few other series over the years, including the Highway 50 roadside geology series. Roadside geology posts are generally linked to under tags for the particular road, highway, or interstate, and also under the broad roadside tag. Tags for particular highways, for example the I-80 tag and highway 50 tag, will include non-geological posts related to that road or highway, as will the roadside tag.

Anyway, I have and have not met my original goals for this blog, and I've found along the way that I prefer tying most posts to photos, which for me generally slows down my posting rate.

And the road goes on...

     The Road goes ever on and on

         Down from the door where it began.

     Now far ahead the Road has gone,

        And I must follow, if I can,

     Pursuing it with eager feet,

        Until it joins some larger way

     Where many paths and errands meet.

        And whither then? I cannot say.

                ...J.R.R. Tolkien, LOTR