FMF - Friends of Minerals Forum, discussion and message board

[Peter Megaw]

I am repeatedly asked about the potential longevity of the azurite-malachite-brochantite zone at the Milpillas Mine, Sonora. (Note, although the mine is near Cananea, it is really in the Municipio of Santa Cruz). According to publicly available materials published by Industrias Penoles, owners of the mine, the mine opened in early 2006 with a total expected mine life of 11 years...so we're 5 years into it already. That does NOT mean we have another 6 years of azurites to look foward to because what they call the "green oxides" (malachite-brochantite-azurite with minor cuprite and native copper) only represent 30-40% of the total copper resource by volume...the remainder is "gray oxides" (primarily supergene chalcocite with neoticite/tenorite mixed with sulfides). Most of the protore (chalcopyrite and pyrite) is not ore grade even at current copper prices.

The green oxides comprise 6 lenticular flat-lying bodies, three of which are fairly large and three of which are quite small. The green oxide orebodies occur at two separate elevations, separated by 30-40 meters. Because these were the highest grade ores (>3% Cu) they were developed for exploitation first, with later years focusing on the gray ores which underlie the green ores. The mine is currently producing a mixture of the two ore types

So, the question is how much of which of the green oxide bodies remain? Simple math suggests that if they're 5/11ths through their mine life and that the green ores are 30-40% of the total resource, they've got to be getting close to running out of green ores...and our beloved azurites. However, there are small lenses of green ores interspersed in the gray ores and if one of these lenses contains good azurites we could be very happy since as collectors we care more about quality than quantity.

Watching the specimens that are coming out is another indication that they are running through the green ores. Significant amounts of brochantite have been emerging (much more stays underground since it's so fragile) and generally sulfates underlie carbonates in oxidizing copper deposits. (This is the pattern seen at Morenci for example). Further, more neoticite and tenorite have been seen (Some of the tenorites are actually pretty well crystallized, which is quite unusual for the species) and some samples are starting to show sulfides in their matrix.

All things considered, my take is that we're looking at between one and two more years of regular production and these dry up.

I would LOVE to be wrong on this...

[Montanpark]

Hi Peter,
thx for the info .. but pls tell me what is neoticite? ... i never heard of that, maybe that i have been out of business too long.

cheers
Roger

[Peter Megaw]

Roger, neoticite is a hydrated manganese-iron-magnesium silicate that commonly occurs in the oxidation and supergene zones of Porphyry Copper Deposits. The name is frequently applied to undifferentiated manganese oxides..but its importance is taht it scavenges copper and other metals from supergene fluids and is a good indicator of secondary enrichment. it is often intermixed with tenorite (sensu latu)

When you see black manganese oxides in this environment as an explorationist you check it for copper content by putting a drop of hydrochloric acid on it and scraping around with a steel needle. If copper is presnt it will plate out on the steel.

If this happens over a significant area...you bring in a drill rig

[Cascavel1]

Any recent updates on the status of the mine and what is still coming out in the way of specimens? I've heard that some nice cuprite crystals have been emerging and very little azurites are being found. It will be a sad day when it's all done.

[Cascavel1]

Do any of you who have been collecting pieces from this mine since the beginning have an accurate log of all the major pockets that were hit and the approximate dates when? I know some of you have followed it pretty closely from the beginning and would love to see this information preserved.

[Peter Megaw]

Recently a large number of distinctive azurite specimens with remnants of wispy malachite on their backsides have emerged. Some are calling this the "spiderweb" or "cobweb" pocket.

It appears to be an oxide-carrying structure...perhaps watercourse...that extends downwards from the base of the oxide zone into the currently active chalcocite "gray copper" mining area. Apparently the pocket/fracture was hit several times in mining and several hundreds of TNs to large plates largely composed of azurite have come out. Many of the pieces are damage free suggesting either that the pocket is wider than seen in other areas (where contacts are common) and/or that the security has slacked off in the sulfide zone and the miners are freer to collect and take care in doing so.

There have also been a few nice Barytes found in and above the zone...Photo of one on matrix attached

[Cascavel1]

Beautiful Baryte specimen Peter. Have you seen any of the yellow Barytes yet? I just recently saw a photo of one but previously only had seen clear, lite blue, greenish and pink varieties of Baryte.

[Fiebre Verde]

[Cascavel1]

Yes there was a huge amount of azurite that was recovered from the "cobweb" pocket so much of what has "flooded" the market has been from there. Several other small pockets produced some unique specimens as well.

[Cesar M. Salvan]

The well known Milpillas mine (Municipio de Santa Cruz, Sonora, Mexico) benefits a porphyry copper deposit, characterized by a potent oxidation zone and enrichment zone with secondary copper sulfides. The primary ore is constituted by chalcopyrite and bornite. The oxidation leads to the formation of a complex secondary mineral assemblage, with a lot of well crystallized and rare species.

Recently, we studied some samples of secondary copper vanadates from the Milpillas mine. Both were labeled as supposed Vésigniéite, a barium and copper vanadate. One of the samples is a centimeter size aggregate of dark green, olive green and yellowish green aggregates of scales and pseudohexagonal crystals forming lamellae.
The analysis revealed that the label was incorrect and the specimen is actually Volborthite, a copper vanadate. This specimen is very good for the species. At least, it is the best Volborthite specimen I saw.

The second sample studied is formed by aggregates of green small crystals and scales on malachite. The analysis results showed that the mineral is Tangeite, also known with the obsolete name calciovolborthite in the literature.
The mineral species were determined using Raman spectroscopy and elemental analysis by energy dispersive X-ray spectroscopy (specifically apxs spectroscopy).

Since the three species are hardly distinguishable without an analysis, and given the tendency to label the copper vanadates from Milpillas as Vésigniéite, it seems to be reasonable to revise the non-analyzed specimens.

[Cascavel1]

Interesting note on the Tangeite. I know there has been a fair amount of mislabeled volborthite out there as of late but glad to hear there is also something else new to the mine. As someone who only collects this locality I appreciate the insight.

[Peter Megaw]

I am delighted to hear of a probable third Copper vanadate from Milpillas.

I say third because we are confident in the analysis of the first material that was analyzed...from the same sample as in the accompanying photo. This was analyzed by XRD with cell refinement and Raman (fix the gap and follow the link to the pattern at RRUFF

https://rruff.info/R120149/display=default/
(link normalized by FMF)

This material was found in mid-2011 on the back of a large specimen of malachite pseudomorphing azurite.

Interestingly, these "crystals" are hollow shells composed of tiny scales of vesignieite...clearly epimorph/pseudomorphs after something. At the time these were found volborthite had not yet been recognized/found, but given the size and morphology of the volborthites that have come out recently it is likely that that is what contributed the form to the pseudomorphs. I got a piece of volborthite from Jordi recently that looks like it has one or two different "parasitic" phases growing like mold on the volborthite. it would not surprise me if these are vesignieite and/or tangeite.

Barium is of course common at Milpillas so there is nothing geochemically surprising in finding vesigneite...and there shouod be enough Ca anywhere for Tangeite.

[Peter Megaw]

Whoops...out of practice from being on the road for too long

The photos were taken by Jeff Scovil.

I could also comment that after the first find of vesignieite there were several more finds of similar looking material...one of which was probably the source of the specimen analyzed as tangeite. I traded some to Alfredo Petrov at Tucson, so he may also have some of this

[Cesar M. Salvan]

I'm in agree with Peter Megaw. Indeed, is perfectly compatible the occurrence of the three vanadate species. The results highlight the convenience to analyze the specimens individually I recommended at the beginning of this thread and avoid in certain cases (as this case illustrates) to label specimens based on the de visu observations.

Just for clarification and completion of this thread, and taking into account that some people tend to call in question the analytical work of other colleagues, I share one of the analysis performed in the tangeite specimen of the picture I post. This spectrum could aid, also, in the future determinations of specimens of this mine.

[Cesar M. Salvan]

I update this thread, now regarding this piece published by Michael Shaw ( https://www.mineral-forum.com/message-board/viewtopic.php?p=52076#52076 )
and others published in mindat.

The story is that some time ago, beautiful specimens of pyrite crystals from Milpillas mine (Mexico) covered by a thin layer of chalcocite (Cu2S), were put on the mineralogy market as Bornite (Cu5FeS4). The crystal shape and color of the crystals didn’t match well with Bornite.

Moreover, the geochemistry of the deposit make more likely that the specimens are composed by chalcocite-covered pyrite crystals (a relatively common phenomenon in the supergene enrichment zone of some copper deposits). Despite these observations, the identification were considered legit after a misinterpreted analysis, performed apparently (according to some picture captions in Mindat) by Marcus Origlieri, analyses for which we didn’t found any reference and we were unable to check it out, so we cannot comment it. Given the interest and quality of the specimens, two of them were submitted to me for analytical confirmation, which we performed by several methods, both non-destructive and destructive.

If someone have one of these specimens in his collection labeled as 'bornite', it should be relabeled to chalcocite-covered pyrite.