Geology & GeoMetallurgy

Geology & GeoMetallurgy 2017-03-23T09:44:23+00:00
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Supergene copper zone (14 replies)

Maya Rothman
1 year ago
Maya Rothman 1 year ago

I am working on lode copper deposit associated with thrust faults hosted in volcanic clastic rock. Copper occur in wavy Qtz-Py-Chalcocite vein and fracture fills. My question is about the copper minerals we have being chalcocite and covelite from surface up to 100m depth. At the same zone pyrite still abundant as disseminated/vein. If that is supergene zone (lots of chalcocite, up to 6% Cu assay) will you still see abundant pyrite? 

Originally maybe it was pyrite-chalcopyrite mineral assemblage then chalcopyrite break down and precipitated as chalcocite-covellite. Pyrite preserved coz it is more resisted than chalcopyrite.

I found the chacocite and covellite as fracture fills within wavy/wormy quartz-pyrite vein, also as fracture fill / vuggs fill on the host rock (no vein). Therefore i see it as secondary copper. Deeper zone is not mineralised, just pyrite. Seems like the copper has fluid pathway along shear and fault plain and leaving compact barren zone in between.

Alan Carter
1 year ago
Alan Carter 1 year ago

It seems to me that if we assume that chalcopyrite was the universal original copper mineral, it will break down to bornite, chalcocite, covellite etc much more readily than pyrite breaks down to hematite, goethite and / or limonite.

And secondary remineralisation of Chalcopyrite to Chalcocite is what happened to the Thaduna deposit in WA; the pyrite at the deposits edge was unaffected.

This could be similar to the Messina copper deposit in South Africa where chalcocite veins close to surface develop into large lode deposits associated with breccia pipes. The lodes were zoned with pyrite and even pyrhotite on the edge.

Bob Mathias
1 year ago
Bob Mathias 1 year ago

Not having the complete picture, one could speculate accepting that mineralization is not primary. It sounds as the pyrite has not been affected be weathering, so it could be that the rock hosting the pyrite has been put in contact with the secondary copper minerals after the weathering event by the thrusting; or the pyrite has been preserved to be in contact with water, for example as inclusion in an mpermeable rock, vs a broken rock (permeable) to act as channel-way and trap for the secondary copper minerals.
Are there any secondary iron minerals (goethite-hematite-jarosite) in the upper sequence as by product of copper sulfide weathering? This could provide a vector of water direction that usually is vertical.

John Koenig
1 year ago
John Koenig 1 year ago

Sulfides minerals will alter according to climate and weathering conditions. The pH, Eh, temperature and salinity of meteoric and ground water will determinate who will be first altered. I've worked in a Ni-Cu-PGE deposit in Brazil, which has chalcopyrite more resistent to weathering than pyrite: "At a depth of ~30m the altered violarite is partially oxidized hosting ~46 wt% O and pyrite is also partially oxidized hosting ~14 wt% O, whereas the chalcopyrite is still unaltered (Fig. 5A)." Additionally not sure, but maybe in a given hydrothermal condition, chalcocite and covelite could form as primary minerals?

I've worked in a sort of lode-copper deposit that indeed chalcopyrite + pyrrhotite were the ore minerals assemblage.

Victor Bergman
1 year ago
Victor Bergman 1 year ago

Chalcocite can definitely occur as a primary mineral but, in general, you would expect that chalcocite and pyrite would occur as seperate events, because if they are precipitating out of the same fluid you would have perfect conditions for the formation of chalcopyrite.

Supergene copper precipitates below the level of oxidation so it is wholly possible that it is replacing chalcopyrite and leaving pyrite untouched. That said, in my experience chalcocite will "onion skin" ie plate the pyrite even though it will not usually replace it. Beware of people telling you that something is not possible; I have seen chalcocite precipitate on and almost completely replace biotite psuedomorphing hornblende.

My advice is to sample the chalcocite bearing material and have an experienced ore petrologist examine the chalcocite in reflected light. How do you find the chalcocite? Diseminated, on fractures, in veinlets? And is it metallic blue, sooty? Also, what do you find deeper in the system?

Maya Rothman
1 year ago
Maya Rothman 1 year ago

The colour is black and some others a re metalic grey.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Wow! You realize that there is a source somewhere nearby?

Maya Rothman
1 year ago
Maya Rothman 1 year ago

I need more geology study and on that. I believe fluids should coming from somewhere right? Not just floating chalcocite zone on the surface. 

Well at this time we focus on calculating resources this shallow zone for possibly mining then we might do development on possible deeper primary zone.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

A prudent strategy. It is possible that you only have the roots of a system here, but a primary source lateral to your shears sounds worth prospecting for.

John Koenig
1 year ago
John Koenig 1 year ago

You are in the primary zone of stratabound copper deposits, look the rounded area, on same stratigraphic level, at deep only you can have barren QSP.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

I'm interested in that proposition. Here in Chile I know of a lot of primary chalcocite, but not as describes earlier. This sounds like it is supergene.

JohnnyD
1 year ago
JohnnyD 1 year ago

We have the same mineralization in the anti- atlas (north Africa), a halo of pyrite and cooper zoning assisted to folds. Difficult to study due to geological setting (Proterozoiq rocks). My question's have you see some deformation in the Cooper deposit.

Maya Rothman
1 year ago
Maya Rothman 1 year ago

Recent hole intersect silica chlorite alteration with strong quartz-py-chalcopyrite vein. Some part of chalcopyrite start turning to chalcocite. I believe this is the primary form of the deposit. Near surface is argilic overprinted with chalcocite mineralisation.

Lots of broken core, clay gouges, shears in the deposit. I think this structure prior to mineralisation event, providing fluid pathway. There might be some late local structure as well though.

JohnnyD
1 year ago
JohnnyD 1 year ago

Drilling through to a relative barren zone below could substantiate your feelings.

Maya Rothman
1 year ago
Maya Rothman 1 year ago

I meant the deeper silica-chlorite altered rock zone with quartz-chalcopyrite is the primary, but on the top of that is argilic altered with same vein but the chalcopyrite already enriched and become chalcocite. Good profile on this hole, help understand the system better.

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