Geology & GeoMetallurgy

Geology & GeoMetallurgy 2017-04-04T06:58:01+00:00
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Genesis of Au-Ag-Mo porphyry-epithermal deposits (2 replies and 1 comment)

John Koenig
2 years ago
John Koenig 2 years ago

This article on the Genesis of Au-Ag-Mo porphyry-epithermal deposits has me a little unclear from the abstract but are they suggesting that part (or all) of the fluid in a porphyry system is vapor  or that the fluid behaves like a vapour? If the latter, why?

Bob Mathias
2 years ago
Bob Mathias 2 years ago

Yes, metals can transfer by vapour and vapour-like fluids in porphyry systems, in addition to brine. For example; I found many chalcopyrite daughter minerals in vapor rich inclusions in porphyry copper systems (approved by laser Raman).

Bob Mathias
2 years ago

I wonder if they have information on polymetallic deposits of narrow veins in volcanic please

Alan Carter
2 years ago
Alan Carter 2 years ago

Very interesting summary about "refining" processes of the system Cu-Mo-Au in the porphyry structures. I could not read the article (than hard).

Although the authors are based on experimental data with fluid composition, T, P, and salinity knew, the result is compatible to geological observations made on the large deposits: Chuquicamata, El Teniente, Bingham etc.

I do some remarks:

In the nature the things happen somehow differently. Forming a porphyry system requires understanding the metallogenic processes, from the moment of splits the hydrothermal phase located in the advanced crystallization (cooling). This process may be greater or less intensity, volume, speed.

In the case of silicate magmas, if incorporation of metals is made at the network crystal level, then we have loe content - and if the crystallization sulfides occurs as micro-inclusions sulfides in and between silicate minerals then the porphyry will have high content - says Kessler,(2002).

In normal conditions a calcoalkaline rock will have Cu content between 20 - 100 ppm.

On the other hand the sulphide crystallization it is proportional and dependent to the SiO2 content of the magma; a felsic magma will not have possibilities to generate significant metal-fluids.

The "seizure" of metals from magma - by hydrothermal phase separation - must occur BEFORE to crystallization of sulfides, indicating the need of MAFIC magma as a metal source.

I do not realize what type of fluid is simulated in the laboratory by Hurtig and Wiliam-Jones...

In other words the secondary boiling will be "pregnant" with sequestration of metals i water-brine and vapor phase.

The sulfides precipitate from water-brine and less from the vapor phase. Assuming that an intrusive body is generated by the fluid to whom it worked in the laboratory, it is unlikely to generate a porphyry deposit.


What shows the authors it refers to a structural control close to paleo-surface and not to the DEPTH of a hydrothermal system. After various opinions the massive hydrothermal phase separation - in a magmatic interconnected system - appears/occurs at 5-6 km depth.

At this paleo-depth the participation of Cu and Mo in brine is essential. The hydrothermal phase will ascend through the same magma-tic system to depth of 2-2,5 km; the rise and dispersion of fluids will be controlled by the permeability conditions (primary and secondary).

On this route the composition of fluids,pH, T, P, will be modified by the successive buffers.

The variations and complications of this mineralization process is more complicated than in laboratory conditions; the mineralization process have multifaceted events with overlapping one another and / or interruptions.

Precisely these aspects are important to the raised deposits reminded at the beginning of discussions.

An great intrusive body which remains immobilized and an metallogenic activity generated only by their fluids - without suffering other magmatic-ht. overlapping processes - it will have all chances to remain at the limit of 0,3 - 0,4% Cu+/- Mo (as many deposits in Eastern Europe).

Contrariwise if the secondary boiling occurs early, its effectiveness in extracting the metal load of magma increases significantly enabling a significant concentration of metals.

Additional, a premature volatile evolution, can cause a violence ascent eruption to the top of volcano-plutonic system.

Even if the emission of fluids is similar to that described by Hurtig and W-Jones it won`t create a porphyry system. In the metal-brine phase (liquid) the Cu behaves in a very compatible manner, increasing from 10-12 ppm (average in granodiorite) up to 10% weight in hypersaline inclusions (~62% NaCl equiv.) and 4,5% NaCl equiv. in the vapor dominated inclusions (specific for epitermalites +/- Au).

In the vapor phase the presence of S cause considerable increase the ability to carry Cu. The secondary boiling is related also to anhydrous minerals (especially anhydrite).

In this way the P of fluids, increases and in this conditions Co and Mo behave in incompatible manner in fluid (<1%) forcing the precipitation of bornite and chalcopyrite.

I think that the transport capacity of Cu and Mo decreases gradually with decreasing water content in magma - reason which the subordinate presence of Cu and Mo in epitermalites.

The Au in porphyritic structures is “brought" by the latest basic intrusions. The geochemical anomalies of Au in these environments consider the, secondary and due to leaching.

Comparing the behavior of endogenous Au with one of Mo, the field data show parallel and divergent issues; Mo prefer felsic rocks but both metals shows affinity for S.

The behavior of these metals is different both in initial stages and in the final stages of hydrothermal process. I noticed the better ability of Mo to migrate in volatile halos in the pneumatolitic alterations.

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