Dear Antoni,

Honestly, I don´t know in the literature any case of a large epithermal deposit whose duration exceeds 2 million years. But I think that epithermal systems are in general, short-lived, and in all probability, less than 2 m.y., maximun that has been suggested for major magmatic hydrothermal systems by Silberman (1985).

Nilda Urbina

Antoni,

Individual mineralizing episodes are, as Nilda says, short lived perhaps up to 10,000 years or so (there are some papers by Stuart Simmons on Ladolam on this and a recent paper in EPSL on Bajo de la Alumbrera (Buret et al). However, many epithermal (and porphyry) systems are emplaced episodically and a single deposit represents mutliple mineralizing episodes. Examples on which I published ages are El Indio (7.8 to ~ 5 Ma) and Veladero (~12 to 11 Ma), whereas Yanacocha, the biggest one, was also emplaced over a long period of time as multiple pulses (Longo et al. 2010). On the low sulfidation side of things mineralizing episodes may occur over an even more extensive period of time. At Cerro Bayo (Poblete et al. 2014, Econ Geol) mineralization occurred episodically between 144 and 111 Ma. 

Saludos

Your insight is very helpful indeed! Thanks! We've just found that some particular intermediate- to low-sulfidation deposit spans nearly 10 million years with continuous volcanism and productive hydrothermal activity. It's an intriguing case.

Multumesc, Ioan!

The reason for my question:

It is important to distinguish the duration of total magmatic and hydrothermal activity in a district from the duration of ore deposition in a zone or a vein, the latter which may be very brief. Reliance on Ar-Ar ages which are easily reset or in the case of alunite, formed under supergene acidic conditions during oxidation of pyrite even as crystalline habits resembling hypogene phases, can incorrectly create the impression of long duration systems. Symmetrical inward growth with common paragenesis in many LS and IS veins from 1. early crustiform phases with the majority of sulphides and Au-Ag phases on vein margins, progressively through to 2.ore-bearing breccias with Au-Ag fragments, cockade breccia, and -or less sulphide rich interstitial crustiform quartz phases, to 3. late amethyst and carbonate with system collapse argue for a continuous and brief syntectonic filling process, like your work at La Guitarra shows Antonio. It is especially difficult to reopen a vein once formed if the system has cooled and sealed in between time (symmetrical and thermally retrograde evolving vein fill is strong evidence for continuous and rapid filling) new vein pulses would be expected to exploit older vein margins and not sealed cores, and cockade textures often show draping and bending or crustiform fragments implying incomplete lithification of earlier vein phases. These, and lack of evidence of superposition of renewed, crosscutting ore phases apart from late, post-ore carbonates in most vein systems are compelling evidence for rapid vein-forming processes. Similar arguments although with less temporal constraints can be applied to HS systems which require relatively continuous generation and regeneration of permeability in the same position to display the common early disseminated pyrite, pyrite-enargite and then sulphide vein to later IS vein networks that are common in these deposits and formed in the same position, a process difficult to maintain in silicified lithocap settings and their roots over a long duration especially if multiple, temporally separate fluid pulses are invoked, and suggesting a continuous evolving hydrothermal system. I'd suggest more reliance on field relationships than potentially contentious age dates, when resetting and error ranges can provide a misleadingly long picture of system duration, or may be assessing veins formed at different times in the same district through different hydrothermal cells and not the duration of an individual vein.

I appreciate the feedback and generally agree with your observations. In this case, though, dating of vein material comes from different veins and the age spectra show no evidence for any significant disturbance or resetting. Now, I would agree that the available ages may not be enough as to draw the detailed evolution of the structures that host epithermal veins in the area; that is why we did not go too far in our interpretation and only described what seems to be a likely pattern—and left some lingering ideas about the subject for further research.

Field relationships are the necessary starting point for research like this—only that, in this case, such relationships among mineralized structures are not evident and we need to lean heavily on sound geochronologic data.

Sounds good Antoni - I was not refering specifically to your Tayoltita-San Dimas conclusions, just more to studies that have implied formation of individual epithermal veins in increments of hundreds of thousands to millions of years, as has been implied in some studies, including Hishikari, which shows the continuous paragenesis I refer to above. I think this would be very hard to do - reopening and maintaining the same fluid channelways and mineralogical-textural evolution without the process being relatively continuous and short-lived for an individual vein or oreshoot, unless clear overprinting relationships under different conditions and renewed hydrothermal activity are apparent, and not just associated with fluid pressure changes that accompany breccia formation. Suddenly reforming and recreating the same hydrothermal depositional setting with similar P-T conditions in a structure with crustiform banding, boiling and fluid pressure cylcing symmetrically with earlier vein generations after a protratcted non-hydrothermal hiatus would be very unlikely. The studies I have seen of Tayoltita area veins look to show the same relationships as other districts. Even with good gechron, the potential short duration of such activity would be difficult to resolve. Would be interesting to discuss this with you in the field at some point!

Again, I agree with you, but I am thinking of the structural evolution of mineralized areas as well. I would not expect any vein to re-open neither randomly nor in the same place over and over. Shallow epithermal systems may "resort" to violent hydraulic brecciation but deeper systems would require the same structures to be reactivated several times. Detailed structural studies are much needed. Some structures, though, may not reactivate but as other faults may form nearby. As long as they are able to act as channelways to upwelling fluids...

Epithermal veins as thick as La Guitarra in Temascaltepec are not common unless you have a giant vein as those in Guanajuato or Fresnillo, which is suggestive of several reactivations of the same fault. Tayoltita and Zcatecas are nice examples of different faults activating diachronically here and there. In some areas, these structures have reactivated differently as well—you may correlate stages of mineralization across veins, and you may have "missing" stages anywhere! Different structural behaviors may then determine mineral zoning at any scale.

I think that I might have digressed a little, but I find this quite fascinating.