In the heart of the gold country of NW Quebec, where "structurally controlled" gold deposits are the norm, there are two deposit types that have been puzzles: at least two "porphyry-type" (disseminated gold in crosscutting veins and stringers within a porphyritic intrusion) that were mined out (only one brief publication on one of them) and a stratigraphic chert unit that was only partially mined (only a brief publication). The mines are now closed.
In Fig 1 the porphyry type intrusions were on a higher elevation half the way between the North and South Malartic Faults. The chert horizon -whose full extent was never explored - and its thicker "wedge" near the Sladen Fault appears to be - in this plan view - the source of the gold metal that created some or all of the other gold deposits along the various faults at that level.
Greenschist to lower amphibolite metamorphism affected this area. Figures are in feet.
Dear Mr. Issigonis,
It is a question, where you can write a book about as to try and give an answer. See these publications, where deposits on a European and global scale a treated:
DILL, H.G., SACHSENHOFER, R.F., GRECULA, P., SASVÁRI, T., PALINKAŠ, L. A., BOROJEVIĆ-ŠOŠTARIĆ S., STRMIĆ-PALINKAŠ S., PROCHASKA, W., GARUTI, G.,
ZACCARINI, F., ARBOUILLE, D., and SCHULZ H.-M. (2008) Fossil fuels, ore – and industrial minerals .- In: T. McCann (Ed.), Geology of Central Europe, Geological Society of London, Special Publication, London, 1341-1449 .
DILL, H.G. (2010) The “chessboard” classification scheme of mineral deposits: Mineralogy and geology from aluminum to zirconium.- Earth Science Reviews, 100: 1-420.
Please be more specific as you speak about porphyry-type Au deposits (they are related to convergent plate margins) and tell us what the age of them is. Faultbound is a very wide term from stockworks to shear-zone-hosted deposits. And your chert horizon with gold could also turn out to be a sinter- related Au mineralization.
In the Harz Mts., Germany, you find the famous syngenetic Rammelsberg pyrite-Pb-Cu-Zn-barite deposit and few kilometers away there are many Pb-Zn vein-type deposits. The first deposit is late Devonian in age and the other vein-type deposits formed after the main tectonic phase during the late Carboniferous. Examples like that you will find across the globe by the thousands.
For a more detailed discussion you need some more information on the age, setting and type of mineralization.
Best regards
Harald G. Dill
Hi Mike. We have a common history somewhere, but you'll have to remind me where it was. Regarding your question, do you have more detailed information, such as a cross-section or stratigraphic column to show where the mineralization occurs? What mines does the question refer to? Most of the mines in the area are synvolcanic lode vein deposits. There has been some local remobilization due to compaction mainly. Most of the porphyries are flows, but there are clear criteria for distinguishing them from intrusive equivalents. The Bourlamaque batholith consists of felsic flows. More specific information would allow a better answer. From my perspective, there are no epigenetic deposits in the area, but one must look at each situation separately.
Hi Ulrich. We were at U of T doing postgraduate. The mines in question are Barnat (porphyry) and East Malartic (chert). Fig. 2 has a section of the so called "#4 porphyry" amongst the various faults( after Fallara) The "felsic flows" idea would explain many observations we made at those mines. These flows could be generated at subduction/collision events: Here we have two continents of different ages, the Malartic Group and the Pontiac Group almost rubbing shoulders with one another.
Fig.3 has a section of the orebody in relation to the surrounding ultramafic flows.
The chert has bend described as felsic tuff by the sole investigator and is enveloped by greywackes.
The age of the porphyries/chert is about 2,686 million years old.
In the 1980's, 1990's and early 2000's I was often ridiculed for claiming syngenetic origins for some of the deposits. It looks like ideas have changed.
Dear Mr. Issigonis,
the situation becomes much clearer now after having taken a look at the cross sections. Nothing points to a porphyry-type deposit; it is an environment with all hallmarks typical of a greenstone belt and the gold mineralization is truly faultbound. It is the Abitibi Greenstone Belt with one of its smaller gold mines.
Best regards
Harald G. Dill
Hi Mike:
I would be happy to carry on this discussion outside of the researchgate forum. If you go to my website: www.syngeneticgold.com, you will find several relevant links. In the Noranda-Malartic-Val d’Or corridor, there are many deposits that are clearly synvolcanic. The work of Dick Hutchinson and students addresses some of these and volcanic-hosted gold deposits in general. The Bousquet deposit is generally accepted as synvolcanic. The websites for Virginia Mines (Akasaba deposit); Alto (Destiny Project); Agnico-Eagle (Lapa Deposit) all contain good grade thickness plots from which you can deduct vent geometry. The Val d’Or-Cadillac-Larder Lake Break is a complex stratigraphic interval and not everywhere a fault with displacement and there are several different types of deposits within it. The talc schist in your Figure 2 looks like an altered komatiite and does not present a compelling case that the Pontiac group is faulted against the Piche Group (Legault & Rabeau). One of the most important papers for exploring for syngenetic mineralization is by Phil Thurston, John Ayer et al (2008) in Ec. Geol. They deal with correlative conformities and stratigraphic correlations between authochthonous panels that separate seven discrete volcanic stratigraphic episodes on the basis of groupings of numerous U-Pb zircon ages. Syngenetic concepts have been virtually expunged from the gold deposit literature by articulate epigeneticists. The arguments have been ongoing for 100 years or more, but as we learn more about modern sea-floor hydrothermal systems, we can better understand the Archean equivalents. Also a major breakthrough has come from understanding the role of “lamprophyres”, which is what has compelled me to persist with research on gold deposits. So, I hope syngenetic ideas will be revived again or at least seriously considered instead of continuing to being arbitrarily dismissed or ignored.
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