Hi Jun and Bruce, a good talk, particularly liked the fluid modelling Bruce. Jun I think you present some very interesting points and the Perkins discontinuity is a great name for a concept we have all seen at one time or another. Your deposit scale grade distribution is a very powerful tool for focusing in on controls to mineralisation, and I think you illustrated that very nicely. On the VMS front I have issues with identifying many VMS as epigenetic or late in the deformation history based on grade distribution alone. Apart from epithermal deposits, they are one of the few deposit styles we can actually observe forming today (if you have a handy submarine) and are thus ore deposit processes that are very well understood. Many of these deposits are intensely deformed as to get from seafloor to current land surface does involve tectonic processes and they tend to wrap around sub-volcanic intrusives (at least kuroko style) so folding is quite natural to see. Deposits are often well zoned, with sulphide mounds and baryte distribution which closely mimic modern examples. I had the pleasure to work with Dr Jim Franklin who drilled some of these modern smoker systems and who was one of the people who used that information to develop our understanding of these deposits. Stratigraphy is important as are plumbing systems and cap seals which are often readily identifiable in the field, even in highly deformed deposits. Syn volcanic faults which form the plumbing systems are rarely visible directly, unless indicated in the stratigraphy by asymmetric deposition (growth faults, grabens etc) and in deformed deposits are often rotated into the direction of strain but can still be inferred by alteration minerals and discordancies on the limbs of folds. A possible explanation for the fact that you do often see these massive sulphides balled in the centre of antiforms, but also synforms and I have seen them squeezed like a tube of toothpaste under high metamorphic strain several hundred metres from their alteration systems. I wonder if this is because they tend to develop into nappe structures in early compressional deformation and form recumbent folds with a massive sulphide core as they fail along the synvolcanic faults which bound the massive sulphide and are then rotated into D2. They are often highly dismembered that is for sure. Which brings us back to models and mapping. I think it is an important point you make about what planar structures look like after deformation on a fold hinge but these structures can be preserved as relatively linear features on fold limbs I should imagine. In the case of VMS I fully agree that there is a temptation to whack a syn volcanic fault in there because the model says you should have one and without appropriate support in terms of alteration or stratigraphy this is something you shouldn't do. Another thing I am wondering about is cognitive bias in that we tend to see faults always as linear planar features and they are drawn as such even in highly deformed terrains. This could be for a number of reasons: 1. people are only seeing late faulting which is often brittle and easy to see, whereas faults controlling mineralisation (in deeper deposits at least) are often ductile and hard to spot or 2. on an outcrop or mine scale we generally only see a dislocation to infer a fault over an area of a few metres and we don't actually know what happens after that - does it bend or deform outside of our field of view so it tends to get drawn as a linear feature. 3. Another possibility is people struggle to put the geological map or section together because they have limited information and tend to draw a fault or two in because they cannot match data between drillholes and this is generally done as a planar feature, without any real or very limited evidence. This could explain why so many of your map examples showed linear faults in deformed terrains.
Hi Jun and Bruce, Great presentation! Thanks for sharing. I came across your video somehow just in time when you uploaded it... What a coincidence! - I see the purpose of Jun's structural geology quiz questions now with all those field examples, specifically when observing them on the "field- or deposit-scale". - I like Bruce's skilled use of hydrogeological models explaining the concentration of mineral deposits along the hinges of anticlines. - Geochemical information of a site might not be very useful in the ore-finding effort without the geospatial or structural geology knowledge. Cheers!
In regards to the comments period, Jun mentioned that structure gets looked at as mystical, & this was in response to the comment that it doesn’t have the amount of numbers and equations that geochem has. It’s really the exact opposite, & it’s something that was reborn in theoretical physics & somewhat in chemistry early in the 20th c. & is a phenomenon that goes back to Plato & the Pythagorean cults of Ancient Greece. It’s the view of knowledge in general being some sort of supernatural thing cutoff from reality & numbers/math is our way of reaching it. They believe that perception of the physical world provides no information relevant to knowledge. So when you’re conceptualizing structural observations based on fundamental principles of geoscience, they view that as some mystical nonsense cuz it doesn’t involve pure numbers untainted by physical observation. It’s truly bizarre, but the most dominant mode of thought since Plato. It’s scarily similar to a delusion, I mean you have scientists that refuse to believe that what they observe has any objective validity.
Hi Jun and Bruce, a good talk, particularly liked the fluid modelling Bruce. Jun I think you present some very interesting points and the Perkins discontinuity is a great name for a concept we have all seen at one time or another. Your deposit scale grade distribution is a very powerful tool for focusing in on controls to mineralisation, and I think you illustrated that very nicely. On the VMS front I have issues with identifying many VMS as epigenetic or late in the deformation history based on grade distribution alone. Apart from epithermal deposits, they are one of the few deposit styles we can actually observe forming today (if you have a handy submarine) and are thus ore deposit processes that are very well understood. Many of these deposits are intensely deformed as to get from seafloor to current land surface does involve tectonic processes and they tend to wrap around sub-volcanic intrusives (at least kuroko style) so folding is quite natural to see. Deposits are often well zoned, with sulphide mounds and baryte distribution which closely mimic modern examples. I had the pleasure to work with Dr Jim Franklin who drilled some of these modern smoker systems and who was one of the people who used that information to develop our understanding of these deposits. Stratigraphy is important as are plumbing systems and cap seals which are often readily identifiable in the field, even in highly deformed deposits. Syn volcanic faults which form the plumbing systems are rarely visible directly, unless indicated in the stratigraphy by asymmetric deposition (growth faults, grabens etc) and in deformed deposits are often rotated into the direction of strain but can still be inferred by alteration minerals and discordancies on the limbs of folds. A possible explanation for the fact that you do often see these massive sulphides balled in the centre of antiforms, but also synforms and I have seen them squeezed like a tube of toothpaste under high metamorphic strain several hundred metres from their alteration systems. I wonder if this is because they tend to develop into nappe structures in early compressional deformation and form recumbent folds with a massive sulphide core as they fail along the synvolcanic faults which bound the massive sulphide and are then rotated into D2. They are often highly dismembered that is for sure.
Which brings us back to models and mapping. I think it is an important point you make about what planar structures look like after deformation on a fold hinge but these structures can be preserved as relatively linear features on fold limbs I should imagine. In the case of VMS I fully agree that there is a temptation to whack a syn volcanic fault in there because the model says you should have one and without appropriate support in terms of alteration or stratigraphy this is something you shouldn't do.
Another thing I am wondering about is cognitive bias in that we tend to see faults always as linear planar features and they are drawn as such even in highly deformed terrains. This could be for a number of reasons: 1. people are only seeing late faulting which is often brittle and easy to see, whereas faults controlling mineralisation (in deeper deposits at least) are often ductile and hard to spot or 2. on an outcrop or mine scale we generally only see a dislocation to infer a fault over an area of a few metres and we don't actually know what happens after that - does it bend or deform outside of our field of view so it tends to get drawn as a linear feature. 3. Another possibility is people struggle to put the geological map or section together because they have limited information and tend to draw a fault or two in because they cannot match data between drillholes and this is generally done as a planar feature, without any real or very limited evidence. This could explain why so many of your map examples showed linear faults in deformed terrains.
Hi Jun and Bruce,
Great presentation! Thanks for sharing. I came across your video somehow just in time when you uploaded it... What a coincidence!
- I see the purpose of Jun's structural geology quiz questions now with all those field examples, specifically when observing them on the "field- or deposit-scale".
- I like Bruce's skilled use of hydrogeological models explaining the concentration of mineral deposits along the hinges of anticlines.
- Geochemical information of a site might not be very useful in the ore-finding effort without the geospatial or structural geology knowledge.
Cheers!
It would be nice to know where the paper is published.
In regards to the comments period, Jun mentioned that structure gets looked at as mystical, & this was in response to the comment that it doesn’t have the amount of numbers and equations that geochem has. It’s really the exact opposite, & it’s something that was reborn in theoretical physics & somewhat in chemistry early in the 20th c. & is a phenomenon that goes back to Plato & the Pythagorean cults of Ancient Greece.
It’s the view of knowledge in general being some sort of supernatural thing cutoff from reality & numbers/math is our way of reaching it. They believe that perception of the physical world provides no information relevant to knowledge. So when you’re conceptualizing structural observations based on fundamental principles of geoscience, they view that as some mystical nonsense cuz it doesn’t involve pure numbers untainted by physical observation.
It’s truly bizarre, but the most dominant mode of thought since Plato. It’s scarily similar to a delusion, I mean you have scientists that refuse to believe that what they observe has any objective validity.