Gold, Faults and Fluids

at the top of the Australian Alps is a tiny working goldmine the red Robins gold has kept Ken Harris going for 34 years hello Ken can I Drive just find your head on those stalls with you coming up I'm on a space I've come here to see the geological structure that contains all the gold bearing veins it's called the Red Robin fold you can see here that's quartz right away across there and that's sort of one that's the football and there's a hanging wall to just take the quartz out and then every now and again it'll narrow lead I've come because I want to explore why faults and veins so often go together so I'll talk to Ken and one of the world's top academics in the field well originally the reef out propped on the surface it is a quartz reef and basically I've just sunk down on a more or less the whole distance but to make sure that it's got gold and you have to take samples to check for gold but basically it comes down to following the course this course lies along a steep fault which extends from the surface about a hundred meters above us to at least 20 meters below and like almost every goldmine if there was no funkier there'd be no quartz and no gold ken struggles and squeezes to find the gold-bearing quartz his work slowly reveals the path that goldrich fluids once followed but the fault is not as simple as it seems the veins pinch and swell and their complex shapes show that fluids invaded the fault repeatedly but even complex systems can make sense when you understand the different elements at play one scientist who understands these elements more than most is professor Rick's Ibsen he realized a key to understanding the relationship between faults and quartz veins is earthquakes because one thing we know now is that you know faults mostly move during earthquakes it's actually quite rare around the world for a fault to move steadily and not in jerks during earthquake Rick came to study earthquakes by chance in the 1970s he left New Zealand to study for a PhD so I ended up in Scotland working in the Outer Hebrides where there was a ancient fault zone that had been neglected because it penetrated some rather uniform rocks the Lewis Ian nice and you couldn't make an ordinary sort of geologic map but what I realized you could do was map bands of different types of fault rock the products a fault movement and a pattern came out of that it was common back then to picture faults as slow creeping structures but Rick found evidence that some faults had moved very rapidly and that made me realize that the rocks held clues - one does go on during an earthquake but as a geologist I became aware that quite a lot of faults that we routinely map have some sort of hydrothermal vane material along the faults and sometimes it's calcite or quartz or gypsum but one of the characteristics is that there is textural evidence that this material was not all deposited in one episode but it came in a series of pulses of fluid and the natural thing was to say well perhaps this incremental deposition is something to do with incremental displacement on the faults it's tied in to earthquake slip episodes Rick realized that to fully understand the vane earthquake relationship he had to factor in something else that earthquakes behave differently at different depths for much of the earth earthquake activity is in the top half of the crust it only goes down to about 15 kilometers maybe 20 kilometers and then it stops and so as a fault zone goes from the top to the bottom of the crust it changes from being brittle or frictional to being more ductile and flowing with depth but one of the fascinations about Victorian gold and the other orogenic gold deposits is that they have formed probably mostly in the range of sort of seven to fifteen kilometers something like that but at these depths the pressure of overlying Rock is enormous and horizontal compression or shortening is often an additional force in active orogenic terrains so how can vane fluids enter fractures and faults under these extreme conditions here the rocks are being squashed so it's counterintuitive isn't it that it's totally counterintuitive you would get vanes yes and in fact you see huge vein systems localized on pretty steep reverse folds in the zones of compression and it raises a very interesting point say why on earth would you get hydrothermal deposition which involves the creation of space on a fold almost perpendicular to the shortening direction so what's the key to understanding that to create space at depth you have to counteract all the overburden pressure with abnormal fluid pressure the huge weight of overlying rock keeps fractures tightly sealed but if gold-bearing fluids become highly pressured they can literally pump open fractures and faults and the fluids will seek the fractures that require the least pressure to open one of the attributes that's notable about these orogenic gold deposits is that you see quite a lot of flat veins associated with them some of which have beautiful fiber structures they've opened vertically and in simple mechanical terms the only way we can really understand this is if we have a fluid pressure driving crack opening that is counteracting the overburden pressure and that requires very high fluid pressure the overburden pressure is a force to be reckoned with the vane fluids had to counteract it and it's now the same for cain he needs to keep the fault from collapsing using closely spaced Timbers there's not enough room for rock bars here Harleen opera room to use use a rock drill so I just put in these Timbers you call them stalls so you have to cut a hole in the football so time for the style measure it accurately and just slam it in with a 70 vote sledge hammer and put in the wooden wedges there's extra support but it's got to be tight otherwise the whole thing can fall into you always checking it solar feed this is reasonable the wire at the moment but it's pretty pretty loose overhead so sort of proceed with caution on due to putting another stall just there actually Ken has only a hundred meters of rock above him but when the vanes formed that may have been kilometers of overburden this weight is called lithostatic pressure this is an immense force but gold-bearing fluids released during metamorphism can counteracted and form vanes nearer the surface it's more likely that hydro static pressure prevails this is basically just the weight of the water itself and is much weaker than lithostatic pressure at depth Rick realized this was a very significant difference and from there developed his fault valve model so the notion is that if you can make a link between these two zones suddenly a fracture permeable pathway associated with a rupturing of a fault then you're likely to get discharged along the fault from the area of abnormal overpressure back towards the hydrostatic condition so faults can act like fluid valves fluid pressure rises below a zone of impermeable rocks near the base of the sized magenta cone if a fault ruptures the impermeable seal over-pressured fluid can escape into the hydrostatic zone above this drop in pressure creates ideal conditions for vane deposition and what we would suggest is that there are endless cycles of fluid pressure accumulation somewhere around the bottom of the sized magenta cone with episodic discharge through the faults every time they fail during earthquake each earthquake is a potential vein forming event so vein textures are good evidence for this cyclic behavior you can see when you look at a vein like this that it's composite and I can see probably at least a dozen episodes and there but microscopically you could probably find even more episodes of fracturing hydro thermal deposition refractory and so on by the late 1980s Rick's interest in veins led to a serendipitous meeting with Canadian gold experts Howard Poulsen and Francois rêveur which led him to fully developed a fault valve model and then we were sitting in a bar in a place called fort Francis watching ice hockey and discussing meza zonal vein systems and Francois was drawing sketches of Sigma mine on beer mats there was a wonderful evening it was one of those evenings you remember as a scientist all your life because everything suddenly clicked and you had the flat veins you had steep reverse folds and I said hang on what's the angle between the two and it turned out to be 70 degrees and I said but that means it can only move if it's wildly over-pressured and that was when valving really took off because we sort of saw that you need this rather exceptional combination of circumstances I mean I look back to an undergraduate lecture our lecturer said the thing to remember is it takes a lot of coincidences to make a mineral deposit it's never one thing and trying to identify those coincidences is very interesting you've alluded to the fact that you've got high fluid pressure and the steeply dipping fault but exactly what is the significance of that relationship well it comes out of some pretty basic mechanics and you can actually do the algebra on an envelope but faults are easiest to move when they are aligned at about 30 degrees to the compressive stress axis and in a compressional regime that's horizontal so there's an angle something like that yes and we see an awful lot of thrust faults that are active at dips of about 30 degrees but if you look at the global distribution of active reverse faults we see that there is in fact a range that goes from sort of 10 degrees up to about 60 and we don't see any reverse folds active at steeper than 60 degrees in terms of modern earthquakes and the significant thing about that is that it turns out from the algebra that if the easiest orientation to make a fault move is 30 degrees then the frictional lock-up angle beyond which it's extraordinarily difficult to make a fault move is 60 degrees and there is a condition if you have fluid pressures a little bit more than lithostatic that you can actually make a fault move when it's 60 degrees or more and that's what you seem to be seeing in a lot of these deposits so the key relationship is that if you raise fluid pressure enough presumably with some seal capping the fluids that even a steep fault that normally could not move will move provided there aren't any better oriented faults in the neighborhood that's a key issue because if they're still a favourably oriented fault that will go in preference Ric has shown that if gold-bearing fluids exert enough pressure they can force their way into faults and fractures even very steeply dipping faults most of all Rick's work shows that vane is formed in very active fault systems I think that's one of the exciting things that we're seeing is that the structures weren't dead when the fluids were moving around and that's something the old-timers knew very well they comment on again again in the old mining literature of the 1890s 1920s they say look it's clear that these structures were active when the hydrothermal fluids are moving through them and it's true and that's exciting

Channel: Geology Films

Views: 78,561

Rating: 4.8816328 out of 5

Keywords: Gold, vein, quartz, mining, fault, fluid, how gold is formed, how gold forms, gold mining, Geology, Ken Harris, Rick Sibson, gold mines

Id: KDexpMBAs6M

Length: 15min 6sec (906 seconds)

Published: Thu Nov 07 2013

Reddit Comments

[u/PM_YOUR_PARASEQUENCE]

Thanks for the series! It seems like most documentaries are made for people with a middle-school understanding of earth science which makes them tends towards repetitive and boring if you come into it knowing more than that. I do feel like this filmmaker does a good job at being clear and straightforward for laypeople but it's also refreshing to not have to stop for 10 minutes while the narrator explains what a fault is.

[u/CaverZ]

Here is another 2 parter on the same topic from hydrothermal fluid specialist Prof Stephen Cox Part 1 https://youtu.be/co8GGqzCzho

Part 2 https://youtu.be/a2iEjXIUJEI

[u/Lady_Zilka]

Oh I have to write a term paper about this, and this gives me a person to search papers from. Awesome!

[u/MrVolcanoes22]

Thank you for finding this channel its amazing!

[u/intergalactic_spork]

Thank you! Watched the first episode and liked it a lot. It’s actually seem quite understandable even for lay people who are just interested in geology.

[u/Juevolitos]

Yeah, this was great. Thanks!