The roles of veins in a deformed rock volume – Part 6
Why did I undertake writing this series of posts? Partly, it came from having some spare time to collate material during COVID lockdown. Before this period of consulting inactivity I spent a lot of time in a lot of places looking at veins in a lot of geological systems, from epithermal and porphyry deposits to sediment-hosted base metal deposits to orogenic gold deposits, plus many others in between.
Mineral System Analysis entails bringing all elements of geology together to understand the environment that can potentially host a world-class mineral deposit. Veins play many roles in the structure-fluid evolution of a deposit, so any Mineral System Analysis is incomplete without a Vein System Analysis. In my posts I have shown how veins record the fluid evolution of a deposit, how they form and control part of the structural architecture of a deposit, how their orientations provide information on the geometry of a deposit, how their distribution is a function of the permeability of the rock encountered by mineralising fluids, how they provide structural time-lines for correlation, how they impact rock quality, and how their geochemistry provides insight into the fluid compositions over times. Most importantly for miners and explorers, and the shareholder, they are commonly the host to mineralisation.
So, the other thing that prompted me to post these articles was constant reinforcement of the unfortunate fact that much of the information that veins can provide goes begging. This is either because it is not captured, the vein systems aren’t interrogated in detail due to inexperience, the people in control of exploration and mining simply don’t think it is important, or all of the above. Simple fact - at the end of the day, the complete understanding of a deposit with veins cannot be achieved if the veins are not studied. Conversely, a detailed study of the veins can identify the ones that aren’t part of the fluid-mineralisation signature, avoiding an ongoing waste of time and money sampling, measuring etc veins that won’t change the resource bottom line or the exploration/mining strategy. Anyway, enough pre-amble and soapboxes!
This is the sixth item in my list of contributions veins make to a deformed rock volume, which I repeat below. So far, I have discussed veins as fluid pathways and depositional sites in Parts 1 and 2 of this series of posts. I followed this with a post (Part 3) on veins as volume compensators, documenting how they infill space that is created during host-rock deformation. In Part 4, I then documented the role of veins in controlling rock permeability and the distribution of alteration by impeding fluid movement. Part 5 was a lengthy discourse on the roles of veins as strain localisors during deformation.
Links to previous posts, including one I wrote on variations in vein morphology, are given at the end of this post.
In this post I discuss, again largely by way of illustration, the sixth item in the list below, namely the role of veins as movement accommodators in hydrothermal mineral systems.
Fluid pathways
Depositional sites
Volume compensators
Fluid barriers
Strain localisers
Movement accommodators
From my previous posts it should be obvious that the vein walls must move relative to each other in order to make space for the deposition of vein material. The action of opening can accommodate this displacement of one vein wall relative to another both during vein deposition and during subsequent deformation i.e. coeval and overprinting deformation can utilise the veins as structures to accommodate significant movement.
The displacement associated with veins can be broken into two broad categories:
- Displacement of vein walls during opening with minor deformation of the vein material. The best examples are extension veins, especially crack-seal veins, that incrementally open, with the opening direction being orthogonal or oblique to the vein margins.
- Displacement of the vein walls during and after opening due to accommodation of movement along the structural weakness occupied by the veins. These examples manifest as vein filled faults that commonly represent products of hydrothermal mineral deposition as part of the fluid-sourcing process. In this case the host fluid pathway will be a structure with variable amounts of brittle and ductile deformation.
I will begin this post with examples of veins that have formed via incremental opening with minor coeval or subsequent deformation. The first example shows fibrous quartz veins from an area of orogenic gold mineralisation in northern Ethiopia. The trajectory of vein opening can be traced by following the geometry of the fibres and it is obvious the vein is a product of many fluid cycles.
Overall, however, the hundreds to thousands of vein opening episodes have resulted in very little lateral displacement of the vein walls. This is an important point to keep in mind. Most vein deposits, in particular orogenic gold deposits, form on low-displacement structures. My colleague Steve Cox has done a phenomenal amount of work into these systems and it seems that lodes seldom occupy areas much greater than 1 km2 of fault surfaces, and net slip accumulated during ore formation is typically less than 150 m. In particular, they indicate that swarm seismicity is the characteristic response to injection of large volumes of overpressured fluids into intrinsically low-permeability rock. Injection-driven swarm seismicity and related permeability enhancement involves repeated sequences of thousands of ruptures.
A variation on this theme can be seen where the zone of displacement comprises a sheeted vein array. Individual veins can each accommodate movement increments that cumulatively produce a zone of displacement. This is shown in the example below from Potato Point in New South Wales, Australia.
Commonly, we see the effects of movement accommodation by veins because of the separations of other marker structures, including earlier veins. In many cases there is no obvious strain accumulation at the vein margins, suggesting all displacement has been accommodated during lateral translation of the vein walls during growth of the displacive vein. This is show in the next three images.
One of the things to appreciate from the next three images is that earlier veins are markers that give the direction of separation (it is not displacement because we don’t have a movement vector and hence the displacement may be apparent). The overprinting relationships are especially important as they contribute to our knowledge of the fluid evolution and geological history of the fluid system.
Stockwork vein systems are also movement accommodators, although overall there is generally minimal wall-rock translation at the scale of the vein populations. The exception is if one of the contributing populations is subparallel to the main fluid pathway and is preferentially reactivated relative to the others. If this does not happen, the main effect of vein opening is to increase the volume of veined rock. Differential transient opening on differently oriented veins will occur, depending on the stress state of the rock, rather than marked displacement in a preferred direction.
In the example below, the overall displacements on vein structures will have been minimal. A relatively greater amount of displacement may have been accommodated by the subhorizontal veins because there are more of them, but this will still have been minor.
In the example below, the subvertical veins have accommodated minor displacement, as indicated by the offsets of the other veins and their sense of curvature at the point of vein intersection. Note that the vertical veins are the oldest set and have been reactivated to deform the younger set.
Orthogonal vein sets, in particular ones that show ladder vein development, must necessarily have some movement parallel to at least one vein set. This is required to explain the lack of continuity of corresponding portions of veins that form the ‘rungs’ of the ladder veins across the veins that form the ‘rails’ of the ladder.
In the example below, the relatively thinner ‘rails’ of the ladder have effectively acted as transfer structures, accommodating movement at the vein boundary to compensate for opening of thicker ‘rung’ veins.
In brittle vein systems the displacive veins are commonly hosted by fractures and the veins appear discontinuous. Vein segments are commonly irregularly shaped and thicker sections can occupy extensional sites such as jogs.
In this example an earlier vein has been progressively steeped down to the right by fractures that host discontinuous veins.
The vein segments can morph into breccia veins that also show displacement. This is common in epithermal systems, as shown in the central section of the core interval from Ada Tepe below where a breccia vein displaces the contact between banded silica and the host-rock.
In the example below the steeply dipping vein has been displaced by a composite vein. The youngest portion of the composite vein is evident as a thin white feature below the composite vein on the left but then traverses to the middle of the earlier vein portion before exiting at the lower side of the composite vein on the right.
In host-rocks that are relatively more ductile, discrete veins can displace country-rock structure and produce curvature of the displaced features at the vein contact. These geometries are important because they provide information on overprinting, fluid evolution and kinematics.
The geometries and continuity (or lack of) of displacive structures with veins in the adjacent wall rock can give information on the relative ages of the vein populations. In the example below, a narrow steeply dipping vein is a zone of displacement but becomes continuous with wall-rock veins in the bottom half of the photo. This indicates evolution of the structure coeval with vein deposition.
In the photo below, a fault zone manifests as an anastomosing network of sinuous, narrow, graphite-filled shears. These shears are also zones of strain enhanced dissolution. Most of the carbonate veins terminate against the shears and show geometries similar to ladder veins. However, some veins have also been emplaced along the shears and show continuity with veins in the wall-rock, indicating that a portion of the shear evolution overlapped that with vein deposition.
A similar relationship to the above photo can be seen in the next one. Here, a stockwork of quartz veins is developed in the footwall of a fault. Some of the stockwork veins are locally continuous with the sheared veins occupying the fault.
Shear veins are ductile movement accommodators that occupy structures that have hosted both deposition of vein material accompanied by coeval or subsequent displacement. The degree of ductile fabric development is a function of many things, including the vein mineralogy, the amount of strain, the strain rate, how much fluid was present, contrasts in host-rock composition, depth of displacement, geothermal gradient etc. The figure below shows a structure hosting shear veins with variable fabric development, which is due to the factors listed above plus the relative ages of the different deposition episodes. The youngest veins are least deformed and locally retain initial extension vein textures.
The example below shows displacement of a carbonate extension vein by a shear vein of variable thickness.
The last two examples show drill core of the same scale but with different manifestations of the shear veins. In the first example the structure is discrete, and the shear veins occupy about half of the structure. In the example shown after that, the zone of displacement is comprised entirely of shear vein material.
And so concludes my posts on the contributions veins make to a deformed rock volume. There are many other topics that are fundamental to understanding vein formation and deformation, but they are for another day.
I have spoken to many of the industry and academic experts on the transfer of fluids at all scales, the controls on the fluid movement and the history of this in the rock, structural geological processes, and how to understand the products that geologists search for and mine. In doing so, I find that there are too many people to name who have influenced my work. However, ones who immediately come to mind, who have made a remark or explained a texture that has sent me off on tangents of confusion and learning, include Tim Bell, Aidan Forde, Julian Stephens, Jon Hronsky, Dick Tosdal, Bill Laing, Roger Taylor, Ken Hickey, Nick Oliver, Steve Cox, Dave Craw, Giuseppe (Joe) Lo Grasso, Rick Sibson, Julian Barnes, Sean Hasson, Scott Halley, Alex Aaltonen, Greg Morrison, Joao Hippertt, Roger Taylor, Peter Pollard, Marcus Tomkinson, Pat Williams, Garry Davidson, Gerard Tripp, James Siddorn, Steve Mickelthwaite, Richard Lilly and Jun Cowan. I’ve no doubt forgotten people and for this I apologise. In the end, the thoughts and mad ramblings present in my posts, correct or otherwise, are my own and I have benefitted from the feedback so far.
So, once again, thanks to readers who have persevered with me this far. Below is a list of links to the posts I have made preceding this one.
Cheers!
Previous posts in this series
The crazed and commonly disjointed thoughts I have presented prior to this one have all been posted on LinkedIn and the links to these are below.
Vein system analysis – some comments on vein morphology
This discusses some of the variations in vein morphology we need to be aware of in order to make informed interpretations on geology at all scales.
The roles of veins in a deformed rock volume – Part 1
This discussed veins as fluid pathways.
The roles of veins in a deformed rock volume – Part 2
This discussed veins as depositional sites.
The roles of veins in a deformed rock volume – Part 3
This discussed veins as volume compensators, infilling space as the rock undergoes deformation.
The roles of veins in a deformed rock volume – Part 4
This documented the role of veins in controlling rock permeability and the distribution of alteration by impeding fluid movement.
The roles of veins in a deformed rock volume – Part 5
This discussed the role of veins as strain localisors in hydrothermal mineral systems.
Consulting geologist at Klondike Exploration Services
4 年For those of you who have been following Bretts excellent set of posts on quartz veins in deformed rocks i thought some of you might be interested in some more general classification interpretation and application of quartz textures for exploration in a range of environments. The papers below are available from SEG, my site on researchgate and Linked in. Application of Quartz Textures to the Classification of Gold Deposits Using North Queensland Examples? Author(s)??Kim Dowling?;?Gregg Morrison DOI: https://doi.org/10.5382/Mono.06.26 Published:?January 01, 1989 Quartz?textures in epithermal veins, Queensland; classification, origin and implication Economic Geology (1995) 90 (6): 1841–1856. DOI:?https://doi.org/10.2113/gsecongeo.90.6.1841 ...Guoyi?Dong; Gregg Morrison; Subhash Jaireth GeoRef, Copyright 2004, American Geological Institute. 1995 ... Epithermal texture guide and model.
Technical Specialist - Structural Geology
4 年Hi Brett, thanks again for another great installment!
Director - Hidden Treasure Consulting; Tech Director, Vultus Pro Mineral Resources & former Associate Consultant Geologist, AMC (AEMS)
4 年Great stuff!
Project Geologist at UDRP-World Bank
4 年PLEASE SUGGEST ANY OF YOUR BOOKS FOR READING
Geologist 2 (Exploration and Resource Modeling) at Freeport-McMoRan
4 年This has been an excellent series, thank you. You mentioned you run a structural geology training course, but have you thought about putting much of this information into kind of a self-published paper or book? I was thinking something along the lines of a short manual you could easily take into field projects or a classroom that could be quickly referenced. Such as when the geologists are looking at veins in drill core or in outcrop and want to cross-reference what they're looking at with other possible analogues from other localities or want ideas to make sense of the veins they're looking at. The idea was more along the lines of this generic ore petrography book I have used in school and on projects where each page goes over the different ore minerals, their properties, habits, pictures, etc, but in this case it would be veins and their structures.