By Jason McIntosh

Modelling multiple thin intersecting veins in 3D can be an arduous task, luckily the Leapfrog vein modelling tool is perfect for visualising thin intersecting vein systems. Complex vein systems are common in many geological settings, but for the purpose of this blog I’m going to focus on shear zone vein systems. So bear with me as I attempt to sum up the characteristics of metalliferous shear zone ore deposits and how they can be modelled using Leapfrog Geo in an easily digestible blog.

A shear zone is a discontinuity surface in the Earth’s crust and upper mantle. Depending on the characteristics of the shear zone genesis and later regional tectonics, shear zones can form economic gold, silver, copper, lead, zinc and molybdenum deposits. However, the formation of large mineral deposits is dependent on a number of factors.

Shear zones form in brittle/ductile transition zones as metamorphic facies are uplifted during orogenic collisions. They are mineralized throughout successive cycles consisting of increased and decreased fluid pressure phases. Metamorphic compression pressurizes the fluid and seismic activity reduces the pressure by allowing the fluid to invade the country rock along grain boundaries and fractures. The successive cycles allow fluid to disperse and regenerate, therefore allowing for incremental precipitation of incompatible elements such as gold within fractures and along grain boundaries.

Characteristic veins within the Brittle-Ductile transition zone. (Image sourced from USGS).

The fluid is generated by partial dehydration of the basement rocks during ductile metamorphism (a result of burial pressure). The water generated in the process leaches out arsenic and mobilises gold from beneath the collisional orogen. This fluid can invade the country rock along grain boundaries and microshears to form disseminated deposits or along strike slip faults, conjugate veins and normal faults to form veined lode deposits.

We have our own small shear zone gold deposit here in New Zealand. The Hyde-Macraes shear zone in Central Otago is a typical orogenic shear zone formed in an uplifted Mesozoic schist belt. The mineralisation occurred during the transition from ductile to brittle deformation as the schist belt was uplifted from greenschist facies metamorphic conditions. The gold enriched fluid was incrementally released during seismic episodes and deposited within concordant veins, shallow lode shears, stockworks and disseminated stockworks.

The Macraes Goldfield.

Shear zone gold deposits are located globally in batholith-hosted quartz veins formed in tectonically uplifted metamorphic basement rocks (Charters Towers Australia), greenstone hosted quartz-carbonate veins formed in Greenstone belts associated with major fault zones (Mt. Charlotte, Australia) and turbidite hosted quartz-carbonate vein sequences formed in folded brittle-ductile transition zones (Ashanti, Ghana).

That was a fly through explanation of shear zones. As you can appreciate there is still plenty left to explore on the subject. Now to model a typical shear zone vein system in leapfrog.

Veined ore deposits can often be difficult to interpret in drillhole data. To make sense of your lithology intervals use ‘Spilt Lithologies’ to separate the vein intervals into individual veins. If you are un-sure about your selection don’t worry, ‘Spilt Lithologies is editable and will be dynamically linked to your Geological model.

Interval selection.

Next create a Geological Model (GM) and use the split lithology as the base lithology column. Within your GM create a ‘Vein System’ by right clicking ‘Vein System’ and selecting ‘New Vein’. To add veins to the vein system right click ‘Vein System’ and select ‘New Vein’ ‘From Base Lithology’. The vein segments are assigned as either footwall or hangingwall based on their proximity to the vein reference surface. If you wish to change the orientation of certain segments right click on the segments icon and select ‘Edit In Scene’. The footwall and hangingwall can also be edited with a polyline or curved polyline by selecting ‘Hangingwall’,’‘Edit’, ‘With Polyline’ or ‘Curved Polyline’.

Vein segment orientation editor window.

You may notice that the veins are unrealistically thick in certain areas. If this is the case you will need to either add pinch outs or adjust the maximum and minimum thickness.

Vein

Un-pinched vein.

Adding pinch outs has the effect of tapering the vein with respect to neighboring drillhole segments that contain different lithologies to the vein lithology. Undesirable pinch outs can be excluded by right clicking ‘Pinch Outs’ in the tree and selecting ‘Edit in scene.

If pinch outs are not sufficient in limiting the vein thickness you can add a maximum thickness limit as well. If pinch outs limit the thickness too much un-select ‘Pinch out’ and add an appropriate minimum thickness.

Pinched vein (pinch outs in purple).

In some cases the vein may not fit the drillhole data as accurately as you would like. If so you can adjust the contact snapping and vein reference surface to improve the accuracy.

To adjust the contact snapping for the entire GM open the GM editor and select ‘Snap surfaces to contact points’ and set the maximum snap distance. The snapping option is also available for individual veins in the vein editor.

The vein reference surface is created with an adaptive resolution a quarter the size of the GM resolution. It is not intended to completely honor all the data, but instead fit the vein as close as possible to the midpoints while maintaining a realistic surface. The minimum distance between the reference surface and the midpoints is controlled by an error margin that is also tied to the GM resolution. Therefore increasing the GM resolution decreases the error margin and fits the vein closer to the base lithology midpoints.

By default the reference surface is curved. This can be changed to an adjustable planar reference surface by opening the ‘Reference Surface’ window and selecting ‘Planar reference surface’.

Curved vein reference surface with vein midpoints.

Lateral extents are another useful tool that can be utilized to constrain the continuity of vein systems. To create a lateral extent simply create a mesh where the veins should terminate and add the mesh as a lateral extent in your GM by right clicking ‘Boundary and selecting ‘New Lateral Extent’.

After you are happy with the individual veins you can start adding interactions within the vein system. To do so open the ‘Vein System tab within your vein system and select the vein you wish to terminate, add a vein which will provide the terminus and select the side which will provide the terminus.

Finally activate the individual veins within the vein system and then activate the vein system in the surface chronology and you should be left with a realistic vein system!

Vein system

 

Further reading:
Case Study – Using implicit modelling to model complex data.