Composite veins

Composite veins are very common veining structures comprising features that are the product of more than one vein-forming event. The term composite vein, and the definition of this structure, as used here, are my own and differ from some historical definitions for the same term.

Terminology

The definition of composite veins used herein fits the context of veins we observe in hydrothermal mineral deposits, and is, in my opinion, markedly more useful than that proposed historically. Much of the previous terminology centres around the definition of veins as syntaxial or antitaxial. In reality, the textural definition of syntaxial and anititaxial, which pertains to vein fibre growth, is never used when trying to resolve the context of the vein in an evolving mineral system. Furthermore, the historical terminology can only be applied to extension veins, severely limiting the usefulness of composite veins as proposed by previous workers. Importantly, their definition cannot be applied to shear veins, where the primary growth textures and history have been destroyed.

The term composite vein has been used by a couple of previous workers in the following contexts:

???????Alex Strekeisen (https://www.alexstrekeisen.it/english/meta/veins.php) defined composite veins as ‘veins in which an antitaxial vein segment is sandwiched between syntaxial vein rims. Such a vein has three median lines.’ Strekeisen’s diagram is shown below because it is a very good one for summarizing antitaxial/syntaxial terminology.

???????Paul Bons (Bons: The formation of veins and their microstructureshttps://www.tectonique.net ? wdmrom_pdf ? bons) used the same definition as Strekeisen for these morphologies, citing Durney and Ramsay (1973, In:?Gravity and Tectonics?(edited by K. A. De Jong & R. Scholten, pp 67-96), and further commenting that?‘The term composite should be reserved for veins where both morphologies and minerals occupy similar proportions of the vein.’

???????The Free Dictionary https://encyclopedia2.thefreedictionary.com/composite+vein defines a composite vein as ‘a large fracture zone composed of parallel ore-filled fissures and converging diagonals, whose walls and intervening country rock have been replaced to a certain degree’.

My definition of composite veins, as used in this document, is as follows:

·??????Composite veins comprise the parallel to subparallel juxtaposition of two or more different veins that differ in terms of one or more factors, including mineralogy, age, morphology or conditions of formation.

·??????Veins commonly develop as composite structures when subsequent vein episodes exploit the weakness represented by pre-existing vein-wallrock contacts, or planar weaknesses internal to the veins (e.g. shear-induced laminations, mineralogical boundaries).

·??????In the case of exploitation of earlier-formed internal weaknesses, the younger veins will commonly track from one side of the vein to the other.

Recognition of composite veins

Composite veins can be recognised by:

·??????An asymmetric distribution of vein minerals

·??????The presence of two sets of minerals that have markedly different formation conditions e.g. lower greenschist-grade metamorphic minerals may comprise a vein that has adjacent bands of amphibolite-grade minerals

·??????An asymmetric distribution of alteration selvedges

·??????Cross-cutting relationships between the component veins that that are developed along strike/dip

·??????Differing overprinting/geometric relationships between the different vein fill minerals e.g. one set of vein minerals may be folded whereas others are not

·??????Differing morphology of the planar structures comprising the vein e.g. fibrous versus massive

Examples of composite veins

This section provides various examples and descriptions of composite veins from hydrothermal mineral deposits.

The first two photos are from the Homestead orogenic gold mine in Western Australia. The top photo shows a laminated quartz-carbonate vein that has been emplaced along the margin of a relatively massive quartz vein. Local cross-cutting relationships and divergence of the two veins provide information on relative ages. The lower photo shows a fibrous quartz extension vein that is juxtaposed against a relatively massive quartz extension vein. Relative ages are difficult to evaluate in this example.

The example below is from the Monty VHMS deposit in Western Australia shows a composite vein formed from the juxtaposition of an early carbonate extension vein against a later epidote-quartz breccia vein.

The next two photos show examples of composite veins displaying an asymmetric distribution of alteration selvedges. In both examples, fine-grained biotite alteration is preferentially developed on one side of the composite vein. In the photo immediately below, the two veins can be seen to diverge at the lower left of the image.

In the photo below, the alteration is associated with relatively thin quartz-carbonate veins that have been emplaced along the contact of the older carbonate-actinolite veins.

The figure below shows development of a composite vein in the upper left of the figure where three veins coalesce. Cross-cutting relationships show the order of emplacement of the veins, from 1 (oldest) to 3 (youngest).

The next example shows a composite vein from the Chatree epithermal gold deposit in Thailand. The older grey quartz vein is overprinted by a subparallel pink laumontite vein that progressively cuts across the older vein along strike, varying in position from one side of the older vein to the other. The yellow inset is shown in detail in the photo following this one.

The detail of the inset from the previous photo shows the pink laumontite vein varying in position from one side of the host quartz vein to another. The occurrence of two distinct vein types is supported by a relatively thin laumontite-only vein below the composite vein.

The photos plus line diagram in the figure below show a composite quartz vein viewed in both sides of diamond core from Quarters gold mine, Western Australia.

A younger quartz extension vein cross-cuts the older vein and then trends parallel to the contact of the older vein.

The example here is from the Twangiza orogenic gold deposit in the Democratic Republic of Congo.

The relatively more steeply dipping, older veins have a mineralogy that is the product of two veining events. The younger veins with a lower dip have cross-cut the older veins, causing deposition of pyrite in the older veins close to the vein intersections.

The photo below is also from the Twangiza orogenic gold deposit in the Democratic Republic of Congo and shows white quartz veins comprising the same population. The contact of the left-hand vein has been exploited by a younger quartz-sulphide vein with a graphite(?) or chlorite(?) alteration selvedge, forming a composite vein.?

In the example below, two parallel, planar veins comprise a composite vein. The earlier vein is a khaki-grey colour and the younger vein is a white colour.

The younger vein can be seen to run parallel to the older vein until the centre of the photo where it swaps sides and the other margin.

The above example is similar to the one below, two parallel veins have different mineralogy and colour. The younger white carbonate vein can be seen to cut across grey quartz-rich vein the on the right-hand side of the image.

The following example shows a composite chalcopyrite-carbonate vein from the Kinsevere copper deposit, Democratic Republic of Congo. Note how the veins diverge on the left-hand side of the photo.

In the following example, the same vein in both sides of a core interval are shown. The images show a composite vein comprised of an older white carbonate vein and a younger quartz-carbonate vein that has overprinted it.

The photo below shows a relatively younger, thinner laminated, quartz extension vein that has been emplaced along the contact of the older, thicker relatively uniform, monomineralic quartz vein.

The next photo shows a black, chlorite-rich shear vein that has bifurcated. The younger carbonate veins have been emplaced dominantly along the contacts of the chlorite veins. However, local splays off the carbonate vein faithfully mimic the older asymmetric shear fabric and the continuous portion of the younger vein locally cuts across the chlorite veins.

The relatively thinner, younger vein in the photo below traverses country-rock and locally cuts across the older vein, producing a composite structure where they intersect.

In the photo below, the white quartz-carbonate vein is at a high angle to pre-existing quartz-carbonate-actinolite veins. However, it locally trends subparallel to the older vein set, forming a similarly local composite structure.

The relatively thinner, younger vein in the photo below traverses country-rock and locally cuts across the older vein, producing a composite structure where they intersect.

In the next photo, the two veins are subparallel, locally contacting to form a composite vein in the lower portion of the photo.

The photo below shows two veins comprising the composite vein are evident by their different colours, mineralogy, and texture. In detail, the thin younger vein is also a composite vein, with a white quartz vein overprinting the pink-brown carbonate-quartz vein.

The following three photos are from the same piece of core.

The upper right photo shows a relatively symmetrically mineralogically zoned quartz-carbonate vein with chloritic margins.

The centre photo shows the quartz-carbonate vein cutting across the chlorite vein.

The lower photo shows the spatial context of the previous two photo as viewed from right to left.

The next two photos show a composite quartz-chlorite vein in granodiorite from Meguet, Burkina Faso.

The composite nature of the vein is indicated by the asymmetric distribution of vein minerals

The lower photo overlaps the top one and shows separation of the composite vein into two discrete veins – a chlorite vein and a fibrous quartz vein.

The composite vein in the photo below can be identified by:

• Difference in mineralogy
• Difference in morphology (breccia veins vs shear veins)
• Cross-cutting relationships

Note that the black vein has been split by both the lower white vein and the upper carbonate breccia vein. Some of the black vein occurs as a remnant selvedge on the upper margin of the carbonate breccia vein and as clasts within it.

The photo below is from the Buritica deposit, Colombia, and shows a composite vein formed by a pyrite vein overprinting the host carbonate vein.

In the example below, also from the Buritica deposit, the younger vein has exploited a fracture that is at an acute angle to the host carbonate vein orientation. It is unlikely that the younger vein was part of the same structure-fluid event as the host vein because it has a white alteration selvedge that has overprinted the host carbonate.

The photo below shows a quartz vein that has been overprinted by a relatively thinner quartz-haematite vein to produce a composite vein.

The photo below shows a composite vein comprising veins of different thickness and colour. Note that a zone of pyrite mineralisation occurs along the contact of the two veins, possibly representing a reaction product between the two structures.

The photo below is from the Buritica deposit, Colombia. The host is a quartz-sphalerite vein with minor carbonate that has been overprinted by a thin carbonate extension vein to produce a composite vein.

In the lower photo the host is a pyrite-carbonate-quartz vein from the Curraghinalt orogenic gold deposit in Northern Ireland. Note that the younger thin carbonate vein has a grey alteration selvedge (fine-grained pyrite?) that overprints the host vein.?

In the photo below, the younger white quartz-pyrite vein has exploited the grey quartz-carbonate vein.

In the photo below, the host is a quartz vein. It is cut by a quartz-pyrite vein that has exploited an acutely cross-cutting fracture.

Composite veins – limitations/conflicts

As with many textures, geometries and overprinting relationships in geology, there are grey areas of interpretation and application.

In the case of composite veins, the question arises as to whether mineralogically different layers in a vein are the products of markedly different prevailing conditions, or if they simply represent the evolution of vein growth under relatively similar conditions. This complexity is especially difficult if the mineralogy of the veins does not change markedly from layer to layer e.g. carbonate-quartz-sulphide bands next to carbonate-quartz bands.

In cases of mineralogical similarity, other vein attributes have to be compared, e.g. there may be extension vein fibres developed adjacent to shear vein textures. Generally, unequivocal cross-cutting relationships will define a vein as being a composite feature. Epithermal veins are the biggest exception in this case, commonly producing multiple cross-cutting relationships between similar mineralogy veins. Note that different features comprising a composite vein don’t necessarily imply a large intervening time period. For example, breccia vein textures internal to chalcedonic layering in an epithermal vein may have formed over a very short time.

The following veins shown here could be defined as composite veins, based on variations in mineralogy and texture of adjacent compositional layering. However, there is enough uncertainty to suggest they may also be products of development of the same veining episode.

The vein below is from Kanowna Belle gold mine, Western Australia, comprises a white quartz vein on top of a quartz-sulphide vein that shows extension textures. However, there is no discernible cross-cutting relationship and the bulk mineralogy of the upper and lower portions of the vein differ only with respect to the presence of sulphide. As such, it is probably best to consider this as part of a single vein population in which the fluid composition has changed over the period of formation i.e. not a composite vein.

In the sample below from the Golden Grove VHMS deposit, Western Australia, there are compositional differences, and the lower fibrous, chlorite-bearing portion of the vein locally diverges from the white vein core. In this case the vein could be either a composite structure, or part of a single vein population in which the fluid composition has changed over the period of formation. This is a good example of the grey area in definition.

The vein below comprises several layers that vary in mineralogy, texture, and wall-rock clast content. Overall, however, the vein is largely quartz, with variations in graphite content. The breccia texture may simply be a product of relatively more explosive opening of the structure but with infill of essentially the same fluids that formed the white, monomineralic layers. As such, this is probably not a composite vein.

The significance of recognizing composite veins

Composite veins, as defined herein, are products of separate veining episodes. Commonly, the mineralogy of the component veins is very similar. It is critically important to recognize the different vein populations because a failure to do so will render the interpreted geological history for a deposit incomplete and/or incorrect.

Establishing a geological history is one of the most important things to do when trying to understand a hydrothermal mineral deposit. It allows recognition of all of the geological entities and identifies the mineralizing structures in time and space. This then allows collection of orientation data for different populations. Critically, it mitigates against the mixing of data populations.

If a composite vein is not recognized, the component veins will be rendered the same in terms of geological age and orientation. This can impact drill program design, correlation of geological bodies, and the shape of resource models.