What are the Differences Between SEDEX and VMS Deposits and how the VMS and SEDEX deposits fit into the overall scheme of things. You remember that nature concentrates the metals by a process of partially melting crustal rocks at depth, letting the metal rise through the crust and cool, dumping the valueless material such as feldspar and amphibole on the way and concentrating the really useful metals in the remaining magma or hydrothermal fluid. Cool, dump the dull stuff and skim off the useful metals.
VMS deposits are some of the last deposits to from in terms of the upward migration of metals and mineralization drops out of the fluid either right at the surface of the seafloor or in a few tens of meters from that. So, they form at even shallower depths than shallow epithermals or carling deposits. SEDEX deposits are not directly related to volcanic activity and so don’t fit in directly to this framework. However, their mode of formation is just so similar to that of the VMS deposits that I’ve clumped them together for these courses.
Comparing VMS and SEDEX deposits both have submarine equivalents to the epithermal hot-spring deposits that I discussed in another part of the geology course. With minerals deposited from geothermally water at or near a rock-water interface. The mineralization in both VMS and SEDEX is deposited pretty much at the same time as the rock that hosts the mineralization. This is termed syngeneic mineralization as opposed to epigenetic mineralization, which is deposited long after the surrounding host rocks. The main difference between the VMS and SEDEX deposits, it’s that VMS deposits are dominantly copper and zinc rich and associated with volcanic activity. Whereas SEDEX deposits are dominantly lead and zinc rich and rely mainly on the heat caused by the depth and burial in deep sedimentary basins to drive the hydrothermal system.
Besshi deposits, named after and area in Japan where these are well-known, are a hybrid of the two kinds, where, the metal mix from the VMS deposits but hosted in sediments like the SEDEX deposits.
- Both types of deposits are products of submarine hot-springs
- VMS deposits are volcanic-hosted, dominantly copper and zinc and they are fairly common
- SEDEX deposits are dominantly sediment-hosted, they are dominantly zinc and lead they are bigger but generally less common than the VMS deposits
- In VMS deposits the metals are usually spatially zoned, with copper near the vent and then zinc and lead more distant to that
- These deposit types are major global sources of base metals, particularly zinc
- Geophysics has a vital role to play in exploration, particularly EM and gravity
- Finally, the next generation of this group of deposits may be from seafloor mining, the deposits are generally small, but are relatively easy to find as they are covered by water and they can also be very high grade.
And now i want to talk a little more about Prospecting & Exploration for VMS and SEDEX, because techniques and strategy are similar for both. Exploration for these is like look for the filling in a sandwich once you identify the break between the slices of bread- that’s the time break you know that the filling will be somewhere along that plain and that allows you to focus your efforts on just a tiny part of the stratigraphic package. The stratigraphic sequence around VMS deposits tend to follow a standard pattern, it generally starts with a thick series of oceanic basalt folds, then during a break in the volcanism a small felsic dome is extruded deriving from the intruding magma chamber of depth. As this called the hydrothermal fluid depth, altering foot wall and depositing massive sulphides arising, the sequence is then cut off by renewal of basaltic flows that which bury and preserve the sulphides. Why is it important? Because it can help your exploration by identifying that time break that you are looking for and the likely vent areas are going to be near felsic domes. This time break in the basalt eruption may host additional VMS deposits elsewhere in the district, also the faults the produced one deposit, often leak fluids over a long time and stack lenses on the same fault or on another time break in sedimentation.
Although the stratigraphic indicators are not as strong in the case of SEDEX deposits the same principle applies once you find the mineralization there are likely to be more occurrences elsewhere on the same time break in the stratigraphy or’ up or down stratigraphy on the same fault system. So in exploration of these deposits, they try to see through the younger deformation and we are trying to identify the early faults. Faults which may have been active at the time of mineralization. Soil and rock geochemistry and electromagnetic geophysics or EM are the two most useful exploration tools for massive sulphides although magnetics and gravity may also be of value in detecting mineralization. Once you are in the proximity of mineralization identification of QSP footwall alteration may help guide you to the massive sulphide plump that you are looking for. As the exploration of all types of deposits, drill bit is the ultimate judge of value.
Let’s walk through a typical exploration programme for one of these deposits. The Tarsis MOR property, in southern Yukon, not because it was a great success, in fact the project has been put on the back burner now, but because it is a simple and straightforward story and Tarsis has also published a nice little plan on their website. On the MOR property, exploration for VMS mineralization began with a series of soil sample traverses across most of the property, with the lines oriented at right angles to strike with an anomaly 100 meter line spacing and a sample interval of 50 meters, those are the black dots on the plan, these sample points were guided and located using GPS. The sample identified large zinc soil anomaly and a coincidence, slightly smaller lead anomaly. The entire property was also flown with VTEM which stands for Versatile Time-domain Electromagnetics, which suspends a electromagnetic pulse from a large coil suspended beneath a low flying helicopter then it records the return signal after its been modified by passage of the earth below. EM is capable of detecting buried conductors, such as massive sulphides on the MOR property, the survey identified three such conductors, one of which was coincident with both the soil lead and zinc anomalies. This then became the top priority target. Six holes were sited to test that central conductor. This is a section through two of those holes and as you can see the intersected three massive mineralized horizons of depth and none of these came to surface, the surface was entirely followed by the geochemistry and particularly the geophysics. In this particular case, the width of these massive sulphides were sub-economic but it shows that the exploration process was effective.
But now the differences, firstly, SEDEX deposits are generally formed in fault-bounded sedimentary basins on continental crust rather than in volcanic piles on oceanic crust.For SEDEX deposits, the basin needs to accumulate several kilometres or tens of kilometres of oxygen lacking sediment, usually shells. Secondly, the heat that drives the hydrothermal system is dominantly from depth and burial rather than a felsic intrusion although there may be a deep mafic intrusion. For the same reason the metals are not derived from felsic magmas, so copper is largely missing, instead the metals are purely derived from leaching of the sediments themselves and lead, zinc and silver dominate instead.
To form SEDEX deposits, you require the deep sedimentary basins so distribution is more limited than that of VMS deposits, however they are still found on all continents and there are about 125 SEDEX deposits on note, and these are marked in red on the map. They vary considerably in both size and grade. This graph shows that range, tonnes per percent metal. Size along the x-axis varies from 1 million tonnes up to a huge 400 million tonnes with the mean standing around about 20 million tonnes. Lead and zinc grades, that’s the y- axis, ranges from 3% up to 30 %, with a mean of about 10-12%. As you can see, SEDEX deposits are both much bigger than VMS deposits and have better grades, making them highly desirable exploration target.
Also more about VMS and SEDEX geology.