Here's some information from Dr DeCosta about 'ska

http://www.science.smith.edu/geoscience ... skarn.html

If you want to go a little deeper into the study of skarns and what to expect when you discover one there’s 2 guys that stand out as the worldwide experts.  They are Einaudi and Meinert.  The link above refers to some of Meinert’s work.  Of particular interest to the LDM skarn might be the paragraph copied below citing the thicker nature of near surface skarns like that at LDM which is actually on surface and how surface skarns have more permeability for the metal-laden hydrothermal fluids due to “hydrofracturing”.  What happens is rain water (“meteoric water”) trickles down and collides with the super hot hydrothermal fluids and magma and an explosion occurs.  The $64 word for this phenomenon is “phreatomagmatism” in case you ever need it while watching Jeopardy.  Sometimes the explosivity is so severe that you can’t even recognize the minerals that typify a skarn like garnet, pyroxene and wollastonite.  This is fine since the metals stay put.

This opens up the porosity of the rocks near the surface.  They can then accept more of the metal-laden hydrothermal fluids.  Near surface the “breading” of this “corn dog” (skarn-porphyry intrusive complex) i.e. the skarn, gets thicker.  Down deep skarns have a narrower “rind” (breading) because the rocks there tend to “fold” under pressure.  The surface skarns are not only larger and richer they’re cheaper to mine.

The reasons the grades in the skarn are so critical is that they’ll roughly parallel those found in the “hot dog” (intrusive porphyry) of this “corn dog” (skarn-porphyry complex).  In essence, finding the skarn at surface gives us a sneak peek at approximately what we can expect to find while drilling (although skarns are less homogenous) into the porphyry.  I couldn’t even hazard a guess as to how many drill holes worth of information “equivalents” the drifting of this adit will provide.  Think of it as a “window” into the mountain the engineers and geologists were lucky enough to find at surface.  The typical homogeneity of porphyry deposits allows this extrapolation.  Drilling still needs to be done in order to design the open pit and calculate reserves and resources.

From Meinert:
“Thus, skarn formed at greater depths can be seen as a narrow rind of small size relative to the associated pluton and its metamorphic aureole. In contrast, host rocks at shallow depths will tend to deform by fracturing and faulting rather than folding. In most of the 13 relatively shallow skarn deposits reviewed by Einaudi (1982a), intrusive contacts are sharply discordant to bedding and skarn cuts across bedding and massively replaces favorable beds, equalling or exceeding the (exposed) size of the associated pluton. The strong hydrofracturing associated with shallow level intrusions greatly increases the permeability of the host rocks, not only for igneous-related metasomatic fluids, but also for later, possibly cooler, meteoric fluids (Shelton, 1983). The influx of meteoric water and the consequent destruction of skarn minerals during retrograde alteration is one of the distinctive features of skarn formation in a shallow environment.”