The below idealized VMS diagram shows the position of the exhalites that accompany VMS deposits. This can be banded jasper (“Jaspillite”), or Jasperoid beds.
There are two types of VMS deposits on the Molly Marie Prospect, and each has different exhalites. One type is the sub-seafloor type, and these occur in the phreatic breccias on the west side of the prospect. Exhalites found for this type are aquifer-hosted Jasperoid beds and Jaspillite breccia. The other type, which formed just beneath pre-existing basalt, are indicated by extensive beds of banded Jasper on top of the basalt.
Aquifer-hosted Jasperoid beds
Although not a VMS deposit, the Magma Mine was a massive sulfide mine, and jasperoid beds were mined up-dip from the sulfide ore for 50 years there at an average grade of .5 opt Au at the LS&A mine. Jasperoid beds were also mined extensively for gold near Wenatchee, Washington.
The many large outcrops of Jasperoid beds at the Molly Marie Prospect are shown in the Ore Genesis Section. As mentioned there, 2 chip samples of Jasperoid at the Molly Marie ran .02 opt Au. Often, the jasperoid beds are bordered by epidote-altered arkose.
Below is a photo of some of the Jasperoid beds at the Molly Marie prospect.
Banded Jasper or “Jaspillite”
Like Black Chlorite, Jaspillite is exclusively found near VMS deposits, with one exception, the iron mines of Minnesota. Jaspillite was found at Jerome, AZ at the huge VMS deposit that was mined there. Although the type shown below is not found in abundance on the Prospect, many pieces can be found. Due to its brecciated texture, it was deposited at a hot spring. The large piece below was discovered mostly buried.
Below is a piece of sawn Jaspillite that was found at the Molly Marie.