VHMS Deposits

In 2001 we published on a simple graphical method for the study of alteration in VHMS deposits, we called the Alteration Box Plot (Large et al., 2001) based on drill hole major-element geochemistry. This approach relates whole rock geochemistry to alteration mineralogy (Fig. 2) by defining alteration trends from least altered volcanics (the coloured boxes on Fig 2) to more altered based on mineralogy and relatable to proximity to mineralisation. Alteration indexes, such as the Ishikawa alteration index (AI) and the chlorite-carbonate-pyrite index (CCPI), were used to measure the intensity of sericite, chlorite, carbonate, and pyrite replacement of feldspars and glass, in the volcanics, associated with hydrothermal alteration proximal to the massive sulfide orebodies. The alteration minerals are plotted as nodes around the edges of the box. We demonstrated that the alteration box plot is a powerful means of understanding the relationship between mineralogy, lithogeochemistry, and intensity of alteration in zoned alteration systems related to VHMS deposits. This method is now in common use by explorers and academics exploring and studying VMS deposits (e.g., Theart et al., 2011; Bichette & Percey, 2016; Gisbert, 2021; Gifkins et al 2005; Pinto et al., 2021; Krushnisky et al., 2023).

Up-date of the VHMS alteration plot.

The alteration plot for VHMS deposits has been upgraded by better defining the fields of least altered volcanics (basalts, dacites and rhyolites). This has been done by using vast databases of least altered volcanic rock compositions and is shown in Fig.3.

Figure 3a: updated alteration box plot for VHMS deposits with the fields for least altered volanics better constrained by the use of large global databases.

With increasing hydrothermal alteration the volcanic rocks move from the least altered field to the RHS as the intensity of alteration increases. They move toward the mineral alteration nodes (chlorite, pyrite and sericite) shown in Fig. 4 depending on which mineral dominates the alteration. With intense footwall alteration most of the samples plot in the red field, chlorite-rich samples at the top right-hand corner, sericite-rich down the RHS and dolomite-rich across the top. Samples very close to ore commonly plot in the RH top corner due to their high chlorite and pyrite content.

Alteration assemblage fields for VHMS deposits

Later I will show you how to produce an alteration diagram for porphyry copper deposit, but here I will use the same diagram to better define mineral assemblage fields in VHMS deposits under greenschist facies metamorphism. The diagram is shown below and is based on the key ratios of (K/K+Al) on the x-axis against K/(K+Ca) on the y-axis.

Passing to the right on the diagram the alteration assemblage is becoming more potassic, whereas passing down the diagram the alteration assemble is more calcic. The least altered volcanics plot diagonally across the diagram from the basalt endmember in the LH bottom corner to the rhyolite end member at the middle top. I will give some examples in the case study section later-on so you can see how the diagram works. I do not have many multielement geochemical data sets from VHMS drilling programs, even though I studied them extensively 20-30 years ago, so if you have some data you want me to examine then send it to rosslarge2@gmail.com.