Total alkalis against silica names a volcanic rock from its analysis alone — the classification of last resort when the grains are too fine to see. It must be recalculated to 100 per cent free of water and carbon dioxide first, or an altered rock slides down the diagram into the wrong name.
Alkalis, iron and magnesium as a triangle. A tholeiitic series drives towards iron before it turns for the alkali corner; a calc-alkaline series never does. That difference is the difference between a mid-ocean ridge and a subduction zone.
Iron is usually given as two oxides. FeO total = FeO + 0.8998 × Fe₂O₃ — the ratio of the formula weights — and the app does that for you from the analysis above.
Elements with an even atomic number are far more abundant than their odd neighbours — a fact about how nuclei are built, not about the rock. Dividing by chondrite removes that zigzag and what is left is geology: the slope, and the europium anomaly.
Hydrothermal alteration moves alkalis out and magnesium and iron in. Two indices measure that, and together they separate diagenesis from a real alteration pipe — which is what you are looking for around a massive sulphide.
These are whole-rock analyses of a rock you believe in. Whether the numbers themselves can be believed — standards, blanks, duplicates — is Assays and QA/QC, and it comes first.