Identification of Diagnostic Alteration Types in the Wallrocks of Archean Volcanic-Exhalative Massive Sulfide Deposits: Stephen D. Amor and Ian Nichol
Manuscript submitted May 1982; published December 1983

The present investigation is concerned with the identification of diagnostic lithogeochemical alteration signatures around volcanic-exhalative massive sulfide deposits in the Superior Province, with the overall objective of deriving lithogeochemical criteria, applicable in the search for new deposits of this type.

Previous work on these deposits has indicated that, in general, the footwall alteration halo is marked by iron and magnesium enrichment, and calcium and sodium depletion. These features are often only detectable if the over-riding effects of igneous differentiation are compensated. It is apparent that the relative contribution of individual elements to the geochemical alteration varies from deposit to deposit, preventing the recognition of any universal geochemical alteration criterion.

The role of discriminant analysis has been examined to establish the possibility of the technique indicating a more reliable expression of geochemical alteration. Discriminant analysis establishes the optimum weighted combination of variables to distinguish two or more populations from each other, in this case mineralized from barren environments. The application of the procedure to the data relating to the composition of wall rock associated with eight volcanic-exhalative massive sulfide deposits has drawn attention to the existence of two distinct types of alteration. The Joutel and Poirier deposits are characterized by Fe2O3. MgO, Zn and Ag enrichment, and CaO and Na2O depletion; this alteration style has been termed the "Joutel" type. At the South Bay, Sturgeon Lake and Mobrun deposits, Na2O is also strongly depleted, but Fe2O3 and MgO are usually depleted and K2O is strongly enriched; this alteration style has been termed the "South Bay" type. Both of these alteration styles are displayed at the Mattabi and East Waite deposits. "Joutel" type alteration appears to be in close spatial association with discharge vents, while "South Bay" alteration is more laterally widespread and is representative of at least one deposit thought to be formed distally from its associated discharge vent.

These geochemical signatures are more strongly expressed in pyroclastic rocks, and andesites, relative to massive rocks, and rhyolites.

The geochemical alteration patterns delineated in this way constute significantly larger exploration targets than the readily observable mineralogical alteration haloes.

Results of the current investigations indicate that the mineralizing processes associated with Archean volcanic-exhalative massive sulfide deposits have given rise to more than one geochemical response in the wall rocks and thus it is an oversimplification to aim exploration at the detection of a single type of response. The application of discriminant analysis provides a potential means of identifying and comparing as many responses as are present at the deposits studied. In this respect, it is superior to any univariate statistical method, and has considerable application in exploration.

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