The Lar deposit is a stratiform Cu-Zn volcanogenic massive sulfide deposit lying within a bimodal but mainly basaltic sequence of lower Proterozoic volcanic rocks located about 60 km southwest of Lynn Lake, Manitoba, within the southern arm of the Lynn Lake greenstone belt. The alteration zone associated with the deposit consists of a semiconformable zone of intense Mg enrichment typified by a coarse-grained, hornfelsic-textured anthophyllite-cordierite + or - garnet assemblage, enveloped by a diffuse zone of K enrichment marked by the presence of biotite. Less prominent alteration facies occur locally throughout the volcanic pile, both stratigraphically above and below the ore horizon.The zone of intense Mg enrichment lies stratigraphically directly below the massive sulfide mineralization. The rocks of this zone are characterized by strong additions of Mg, B, S, and the chalcophile elements in addition to strong depletions of Na, Ca, Sr, and Eu. Within the altered envelope of the ore lenses the Cu/Zn ratio decreases upward and outward. Mass balance calculations indicate mass change ratios ranging from 0.77 (23% mass loss) to 1.70 (70% mass increase) were produced in the rocks during alteration. Spatial distributions of alteration effects corrected for mass change are illustrated on three-dimensional mesh diagrams.Applications of the study to mineral exploration are evaluated. Outer limits of alteration can be detected more sensitively by their geochemical signatures than by their mineralogical expression, as can alteration zone boundaries. The value of mass balance-corrected flux data for these purposes is demonstrated by the wide range of mass change ratios observed in these rocks which would be represented as "noise" in raw data.