GEOLOGY OF THE B-BASE-LINE ZONE, WALTON CU-PB-ZN-AG-BA DEPOSIT, NOVA-SCOTIA, CANADA

The Walton Ba-Cu-Pb-Zn-Ag deposit, Nova Scotia (production 4.5 million metric tons (Mt) >90% BaSO4 and 412,850 t @ 0.52% Cu, 4.28% Pb, 1.29% Zn, and 350 g/t Ag), is hosted by Visean age carbonate rocks of the M acumber Formation and its associated breccia. The Macumber Formation i n the Walton area is divided into two units, the lower Macumber, a lam inated carbonate unit 9 to 12 m thick and the upper;Macumber, a synsed imentary carbonate slump breccia that forms two mounds up to 21 m thic k. Outside the immediate mine area the carbonate is either limestone o r dolostone; the Macumber Formation surrounding the deposit has been a ltered to manganiferous siderite. Although three textural types of sid erite are recognized, little variation in bulk chemical composition is evident among them. Minerals were deposited in the sequence: barite, pyrite-marcasite, sphalerite, galena, tennantite, chalcopyrite, and (p ara)rammelsbergite. Each mineral replaces earlier minerals to varying degrees. Based on barite and metal content two main ore types are defi ned. Type I, with a bulk composition of 0.3 percent Cu, 2.5 percent Pb , 0.2 percent Zn, and 31.2 percent BaSO4, comprises barite and variabl e sulfide contents. Type II, comprised of sulfides only, has a composi tion of 0.5 percent Cu, 0.5 percent Pb, 0.1 percent Zn, 51 g/t Ag, and no barite. Type I ore forms a roughly conformable sheet which straddl es the upper Macumber-lower Macumber contact and contains lenses of Pb - and Cu-rich ore. Type II ore is most commonly associated with the up per Macumber mounds where it forms large pods which cut the type I ore zone. Type II ore also occurs within the underlying Horton Group sand stones but only where the upper Macumber mounds have developed. There is also a stratigraphic zonation in metal content inasmuch as all meta ls are higher, on average, in the lower Macumber relative to the upper Macumber. The host stratigraphy is steeply dipping and faulted and bo th features are considered to be postore events. Karsting, which also postdates mineralization and probably folding, has removed portions of the stratigraphic upper parts of the deposit. The cavities produced b y this process are now partially filled with sand and limestone brecci a.