Abstract

The Jardin arsenian Cu-Ag deposit of the Copiapo mining district, northern Chile, is in many respects representative of the volcanic-associated, disseminated, cupriferous manto mineralization characteristic of this segment of the central Andean orogen. Containing ca. 1 to 2 million metric tons of ore with a grade of 1.75 percent Cu and 140 g/metric tons Ag, the crudely stratiform deposit is hosted by the upper, brecciated and unwelded part of a approximately 75-m-thick, continental rhyolitic ignimbrite and by an overlying approximately 2-m succession of tuffaceous lacustrine sedimentary rocks. The ignimbrite represents the local basal unit of the Paleocene Hornitos Formation. The reduced, ore-hosting, sedimentary horizons were deposited in an ephemeral saline pond and comprise coarse and fine breccias, tuffaceous sandstones and silt-stones, and calcareous-carbonaceous shales containing thin coaly seams and accumulations of plant debris.The greater part of the mineralization occurs as disseminations, discontinuous veinlets, thin concordant lenses and, most strikingly, cylindrical bodies, 1 to 8 mm in diameter, oriented perpendicular to bedding. Chalcocite, bornite, and tennantite (ca. 1-4.3% Ag) are the major ore minerals and are associated with minor sphalerite, digenite, covellite, chalcopyrite, wit-tichenite, native silver, acanthite, stromeyerite, mckinstryite, jalpaite, and the ruby silvers. Pyrite and marcasite are subordinate constituents of the ore and widely display evidence of replacement by the Cu and Ag minerals. Mineralization in both the ash flow and sediments is associated with moderate argillic and carbonate alteration. The hanging wall of the orebody lies within a red fanglomerate, encroachment of which terminated the accumulation of the lacustrine strata.The sulfidic pipes commonly display concentric mineral zonation suggestive of the successive, in part open-space, precipitation of pyrite and the Cu-Ag minerals. Textural evidence suggests that at least the introduction of pyrite into these bodies occurred prior to the deposition of the immediately overlying strata. The pipes, both individually and in aggregate, do not resemble rhizoliths or dewatering structures. More probable origins are as Scolithus sp. lebensspuren or as gregarious hydrothermal conduits directly reflecting the upward penetration of ore-forming fluids. Sulfide-free pipes are extremely rare, and we favor the latter interpretation.The Jardin mineralization displays features characteristic of several ore deposit types, particularly of the red-bed and epithermal clans. Although the temporal relationships of Cu-Ag mineral deposition and lacustrine sedimentation are uncertain and direct evidence of the conditions of mineralization unavailable, our preferred genetic model involves the convective flow of metal-bearing ground waters through the permeable upper zone of the ignimbrite into the mantling lacustrine strata, during both the fumarolic stage of the tuff and the deposition of the sediments. If this be the case, the emplacement of the disseminated mineralization may have taken place in less than a century; the temporal constraints would be reduced if the Cu-Ag minerals formed largely through replacement of early diagenetic pyrite. The hydrothermal fluids are inferred to have been Cl rich, low temperature, neutral to weakly alkaline, and with an f (sub O 2 ) exceeding the hematite-magnetite buffer; they probably derived their metals and arsenic from nearby, weathered, Upper Cretaceous base and precious metal deposits exposed by the sub-Hornitos Formation erosion surface. Ore deposition occurred in response to reduction, neutralization and moderate cooling on contact with the roof zone of the ignimbrite and the carbon- and sulfur-rich sediments; there is no evidence for sulfide replacement of organic detritus.The Jardin manto may perhaps best be interpreted as an unusual, Ag-rich, red-bed copper deposit nucleated in part by an ash flow cooling on the floor of an intermontane basin. However, the Paleocene epoch in the Copiapo district saw the development of numerous Ag-Cu-As epithermal vein systems, and the deposit could also represent an analogous center in which ore deposition was controlled by a sequence of permeable reduced sediments rather than by faults.

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