GEOLOGY AND GENESIS OF BI-CU MINERALIZED BRECCIA-PIPE, SAN FRANCISCO DE LOS ANDES, SAN JUAN, ARGENTINA

San Francisco de los Andes is on the eastern flank of the Cordillera Frontal in San Juan province, Argentina. Close to San Francisco de los Andes are outcrops of marlne sandstones, shales, and siltstones striking N-S and dipping at high angles eastward, probably of Carboniferous age. These have been intruded by the Chorrillos granodiorite and Amancay granodiorite. The intrusives are discordant; erosion uncovered their upper parts but in places the contact of the roof with the country rock is preserved. The plutons show evidence of shallow intrusion. Around the intrusives is a halo formed by strong tourmalinization accompanied by quartz, K-feldspar, biotite, sericite and subordinate pyrite, apatite, chlorite, rutile and cassiterite. Both sedimentary and granodioritic rocks are cut by NW-SE joints that formed soon after the solidification of the upper part of the plutons. Several tourmaline breccia-pipes are in the area, but San Francisco de los Andes is the only one mineralized. This is a vertical columnar body elliptical in plan, emplaced in the sedimentary rocks. The long axis of the ellipse is 70 m and trends NW-SE parallel to the major joint system of the country rock. The rock fragments are the same as the wall rock; they are well oriented and dip at low angles, which contrasts with the rim where the fragments dip inward at higher angles. Three stages of mineralization are recognized: alteration, filling, and replacement. The mineralogy of the alteration stage is similar to that' of the metasomatic halo around the intrusives. During the filling stage the rock fragments were cemented by tourmaline, quartz, and pyrite. In the replacement stage the ore body was formed in the rim. Arsenopyrite and tourmaline were followed by bismuthinite, emplectjte, chalcopyrite, bornite, digenite, tetrahedrite, sphalerite, chalcocite, luzonite and covellite. Weathering of the deposit has been controlled by the Py:As ratio; where this ratio is high, as in the core of the pipe, weathering is deeper than in the rim where the Py:As ratio is low. The weathering zone of the rim is characterized by abundance of rare arsenates:rooseveltite, a new Bi-arsenate, olivenite, conichalcite, clinoclase, beudantite and scorodite. © 1969 Society of Economic Geologists, Inc.