LITHOGEOCHEMICAL AND RELATED TRENDS THROUGH THE LOWER MINERALIZED ZONE OF THE DON MARIO GOLD-COPPER DEPOSIT, SANTA CRUZ, BOLIVIA
COOK, Robert B., Dept. of Geology, 210 Petrie Hall, Auburn University, AL 36849;THOMSON, Ian, Orvana Minerals Corp., 710-1177 West Hastings St., Vancouver, B.C., Canada V6E 2K3; and BRAZELL, Thomas W., 1710 Olde Coalmine Rd., Midlothian, VA., 23113
The lower mineralized zone (LMZ) of the Don Mario deposit consists of a northwest-striking, northeast-pitching zone of locally high-grade, gold-copper-bismuth mineralization within a sheared, silicified sequence of regionally metamorphosed metasediments, metavolcanics and related chemical sediments. Geochemical trends derived from a 230 meter composite section indicate that as the LMZ is approached there is a dramatic initial increase in SiO2 content. Within the LMZ proper there is a sudden decrease in SiO2 to about 50% and a coincident sharp increase to 30% total FeO + MgO. Na2O and CaO contents of the immediately adjacent host units are quite low but exhibit slight enrichment in the hanging wall. K2O exhibits erratic distribution in the hanging wall and a marked enrichment in footwall schists. TiO2 is anomalously depleted in the hanging wall and dramatically enriched in the footwall to 0.8%. Cr2O3 displays no clear trends but does exceed approximately 0.1% in any part of the section.
The LMZ is characterized by anomalous enrichment in Au, Ag, Cu, Pb, Zn, Bi, W, and Mo with little or no enrichment of these elements in immediately adjacent units. Primary ore minerals include electrum, chalcopyrite, galena, sphalerite, bismuthinite, bismuth, scheelite, molybdenite and ubiquitous iron sulfides. A late oxidizing event has resulted in the rimming of sulfides by magnetite, the remobilization of gold and bismuth, and the development of a suite of chlorite and serpentine minerals. The available data suggest that the LMZ is a structurally controlled ore shoot formed by local remobilization of metals contained in an iron-formation-related chemical sediment in response to regional matamorphism and the intrusion of the nearby Senoritas granodiorite.