Date of Award
1987
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Abstract
The Crixas Gold Deposit is located in Crixas Greenstone Belt, Goias. The area is underlain by metamorphosed Archean rocks consisting of a basal unit of ultramafic rocks. This is overlain by tholeiitic metabasalts which are in turn overlain by meta-pelitic rocks.;Structural hanging wall to footwall lithologic sequence of the deposit consists of: Foliated Amphibolite transitional to Ferroan Dolomite-Chlorite-Biotite-Quartz Schist (FDCBQS), veined Ferroan Dolomite-Chlorite-Sericite-Biotite-Quartz Schist (vFDCSBQS) and Massive Ferroan Dolomite (MFD). These rocks envelope Sericite-Chlorite Schist (SCS) and Chlorite-Magnetite Schist (CMS). Silicified Dolomite and Graphitic Pelite occur below these units. Banded Chlorite-Sericite-Garnet Schist (BCSGS) is in sharp upper contact to Graphitic Pelite and sharp lower contact to Banded Quartz-Biotite-Chlorite-Plagioclase Schist.;Gold occurs associated with arsenopyrite and pyrrhotite within an Upper Ore Zone of FDCBQS, vFDCSBQS, MFD, SCS and CMS and a Lower Ore Zone of Quartz Cemented Graphitic Pelite Breccia (QCGPB) located at or near the contact of Graphitic Pelite and BCSGS.;The Upper and Lower Ore Zones are interpreted as metasomatised parent lithologies. Metasomatism post-dates a maximum regional metamorphism of Epidote-Amphibolite Facies and consists of carbonatization, sericitization, Fe-metasomatism and sulphidization. The Foliated Amphibolite is the carbonatized parent to FDCBS, vFDCSBQS and MFD and sericitized, Fe-metasomatised and sulphidized parent to SCS and CMS. The Graphitic Pelite is the sericitized and sulphidized parent to QCGPB.;Gold deposition resulted from reduced f{dollar}\sb{lcub}\rm S2{rcub}{dollar} due to the sulphidation reaction of S and Fe-silicate minerals to form sulphides. f{dollar}\sb{lcub}\rm O2{rcub}{dollar} is constrained by the presence of graphite and magnetite.;Geothermometric calculations of coexisting garnet and biotite, garnet inclusion textures indicate that the QCGPB represents a major structural and geothermal discontinuity. It is proposed that shearing related low angle thrusting accounts for the development of both the Lower and Upper Ore Zones. The age of this thrusting is not known, but is speculated to be Brasiliano Cycle in age (570 Ma).
Recommended Citation
Thomson, Margaret Lee, "The Crixas Gold Deposit, Brazil: Metamorphism, Metasomatism And Gold Mineralization" (1987). Digitized Theses. 1716.
https://ir.lib.uwo.ca/digitizedtheses/1716