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Geochemical signature (bedrock and saprolite) of gold mineralization and associated hydrothermal alteration at Dorlin, French Guyana
Journal of Geochemical Exploration, 32 (1989) 59-60
59
ElsevierSciencePublishersB.V., Amsterdam-- Printed in The Netherlands
Geochemical signature (bedrock and saprolite) of gold mineralization and associated hydrothermal alteration at Dorlin, French Guyana G.H. TAYLOR1,B. COSTE2, A. LAMBERT2and H. ZEEGERS2 1BHP-Utah International, Inc., 550 California Street, San Francisco, CA 94104, U.S.A. 2Bureau de Recherches Gdologiques et Mini~res (BRGM), B.P. 6009, 45060 Orleans Cedex, 2, France
(ReceivedJanuary 21, 1988;Revisedand acceptedAugust18, 1988) EXTENDED ABSTRACT The Dorlin Au prospect in central French Guyana is located within a Proterozoic volcano-sedimentarybelt where numerous old Au workings are known. Several coincident Au anomalies in soils and stream sediments were defined in a 1975 regional reconnaissance program. These were followed up by soil and saprolite auger-sampling surveys. As defined by the auger survey the prospect comprises several strongly anomalous zones (Au and associated elements). Weathering is intense and exposure poor, and this, coupled with remobilization and leaching of Au in the upper part of the weathering profile, makes ranking of Au anomalies at Dorlin a difficult task. A deep diamond drilling program (150-200 m) was undertaken in 1986 by a B R G M / B H P - U t a h International joint venture along a profile perpendicular to a large saprolite Au anomaly. Study of the mineralized and barren samples using petrology, mineralogy and geochemistry (Au by AAS and 34 major and trace elements by ICP) showed that Au is principally associated with sulphides (pyrite, arsenopyrite) included in a complex hydrothermal assemblage, with chlorite, sericite, quartz and tourmaline. High contents of elements such as K20, As, B and MgO distinguished mineralized from unmineralized samples. The changes in geochemical signature as weathering intensity increases (with decreasing sampling depth) were studied for two distinctly different mineralized intersections from two drill holes. In the first hole, fresh mineralized samples are high in the key elements MgO (3-6%), K20 (1-2%), Fe203 (15-20%), and As (500-1000 ppm), but relatively low in B. From 1-8 m depth in saprolite a significant loss is seen in MgO contents, indicating that Mg is contained in minerals that are unstable in the weathered zone. Elements such as Fe203 from pyrite and As from arsenopyrite accumulate in the weathering zone as second0375-6742/89/$03.50
© 1989ElsevierSciencePublishersB.V.
60 ary Fe-oxides (hematite, goethite) and products scavanged by the Fe-oxides respectively. The second mineralized section is dominated by tourmaline, which is reflected in both fresh and weathered samples by high B contents (several thousands of ppm) and relatively high MgO concentrations (up to 3% ). Here at least part of the magnesium content of the rocks is retained in the weathered profile as a constituent of resistant tourmaline. Each bedrock Au occurrence is characterized by a unique geochemical signature. The type of mineralization, can be readily predicted even in deeply weathered saprolite by inexpensive hand-auger drilling and analysis of samples for selected elements. Results can play a significant role in planning future drilling programs.
59
ElsevierSciencePublishersB.V., Amsterdam-- Printed in The Netherlands
Geochemical signature (bedrock and saprolite) of gold mineralization and associated hydrothermal alteration at Dorlin, French Guyana G.H. TAYLOR1,B. COSTE2, A. LAMBERT2and H. ZEEGERS2 1BHP-Utah International, Inc., 550 California Street, San Francisco, CA 94104, U.S.A. 2Bureau de Recherches Gdologiques et Mini~res (BRGM), B.P. 6009, 45060 Orleans Cedex, 2, France
(ReceivedJanuary 21, 1988;Revisedand acceptedAugust18, 1988) EXTENDED ABSTRACT The Dorlin Au prospect in central French Guyana is located within a Proterozoic volcano-sedimentarybelt where numerous old Au workings are known. Several coincident Au anomalies in soils and stream sediments were defined in a 1975 regional reconnaissance program. These were followed up by soil and saprolite auger-sampling surveys. As defined by the auger survey the prospect comprises several strongly anomalous zones (Au and associated elements). Weathering is intense and exposure poor, and this, coupled with remobilization and leaching of Au in the upper part of the weathering profile, makes ranking of Au anomalies at Dorlin a difficult task. A deep diamond drilling program (150-200 m) was undertaken in 1986 by a B R G M / B H P - U t a h International joint venture along a profile perpendicular to a large saprolite Au anomaly. Study of the mineralized and barren samples using petrology, mineralogy and geochemistry (Au by AAS and 34 major and trace elements by ICP) showed that Au is principally associated with sulphides (pyrite, arsenopyrite) included in a complex hydrothermal assemblage, with chlorite, sericite, quartz and tourmaline. High contents of elements such as K20, As, B and MgO distinguished mineralized from unmineralized samples. The changes in geochemical signature as weathering intensity increases (with decreasing sampling depth) were studied for two distinctly different mineralized intersections from two drill holes. In the first hole, fresh mineralized samples are high in the key elements MgO (3-6%), K20 (1-2%), Fe203 (15-20%), and As (500-1000 ppm), but relatively low in B. From 1-8 m depth in saprolite a significant loss is seen in MgO contents, indicating that Mg is contained in minerals that are unstable in the weathered zone. Elements such as Fe203 from pyrite and As from arsenopyrite accumulate in the weathering zone as second0375-6742/89/$03.50
© 1989ElsevierSciencePublishersB.V.
60 ary Fe-oxides (hematite, goethite) and products scavanged by the Fe-oxides respectively. The second mineralized section is dominated by tourmaline, which is reflected in both fresh and weathered samples by high B contents (several thousands of ppm) and relatively high MgO concentrations (up to 3% ). Here at least part of the magnesium content of the rocks is retained in the weathered profile as a constituent of resistant tourmaline. Each bedrock Au occurrence is characterized by a unique geochemical signature. The type of mineralization, can be readily predicted even in deeply weathered saprolite by inexpensive hand-auger drilling and analysis of samples for selected elements. Results can play a significant role in planning future drilling programs.