Zinc, sulfur and lead isotopic variations in carbonate-hosted Pb–Zn sulfide deposits, southwest China

Abstract

The Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province in the western Yangtze Block, southwest China, containsmorethan four hundred Pb–Zndeposits with more than200 million tons of Pb–Znoresatmean grades of 5 wt.% Pb and 10 wt.% Zn. These deposits are hosted in Sinian (Ediacaran) to Permian carbonate rocks and are structurally controlled by thrust fault–fold structures, and are spatially associated with the late Permian ~ 260 Ma Emeishan flood basalts. Two representative low temperature hydrothermal Pb–Zn sulfide deposits, the Tianqiao and Banbanqiao deposits in the southeastern part of the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province are selected for Zn–S–Pb isotopic analyses. Sphalerite from the Tianqiao deposit has δ 66 Zn values ranging from −0.26 to +0.58‰ relative to the JMC 3–0749L zinc isotope standard, whereas δ 66 Zn values of sphal- erite from the Banbanqiao deposit range from +0.07 to +0.71‰. The zinc isotopic composition of sphalerite from both deposits increase from early to final mineralization stage. In addition, sphalerite from the center (near to bottom) part of the No. 1 ore body in the Tianqiao deposit has lower δ 66 Zn values (−0.01 to +0.43‰) than those (+0.11 to +0.57‰) in the periphery (near to top). Sinian to Permian sedimentary rocks and Permian Emeishan flood basalts, the potential zinc metal source rocks, have δ 66 Zn values range from −0.24 to +0.17‰ and from +0.32 to +0.44‰, respectively. The majority of the hydrothermal sphalerite has heavier zinc isotope thanthe countryrocks, precluding the mixingof multiplezincsourcesasthe key factorcon- trollingthespatialandtemporalvariationsofzincisotope.Therefore,theincreasedδ 66 Znvaluesfromtheearlyto late stage and from the center to top could be due to kinetic Raleigh fractionation. Sphalerite from the Tianqiao andBanbanqiaodepositshasδ 34 Svaluesrangingfrom+10.9to+14.8‰andfrom+3.9to+9.0‰,respectively, lower than Cambrian to Permian marine sulfates (+15 to +35‰) and sulfate-bearing evaporates (+15 to +28‰) in the Devonian to Permian carbonate host rocks. Sulfur of the Pb–Zn ores from both deposits is interpretedastheresultofthermalchemicalsulfatereductionofevaporatesinthesedimentaryrocks,mostlikely the host rocks. Sphalerite from the Tianqiao deposit has Pb isotope similar to that of age-corrected Devonian to Permian carbonate host rocks, whereas sphalerite from the Banbanqiao deposit has Pb isotope similar to that of age-corrected underlying Precambrian basement rocks. Therefore, at least lead in the Tianqiao and Banbanqiao deposits was mainly originated from the host rocks and the underlying basements, respectively. Zn–S–Pb isoto- pic studies of sphalerite from both deposits indicate that sources of metals and sulfur in the hydrothermal fluid for the Tianqiao deposit are the Paleozoic carbonate host rocks, whereas for the Banbanqiao deposit the sources are the Precambrian basements and the Paleozoic carbonate host rocks, respectively.