Quantifying the release of base metals from source rocks for volcanogenic massive sulfide deposits: Effects of protolith composition and alteration mineralogy

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• 作者：Simon M. Jowitt^a ; ^b ; ^{simon.jowitt@monash.edu} ; Gawen R.T. Jenkin^a ; Laurence A. Coogan^c ; Jon Naden^d
• 关键词：Troodos ophiolite ; VMS ; Epidosite ; Hydrothermal alteration ; Source rocks
• 刊名：Journal of Geochemical Exploration
• 出版年：2012
• 期刊代码：44_03756742
• 类别：ear
• 出版时间：July, 2012
• 卷：118
• 期：Complete
• 页码：47-59
• 文件大小：2179 K

摘要

This detailed study of the release of base metals during hydrothermal alteration from the sheeted dike complex of the Troodos ophiolite, Cyprus, aims to better understand the source of these elements in ore-forming hydrothermal fluids. The study area, ~ 10 km² between the villages of Spilia and Kannavia, has previously been recognized as a region in which the abundance of epidote in the altered sheeted dikes is higher than average 鈥?a so-called epidosite zone. The originally basaltic to andesitic sheeted dikes have been variably altered, but the secondary mineralogy is independent of the protolith composition. Four alteration facies have been identified in the epidosite zone. With progressively increasing modal epidote, decreasing modal amphibole, and decreasing bulk-rock Mg these are: (i) diabase, which is composed of amphibole + chlorite + albitic plagioclase 卤 epidote 卤 quartz, (ii) transitional diabase-epidosite, (iii) intermediate epidosite, and (iv) rare (< 15%of the study area) end-member epidosite which consists largely of quartz + epidote. Comparing protolith base metal differentiation trends, defined by new analyses of cogenetic volcanic glass, with these altered samples indicates that the rocks originally contained 47-99 ppm Zn, 1030-1390 ppm Mn, 19-28 ppm Co, 19-57 ppm Cu and 7-50 ppm Ni. The vast majority of the altered rocks within the epidosite zone studied have low Cu (averaging 3 ppm) irrespective of alteration facies. This uniform and large depletion suggests that Cu was originally largely present in sulfides that were completely destroyed during hydrothermal alteration. With the exception of Co, the other base metals have substantially lower concentrations in the altered rocks than in their protoliths and show increasing base metal depletion with increasing modal epidote abundance. This suggests that breakdown of silicate minerals was important in controlling the release of these metals. Cobalt is enriched in the diabase and transitional diabase-epidosite alteration facies, and depleted in the end-member epidosite alteration facies, relative to protolith concentrations. This suggests that Co was redistributed within the sheeted dike complex rather than substantially leached out; the same is probably true of Mg. Mapping across the steep topography of the study area indicates that the Spilia-Kannavia epidosite zone has a volume of ~ 2 km³. Based on this estimate, hydrothermal fluids leached ~ 369 kt of Zn, ~ 52 kt of Ni, ~ 3647 kt of Mn and ~ 162 kt of Cu. These Zn and Cu losses are similar to the masses of these metals present in the largest volcanogenic massive sulfide deposits on Cyprus. Based on the differences between protolith and altered rock compositions it is predicted that alteration of primitive protoliths will tend to produce fluids with higher ratios of Cu and Ni to Zn and Mn, whereas alteration of more evolved protoliths will produce fluids with lower ratios.