Identification of spectrally similar materials using the USGS Tetracorder algorithm: the calcite–epidote–chlorite problem
58
Citation
40
Reference
10
Related Paper
Citation Trend
Keywords:
Sericite
Endmember
Sericite
Illite
Silicate minerals
Pyrophyllite
Cite
Citations (105)
Considerable geological and geophysical evidence now exists to support the hypothesis that seawater circulates through freshly intruded basalt at the mid-ocean ridges. As a consequence of this process, reactions between basalt and seawater take place at elevated temperatures. The mineralogy and chemistry of hydrothermally altered pillow basalts dredged from the Mid-Atlantic Ridge, and belonging to the greenschist facies, have been studied in order to determine the mineralogical changes that result from hyrdrothennal alteration, and to assess the chemical fluxes that result from these reactions in terms of their possible significance in elemental geochemical budgets as potential sources and sinks for elements in seawater. Where possible, pillow basalts were studied that showed varíous degrees of a1teration within a single rock. Such samples provide the best evidence that they have been affected by hydrothermal alteration, rather than regional burial metamorphism, and provide the most useful information for elemental flux calculatìons. During hydrothermal alteration, plagioclase is generally albitised, sometimes with the formation of epidote, and albite may be subsequently a1tered to chlorite. Plagioclase, in association with skeletal clinopyroxene, alters to chlorite and epidote. Olivine is pseudomorphed by chlorite, and clinopyroxene alters to actinolite. The glassy matrix alters to an intergrowth of actinolite and chlorite. Vein minerals irclude chlorite, actinolite, epidote, quartz, and sulphides. On the basis of their minaralogy, the samples may be subdivided into chlorite-rich (>15% chlorite and <15% epidote) and epidote-rich (>15% epidote and <15% chlorite) assemblages. The chlorite-rich assemblages lose CaO and gain MgO, while the epidote-rich samples show very little change in composition compared with their basalt precursor. The epidote-rich samples are more oxidised than their precursors, while the chlorite-rich rocks can be further suhdivided into those that maintain the same proportions of fetrous and ferric iron, and those that show an increase in ferrous iron due to the precipitation of pyrite. The major chemical changes that occur during hydrothermal alteration of pillow basalts are uptake of MgO and H2O, and loss of SiO2 and CaO. The concentrations of Na2O and K2O are apparently not greatly changed, although. they do show some variations in the core-to rim analyses. Consideration of the elemental fluxes in terms of steady-state geochemical mass balances indicates that hydrothermal alteration provides a sink for Mg, which is extremely important in solving the problem of apparent excess magnesium input to the oceans. The amount of calcium that is leached from the rock may be of significance in the geochemical budget of calcium. The concentration of silica in the circulating fluid is probably controlled by the solubility of quartz, and considerable redistribution of silica takes place within the basaltic pile. The changes in the redox conditions during hydrothermal alteration do not affect the present-day oxidation states of the atmosphere and hydrosphere. Trace element analyses indicate that copper and strontium are leached out of the rock and migrate in the circulating fluid, with local precipitation of Cu as sulphides in veins. Li, B, Mn, Ba, Ni and Co show sufficient variation in concentration and location within the altered basalts to indicate that some leaching does take place, and hence hydrothermal alteration of basalts could produce a metal-enriched solution, which may be important in the formation of metalliferous sediments at active mid-ocean ridges.
Greenschist
Actinolite
Cite
Citations (14)
The metavolcanics of the Miocene Tanzawa Group of the southern Tanzawa Mountains can clearly be divided into six mineral zones ; stilbite, laumontite, prehnite-pumpellyite, epidote, epidote-amphibole and amphibole zones. Four phases of mafic phyllosilicates in the five lower grade zones are identified by systematic X-ray diffraction studies; (1) smectite in the stilbite and laumontite zones, (2) randomly mixed-layer chlorite/smectite mineral which is ubiquitous in the stilbite, laumontite and prehnite-pumpellyite zones, (3) regularly mixed-layer chlorite/smectite in the stilbite zone, (4) chlorite occurs first in the laumontite zone. With increasing metamorphic grade, smectite layers progressively transform to chlorite. Chlorite is the predominant phyllosilicate in the prehnite-pumpellyite zone and is a unique phyllosilicate in the epidote and epidote-amphibole zones. Variations in Fe/(Fe+Mg) of chlorite are positively correlated with Fe/(Fe+Mg) ratios of whole rocks, but the other chemical variations are clearly unrelated to whole rock compositions. The content of Si decreases with increasing metamorphic grade.
Amphibole
Cite
Citations (1)
Fugacity
Cite
Citations (5)
Sericite
Microprobe
Cite
Citations (8)
Abstract Propylitic alteration, characterized by the occurrence of chlorite and epidote, is typically the most extensive and peripheral alteration facies developed around porphyry ore deposits. However, exploration within this alteration domain is particularly challenging, commonly owing to weak or nonexistent whole-rock geochemical gradients and the fact that similar assemblages can be developed in other geologic settings, particularly during low-grade metamorphism. We document and interpret systematic spatial trends in the chemistry of chlorite and epidote from propylitic alteration around the E48 and E26 porphyry Cu-Au deposits of the Northparkes district, New South Wales, Australia. These trends vary as a function of both distance from hydrothermal centers and alteration paragenesis. The spatial trends identified in porphyry-related chlorite and epidote at Northparkes include (1) a deposit-proximal increase in Ti, As, Sb, and V in epidote and Ti in chlorite, (2) a deposit-distal increase in Co and Li in chlorite and Ba in epidote, and (3) a pronounced halo around deposits in which Mn and Zn in chlorite, as well as Mn, Zn, Pb, and Mg in epidote, are elevated. Chlorite Al/Si ratios and epidote Al/Fe ratios may show behavior similar to that of Mn-Zn or may simply decrease outward, and V and Ni concentrations in chlorite are lowest in the peak Mn-Zn zone. In comparison to porphyry-related samples, chlorite from the regional metamorphic assemblage in the district contains far higher concentrations of Li, Ca, Ba, Pb, and Cu but much less Ti. Similarly, metamorphic epidote contains higher concentrations of Sr, Pb, As, and Sb but less Bi and Ti. These chlorite and epidote compositional trends are the net result of fluid-mineral partitioning under variable physicochemical conditions within a porphyry magmatic-hydrothermal system. They are most easily explained by the contribution of spent magmatic-derived ore fluid(s) into the propylitic domain. It is envisaged that such fluids experience progressive cooling and reduction in fs2 during outward infiltration into surrounding country rocks, with their pH controlled by the extent of rock-buffering experienced along the fluid pathway.
Paragenesis
Cite
Citations (51)