V-RICH MINERALS IN CONTACT-METAMORPHOSED SILURIAN SEDEX DEPOSITS IN THE POBLET AREA, SOUTHWESTERN CATALONIA, SPAIN
The Lower Llandoverian metasedimentary rocks of the Prades Mountains, southwestern Catalonia, Spain, consist of interstratified anorthite-rich beds, chert beds, phosphate beds, sulfide-rich black shale (with mainly pyrrhotite and minor chalcopyrite), massive sulfide lenses and calc-silicate beds. These metasedimentary rocks are anomalously enriched in V and Cr, and have disseminated minerals of precious metals: mainly sperrylite and palladian lollingite. The V- and Cr-rich associations are the result of an isochemical metamorphism of V- and Cr-rich protoliths. The contact metamorphism of V-rich shale produced metapelitic hornfels with V-rich aluminosilicates and V–Cr oxides. The occurrence of V oxides in apparent equilibrium with V-rich silicates suggests a limit for the V and Cr contents in these aluminosilicates at the conditions of thermal metamorphism. On the other hand, metamorphism of anorthite- and V-,Cr-rich sedimentary rocks produced V- and Cr-rich silicates, such as goldmanite, V-rich amphiboles, V-rich titanite and V-rich allanite.
VANADIAN-CHROMIAN TOURMALINE AND VANADIAN MUSCOVITE IN CONTACT.METAMORPHOSED CARBONACEOUS ROCKS, PRIMORYE, RUSSIA
Vanadian-+hromian tourmaline (as much as 4.06 wl.Vo Y 2O3 and 2.387o Cr2O3) and vanadian muscovite (w much as 2.13Vo V2O3) occur in contact-metamorphosed carbonaceous metasediments of the hibrezhnaya anticlinal zone in himorye, Russia. Cfus'ias are compositionally zoned, showing depletion in V and Cr, and enrichment in Al, Mg and Fe, from core to-rim. Zoning is attributed to progressive decrease of VindCr contents in the host rock durilg the coune of crystal growth. Cr and V are incorporated according to the schemes: Cf* <+ Al3+ and V3* <-+ 1Mg2*,Fe2*,Mn2*;. The charge increase that results from the latter exchangeisconsideredtobecompensatedbytheconcomitantsubstitutionofrvAlforSiandbytheappearanceofvacancies in the X site. The associated vanadian muscouite contains minor amounts of Mg, Fe, Ti and Na. An empirical formula calculated from results of an electron-microprobe analysis is (K1.9aNas.69)r2.03(Al3.34v0.29lvlgn.26Tis.65Fe0.0rx,96 (Si6.27Alr.i3)x.00O22(OH)2. Both minerals are products of contact metamorphism of V-Cr-bearing carbonaceous rocks.
Stratiform replacement lead-zinc deposits; a comparison between Mount Isa, Hilton, and McArthur River
A comparison of three stratiform lead-zinc-silver deposits from northern Australia, at Mount Isa, Hilton, and McArthur River, reveals significant differences between them, but many critical features in common. As suggested many times in the past, they appear to share a common origin. This common origin, however, seems not to be that of the current paradigm whereby they form integral components of the sequences in which they occur. Instead, all are better interpreted as replacement deposits, formed during an east-west-shortening event, which is at least the second deformation event that each sequence underwent. The McArthur River deposit is distinctly different from the others, in that sphalerite and its included galena has directly replaced laminated bituminous bands. In contrast, sphalerite and galena in the other two deposits have replaced either bedding-parallel or breccia-matrix metasomatic alteration minerals, mostly dolomite.All deposits occur adjacent to major reverse faults rather than, as commonly supposed, normal growth faults. They are associated with major carbonate alteration systems that are dolomitic in the immediate deposit environment. The degree of deformation of the host rocks associated with each deposit is variable, with McArthur River being in the least deformed sequence, Mount Isa significantly more so, and Hilton associated with a quite deformed sequence of a different style.The interpretation that these deposits share a similar origin, in spite of significant differences in many characteristics, reinforces the necessity to reexamine other stratiform deposits around the world to see whether they are integral components of the sediment, or whether they may also be of late replacive origin.