Zincochromite from the Guaniamo river diamondiferous placers, Venezuela: evidence of its metasomatic origin

Zincochromite associated with chromite and magnesiochromite was found in heavy-mineral concentrates from the Quebrada Grande placer deposit in the Guaniamo River diamondiferous district, Venezuela. It occurs in subhedral to anhedral grains from 0.1 to 0.7 mm in size. Reflectance measurements show an increase of reflectance values with FeFe 2 O 4 content in spinel solid solution. The Vickers microhardness (load 100 g) is 1367(84) kg/mm 2 . Zincochromite containing 60 mol.% ZnCr 2 O 4 has a unit cell with a value of a equal to 8.271(3) A. As revealed by EPMA, zincochromite grains are markedly zoned, with the rim and microfissures enriched in Zn. Furthermore, the chromite crystals partly replaced by zincochromite indicate high-temperature metasomatic processes involving a Zn- and Mn-bearing fluid phase. All chromian spinel compositions observed underwent, more or less intensely, this replacement. The chemical formula of the zincochromite richest in Zn from Guaniamo can be written: (Zn 6.499 Fe 2+ 0.613 Mn 2+ 0.511 Mg 0.434 Ni 0.022 ) ∑8.079 (Cr 9.639 Al 4.742 Fe 3+ 1.262 Ti 0.213 V 3+ 0.040 ) ∑15.921 O 32 . A large extent of Mn-for-Fe 2+ substitution has been observed. In some grains, the number of Mn atoms per formula unit exceeds Fe 2+ . Statistically, taking into account the Fe 3+ concentration, zincochromite was formed at higher oxygen fugacities than its chromite and magnesiochromite precursors. EPMA traverses of zoned zincochromite and Zn-bearing magnesiochromite crystals indicate the substitution mechanisms 3Zn → 2(Fe 2+ + Mn) + Mg and 4Zn → (Fe 2+ + Mn) + 3Mg, besides the Zn → (Fe 2+ + Mn) and Zn → Mg replacements. Chromite and magnesiochromite contain low, but consistent Zn concentrations, which demonstrate an early Zn enrichment of the magmatic system. Their chemical compositions are compatible with chromian spinels occurring in diamondiferous kimberlites and lamproites. Considering the presence of Zn-bearing chromites in several kimberlite-bearing areas on a worldwide scale, the occurrence of zincochromite at Guaniamo points to its use as a mineral indicator of kimberlites.

[1]  P. Roeder,et al.  Crystallization of Groundmass Spinel in Kimberlite , 2008 .

[2]  S. Arai,et al.  Genesis of peculiarly zoned Co, Zn and Mn-rich chromian spinel in serpentinite of Bou-Azzer ophiolite, Anti-Atlas, Morocco , 2007 .

[3]  W. Griffin,et al.  Diamond from the Los Coquitos area, Bolivar State, Venezuela , 2006 .

[4]  Z. Johan.  Platinum-group minerals from placers related to the Nizhni Tagil (Middle Urals, Russia) Uralian-Alaskan-type ultramafic complex: ore-mineralogy and study of silicate inclusions in (Pt, Fe) alloys , 2006 .

[5]  P. Spry,et al.  ZINCIAN SPINEL ASSOCIATED WITH METAMORPHOSED PROTEROZOIC BASE-METAL SULFIDE OCCURRENCES, COLORADO: A RE-EVALUATION OF GAHNITE COMPOSITION AS A GUIDE IN EXPLORATION , 2005 .

[6]  N. Sobolev,et al.  Significance of Accessory Chrome Spinel in Identifying Serpentinite Paragenesis , 2005 .

[7]  P. Roeder,et al.  The Range of Spinel Compositions in Terrestrial Mafic and Ultramafic Rocks , 2001 .

[8]  W. Griffin,et al.  DIAMOND FROM THE GUANIAMO AREA, VENEZUELA , 2000 .

[9]  D. A. Kelly,et al.  Platinum nuggets from the Kompiam area, Enga Province, Papua New Guinea: evidence for an Alaskan-type complex , 2000 .

[10]  S. Barnes.  Chromite in Komatiites, II. Modification during Greenschist to Mid-Amphibolite Facies Metamorphism , 2000 .

[11]  P. Chattopadhyay.  Zn-spinel in the metamorphosed Zn-Pb-Cu sulphide deposit at Mamandur, southern India , 1999, Mineralogical Magazine.

[12]  P. N. Anderson,et al.  Unusual upper mantle beneath Guaniamo, Guyana shield, Venezuela: Evidence from diamond inclusions , 1998 .

[13]  T. Weiser,et al.  Zinc-rich chromite from Paleoproterozoic conglomerates at Tarkwa gold mine, Ghana , 1997 .

[14]  M. Jakšić,et al.  A Zincian Chrome-Spinel from the Cr-Ba-Fe-Cu-Zn Deposit near Busovaca (Bosnia and Herzegovina) , 1994 .

[15]  R. O. Moore,et al.  A review of the use and application of mantle mineral geochemistry in diamond exploration , 1993 .

[16]  D. Béziat,et al.  Les spinelles chromozinciferes du district aurifere de Salsigne (Montagne Noire, France) , 1991 .

[17]  Z. Johan,et al.  Platinum mineralization in the Alaskan-type intrusive complexes near Fifield, N.S.W., Australia. Part 2. Platinum-group minerals in placer deposits at Fifield , 1991 .

[18]  L. Cabri,et al.  Platinum-group-element mineralization in lode and placer deposits associated with the Tulameen alaskan-type complex, British Columbia , 1990 .

[19]  M. Ayad,et al.  Mise en évidence de métabasites alcalines Cambrien inférieur dans le district aurifère de Salsigne (Versant Sud-Ouest de la Montagne Noire) , 1988 .

[20]  P. Spry.  The chemistry and origin of zincian spinel associated with the Aggeneys Cu-Pb-Zn-Ag deposits, Namaqualand, South Africa , 1987 .

[21]  P. Candela,et al.  Compositional zoning in unusual Zn-rich chromite from the Sykesville District of Maryland and its bearing on the origin of ferritchromit , 1987 .

[22]  J. Craig,et al.  Gahnite in the metamorphosed stratiform massive sulfide deposits of the Mineral District, Virginia, U.S.A. , 1986 .

[23]  E. Condliffe,et al.  Some mineralogical and geochemical aspects of chromium-bearing skarn minerals from northern Karelia, Finland , 1986 .

[24]  P. Spry,et al.  Zincian spinel and staurolite as guides to ore in the Appalachians and Scandinavian Caledonides , 1986 .

[25]  C. Wagner,et al.  Mineralogy of two peralkaline, arfvedsonite-bearing minettes. A new occurrence of Zn-rich chromite , 1985 .

[26]  R. Batchelor,et al.  Gahnite compositions compared , 1984, Mineralogical Magazine.

[27]  H. Dick,et al.  Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas , 1984 .

[28]  P. Picot.  Atlas of Ore Minerals , 1982 .

[29]  A. Tulloch.  Gahnite and columbite in an alkali-feldspar granite from New Zealand , 1981, Mineralogical Magazine.

[30]  P. Treloar,et al.  Metamorphic development of cordierite-amphibole rocks and mica schists in the vicinity of the Outokumpu ore deposit, Finland , 1981, Transactions of the Royal Society of Edinburgh: Earth Sciences.

[31]  J. Bevan,et al.  Zinc- and manganese-bearing chromites and associated grossular from Zimbabwe , 1980, Mineralogical Magazine.

[32]  E. F. Stoddard,et al.  Zinc-rich hercynite in high-grade metamorphic rocks; a product of the dehydration of staurolite , 1979 .

[33]  R. Mitchell.  Mineralogy of the Elwin Bay kimberlite, Somerset Island, N.W.T., Canada , 1978 .

[34]  A. Moore.  Zinc-bearing chromite (donathite?) from Norway: a second look , 1977, Mineralogical Magazine.

[35]  D. B. Clarke,et al.  Oxide and sulphide mineralogy of the Peuyuk kimberlite, Somerset Island, N.W.T., Canada , 1976 .

[36]  P. H. Nixon,et al.  Origins of the ultramafic nodules from some kimberlites of northern Lesotho and the Monastery Mine, South Africa , 1975 .

[37]  M. Schidlowski.  Untersuchungen zur Metallogenese im südwestlichen Witwatersrand-Becken (Oranje-Freistaat-Goldfeld, Südafrika) , 1970 .

[38]  Alexandra Navrotsky,et al.  The thermodynamics of cation distributions in simple spinels , 1967 .

[39]  T. Irvine.  CHROMIAN SPINEL AS A PETROGENETIC INDICATOR: PART 2. PETROLOGIC APPLICATIONS , 1967 .

[40]  H. Saalfeld.  Strukturdaten von Gahnit, ZnAl2O4 , 1964 .

[41]  J. Dinnin,et al.  Zincian chromite from outokumpu, Finland , 1964 .

[42]  G. D. Nicholls.  The mineralogy of rock magnetism , 1955 .