TY - JOUR
T1 - Distribution of platinum-group elements and Os isotopes in chromite ores from Mayarí-Baracoa Ophiolitic Belt (eastern Cuba)
AU - Gervilla, F.
AU - Proenza, J. A.
AU - Frei, R.
AU - González-Jiménez, J. M.
AU - Garrido, C. J.
AU - Melgarejo, J. C.
AU - Meibom, A.
AU - Díaz-Martínez, R.
AU - Lavaut, W.
PY - 2005/12
Y1 - 2005/12
N2 - The Mayarí-Baracoa ophiolitic belt in eastern Cuba hosts abundant chromite deposits of historical economic importance. Among these deposits, the chemistry of chromite ore is very variable, ranging from high Al (Cr#=0.43-0.55) to high Cr (Cr#=0.60-0.83) compositions. Platinum-group element (PGE) contents are also variable (from 33 ppb to 1.88 ppm) and correlate positively with the Cr# of the ore. Bulk PGE abundances correlate negatively with the Pd/Ir ratio showing that chromite concentrates mainly Os, Ir and Ru which gives rise to the characteristic negatively sloped, chrondrite-normalized PGE patterns in many chromitites. This is consistent with the mineralogy of PGEs, which is dominated by members of the laurite-erlichmanite solid solution series RuS2-OsS2), with minor amounts of irarsite (IrAsS), Os-Ir alloys, Ru-Os-Ir-Fe-Ni alloys, Ni-Rh-As, and sulfides of Ir, Os, Rh, Cu, Ni, and/or Pd. Measured 187Os/188 Os ratios (from 0.1304 to 0.1230) are among the lower values reported for podiform chromitites. The 187Os/188Os ratios decrease with increasing whole-rock PGE contents and Cr# of chromite. Furthermore, γOs values of all but one of the chromitite samples are negative indicating a subchondiritc mantle source. γOs decrease with increasing bulk Os content and decreasing 187Re/ 188Os ratios. These mineralogical and geochemical features are interpreted in terms of chromite crystallization from melts varying in composition from back-arc basalts (Al-rich chromite) to boninites (Cr-rich chromite) in a suprasubduction zone setting. Chromite crystallization occurs as a consequence of magma mixing and assimilation of preexisting gabbro sills at the mantle-crust transition zone. Cr#, PGE abundances, and bulk Os isotopic composition of chromitites are determined by the combined effects of mantle source heterogeneity, the degree of partial melting, the extent of melt-rock interactions, and the local sulfur fugacity. Small-scale (μm to cm) chemical and isotopic heterogeneities in the platinum-group minerals are controlled by the mechanism(s) of chromite crystallization in a heterogeneous environment created by the turbulent regime generated by successive inputs of different batches of melt.
AB - The Mayarí-Baracoa ophiolitic belt in eastern Cuba hosts abundant chromite deposits of historical economic importance. Among these deposits, the chemistry of chromite ore is very variable, ranging from high Al (Cr#=0.43-0.55) to high Cr (Cr#=0.60-0.83) compositions. Platinum-group element (PGE) contents are also variable (from 33 ppb to 1.88 ppm) and correlate positively with the Cr# of the ore. Bulk PGE abundances correlate negatively with the Pd/Ir ratio showing that chromite concentrates mainly Os, Ir and Ru which gives rise to the characteristic negatively sloped, chrondrite-normalized PGE patterns in many chromitites. This is consistent with the mineralogy of PGEs, which is dominated by members of the laurite-erlichmanite solid solution series RuS2-OsS2), with minor amounts of irarsite (IrAsS), Os-Ir alloys, Ru-Os-Ir-Fe-Ni alloys, Ni-Rh-As, and sulfides of Ir, Os, Rh, Cu, Ni, and/or Pd. Measured 187Os/188 Os ratios (from 0.1304 to 0.1230) are among the lower values reported for podiform chromitites. The 187Os/188Os ratios decrease with increasing whole-rock PGE contents and Cr# of chromite. Furthermore, γOs values of all but one of the chromitite samples are negative indicating a subchondiritc mantle source. γOs decrease with increasing bulk Os content and decreasing 187Re/ 188Os ratios. These mineralogical and geochemical features are interpreted in terms of chromite crystallization from melts varying in composition from back-arc basalts (Al-rich chromite) to boninites (Cr-rich chromite) in a suprasubduction zone setting. Chromite crystallization occurs as a consequence of magma mixing and assimilation of preexisting gabbro sills at the mantle-crust transition zone. Cr#, PGE abundances, and bulk Os isotopic composition of chromitites are determined by the combined effects of mantle source heterogeneity, the degree of partial melting, the extent of melt-rock interactions, and the local sulfur fugacity. Small-scale (μm to cm) chemical and isotopic heterogeneities in the platinum-group minerals are controlled by the mechanism(s) of chromite crystallization in a heterogeneous environment created by the turbulent regime generated by successive inputs of different batches of melt.
UR - http://www.scopus.com/inward/record.url?scp=28344431853&partnerID=8YFLogxK
U2 - 10.1007/s00410-005-0039-2
DO - 10.1007/s00410-005-0039-2
M3 - Article
AN - SCOPUS:28344431853
SN - 0010-7999
VL - 150
SP - 589
EP - 607
JO - Contributions to Mineralogy and Petrology
JF - Contributions to Mineralogy and Petrology
IS - 6
ER -