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Contrasting fluids and reservoirs in the contiguous Marcona and Mina Justa iron oxide-Cu (-Ag-Au) deposits, south-central PerA(0)

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Chen, H and Kyser, TK and Clark, AH, Contrasting fluids and reservoirs in the contiguous Marcona and Mina Justa iron oxide-Cu (-Ag-Au) deposits, south-central PerA(0), Mineralium Deposita, 46, (7) pp. 677-706. ISSN 0026-4598 (2011) [Refereed Article]


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Copyright 2011 Springer-Verlag

DOI: doi:10.1007/s00126-011-0343-x

Abstract

TheMarcona每Mina Justa deposit cluster, hosted by Lower Paleozoic metaclastic rocks and Middle Jurassic shallow marine andesites, incorporates the most important known magnetite mineralization in the Andes at Marcona (1.9 Gt at 55.4% Fe and 0.12% Cu) and one of the few major iron oxide每copper每gold (IOCG) deposits with economic Cu grades (346.6 Mt at 0.71% Cu, 3.8 g/t Ag and 0.03 g/t Au) at Mina Justa. TheMiddle JurassicMarcona deposit is centred in Ica Department, Per迆, and the Lower Cretaceous Mina Justa Cu (Ag, Au) prospect is located 3每4 km to the northeast. New fluid inclusion studies, including laser ablation time-of-flight inductively coupled plasma mass spectrometry (LA-TOFICPMS) analysis, integrated with sulphur, oxygen, hydrogen and carbon isotope analyses of minerals with well-defined paragenetic relationships, clarify the nature and origin of the hydrothermal fluid responsible for these contiguous but genetically contrasted deposits. At Marcona, early, sulphide-free stage M-III magnetite每biotite每calcic amphibole assemblages are inferred to have crystallized from a 700每 800∼C Fe oxide melt with a 汛18O value from +5.2㏑ to +7.7㏑. Stage M-IV magnetite每phlogopite每calcic amphibole每 sulphide assemblages were subsequently precipitated from 430每600∼C aqueous fluids with dominantly magmatic isotopic compositions (汛34S=+0.8㏑ to +5.9㏑; 汛18O= +9.6㏑ to +12.2㏑; 汛D=−73㏑ to −43㏑; and 汛13C= −3.3㏑). Stages M-III and M-IV account for over 95% of the magnetite mineralization at Marcona. Subsequent noneconomic, lower temperature sulphide每calcite每amphibole assemblages (stage M-V) were deposited from fluids with similar 汛34S (+1.8㏑ to +5.0㏑), 汛18O (+10.1㏑ to +12.5㏑) and 汛13C (−3.4㏑), but higher 汛D values (average −8㏑). Several groups of lower (<200∼C, with a mode at 120∼C) and higher temperature (>200∼C) fluids can be recognized in the main polymetallic (Cu, Zn, Pb) sulphide stage M-V and may record the involvement of modified seawater. At Mina Justa, early magnetite每pyrite assemblages precipitated from a magmatic fluid (汛34S=+0.8㏑ to +3.9㏑; 汛18O=+9.5㏑ to +11.5㏑) at 540每600∼C, whereas ensuing chalcopyrite每 bornite每digenite每chalcocite每hematite每calcite mineralization was the product of non-magmatic, probably evaporitesourced, brines with 汛34S≡+29㏑, 汛18O=0.1㏑ and 汛13C= −8.3㏑. Two groups of fluids were involved in the Cu mineralization stage: (1) Ca-rich, low-temperature (approx. 140∼C) and high-salinity, plausibly a basinal brine and (2) Na (每K)-dominant with a low-temperature (approx. 140∼C) and low-salinity probably meteoric water. LA-TOF-ICPMS analyses show that fluids at the magnetite每pyrite stage were Cu-barren, but that those associated with external fluids in later stages were enriched in Cu and Zn, suggesting such fluids could have been critical for the economic Cu mineralization in Andean IOCG deposits.

Item Details

Item Type:Refereed Article
Keywords:Fluid sources and evolution; Fluid inclusions; Stable isotopes; IOCG deposits; Central Andes; Peru
Research Division:Earth Sciences
Research Group:Geology
Research Field:Ore Deposit Petrology
Objective Division:Mineral Resources (excl. Energy Resources)
Objective Group:Mineral Exploration
Objective Field:Copper Ore Exploration
Author:Chen, H (Dr Huayong Chen)
ID Code:78063
Year Published:2011
Web of Science® Times Cited:15
Deposited By:Centre for Ore Deposit Research - CODES CoE
Deposited On:2012-06-13
Last Modified:2012-08-22
Downloads:0

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