Davies, Richard Charles Idris (2002) Tectonic, magmatic and metallogenic evolution of the Cajamarca mining district, Northern Peru. PhD thesis, James Cook University.

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Abstract

In the Cajamarca region of northern Peru periods of peak Tertiary magmatism had a close association with orogenic episodes and high plate convergence rates. New 40Ar/39Ar dates show magmatism in the region had commenced by late Palaeocene times, some 15 m.y. earlier than suggested by previous geochronological studies. Palaeogene (57-43 Ma) intrusive and volcanic rocks are intermediate in composition with flat REE profiles and primitive isotope compositions. These magmas were derived from an immature sub-Andean mantle dominated by pyroxene and olivine. This magmatic interval coincided with development of an early fold-thrust fabric in deformed sedimentary rocks. Early Miocene onset of high plate convergence rates triggered the generation of oxidised hydrous melts from the breakdown of a sub-Andean amphibole-rich upper mantle to lower crust. These melts rose into large magma ponds deep within the crust. Sr, Nd and Pb isotope compositions indicate synmineralisation magmas and metals were derived from a common deep source and that magmas underwent minimal upper crustal contamination. During brief changes in the tectonic stress, primitive hydrousrich magmas were released from these chambers and ascended rapidly along deeply tapping faults. Dioritic intrusions with HREE-depleted profiles were emplaced during periods of extension in a highly fractured upper crust. New 40Ar/39Ar dates indicate this occurred from 23.2 to 16.5 Ma. Mineralised stocks are commonly located in the hanging wall of a regional thrust fault and situated at structural intersections, such as oblique secondary structures superimposed on pre-existing regional-scale faults. Mineralisation-controlling structures, e.g. fault, vein and fracture arrays, at the porphyry deposits have subparallel NNW and NE-NNE trends that suggest they were directly controlled by a regionally extensive stress regime. The physiochemical conditions that prevailed during early stage hypogene mineralisation strongly influenced the Au enrichment at the various porphyry deposits. Au-rich deposits are typically hosted in carbonates, tend to have well-developed potassic alteration zones, high temperature and oxygen fugacity hypogene sulphide mineral assemblages (bornite + chalcopyrite) and abundant hydrothermal magnetite. In contrast, mineralised stocks in contact with fractured quartzites ± carbonates are Cu-Au-Mo deposits with lower temperature hypogene sulphide assemblages of chalcopyrite and pyrite, and potassic alteration zones overprinted by low-grade pyritic phyllic alteration. Late Miocene high-sulphidation deposits (~11 Ma) near Cajamarca formed during the cessation of intense crustal thickening and uplift that was associated with shallowing of the slab dip angle. Location of ore bodies at the Yanacocha mine was largely controlled by WNW structures, indicating rotation of the dominant fault orientation from NNE-NNW to WNW with time. A mineralised dioritic-tonalitic intrusion beneath the Yanacocha high-sulphidation system has a steep HREE-depleted profile and more evolved radiogenic Sr-Nd isotope compositions than the early Miocene intrusions. However, a pyrite Pb isotope composition from this intrusion is significantly less radiogenic than sulphides from early Miocene deposits. These features indicate late Miocene magmas were formed beneath a thickened crust, similar to that at the present day, and require a higher garnet content in the source.

Item ID:	10
Item Type:	Thesis (PhD)
Keywords:	Geology, Magmetism, Structural, Peru, Cajamarca Region
Date Deposited:	17 Aug 2006
FoR Codes:	04 EARTH SCIENCES > 0403 Geology @ 0% 04 EARTH SCIENCES @ 0%
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