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Abstract

The Rosario Cu-Mo-Ag deposit is located in the Collahuasi district of northern
Chile. It comprises high-grade Cu-Ag-(Au) epithermal veins, superimposed onto the
core of a porphyry Cu-Mo orebody. Rosario has mining reserves of 1,094 Mt at 1.03%
Cu. An additional 1,022 Mt at 0.93% Cu occurs in the district at the Ujina and
Quebrada Blanca porphyry deposits. The Collahuasi district constitutes the
northernmost known occurrence of late Eocene-early Oligocene porphyry deposits in
Chile.
The Rosario deposit is hosted within Early Permian basement comprising a
sequence of northeast dipping volcanic and sedimentary rocks. This package is
characterised by basal submarine andesitic and upper subaerial dacitic to rhyodacitic
(U/Pb age = 293 ± 14 Ma) facies. Graded feldspathic sandstones are interbedded with
the andesites and dacites. Limestone lenses, interfingered with the dacite, are
consistent with emergence of the submarine sequence, and a shallow subaqueous to
subaerial volcanic arc depositional setting is inferred for the Permian host rocks. The
volcanic sequence was intruded by Early Triassic granodiorite (U/Pb age = 245 ± 12
Ma) and late Eocene porphyritic quartz-monzonite (40 Ar/30 cooling age of primary
biotite = 34.4 ± 0.4 Ma).
High-grade copper ore is hosted by the Rosario fault system, a series of
northwest-trending faults that dip 50° southwest. Detailed structural mapping indicates
that the main mineralising events at Rosario coincided with normal movement on this
fault system. The main faults were reactivated as dextral wrench faults after the period
of copper deposition. At Cerro La Grande, similar high-grade copper veins are hosted
in north-northeast-trending, sinistral wrench faults. Normal movement on the Rosario
fault system is inferred to have been synchronous with sinistral strike-slip deformation
at La Grande. Stress modelling indicates that this brittle deformation event occurred
under southeast-directed compression.
The Permian basement has been uplifted, relative to Mesozoic rocks west of
Collahuasi, along the north-striking Domeyko Fault System. This 2000 km long, arcparallel
fault system consists of a series of fault segments, which are interpreted to be
inverted basin-margin normal and strike-slip faults with highly varied movement
senses. The orientation of the principal compressive stress axes in post-Eocene strike- slip deformation at Rosario is consistent with that inferred for sinistral movement,
during the same period, on the Domeyko fault system. However, this local paloestress
regime is inconsistent with the regional stress expected for north-northeast-directed
convergence during the late Eocene-early Oligocene, and implies that upper and lower
crustal stresses were decoupled. The calc-alkaline Rosario Porphyry is characterised by concave upwards REE
profiles, and high La/Yb (>20) and Sr/Y (>20) values. Lead isotope data define a
limited range bracketed by values of the Paleozoic host rocks. Initial 87 Sr/86Sr and
I43 Nd/144 Nd values range from 0.7050 to 0.7053 and 0.5124 to 0.5126, respectively.
Although high Sr/Y values are a characteristic of adakites, the radiogenic isotopic
signatures and high La/Yb values, indicate the ore-related magmas were derived from
garnet-fractionated MASH zones at the mantle-crust interface, rather than from partial
melting of subducted oceanic crust. Although high Sr/Y values are a characteristic of adakites, the radiogenic isotopic
signatures and high La/Yb values, indicate the ore-related magmas were derived from
garnet-fractionated MASH zones at the mantle-crust interface, rather than from partial
melting of subducted oceanic crust. Hydrothermal alteration at Rosario is characterised by a K-feldspar altered core,
focused on the Rosario Porphyry, that grades out to a secondary biotite-albitemagnetite
assemblage. Paragenetic relations indicate that magnetite was the earliestformed
alteration product, but has been replaced by biotite-albite. Vein cross-cutting
relationships indicate that K-feldspar formed during and after biotite-albite alteration.
Chalcopyrite and bornite were deposited in quartz veins associated with both Kfeldspar
and biotite-albite assemblages. The hydrothermal fluid responsible for early
alteration and mineralisation was a hypersaline brine (40 to 45 wt. % NaC1) that
coexisted with vapour between 400° and >600°C. Weakly mineralised illite-chlorite
(intermediate argillic) alteration of the early K- and Na-silicate assemblages was
caused by moderate temperature (2500 to 350°C), moderate salinity brines (10 to 15
wt.% NaCl). Molybdenite was precipitated in quartz veins during the transition
between potassic and intermediate argillic alteration. These fluids were 350° to 400°C
with salinities between 10 and 15 wt. % NaCI.

Item Type:	Thesis - PhD
Authors/Creators:	Masterman, GJ
Keywords:	Gold ores, Copper ores, Silver ores, Hydrothermal deposits, Porphyry
Copyright Holders:	The Author
Copyright Information:	Copyright 2003 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	No access until 1 January 2004. Thesis (Ph.D.)--University of Tasmania, 2003. Includes bibliographical references
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