Andacollo Mine
Andacollo Mine |
Location: Elqui,
Coquimbo, Chile.
Products: Copper
& Gold.
Owner: Royal Gold,Inc.
Ore Type: Porphyry copper-gold
deposit, hosted by altered andesitic and dacitic volcanic rocks, and small
stocks and irregular dykes of potassium-rich tonalitic porphyry.
Overview
The Andacollo mining district is
located in the Coquimbo region of Chile at 30°14’ south, 71°06’ west, some 55
km southeast of La Serena, at a mean elevation of 1030 m within a semi-arid
hilly landscape. Current mining activity in the district is concentrated on
copper and gold. These metals are mined, respectively, from a porphyry copper deposit
and epithermal, manto and vein gold deposits of adularia–sericite type.11,13
Other types of mineralization include mercury veins hosted by carbonate rocks.
The gold veins are controlled by a northwest-trending set of normal faults, whereas
the manto-type mineralization is strata-bound and largely confined to andesite
breccias, dacites and sites of strong fracturing. The lateral and vertical
continuity of the mantos is strongly controlled by rock type, faulting and
intensity of fracturing. The gold deposits have been the focus of a recent
study,11 but comparable information on the Andacollo porphyry has not become
available.
Andacollo’s operating profit from August
22 to December 31, 2007 was $27 million before the effects of the revaluation
of copper inventory to fair value on acquisition and negative pricing adjustments.
The revaluation established a higher value for copper inventories, based on
market prices at the date of acquisition. This increased our cost of sales by
$24 million and the subsequent decline in metal prices resulted in a loss on
the sale of these inventories. In addition, the mine recorded negative pricing
adjustments of $2 million since they acquired it in August 2007. After these adjustments,
Andacollo’s operating profit was $1 million. Copper cathode production in 2008
is expected to be approximately 20,000 tonnes and capital expenditures are
planned at US$190 million, including US$185 million on the hypogene
development.
Geological setting and
Mineralization
The Andacollo deposits are the products of a complex hydrothermal system
and consist of a porphyry copper-gold deposit and peripheral strata-bound manto
gold deposits and veins with minor associated base metals. The hydrothermal
system was part of the Pacific porphyry copper belt which was generated during
development of an Early Cretaceous magmatic arc displaying shoshonitic
petrochemical affiliations. Rocks that crop out in the area include a volcanic
sequence, the Arqueros and Quebrada Marquesa Formations, consisting of andesitc
and dacite flows, volcanic breccias, and pyroclastic rocks of Early Cretaceous
age. Intrusive rocks range from diorite to granodiorite in composition and date
between 87 and 130 Ma. The porphyry copper-gold deposit is zoned vertically
downward from a leached capping through a supergene enrichment blanket to a
hypogene sulfide zone. Alteration is characterized by central potassic (K
feldspar-biotite), phyllic, and peripheral propylitic zones. Abundant northwest-trending
tensional fractures were superimposed on the porphyry copper-gold deposit and
surrounding areas during the later stages of the evolving mineralized system.
The fractures channeled mineralizing fluids from the central parts of the
porphyry copper deposit outward for up to 5 km. Replacement by adularia and
sericite took place together with deposition of gold-bearing pyrite and minor
amounts of zinc and copper where these fluids encountered permeable dacite
flows and andesite flow breccias. The alteration process caused remobilization
of aluminum and alkalies and addition of K 2 O, which attains values of 12 to
13 wt percent. The Andacollo system is interpreted to be a porphyry copper-gold
deposit that is transitional outward to distal epithermal,
adularia-sericite-type contact metasomatic gold orebodies.
0 التعليقات:
Post a Comment