Abstract | dc.description.abstract | The Andean IOCG clan in the Coastal Cordillera of northern Chile, comprises the iron oxide-Cu-Au sensu stricto,
the iron oxide-apatite (IOA), and the stratabound Cu(-Ag) deposits, also known as Manto-type Cu(-Ag) deposits.
IOCGs and Manto-type deposits constitute the second source of copper in Chile, after porphyry Cu-Mo systems,
whereas IOAs are an important source of iron. Regardless of their economic importance, little is known about
their formation and contrasting genetic models have been proposed i.e., an origin from magmatic-hydrothermal
fluids exsolved froma cooling intrusion, or formation fromnon-magmatic basinal brines heated by plutonic bodies.
Herewe report Re-Os data for five IOCG deposits: Candelaria,Mantoverde, Casualidad, Diego de Almagro, and
Barreal Seco, three IOAs: Los Colorados, El Romeral, and Carmen, and two Manto-type deposits: Altamira and
Franke. Calculated Re-Os ages for some of these deposits are consistentwith previously published geochronological
data, and indicate an Early to Late Cretaceous age. The calculated ages for the Altamira Manto-type Cu deposit
yield a ~90 Ma age, indicating that the Cretaceous Manto-type Cu belt of central Chile extends further north than
previously thought. Additionally, the calculated initial Os ratios using reported ages showthat IOCGs have a more
radiogenic signature (2.0–2.4) than IOAs (1.0–1.2) and Manto-type Cu deposits (~1.1). A comparison with previously
published Os initial ratios for the Mantoverde (0.2) and Candelaria (~0.4) IOCGs suggests that the mineralization
in Chilean IOCGs is the result of different events with metals derived from diverse sources i.e., the
magmatic-hydrothermal system and basement/host rocks.
Rhenium and Os concentrations for the various studied deposits vary over a wide range of values at the ppb and
ppt levels, respectively, with many sulfides and magnetite bearing appreciable no common Os. The Re and Os
contents in sulfides (pyrite, chalcopyrite) from IOCGs are low, ranging from 2 to 149 ppb Re and up to 49 ppt
Os. Pyrite from IOA deposits shows Re and Os concentrations ranging from 10.8 to 214 ppb Re, and Os
b156 ppt, whereas magnetite has lower Re and Os contents (0.7–129 ppb Re, 0.2–3.6 ppt Os). However, rhenium
and Os concentrations in magnetite might be attributed to small sulfide inclusions. Chalcocite from Manto-type
Cu deposits shows the widest range and highest Re and Os concentrations; 15 to 253 ppb and 34 to 639 ppt, respectively.
The Os present in all these deposits is dominated by radiogenic 187Os.
A comparison of Re and Os data fromdifferent deposit types, shows that Andean IOCG and IOAdeposits have similar
characteristics to Chilean porphyry Cu-Mosystemswith lowto moderate Re content (b250 ppb), lowOs concentration
(b300 ppt total Os) dominated by radiogenic Os (N90%Osr), and variable initial Os ratios. Our data
supports the idea that Andean IOCG and IOA deposits formed by magmatic-hydrothermal processes with little
involvement of basinal brines. In contrast, published Re-Os information fromIOCGs in China and Australia argues
for the involvement of a much more radiogenic crustal source possibly related to non-magmatic oxidized saline
brines that leached basement rocks.
We conclude that Andean IOCG (and IOA) deposits in northern Chile were formed mainly by magmatic-hydrothermal
processes related to the formation and emplacement of plutonic rocks with moderate contribution
from leaching of basement and/or volcanic host rocks. Surface or basin-derived brines as well as sediments appear
to play only a minor role in the formation of the Chilean IOCG clan. | |