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Spence is a major porphyry copper deposit in northern Chile, located 150 km northeast of Antofagasta, adjacent to the road to Calama and Chuquicamata (#Location: 22° 47' 50"S, 69° 15' 10"W).

This deposit, with the Cerro Colorado mine to the north (east of Inquique) is managed as part of the Pampa Norte Division. It was discovered by RioChilex in 1996, and is located along the prominent Antofagasta-Calama Lineament. It contains both supergene enriched and hypogene ore.

Mineralisation is associated with a complex of granodioritic to quartz-monzonite stocks with accompanying Paleocene dykes dated at 57 Ma. The long axis of porphyry intrusion and mineralisation is orientated NNE, sub-parallel to the Antofagasta-Calama Lineament, with the mineralised system having plan dimensions of ~3700 x 900 m. The emplacement of these intrusives into an upper Jurassic to lower Cretaceous suite of largely andesitic volcanics and volcaniclastics was controlled by a normal, NE-SW trending structural system which resulted in at least three distinct intrusive events and hydrothermal breccias.

The first intrusive pulse is represented by a quartz-feldspar porphyry (QFP1), accompanied by a late magmatic potassic alteration core of biotite and K feldspar with associated A and B type veins, fringed by a propylitic halo of chlorite-pyrite alteration.   This pulse produced the main hypogene chalcopyrite rich mineralisation which is also the focus of the development of supergene enrichment.   The second intrusive pulse (QFP2) of very similar composition was concentrated in the southern part of the deposit, forming a more compact centre with little fracturing, surrounded by intrusive breccias with clasts of QFP1 produced by explosive volatile components of the system.   Subsequent activity, still mainly in the southern part of the deposit produced further hydrothermal breccias which are classified on the composition of their matrix into molybdenum breccias (with a sulphide rich matrix of molybdenite, pyrite and chalcopyrite) and a tourmaline breccia with a tourmaline-quartz matrix.

A major episode of phyllic (quartz-sericite-pyrite) D-type vein alteration with strong associated pyrite commenced from before the introduction of the QFP2 porphyry and continued through the breccia phase.   This additional sulphide was important to the subsequent development of supergene leaching and enrichment.   A third, limited intrusive phase of feldspar porphyry, subsequent to the breccias, is also in the southern part of the deposit and is characterised by the absence of copper and of quartz.

The deposit is entirely masked by 50 to >100 m of gravel which, in general, overlies a leached cap that passes down into a zone of supergene oxides, an enriched chalcocite blanket and then to hypogene sulphides.   Several episodes of supergene blanket development and leaching are indicated.

Copper minerals within the oxide zone are atacamite and brochantite, underlain by a blanket of chalcocite, covellite and pyrite at the interface between the oxide and hypogene sulphide zones. Hypogene sulphides are chalcopyrite, bornite, molybdenite, tennantite and pyrite (Cameron et al., 2006).

On the basis of laser 40Ar-39Ar measurements of supergene alunite, Rowland and Clark (2001) identified a long period of supergene activity from 44.44±0.54 to 27.74±5.42 Ma, which was extended by a K-Ar date of 20.9±2.2 Ma. The deposit is covered by piedmont gravels of Miocene age, which are indurated, and for the most part, are poorly sorted with a fine-grained matrix that makes them impermeable, except where fractured, or in better sorted layers, as near their base (Cameron et al., 2006).

The atacamite-brochantite assemblage is locally continuous through the leached cap into the basal gravels, indicating oxide deposition continued after supergene oxidation and gravel deposition. Mostly the gravels rest on a leached cap. The preserved leached cap is ~20 to 100 m thick, while the atacamite-brochantite oxide layer is generally ~45 m thick. The supergene sulphide blanket is 50 to 100 m thick, thinning at the margins. The supergene sulphide zone contains irrregular patches of mixed supergene and hypogene sulphides at the transition to the hypogene zone (Cameron et al., 2006).

Published JORC compliant ore reserves and minerals resources at 30 June 2012 (BHP Billiton, 2012) include:
  Total measured + indicated + inferred resources, for,
      Oxide - 62 Mt @ 0.92% Cu;
      Low-grade Oxide - 143 Mt @ 0.20% Cu;
      Supergene Sulphides - 219 Mt @ 0.86% Cu;
      Transitional Sulphides - 33 Mt @ 0.65% Cu;
      Hypogene Sulphides - 2.351 Gt @ 0.43% Cu.
  Total proved + probable reserves, (included in resources) for,
      Oxide - 36 Mt @ 0.85% Cu;
      Low-solubility Oxide - 143 Mt @ 0.20% Cu;
      Supergene Sulphides - 23 Mt @ 1.04% Cu;
      Sulphide - 165 Mt @ 0.94% Cu.

This summary was based in part on Tapia (2003), Cameron et al. (2006) and the BHP Billiton 2012 Annual Report.

The most recent source geological information used to prepare this summary was dated: 2010.     Record last updated: 22/3/2013
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


Spence

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Cameron E M, Leybourne M I and Palacios C,  2006 - Atacamite in the oxide zone of copper deposits in northern Chile: involvement of deep formation waters?: in    Mineralium Deposita   v.42 pp. 205-218
Tapia M,  2003 - Spence Copper Porphyry, Geology of the Deposit: in    ProExplo 2003 Conference Abstract,    p. 1


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