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Centre for Earth and Environmental Science Research

Magmatic and fluid processes responsible for the localization and development of epithermal Ag mineralization within the Sierra Madre Occidental silicic large igneous province, Mexico

Executive Summary
ResearchersMr. Aldo Ramos, CEESR
Prof. Andy Rankin, CEESR
Dr. Scott Bryan, CEESR
Dr. Luca Ferrari, UNAM, Mexico
Dr. Antoni Camprubi i Cano, UNAM, Mexico
Dr. Jamie Wilkinson, Imperial College, London
Funding Body/SourceCONACyT and Kingston University
DurationJuly 2007 - ongoing
Project SummaryThe volatile and metal contents of specific magmas and potentially, their mode of emplacement, play important roles in the development of epithermal ore-forming systems. This study will investigate genetic relationships between intrusive and extrusive volcanic rocks and epithermal mineralization in the southern section of the Tertiary Sierra Madre Occidental silicic large igneous province of Mexico. Previous work has shown a close spatial and temporal association between major pulses of silicic volcanism and epithermal mineralization, but that mineralization may be more directly related to late stage crystallisation of intrusions (Camprubi et al. 2003). Understanding magma processes from generation to emplacement, and the processes controlling the compositions of magmatic volatile phases are keys to understanding the transfer of metals to the volatile phase and hydrothermal system, and formation of economic mineralization.

Project objectives

The proposed project will be an integrated volcanic-geochemical study to:

  1. Constrain the volcanic-structural setting to mineralisation of key areas where fault activity appears linked to magma emplacement and volcanism;
  2. Define intrusive/eruptive centres and identify any key igneous units via structural/stratigraphic relationships that are integral to the localisation and development of mineralisation;
  3. Constrain genetic-temporal relationships between hypabyssal intrusions, volcanic rocks and mineralisation via Pb isotopic and geochronologic (U/Pb, U-Th/He, Ar/Ar) studies and fluid and melt inclusion studies;
  4. Assess the thermal influence of different magmas/intrusions in the generation of ore-forming hydrothermal systems via detailed geochemical and thermometric analyses.

Background

The mid-Tertiary Sierra Madre Occidental (SMO; Fig. 1) is the largest silicic igneous province in North America (McDowell & Clabaugh, 1979; Swanson & McDowell, 1984; Ward, 1995), and has received attention from several US and Mexican groups who have documented aspects of the volcanic geology, rhyolite chemistry, and eruptive and tectonic history of the province over the last three to four decades. Aguirre-Diaz and Labarthe-Hernandez (2003) pointed out that the area of continuous ignimbrite outcrop in the SMO (Fig. 1) would cover the equivalent area of peninsular Italy. Due largely to its huge areal extent and general remoteness, substantial sections of the province have had little scientific investigation.

In Mexico, low-sulphidation epithermal gold deposits are mostly Tertiary in age and are usually in close spatial association with the rhyolitic SMO volcanic province (Staude & Barton, 2001). In detail, the timing relationships between the SMO volcanism and epithermal deposits is not well understood, with high-quality age data available for only a few deposits (Camprubi et al., 2003). In some cases, epithermal mineralisation appears to be younger than the associated volcanic rocks by ~2 myrs suggesting that the mineralisation was related to intrusive rocks crystallising during eruptive hiatuses, which provided heat to drive the hydrothermal system, and possibly fluids, metals and ligands (Camprubi et al., 2003).

An early Miocene volcanic and mineralisation event is indicated for the southern SMO following new ages provided by Ferrari et al. (2002; see Fig. 1B). Ongoing studies by Bryan and Ferrari on early Miocene rhyolitic ignimbrites from the Southern SMO indicate that their origin may be entirely related to remelting of igneous materials and underplate formed during the previous Oligocene phase of magmatism in the SMO, and have a very different petrogenetic histories and crustal source regions to the Oligocene rhyolite ignimbrites.

Fig. 1 A) Map of Mexico showing present configuration of plates, location of Sierra Madre Occidental (SMO), distribution of Oligocene to Early Miocene rhyolitic volcanic rocks, and limits of area affected by Basin and Range extension (Henry & Aranda- Gomez, 2000). Hatched area is the Trans-Mexican Volcanic Belt (TMVB), the current location of supra-subduction zone volcanism. B) Geographic distribution of Oligocence to Early Miocene silicic volcanics in central Mexico. Cross patterns indicate extent of late Cretaceous batholiths. From Ferrari et al. (2002).

The hypotheses to be tested are that:

  1. crustal source regions for rhyolitic magmatism may be a critical factor in generating metal-rich magmas and epithermal mineralisation provinces; and
  2. volatile and metal contents of specific magmas and their mode of emplacement play important roles in the development of the epithermal ore-forming systems.

A major portion of the project will be a detailed compositional and thermometric study of melt inclusions in rhyolitic rocks and of fluid inclusions in weak and extensive zones of epithermal alteration and mineralisation from specific field localities, and to relate fluid compositions to magmatic evidence for crustal remelting. Melt inclusion studies would follow evaluations from field & geochronologic/geochemical studies identifying temporally and/or genetically related units. This will mainly involve state-of-the-art microprobe methods (notably Laser ICP-MS, Laser-Raman and micro FTIR spectroscopy), followed by modelling of the volatile, Cl, metal contents and thermal influence of different magmas/intrusions in the generation of the ore-forming hydrothermal systems.

REFERENCES

Aguirre-Diaz G.J., Labarthe-Hernandez G. (2003) Fissure ignimbrites; fissure-source origin for voluminous ignimbrites of the Sierra Madre Occidental and its relationship with basin and range faulting. Geology, 31, 773-776.

Camprubi A., Ferrari L., Cosca M.A., Cardellach E., Canals A. (2003) Ages of epithermal deposits in Mexico: regional significance and links with the evolution of Tertiary volcanism. Economic Geology, 98, 1029-1037.

Ferrari L., Lopez M.M., Rosas E.J. (2002) Ignimbrite flare-up and deformation in the southern Sierra Madre Occidental, western Mexico; implications for the late subduction history of the Farallon Plate. Tectonics, 21, 17, 1-23.

McDowell F.W., Clabaugh S.E. (1979) Ignimbrites of the Sierra Madre Occidental and their relation to the tectonic history of western Mexico. Geological Society of America Special Paper, 180, 113-124.

Staude J.-M., Barton M.D. (2001) Jurassic to Holocene tectonics, magmatism and metallogney of northwestern Mexico. Geological Society of America Bulletin, 113, 1357-1374.

Swanson E.R., McDowell F.W. (1984) Calderas of the Sierra Madre Occidental volcanic field western Mexico. Journal of Geophysical Research, 89, 8787-8799.

Ward P.L. (1995) Subduction cycles under western North America during the Mesozoic and Cenozoic eras. In: Miller D.M., Busby C. (eds), Jurassic Magmatism and Tectonics of the North American Cordillera. Geological Society of America Special Paper, 299, 1-45. Publications

Bryan SE, Ferrari L, Reiners PW, Allen CM, Petrone CM, Ramos-Rosique A, Campbell IH. (2008) New Insights into Large Volume Rhyolite Generation in the Mid-Tertiary Sierra Madre Occidental Province, Mexico, Revealed by U-Pb Geochronology. Journal of Petrology, v. 49, pp. 47-77

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