Steam-heated alteration: Identification, genesis, and application in exploration for epithermal deposits [pdf]


Supervisor: Dr. Zhaoshan Chang, Prof. Noel White, and Prof. Tom Blenkinsop



Steam-heated alteration consists of a blanket of altered rocks near present-day or paleo-surface. Around the water table it is typically composed of fine-grained porous to massive silicic rocks whereas the upper part is typically friable, consisting mainly of fine-grained alunite, kaolinite and quartz. Ancient steam-heated blankets typically have their friable upper parts weathered away, leaving behind only the siliceous layer.

Steam-heated blankets formed from acid water produced by condensation of vapors boiled from geothermal waters in the vadose zone (e.g, Hedenquist et al., 2000). Steam-heated blankets may occur above high-sulfidation epithermal deposits (HS), and intermediate- or low-sulfidation epithermal deposits (IS/LS; e.g., Hedenquist et al., 2000). If there were fluctuating levels of the water table and multiple epithermal mineralisation events, e.g., at Yanacocha, Peru (Longo et al., 2010), there may be multiple levels of steam-heated blankets.

Stream-heated blankets cause difficulties in exploration for several reasons: 1) Steam-heated blankets may overlie altered rocks related to epithermal deposits, and so prevent ore-related alteration from being detected; 2) the very fine-grained silicic rocks may appear similar to massive silicic rocks closely associated with mineralisation in high-sulfidation epithermal deposits (e.g., Chang et al., 2011), or to silica sinter, thereby causing confusion in interpretation; and 3) they may occur above epithermal deposits of any type, making it difficult to discriminate the potential for different types of epithermal mineralisation.

Most of the difficulties arise because of difficulties discriminating massive silicic rocks related to different epithermal deposit types. However in the recent years, there has been significant progress in technology, such as cathodoluminescence imaging (CL) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), that allow us to examine steam-heated blankets with better methods and lower detection limits for trace elements (e.g., Gotez et al., 2005; Landtwing et al., 2005; Rusk et al., 2006; Rusk et al., 2008; Breiter and Muller, 2009; Jourdan et al., 2009; Muller et al., 2009). With the new technologies we will be able to investigate more primary rock features, particularly cryptic textures and trace element compositions that are related to physicochemical parameters such as temperature, pressure, pH and redox state, thereby helping tackle the problems.

Aims and Objectives

In this study we aim to:

  1. Find reliable ways to identify steam-heated silicic rocks, and to distinguish them from silicic rocks associated with different types of epithermal systems (both productive and barren)
  2. Evaluate the potential to use steam-heated blankets to vector towards mineralisation beneath them
  3. Improve the understanding of the genesis of steam-heated alteration based on new data obtained during the project.


This study will develop criteria to help explorers to distinguish massive silicic rocks from various epithermal environments, thereby helping the industry to recognise covered targets. The project will also endeavour to define vectors in steam-heated blankets to identify mineralised areas beneath the blanket. If successful, these will enhance exploration efficiency and success rates. The data obtained will also help to constrain hypotheses about the formation of steam-heated blankets, thereby refining and improving our understanding of their genesis.


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Chang, Z., Hedenquist, J.W., White, N.C., Cooke, D.R., Roach, M., Deyell, C.L., Garcia, J. Jr., Gemmell, J.B., McKnight, S., and Cuison, A.L., 2011, Exploration tools for linked porphyry and epithermal deposits: Example from the Mankayan intrusion-centered Cu-Au district, Luzon, Philippines: Economic Geology, p. 1365-1398.

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