11. Hydrothermal Alteration By W.C. Pat Shanks III 11 of 21 Volcanogenic Massive Sulfide Occurrence Model Scientific Investigations Report 2010–5070–C U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2012 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order this and other USGS information products, visit http://store.usgs.gov Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 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Pat, 2012, Hydrothermal alteration in volcanogenic massive sulfide occurrence model: U.S. Geological Survey Scientific Investigations Report 2010–5070 –C, chap. 11, 12 p. 167 Contents Relations among Alteration, Gangue, and Ore .....................................................................................169 Mineralogy, Textures, and Rock Matrix Alteration...............................................................................170 Mineral Assemblages and Zoning Patterns ..........................................................................................170 Alteration in Modern Seafloor Volcanogenic Massive Sulfide Systems ................................170 Alteration Zoning in Ancient Volcanogenic Massive Sulfide Deposits ...................................173 Lateral and Vertical Dimensions .............................................................................................................175 Alteration Intensity ....................................................................................................................................175 References Cited........................................................................................................................................178 Figures 11–1. Representative cross sections of alteration related to hydrothermal activity or fossil hydrothermal activity on the modern seafloor .....................................................172 11–2. Representative examples of alteration zoning in volcanogenic massive sulfide deposits .........................................................................................................................174 11–3. Fluid flow modeling showing water/rock ratios during hydrothermal circulation at A, the Panorama volcanogenic massive sulfide district and B, beneath the seafloor at the Lau Basin hydrothermal system ...............................176 11–4. Graphs showing geochemical techniques for quantifying hydrothermal alteration effects using (A) the Gresens mass balance approach and (B) the alteration box plot approach .....................................................................................................................177 Table 11–1. Diagnostic minerals in hydrothermally altered volcanogenic massive sulfide deposits at different metamorphic grades ..........................................................................171 11. Hydrothermal Alteration By W.C. Pat Shanks III The geochemical reactions that produce hydrothermal Relations among Alteration, Gangue, alteration in host rocks of VMS deposits are critically important for a number of reasons. First, three-dimensional and Ore distributions of hydrothermal alteration zones are produced by circulating hydrothermal fluids and thus provide evidence Ore is traditionally defined as a valuable mineral or for pathways of fluid travel and geochemical evidence for the chemical commodity that can be extracted at a profit. In VMS physical and chemical conditions of alteration. The chemical deposits, ore generally consists of sulfide or sulfosalt minerals and mineralogical distributions of hydrothermal alteration that contain Cu, Pb, Zn, Ag, and (or) Au. Gangue is defined zones are generally the only direct evidence of fluid circulation as any noneconomic mineral deposited together with ore; in patterns related to VMS ore formation. Second, systematic VMS deposits this means essentially all nonsulfide miner- arrangement of hydrothermal alteration zones, and recognition als (see “Hypogene Gangue Characteristics,” Chapter 9, this of this arrangement, may provide information useful in volume) and some hydrothermal sulfide minerals (typically mineral exploration and may in some cases provide vectors pyrite or pyrrhotite) that lack economic value. Hydrothermal to undiscovered deposits. Third, hydrothermal alteration can alteration is defined as any alteration of rocks or minerals provide key information on the origin of metallic elements by the reaction of hydrothermal fluid with preexisting solid in VMS deposits. For example, depletion of key elements in phases. Hydrothermal alteration can be isochemical, like meta- altered rocks, combined with measured or inferred estimates morphism, and dominated by mineralogical changes, or it can of the volume of altered rock, can constrain possible be metasomatic and result in significant addition or removal of sources of ore metals. Finally, identification and recognition elements. Where alteration is intense, it can result in signifi- of hydrothermal alteration assemblages and their zonal cant volume changes such that mass balance approaches using relationships in the field may provide important evidence that immobile elements are required to fully understand the altera- a terrane under assessment is favorable for occurrence of VMS tion process (Gresens, 1967). deposits. Some authors include gangue and hydrothermal alteration Hydrothermal alteration varies widely from district to together (Beane, 1994) but, in the context of VMS deposits, it district and among individual deposits, and the literature on seems useful to continue distinguishing gangue from altera- this topic is voluminous. A detailed review is beyond the scope tion. Alteration is by definition an epigenetic process that of this report; the interested reader is referred to the following modifies preexisting rocks (or sediments), whereas gangue summary references and additional citations therein: Slack is generally a syngenetic mineral deposited on or near the (1993), Ohmoto (1996), Carvalho and others (1999), Galley seafloor along with the ore minerals. However, distinctions and Koski (1999), Large and others (2001a), Herrington and between gangue and alteration become difficult in cases where others (2003), Gifkins and others (2005), Peter and others VMS mineralization occurred by replacement and open space (2007), Galley and others (2007), Gibson and Galley (2007), filling in porous and permeable rocks in shallow zones beneath and Goodfellow (2007). the seafloor. Fortunately, alteration zones related to VMS deposits In some cases, the bulk of the massive sulfide ore may do show characteristic zonal arrangements (proximal, distal, be deposited in shallow subseafloor environments. In this comformable) that may be related to fluid flow and water/ mode of mineralization, hydrothermal fluids flow into highly rock interaction processes (upflow, recharge, burial metamor- permeable, high-porosity rocks, where sulfide precipitation is phism). In addition, fairly standard alteration assemblages, triggered by mixing with cold ambient seawater in the pore defined by alteration mineralogy, also occur in country rocks space. The prime example of this style of mineralization is the around many deposits, and even metamorphosed alteration giant Kidd Creek deposit, where most mineralization occurred zones produce predictable assemblages. below the seafloor in permeable, fragmental felsic volcanic 170 11. Hydrothermal Alteration rocks by infill and replacement (Hannington and others, 1999). (Zierenberg and others, 1988). The completely silicified A similar origin is inferred for the Horne mine (Kerr and Gib- rocks were originally mapped as “chert exhalite,” but careful son, 1993), the Ansil deposit where laminated felsic ash flows/ mineralogical, geochemical, and isotopic studies showed the turbidites were replaced by sulfides and silica (Galley and presence of relict igneous chromium spinel, proving massive others, 1995), and for the Turner-Albright ophiolitic deposit hydrothermal replacement. Similar extreme alteration has where massive sulfide formed below the seafloor within basal- been documented locally in the amphibolite-facies wall rocks tic hyaloclastite (Zierenberg and others, 1988). Hannington to the Elizabeth VMS deposit, Vt., where assemblages such and others (1999) suggested that subseafloor mineralization as quartz-white mica-calcite and quartz-white mica-albite- also formed some of the Bathurst (New Brunswick) deposits. staurolite-garnet-corundum contain uniformly high Cr/Zr and Ti/Zr ratios, which reflect protoliths of tholeiitic basalt that were pervasively metasomatized during seafloor hydrothermal Mineralogy, Textures, and Rock Matrix mineralization (Slack, 1999; Slack and others, 2001). These studies underscore the importance of under- Alteration standing the nature and effects of hydrothermal alteration in VMS systems, including overprinting