Sericitic alteration

The formation which is mostly composed of dacite lavas, tuff breccia and mudstone (Hanaoka, Yukisawa, Uwamuki formations) conformably overlies the Hotakizawa and Sasahata formations. The thickness of these formations is 300-400 m. Kuroko ore deposits occur at the upper part of this formation. White rhyolite lava domes characterized by intense sericite alteration have a close spatial relationship with Kuroko deposits. 

Abstract Two distinct mineralizing fluids formed the Hollinger-Mcintyre-Coniaurum (HMC) deposit. The earliest fluid was associated with emplacement of a disseminated Cu-Au-Mo zone in the Pearl Lake Porphyry (PLP). The alteration pattern of the felsic rocks in the PLP is characterized by increased concentrations of K2O, Au, Cu, Mo, W, and Sn, and K/AI, Sericite / Chlorite (SCI) and Sericite Alteration Indexes and the removal of CaO, relative to nearby unaltered rocks. The H20-C02-NaCI mineralizing fluid that altered the PLP had a temperature between 340° and 390°C, and a 5 Owater composition of 11.7 to 12.7 %o. 

The interpretation of lithogeochemical data from basaltic structural domes is complicated by lithological changes associated with the transition from basalt to overlying siliciclastic rocks, as well as by the polydeformed nature of the host sequence. Ferroan carbonate alteration is well developed, and low-level Au enrichment extends for a considerable distance away from zones of economic interest. Arsenic and Sb/AI anomalies are restricted to within approximately 10 m of mineralized zones. Sericite alteration is indicated by Na depletion and K enrichment in basalt within 20 to 40 m of mineralized zones. A number of other elements, including Mn, P, S, Zn, Mo, Cu, Se and Ba, are variably enriched within the rocks hosting Au mineralization, but it is not clear whether elevated concentrations of these elements are a product of syn-sedimentary exhalative activity or result from later hydrothermal alteration. 

Hydrothermal alteration at Pebble consists of a central, strong K-silicate assemblage with sparse magnetite, and peripheral sericitic alteration that overprints the deposit, with propylitic and illite assemblages present locally (Rebagliati Lang 2008). 

Mineralization is present mostly in strong K-silicate altered rocks dominated by K-feldspar, and in multi-generational stock works of quartz-carbonate-sulphide veins. Laterally extensive sericite-altered rocks are peripheral to and overprint the... 

Hydrothermal alteration is classified into several types depending on the alteration minerals (Hemley and Jones 1964 Rose and Burt 1979). Important hydro-thermal alterations include propylitic alteration, argiUic alteration, advanced argUlic alteration and sericitic alteration (Meyer and Hemley 1967). 

The Y, C and B sub-types roughly correspond to types 1, 2 and 3 as defined by Urabe (1974a), who classified Kuroko deposits based on hydrothermal alteration and ore mineral assemblages type 1, kaolinite-pyrophyllite-diaspore-type type 2, sericite-chlorite-type type 3, sericite—chlorite-carbonate-type. Hydrothermal alterations in the Kuroko mine area are described in section 1.3.2. 

Mixed layer clay mineral (sericite/smectite) is found in Kuroko ore bodies and altered dacitic rocks underlying the ore. This mineral is thought to have formed by the... 

For example. Date et al. (1983) recognized the following alteration zones in the Fukazawa Kuroko mine area of Hokuroku district from the centre (near the orebody) to the margin (1) sericite-chlorite zone (zone 111 in Figs. 1.20-1.22) characterized by quartz + sericite Mg-rich chlorite (2) montmorillonite zone (zone 11 in Fig. 1.20) characterized by Mg-Ca-type montmorillonite + quartz kaolinite calcite sericite Fe-rich chlorite and (3) zeolite zone (zone 1 in Fig. 1.20) characterized by clinoptilolite + mordenite + Mg-Na-type montmorillonite cristobalite calcite or analcime + Mg-Na-type montmorillonite + quartz calcite sericite Fe-rich chlorite (Fig. 1.20). 

Pyrophyllite and diaspore alterations were reported from several Kuroko deposits, although they are not common (Urabe, 1974a). This type of hydrothermal alteration is thought to have occurred at a later stage than the hydrothermal alterations associated with Kuroko mineralization (sericite, chlorite, and zeolites) (Utada, personal communication, 1995). 

Figure 1.28. Whole-rock 0 values of Miocene volcanic and sedimentary rocks from the Hokuroku district, grouped by alteration zones. Each square represents one sample. Mont. = montmorillonite, Ser. = sericite, Chi. = chlorite, av. = average (Green et ah, 1983).

Negative Eu anomaly is also found in the fresh and altered dacitic rocks (Dudas et al., 1983). Therefore this negative anomaly in anhydrite is also explained in terms of an influence of sericitization of dacite accompanied by the depletion of Eu. 

The age of formation of epithermal vein-type deposits can be estimated from K-Ar ages of K-bearing minerals (adularia, sericite) in veins and in hydrothermal alteration zones nearby the veins. A large number of K-Ar age data have been accumulated since the work by Yamaoka and Ueda (1974) who reported K-Ar age data on adularia from Seigoshi Au-Ag (3.7 Ma) and Takadama Au-Ag deposits (8.4 Ma). Before their publication on the K-Ar ages of these deposits it was generally accepted that epithermal... 

Representative propylitie alteration minerals inelude epidote, albite, earbonates, quartz, chlorite, sericite, and smectite. The less common minerals are mixed-layer elay minerals such as chlorite/smectite and sericite/smectite and zeolite minerals. 

Oxidation of H2S (reaction (1-35)) occurs under the near-surface environment. Oxygen may be supplied from oxygenated groundwater. These oxidation reactions liberate H+ ion, leading to a decrease in pH. Under low pH conditions intermediate argillic alteration minerals (e.g., kaolinite, sericite) are stable. 

K-Ar ages data on adularia and sericite in the veins and altered host rocks indicate that ages of mineralization vary widely, ranging from 1 Ma to 68 Ma and from 1 Ma to 24 Ma for the Se-type and Te-type, respectively (Tables 1.17 and 1.18). 

Supergroup rocks in the Hishikari district suffered hydrothermal alteration. Chlorite, quartz and sericite occur abundantly near the veins. The other constituents are pyrite, albite, calcite and organic matter. 

Izawa et al. (1990) recognized the following alteration zones from the vein towards margin of the Hishikari Au-Ag mine area, chlorite-sericite zone (zone IV), interstratified clay mineral zone (zone III), quartz-smectite zone (zone II) and cristobalite-smectite zone (zone I) and least altered zone (L.A. (least altered) zone) (Fig. 1.131). 

The ages of Neogene mineralization and hydrothermal alteration in and around the Northeast Honshu and Hokkaido have been determined by K-Ar data on K-minerals (K-feldspar, sericite). These data are summarized in Fig. 1.147 and Table 1.26. 

The deposits are characterized by conspicuous alteration zoning from the centre (orebody) to margin (Tokunaga, 1955 Doi, 1972 Urashima et al., 1981, 1987). They are siliceous zone, alunite zone, kaolinite zone, sericite zone and montmorillonite zone. 

The dominant alteration minerals at the deeper part of the well include anhydrite, epidote, sericite, chlorite, calcite, dolomite, rhodochrosite, kutnahorite, zeolites (mordenite, clinoptilorite), chlorite and sericite/smectite interstratified clay mineral with subordinate amounts of kaolinite in the shallower part (Imai et al., 1996). 

Secondary minerals formed via alteration are epidote and sericite. 

Hydrothermal alteration of the Cap d Ours is typified by silica, chlorite, sericite, epidote and carbonate phases. In order to quantify the intensity of alteration within the volcanic edifice, a new approach was adopted for both sampling and analyses. 

The HMC deposit has four alteration mineral assemblages that grade from the background greenschist facies to quartz-albite-ankerite-sericite proximal to individual veins (Smith Kesler 1985). Studies of the HMC mine area have shown that higher concentrations of As, Ba, CO2, Rb, K2O, and As occur near gold-bearing zones (Whitehead et al. 1981). Specifically, either the C02/Ca0 molar ratio, or a combination of K2O and As concentrations can be used to discern mineralized from barren zones. 

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