Skam deposits

Figure 1.16. Chemical composition of tetrahedrite-tennantite (Shikazono and Kouda, 1979). A Au-Ag vein-type deposits, B Kuroko deposits, C Taishu-Shigekuma Pb-Zn vein-type deposits, D Skam deposits (Kamioka).

Shimazaki, H. and Yamamoto, M. (1983) Sulfur isotope ratios for the Akatani, lide and Waga Sennin skam deposits, and their bearing on mineralization in the Green tuff region, Japan. Geochem. J., 17, 197-207. 

Abstract The San Miguei skam deposit is located in the Tandilia Belt, in the southern part of the Buenos Aires province in Argentina. It has developed in an igneous metamorphic complex, which exhibits a long geological evolution and is associated with a Proterozoic granitic magmatism. 

Therefore the formation of magnetite in that way could hardly be of essential importance in the metamorphism of iron-formations, and martitiza-tion is still less hkely. However, in deposits of other genetic types, for instance skam deposits, oxidation of iron silicates to magnetite at the contact with large masses of carbonate rocks (dolomite, magnesite) can be considered an ore-forming process. The last conclusion is still feasible because the carbon dioxide released in the dissociation of carbonates probably had an undisturbed CO O2 ratio. 

The Funiushan copper deposit is a skam deposit on the inverted northern limb of the western Tang-Lun anticline. It occurs within the contact zone of a quartz diorite and comprises chalcopyrite, subordinate pyrite and sphalerite, as well as minor amounts of magnetite and martite. The ore body lies beneath sedimentary formations and 20-30 m of eluvium and alluvium. Thermally-released Hg was determined along three soil traverses. As the traverse illustrated in Fig. 13-4 shows, the Hg, pattern picks out the mineralisation better than the Hg, pattern. The shape of the Hg, anomaly is characterized by twin peaks over the margins of the ore body. A large thrust fault seems to have a role in anomaly development. 

The Tongshan lindgrenite skam deposit lies on the northern margin of the Ning-Zheng Dome folded zone. The ore bodies are fault-controlled along the contact between... 

Comprising mainly magnetite, sphalerite, galena and a little chalcopyrite, the Xiaomaoshan polymetallic skam deposit occurs at the contact of porphyritic granite and limestone, at a depth of 110 m. The surface of the area is covered by exotic overburden, on which rice is cultivated. Soil samples taken at 20 m intervals along a traverse yielded twin peaks of Hg, above the hangingwall and footwall contacts of the mineralisation (Fig. 13-7). 

The Shanbaidu polymetallic deposit comprises genetically related but spatially separate iron and lead-zinc skam deposits in the contact zone of quartz-diorite and limestone. The ore minerals are mainly magnetite, hematite, galena and sphalerite. The ore bodies are found at depths of 5-40 m, and the surface of the area is covered by 5-15 m of transported exotic overburden. Thermally-released Hg, data from samples collected at depths of 30, 60 and 80 cm in the soil along a traverse crossing both ore deposits are shown in Fig. 13-9. In the samples from 60 and 80 cm, clear anomalies mark the positions of the mineralisation. In the samples from 30 cm, however, the background is... 

Shelton KL (1983) Composition and origin of ore-forming flnids in a earbonate-hosted porphyry eopper and skam deposit A fluid inelusion and stable isotope study of Mines Gaspe, (Jnebee. Eeon Geol 78 387-421... 

Einaudi M. T. and Burt D. M. 1982. A special issue devoted to skam deposits introduction, terminology, classification and composition of skam deposits, Econ. Geol, 77,4, 745-754. 

Important metallic ore deposits include Besshi (Kieslager)-type (strata-bound cupriferous pyritic deposits), strata-bound Mn-Fe-type, skam-type, Kuroko-type and vein-type. Dominant non-metallic deposits are limestone, clay, native sulfur, zeolite, silica and gypsum deposits. The deposits are divisible into three groups, based on their ages of formation Carboniferous-Jurassic, Cretaceous-Paleogene and Tertiary-present. 

The relationship between the iron content of stannite in equilibrium with sphalerite and pyrite or with sphalerite and pyrrhotite was derived based on thermochemical data by Scott and Barnes (1971), Barton and Skinner (1979) and Nakamura and Shima (1982). These types of deposits are skam-type polymetallic (Sn, W, Cu, Zn, Pb, Au, Ag) vein-type and Sn-W vein-type deposits. As shown in Fig. 1.181, the /s -temperature range for each type of deposits is different at a given temperature, /sj increases from Sn-W vein-type through skam-type to polymetallic vein-type deposits. It is interesting to note... 

As mentioned already, Shimizu and Shikazono (1985) have estimated the /s2 temperature range for stannite-bearing assemblages from Japanese vein-type and skam-type tin deposits. This estimated /sj-temperature region is also shown in Fig. 1.183. The /s2-temperature range for the formation of these two types of tin sulfides is different. 

Figure 1.181. Temperature-log /s diagram. Skam-type deposits (solid squares) are considered to be formed under lower /s, condition than vein-type deposits (open squares). Abbreviations are the same as in Fig. 1.178 (Shimizu and Shikazono, 1985).

Heated meteoric waters are a major constituent of ore-forming fluids in many ore deposits and may become dominant during the latest stages of ore deposition. The latter has been documented for many porphyry skam-type deposits. The isotopic variations observed for several Tertiary North American deposits vary systematic with latitude and, hence, palaeo-meteoric water composition (Sheppard et al. 1971). The ore-forming fluid has commonly been shifted in 0-isotope composition from its meteoric 5 0-value to higher 0 contents through water-rock interaction. Meteoric waters may become dominant in epithermal gold deposits and other vein and replacement deposits. 

This group of deposits is closely associated in space and time with magmatic intrusions that were emplaced at relatively shallow depths. They have been developed in hydrothermal systems driven by the cooling of magma (e.g., porphyry-type deposits and skams). From 8D- and 8 0-measurements, it has been concluded that porphyry copper deposits show the clearest affinity of a magmatic water imprint (Taylor 1974) with variable involvement of meteoric water generally at late stages of ore formation. 

Vazquez R, Vennemann TW, Kesler SE, Russell N (1998) Carbon and oxygen isotope halos in the host hmestone, El Mochito Zn, Pb (Ag) skam massive sulfide/oxide deposit, Honduras. Econ Geol 93 15-31... 

Numerous xenoliths of magmatic, metamorphic and sedimentaiy origin occur in the Vulsini rocks, especially in the pyroclastic deposits. Most ejecta represent bedrocks others are skams, intrusive equivalents of erupted lavas, or cumulate lithologies (e.g. Turbeville 1992b Di Battistini et al. 1998). 

The skam copper deposits at Huaitongshan, Gansu province, which are accompanied by some minor lead-zinc veins, lie of depths of 5-80 m. The area is arid and the surface is covered by Quaternary sediments on which a poor sandy and rudaceous soil, 3-50 cm thick, has developed. A traverse of soil samples revealed CO2 anomalies reaching 285 ppm over the main copper mineralisation, a contrast of about 4.4 (Fig. 4-1). The weaker anomaly further north is thought to lie over minor lead-zinc veins. 

Traverses were made across four copper-molybdenum deposits and soil gas was collected from holes drilled to a depth of 1.0-1.8 m. The soil air was analysed titrimetrically after absorption in an iodine solution. Control analyses were performed by mass spectrometry. The ore deposits, with widths of between 10-400 m, are associated with skams and granite porphyries. The thickness of the overburden is between 0.6-3 m. The ore minerals are principally chalcopyrite, molybdenite, chalcocite, bomite, galena, sphalerite, magnetite and pyrite. Weathering and secondary enrichment have affected the ores to a maximum depth of 95 m. 

Surveys for dispersion patterns of sulphide anions and compounds determined as acid-released HjS in soil have been carried out over 30 mineral deposits in China, including skams, porphyry copper deposits and porphyritic iron deposits (sulphur contents higher at the margin of the ore body), altered-brecciated gold deposits and lead-zinc deposits in volcanic breccia. Three successful case histories are summarised here. Of the remainder, only four failed to yield anomalies over mineralisation. 

One test of the method was conducted over a medium-sized iron-copper deposit near the southeastern coast of China. The ore body occurs in a skam near to the contact of granodiorite with interlayered marbles and homfels. The attitude of the ore body is controlled by the contact, which generally is steep. The strike of the ore zones is approximately east-west, plunging west, and their western contact is westerly convex. The top of the ore body, which may have been eroded, is now covered by 140 to 160 m of transported sediments. There is a thick soil cover the ground surface is quite flat and suitable for arable farming. 

Fig.9-6. Section along traverse a showing acid-released H2S content of soil over an iron-copper deposit in southeastern China Hf = brecciated schist, r8 = granodiorite, SK = skam, Q = Quaternary sediments.

In an exploration area in northern Peru, igneous rocks cover an area of several square km within a metamorphosed series of quartzites and limestones. The younger sequence consists of ignimbrites, tuffs and tuffites of approximately 1000 m thickness. The metamorphism produced skams, which are connected with the ore body. A porphyry Cu deposit occurs in the metamorphosed rocks and is characterised by intense hydrothermal alteration (quartz-kaolinite-sericite) which makes determination and classification of magmatic and sedimentary rocks at the surface very difficult. A zone of propylitic alteration can locally be followed up to 500 m into the andesitic-dioritic wall rocks. 

In contrast to the very restricted size of the soil-gas anomaly at Jabiluka, Zonghua and Yangfen (1981) describe broad anomalies associated with a buried skam Cu deposit near Shanghai, China. The ore body is confined to the contact zone of granodiorite and Palaeozoic and Mesozoic sedimentary rocks. The major sulphide minerals in the ore body are chalcopyrite, pyrite and molybdenite. Part of the deposit has been oxidised to limonite. The area is covered by 140-180 m of alluvium. A soil-gas survey of the area was complemented by a multielement study of soils including the determination of Hg. Significant anomalies of Hg in soil and soil gas occurred above the buried sulphide... 

Mierenshan, China, 444 Copper-gold-uranium deposits Olympic Dam, Australia, 331 Copper-molybdenum deposits Skam... 

Whether apatite represents the tail or the dog in halogen mass balance during metamorphism most likely depends on the environment. In high fluid/rock environments where fluid composition controls rock composition (e.g., veins, skams and ore deposits). 

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