26Models of Origin for Carlin.docx

1、26 Models of Origin for Carlin26 Models of Origin for Carlin Gold Deposits, Nevada, USA生词1：trap 捕获 density 密度，比重lithostatic 净岩压的gradient 梯度overpressured 超压的laterally 从侧向passive 被动的hydrostatic 净水压的veinlet 小脉burial 埋藏documented 有文件证明的excess 过度circulate 循环meteoric 大气降水的juxtaposition 并列regime 政权制度，体制，状态

2、throttling 扼杀abrupt 突然的expansion 膨胀paleodepth 古深度stratiform 层状的permeability 渗透性，渗透率bioclastic 生物碎屑的horizon 水平，层位hydrologic 水文的sustainable 可以忍受的brecciation 角砾岩化strength 强度postulate 假定，要求，基本条件This section incorporates previous geologic, geochemical, fluid inclusion, and stable isotope with those of th

3、e present study into a model for the formation of the Carlin gold deposit in a deep geologic setting of fluid mixing between hydrostatic and normally pressured fluids.Depth of ore formationA trapping pressure of 8001400 bars is required by the high-density CO2-rich fluid inclusions present in MGO st

4、age quartz samples, given the temperatures indicated by coexisting H2O-rich inclusions. Typical lithostatic pressure gradients of 250 bars/km require depths of at least 3.21.8km to reach this pressure. At a more likely gradient of 80 to 85 percent of lithostatic, which is a typical gradient for form

5、ation of vertical fractures by overpressured fluids in laterally stressed rocks (Gretener, 1981, Engelder and Lacazette, 1990), the depth would be about 3.81.9km. At the other extreme, passive hydrostatic load conditions involving open communication with the surface (typical of hot springs and meteo

6、ric water circulation) would require minimum formation depths of 84 km. Depths of 6 km or more appear Geologically unreasonable given the fact that MC stage veinlet orientation and fluids inclusion data define maximum depths of only about 6.1 km under conditions of lithostatic load (Kueth, 1989), an

7、d significant burial is not documented in north-central Nevada after HC stage time (Early Cretaceous). Similarly, the 0 to 1.0-kbar pressures reported by Hoistra et al. (1987) at Jerritt Canyon, Nevada, for gas-rich (event IA) fluids responsible for gold mineralization define depths of 5 to 10 km un

8、der hydrostatic conditions. Pressure gradients appreciably in excess of hydrostatic are required to account for the 1-to 5-km depths of mineralization proposed for Jerritt Canyon (Hofstra at al., 1988).Formation of the Carlin deposit by a circulating meteoric water at 150 to 220 under conditions app

9、roaching lithostatic seems unlikely, because a lithostatically pressured fluid could not circulate directly from the surface and because of the likely increase of the 18O value of the water during even a short residence at 3.91. 9 km and 150 to 220. Therefore, juxtaposition of a hydrostatic fluid wi

10、th a near-lithostatic (overpressured) hydrothermal system is indicated.Flow of the deep fluid from the overpressured to normally pressured regime would have involved throttling of the fluid, defined by abrupt pressure decrease and fluid expansion, as discussed by Barton and Toulmin (1961), Toulmin a

11、nd Clark (1967), and Sims and Barton (1962). Flow is focused through the throttling zone, and active mixing of the two fluids is expected on the low-pressure side.Under this model, the deep hot overpressured CO2-rich fluid would ascend along fault structures to the site of the present orebody at pal

12、eodepths of 3.81.9 km. On reaching the Roberts Mountain Formation, it would encounter an extensive zone of stratiform permeability defined by the bioclastic horizon. This zone is postulated to have contained fluid at near-hydrostatic pressure with good hydrologic connection to the surface. A pressur

13、e drop of about 500 bars is sustainable by rock strength at this depth (T. Engelder). Some of the brecciation and faulting may reflect the rapid pressure transition and the accompanying rapid flow.生词2isotopic 同位素的episode 幕salinity 盐度country rock 围岩jasperoid 碧玉状的Paleozoic 古生代silicate 硅酸盐Jurassic 侏罗系e

14、xotic 外来的analogue 类似物consumptionmolalities 摩尔浓度permissive 许可的volatilization 挥发作用elevated 提高的retention 保持力channel 通道outcrop 露头 footwall 下盘FLuid characteristicsFluid inclusion and isotopic results also indicate the presence of two fluids during the gold ore episode (Table 4): (l) a gas-rich (primarily

15、 CO2 and H2S), moderate-salinity fluid which was 18O enriched (6.5 -10, at 225) as a result of considerable oxygen isotope exchange with surrounding country rocks, but is apparently dominantly of meteoric origin based on its D values of -87 to -165 per mil, and (2) a very low salinity, relatively ga

16、s-poor fluid with bothD and 18O values typical of unevolved meteoric water. These two fluids are required to account for the wide range of18O values in MGO and LGO stage jasperoid and quartz and calcite veins, as well as the variable fluid inclusion characteristics of these same features.Possible or

17、igins of the high CO2 content of MGO stage fluids are (1) tapping of low-to moderate-grade metamorphic fluids by deep-seated structures, (2) contact metamorphism of carbonates lower in the Paleozoic section to form calc-silicates adjacent to buried intrusions, or (3) direct magmatic contribution fro

18、m as yet unidentified intrusions at depth (i.e., not the Jurassic intrusive rocks exposed within about 10 km of Carlin).An exotic source for the CO2 is proposed because the relatively high gas contents (5-10 mole%) far exceed any contemporary geothermal analogue and require very high confining press

19、ures not available in a near-surface meteoric water. The acquisition of CO2 at these concentrations by ordinary water-rock interactions would require consumption of similar molalities of acid or oxidizing components (i.e. CaCO3+2H+=Ca2+CO2+H2O; CO+O2=CO2). Such high molalities of acid or oxidizing s

20、pecies are far higher than those recorded in normal geothermal systems. Therefore, thermal breakdown of carbonates, or an igneous source, seems necessary to furnish the CO2. The C isotope data for CO2-rich fluid inclusions are also inconsistent with a dominantly organic origin of the CO2 but are per

21、missive of the other sources. Any of the above sources of CO2 might also furnish elevated levels of H2S by pyrite breakdown accompanying carbonate breakdown or by magmatic volatilization.The second distinctive characteristic of MGO stage fluids is their high18OH20 values, which require significant i

22、sotopic exchange with sedimentary rocks at high temperatures if the initial water is meteoric. Because the CO2-rich fluids would be carbonate and silicate destructive, the original meteoric fluid was evidently heated and exchanged 18O with sedimentary materials prior acquiring the CO2. The retention

23、 of a carbonate destructive character despite passage through underlying carbonates before reaching the Roberts Mountain Formation seems to require flow along restricted channels bordered by silicified country rocks, as suggested by the large outcrops of jasperoid seen in surface exposures of footwa

24、ll rocks.Inclusions of LGO stage fluids in calcite and barite veins containing As Sb Hg Tl phases have filling temperatures of 150 to 220,but require an unknown pressure correction. If ore deposition occurred in a throttle between shallow hydrostatic and deeper overpressured environments at 3.81.9 k

25、m, then for an approximate pressure correction of 10/km, the trapping temperature for low-temperature LGO stage inclusions might be as low as 17.Possible geologic settings生词3illustrat 图示diagram 图解seal 封闭（构造）diagenetically 成岩期cemente 胶结 boundary 边界compaction 压实，压缩petroleum 石油leakage 泄漏episodic 分幕式的 O

26、rdovician 奥陶系Silurian 志留系Cambrian 寒武系argillaceous 泥质的section 剖面bitumen 沥青porosity 孔隙度gas window 生气窗anticline 背斜emanation 放出obscure 模糊的soluble 可溶的scavenge 清扫channel 引导jogs 漫步，轻推immiscibility 不混溶性The possible deep mixing environment is illustrated in Figure 12. This diagram shows a magmatic or skarn s

27、ource for the CO2 and H2S, or an alternative source of metamorphic fluids from the middle or deep crust (Cameron 1988). Mixing with meteoric fluids is interpreted to occur at a pressure seal separating the overpressured and normally pressured environments.Overpressuring (geopressuring) to values exc

28、eeding hydrostatic is reported to occur in most deep sedimentary basins (Hunt, 1990, Powly, 1990). Typically the change from overpressured to normally pressured occurs at a depth of about 3,000 m, at a seal zone with a thickness of a few hundred meters or less. The seal zone may be a relatively impe

29、rmeable stratigraphic unit such as a shale or evaporite, or it may be a zone of diagenetically cemented sandstone that cuts across stratigraphic units. In addition to subhorizontal or stratigraphic boundaries, the overpressured zone may be laterally limited by steeply dipping sealed fault zones. A w

30、ide variety of causes is suggested for overpressuring, including a sedimentation rate exceeding the rate at which fluid can escape to allow compaction, tectonic compression, thermal expansion of pore fluids, and volume increase on generation of gas or oil. Evidence from petroleum fields indicates th

31、at the seal commonly undergoes episodic leakage caused by pressures exceeding the fracture strength of the seal. During these leakage periods, oil and gas generated below the seal leak into reservoirs above the seal, presumably accompanied by water in some cases.The formation of the Carlin deposit i

32、n a deep sedimentary basin by the involvement of two fluids with markedly different pressures, with the higher pressure fluid inferred to be overpressured relative to hydrostatic, is strongly suggestive that the deposit was formed at or just above a pressure seal. Beneath this seal lies at least 1 km of Ordovician through Lower Silurian carbonate and quartzite (Radtke, 1985), plus probably another 2 km of Cambrian rocks of similar lithology (Roberts et al., 1958). Thus, in th