Submarine Hydrothermal/Hot Spring Deposition of Early Cambrian Niutitang Formation in South China

INTRODUCTION

The relationship between life and environmental processes in critical geological periods is one of the significant geological issues and remains a major concern. Not only environment events led to life evolution, but also biological processes changed the environment and climate which in turn influenced sedimentation and ore forming. In Early Cambrian, a diversity of phyla of skeletal metazoan synchronously appeared, and important phosphorite, Ni-Mo polymetallic ores and hydrocarbon source rocks were formed. Investigations on the Early Cambrian sedimentary rocks can provide an insight into the dynamics of life origin and the Cambrian explosion, and the sedimentary settings of the formation of metallic ores and hydrocarbon source rocks.

In South China, Early Cambrian Niutitang Formation, extending more than 1 600 km, is characterized by the presence of black rock series. These rocks witnessed the Cambrian life explosion, and are abundant in Ni, Mo, V, PGE and gold, and proposed to be hydrocarbon source rocks. Previous investigations were conducted on the fossils, strata, occurrence of metallic elements, the genesis and metallization time of polymetallic sulfide ores, sedimentary environmental conditions and the paleogeographic pattern. Several viewpoints were proposed about the origin of polymetallic sulfide ores; they include the normal sedimentation of ore-forming metallic elements from sea water either in reductive condition or during the upwelling of deep seawater (Lehmann et al., 2007; Mao et al., 2002), the submarine hydrothermal or eruptive sedimentation (Chen et al., 2009; Yang et al., 2008; Jiang et al., 2007; Pašava et al., 2007; Steiner et al., 2001; Lott et al., 1999), and the universe or biologically-induced sedimentation (Orberger et al., 2007; Fan et al., 2004). We present here the petrologic and geochemical features of these sediments to show the origin of submarine hydrothermal deposition.

STRUCTURE AND PETROLOGIC FEATURES

Pangea disruption in Early Cambrian led to the existence of Yangtze plate and Huaxia plate as result of the strong tectonic extension. Lower Cambrian Niutitang Formation was formed in Jiangnan shale basin along the south marginal shelf. Niutitang Formation is composd of successively siliceous rocks, phosphorites, breccia siltstones, baritic rocks, polymetallic ores and black shales upward. There exposed crystalline limestones below polymetallic ores in Zunyi and Nayong of Guizhou and in Zhangjiajie of Hunan. Vertical fracture zones were identified in these rocks in Wengan of Guizhou. Massive baritic rocks are present in Tianzhu of Guizhou and in Xinhuang of Hunan. Lenticular baritic rocks are made up of megaspore and small porphyrotope of barite. These petrologic features are indicative of an eruptive deposition. Chimney-like silicalite bodies appear in Tianzhu barite ores of Guizhou and in Zhangjiajie polymetallic ores of Hunan. Brecciform silt was found to bear the rubbles of phosphorite and siliceous rocks, supportive to the product of the hydrothermal eruptive process. Barite veins were observed to pass through phosphorites. Some fissures in phosphorites were filled with pyrite veins. Macroscopic structure features and petrologic characteristics are of origin of the hydrothermal eruption.

ORE MINERAL ASSOCIATION

Ni-Mo polymetallic sulfide ores occurred in layer, bedded and lenticular forms at the bottom of black shales in Niutitang Formation. The polymetallic sulfide ores distributes discontinuously in space. There are bedded, banded and brecciform ores. The brecciform ores have a lot of syngenetic rubbles. The rubbles are composed of black carbonaceous matter and clastic pyrite, millerite and jordisite, showing the feature of the submarine eruptive deposition. Chalcopyrite veins were observed to intercalate colloform pyrite in the polished thin sections of ores. Mineral association includes chalcopyrite, sphalerite with some bearing emulsion texture of chalcopyrite, galena, millerite, jordisite, vaesite, bravoite, gersdorffite, cobaltite and tennantite. The natural interlocking assemblages of pyrite, sphalerite and chalcopyrite show that they were formed in the same physiochemical conditions and represent the product of hydrothermal activity.

GEOCHEMICAL FEATURES

The Ni-Mo polymetallic sulfide ores are abundant in the elements of Ni, U, Au, Co, Cr, Cu, Pb, Zn, Ba, and PGE. The Eu in REE distribution pattern of barite in Niutitang Formation in Wengan shows a positive abnormity (Fig. 1).

Figure  1.  REE distribution patterns of barites in Niutitang Formation in Wengan, Guizhou.

DownLoad: Full-Size Img PowerPoint

The enrichment of the metallic elements and the REE distribution patterns are suggestive of hydrothermal origins. The values of the Y/Ho ratio range from 25 to 57, comparable with the samples in modern submarine hydrothermal systems (Table 1). In the triangular diagram of Fe-Mn-(Co+Ni+Cu)×10, the ore samples fall in the range for hydrothermal sedimentation or submarine hot water sedimentation (Fig. 2). The ratio of U/Th in Wengan's samples is more than unity, leaving in the range of ancient hydrothermal sedimentation (Fig. 3).

Table  1.  Compiled data of the content (10^-6) of Ho and Y, and the ratio of Y/Ho for Ni-Mo sulfide ores in Niutitang Formation in South China

Sample No.	Ho	Y	Y/Ho	Locations andreferences
DT-3	7.300	306.000	41.92	Wengan, Guizhou, Fu et al., 2009
YH-3	5.390	222.000	41.20
WJY-3	5.940	253.000	42.59
GZ-1a	1.060	62.300	58.77	Sancha in Zhangjiajie, Hunan, Jiang et al., 2006
GZ-1b	0.070	3.620	51.72
GZ-1c	0.060	3.160	52.67
GZ-1d	0.040	1.940	48.5
HN-1a	0.670	38.200	57.01	Zunyi, Guizhou, Jiang et al., 2006
HN-1b	0.030	1.680	56
HN-1c	0.040	1.470	36.75
HN-1d	0.010	0.220	22

 | Show Table

DownLoad: CSV

Figure  2.  Diagrams of Fe, Mn and (Cu+Ni+Co)×10 of hydrothermal mineral deposits from oceanic spreading centers (modified from Bostrom, 1983) and the Early Cambrian samples in South China. RH. hydrothermal sediment in Red Sea; HD. hydrothermal sediment; HN. hydrogenous sediment; ED. hydrothermal sediment in East Pacific ridge; WDT and ST. Early Cambrian samples from Wengan, Guizhou.

DownLoad: Full-Size Img PowerPoint

Figure  3.  The lgU versus lgTh plot showing different sediments (modified from Bostrom, 1983). EH. hydrothermal sediment in East Pacific Ridge; RH. hot brine sediment in Red Sea; AH. bauxite; OS. marine sediment in sea water; MH. ironmanganese crust in Deep Ocean; FH. fossiled hydrothermal sediment, and the Early Cambrian samples in Wengan, Guizhou.

DownLoad: Full-Size Img PowerPoint

CONCLUSIONS

On the basis of sedimentary structures, the characteristics of Ni-Mo polymetallic sulfide deposit, the mineral association and the geochemical features, it is concluded here that Niutitang Formation of Early Cambrian in South China was formed due to the hydrothermal activity, which occurred before the Cambrian life explosion. Whether the hydrothermal/ hot water activity stimulated the Cambrian life explosion awaits further investigation.