J. Cent. South Univ. Technol. (2008) 15: 89-94
DOI: 10.1007/s11771-008-0018-7
Formation mechanism of breccia pipe type in Yixingzhai gold deposit
SHAO Yong-jun(邵拥军), ZHANG Yi-zhou(张贻舟), ZHANG Jian-dong(张建东),
DING Zong-wei(丁宗炜), CHEN Xing-lin(陈星霖), LIU Zhong-fa(刘忠法)
(School of Geoscience and Environmental Engineering, Central South University, Changsha 410083, China)
Abstract：Based on the basic geological features of Yixingzhai gold deposit, the geological features, classification and relation to mineralization of breccia pipes were discussed by surveying the xenoliths and breccia’s mineral composition, structure and construction, typical mineralization alteration phenomenon on field and microscopic anatomy in breccia pipes. And the ore-controlling mechanism and the formation mechanism of the blasting breccia pipes were investigated. The main conclusion shows that Hewan and Nanmenshan breccia pipes nearby the NW-striking deep fault are shaped earlier, belonging to the acidic siliceous ingredient production of ultra-hypabyssal magmatic in the late magma stage; Tietangdong and Nanmenshan breccia pipes that are shaped latter are the ultra-hypabyssal auriferous fluid production composed mainly of the skarn in the hydrothermal stage after the magma period. Both root in the unified deep magmatic chamber, belonging to differentiation derivatives at different stages in the deep magmatic chamber.
Key words: formation mechanism; Yixingzhai gold deposit; auriferous quartz veins; breccia pipes
1 Introduction
Yixingzhai gold deposit is formed in the northwest of Sunjiazhuang complex that is located in the Mesozoic NW-striking structural magmatic zone. There cropped out four blasting breccia pipes with the strong structure－magmatic activities in the ore field. These pipes distribute in diamond pattern[1].
The NW-striking fault(Yixingzhai fault and Longshan fault) and the NEE-striking fault are main structures in the region structure. They form the basic structure pattern. The NW-striking fault controls the output of Yanshanian ore-forming body and the distribution of Yixingzhai-Xinzhuang gold belt.
The Yanshanian intermediate-acid magmatic activity is the dominant factor in the formation of the deposit(ore bodies). Spatial distribution of mineralization has the closest relation with ultra-hypabyssal or subvolcanic acid rock in the southern of ore field. Ore bodies are formed in belt of NNW-striking brittle fault in biotite hornblende plagiogneisse with the formation of auriferous sulfide quartz veins. It is the biggest shallow-level meso-epithermal quartz vein gold deposit that is related to Yanshanian intermediate-acid magmatic activities in the zone of magmatic activities in the northeast of Shanxi Mesozoic tectonics[2].
There are many previous studies on geotectonic background[3], regional metallogeny[4], source o ore-forming materials[5-6], deposit geochemical characteristic[7-8], controlling factors[9-10], ore-forming mechanism and deposit origin[11-13]. However, systematic research and anatomy relation to breccia pipe which has the closest relation with mineralization have not been done. Therefore in this work, based on the basic geological features of on the gold deposit, on the geological features, classification and relation to mineralization of breccia pipes were discussed by surveying the xenoliths and breccia’s mineral composition, structure and construction, typical mineralization alteration phenomenon on field and microscopic anatomy in breccia pipes.
2 Characters of auriferous quartz vein belts
The auriferous quartz vein belt is a general designation on all kinds of the ore-bearing structure and crack zones with concentrate output of auriferous quartz veins in the mine field. The occurrence and size of gold orebodies are obviously controlled by the auriferous quartz vein belts. The main 13 vein belts have been found in the ore field. From east to west, the serial numbers are concealed zero, zero, unnamed first, unnamed second, first, concealed the first, the second, the third, concealed the third, the fourth, the fifth, the sixth, the tenth. Their occurrence and size are listed in Table 1.
The main trend of auriferous quartz veins belt is
NW355?. The minor trend is NW325? (concealed first, unnamed first, unnamed second and fourth vein belt). Some auriferous quartz veins are mainly distributed between the Hewan volcanic neck and Tietangdong breccia pipe.
Table 1 Parameters of main auriferous quartz veins
Another some auriferous quartz veins are mainly distributed near the Hewan volcanic neck and Tietangdong breccia pipe. The NNW-striking vein belts generally show a sinistral echelon pattern as “λ”, in which the fifth vein belt likes a bellwether. The main associated mineral products in veins belts are Mo, Ag, Cu, Pb, Zn and S and so on.
3 Geological features and classification of types and formation mechanism of breccia pipes
3.1 Geological features of breccia pipes
There cropped out four breccia pipes in ore field, which distribute in diamond pattern. From northwest to southeast, they are Hewan, Tietangdong, Nanmenshan and Jinjiling. The four breccia pipes are mainly distributed in the zone of dense faults in the middle of ore field, which are obviously controlled by the NW-striking fault, NNW-striking fault, NE-striking fault and NEE-striking fault(Fig.1).
The breccia pipe of Hewan is mainly distributed in the river bed in the east of Yixingzhai village, liking an irregular pipe body, with a peripheral to inner, the dip angle of 60?-85?, the average diameter of 550 m, the area of around 0.196 km2, and obviously controlled by the
Fig.1 Location map of veins in Yixingzhai gold deposit: 1—Cambrian; 2—Changcheng system; 3—Basalt; 4—Diabase-prophyrite; 5—Diorite porphyry; 6—Diorite (Sunjiazhuang complex); 7—Hypabyssal acid rock; 8—Quartz porphyry; 9—Feldspar quartz porphyry; 10—Skarnized blasting breccia pipes; 11, 12, 13—Diabase; 14—Plagioclase gneiss; 15—Gold vein and its serial number; 16—Magnetite body; 17—Blasting breccia pipes; 18, 19—Fault; 20—Line of geological limitation
NW-striking fault, NNW-striking fault and NEE-striking fault. The surrounding rocks are hosted mainly by the biotite hornblende plagiogneisse, the pipe is mostly filled with feldspar quartz porphyry. Partly it is obvious fluxion structure, obvious accidental inclusion of biotite hornblende plagiogneisse, a few alteration breccia.
The breccia pipe of Tietangdong is located in the east of Tietangdong hill in ore field, appeared the rhomboid on the surface, about 225 m long from north to east, about 240 m long from north to west, the area of around 0.054 km2, a steep peripheral to inner, the average dip angle is above 80?, appeared the steep funneled to inner in the space and controlled by the fault structure crossing among NW-striking, NNW-striking and NEE-striking obviously. The surrounding rocks are hosted mainly of the biotite hornblende plagiogneisse, the pipe is composed mainly of the skarnized crypto explosive breccia and the residual limestone, having the phenomenon of shattered time after time, phenomenon of repeatedly alteration(from high temperature to low temperature), phenomenon of stack multi-period hydrothermal mineralization.
The breccia pipe of Nanmenshan is located in the south of Nanmenshan in the southwest of ore field with 900 m long from east to west, the average width of 150 m, the area of around 0.135 km2, the rock mass leaning to the north, the dip angle of 40?-70? and controlled by the fault structure crossing among NW-striking, NNW-striking and NEE-striking obviously. The surrounding rocks are hosted mainly of the biotite hornblende plagiogneisse and remain a few limestone which almost has been marmorized, the pipe is composed mainly of the early feldspar quartz-porphyry, the middle quartz-porphyry, a few alteration breccia and the surrounding rock fragment.
The breccia pipe of Jinjiling is located at the trend of 176? to Tietangdong breccia pipe with the direction about 975 m, appeared the rhomboid on the surface, the diameter about 110 m, the area of 0.009 5 km2, a steep peripheral to inner, appeared the steep funneled to inner in the space and controlled by the fault structure crossing among NW-striking, NNW-striking and NEE-striking. The surrounding rock is hosted mainly by the biotite hornblende plagiogneisse, the pipe is composed mainly of the skarnized crypto explosive breccia and the residual limestone, the phenomenon of hydrothermal alteration and the stack phenomenon of the metallic sulfide mineralization are obvious.
3.2 Classification of breccia pipes type
There cropped out four breccia pipes in ore field. They can be grouped into two categories from the analysis of occurrence character, as described below.
One type takes the breccia pipe of Hewan and Nanmenshan as the representative, the main characters are as follows: the filling materials are mainly Yanshanian ultra-hypabyssal feldspar quartz-porphyry, the medium-erm quartz-porphyry and acid dykes, the local parts of the rock mass have the fluidal structure，the scale of the formed breccia is not large and the formed breccia grows near the side of the pipe and at the top on the upside of the pipe, the genesis of the pipe is from the invading breccia of the magmatic cementation, there crops out a few alteration breccia on the side of the pipe, the rubble ingredients have the gneiss surrounding rock fragment and the xenolith primarily and partly contain the hydrothermal alteration rubble related to the fluid alteration of the rock mass local condensation differentiation in the local and the early condensation magmatic rock rubble.
The other type takes the breccia pipe of Tietangdong and Jinjiling as the representative, the main characters are as follows: the filling materials are mainly skarnized cryptoexplosive breccia and the residual limestone, the scale of the breccia is large, the majority is skarnized breccia rockbody except for few acid dykes on the upside of the pipe, the genesis of the pipe is from the hydrothermal breccia of the skarn cementation. The rubble ingredients are in diversity, which mainly contain the skarn and replacement remnant limestone, posteriorly containing biotite hornblende plagiogneisse, leucoleptite, metamorphic diabase, quartz porphyry, eurite and chert gobbets and so on. Hydrothermal alteration in the pipe is mainly skarnization in early gas-liquid metasomasis in crytoexplosive mechanism, taking silicide, carbonation and chloritization that are formed in filling along cracks as the representative in late stage.
Main skarn minerals in early stage are garnet(Fig.2) and diopside(Fig.3). Main skarn minerals in late stage are chlorite, epidote(Fig.4) and actinolite. The typical metasomatic rocks in early dry skarn stage and in late wet skarn stage separately take the diopside garnet skarn and chlorite epidote actinolite skarn as the representative.
Fig.2 Isometric crystal garnet (-) (taken in plane-polarized light)
Mineralization is common in pipes, which can be divided into three stages: the early skarn stage, metaphase quartz sulfide stage and late carbonate stage. The main metallic minerals in early skarn stage are specular hematite (Figs.5 and 6), a few less magnetite. Having the integrity of crystal form, specular hematite has the suhedral crystal-uhedral crystal texture in the form of dissemination, nodular and a few veinlet in the skarn. In partial enrichment zone, specular hematite become the industrial orebody. The quartz sulfide stage takes quartz sulfide veins which occur largely as the typical representative. The main formed metallic minerals are pyrite, chalcopyrite, bornite, cyprite, sphalerite, galena, native gold and electrum and so on.
Fig.3 Orthometric height process, a group cleavage of diopside (-) (taken in plane-polarized light)
Fig.4 A group cleavage of parallel-axial extinction of epidote (-)(taken in plane-polarized light)
Fig.5 Folia specular hematite in skarn (-) (taken in plane- polarized light)
Fig.6 Fokiated specular hematite in skarn (-)(taken in plane- polarized light)
The quartz sulfide stage is the uppermost mineralization stage. The carbonate stage takes calcite veinlets which are cropped out largely as the typical representative. The main formed metallic minerals are sphalerite, galena and electrum and so on.
Fissure and irregular caves that are good hosted space precipitated by mineralization elements in late stage grow specially in the breccia pipes. Sulfide veinlet, auriferous quartz sulfide veinlet, quartz calcite veinlet and chlorite calcite veinlet and so on penetrate in scissure. In partial zone, the big miarolitic cavity can be seen. In the miarolitic cavity hall there are calcite films(Fig.7).
Fig.7 Irregular shape of calcite miarolitic cavity in skarn
3.3 Formation mechanism of breccia pipes
Cryptoexplosive breccia mechanism can be expounded as follows: containing much siliceous constituent, the high-pressure molten mass from the magmatic chamber in depth ascends from NNW-striking deep fault which has multi-phase activities, controlled by the fault structure crossing among NW-striking, NNW-striking and NEE-striking obviously in perching in its shallow, nearby the concentrative zone of NW-striking deep fault such as the Hewan fault and Nanmenshan fault. The pressure decreases suddenly, molten mass ascends to adjacent surface nearby quickly. Because of the countercheck of condensative crust in early crystallized stage and the countercheck of the adjacent overlying rock, internal pressure rises sharply, the molten mass will break through the bondage of the adjacent overlying rock, leading to cryptoexplosion when it aggregates to a certain extent. On the side of the rockbody and at the top of the rockbody, the magmatic gummy intrusion breccia is formed. Because the pressure of the later molten mass has been released at the top of the rockbody, feldspar quartz porphyry, mesogenetic quartz porphyry and eurite with ultra-hypabyssal are shaped below the breccia pipe. The porphyritic texture, felsitic texture, cryptocrystal-microcrystal texture and fluxion structure are the typical evidences that the quick ascending molten mass and condensation cryptocrystal rockbody are shaped. The little altered breccia on the side of the ultra-hypabyssal rockbody is related directly to the little releasing hydrotherm in condensation at the top of the cryptocrystal rockbody.
After perching of rockbody in condensation, along with magmatic activities, the high-pressure auriferous fluid that contains a large number of volatile and mineral matters from the deep magmatic chamber differentiation continues to ascend from multiple active NW-striking deep fault. It is controlled by the fault structure crossing among NW-striking, NE-striking, NNW-striking and NEE-striking in perching in its shallow. The high-pressure auriferous fluid is also restricted by the space of formed condensation rockbody in early stage. Along with another channel, it will upwell largely in the zone of Tietangdong fault and Jinjiling fault and so on. Because of the bondage of adjacent overlying rock, the internal accumulated pressure rises sharply. At the same time, because of the strong two-metasomatism between the high-pressure auriferous fluid and the overlying limestone, releasing the gas of CO2 largely, the internal pressure of the auriferous fluid creases further. The high-pressure auriferous fluid breaks through the bondage of the adjacent overlying rock at last, leading to crypto explosion strongly, and the skarned gummy hydrothermal breccia that distributes cosmically has been shaped.
In the pipes, the size of breccia is various, the main cementing materials are hydrothermal mineral-skarn minerals in alteration. The skarn and limestone are the main ingredients in the breccia, minor ingredients are gneiss and so on. The massive limestone has not been altered, it is more fresh. The concentrated development zone is basically the zone where the early limestone is distributed. The borderline between the pipes and the adjacent rock—gneiss all around is clear. The alteration and the cataclasm in gneiss body is not obvious. The relation between breccia body and gneiss body brecciation is the sudden transition, having not the macroscopical zonality from strong to weak basically. The cracks and irregular holes are developed. The phasic characteristic sooner or later in alteration and in mineralization type is obvious. They are the typical evidences for magmatic auriferous fluid to ascend rapidly in short time. Imperfect two- metasomatism between auriferous fluid and limestone takes place relatively in small space, leading to crypto explosion, finally forming the skarned gummy hydrothermal breccia.
4 Conclusions
1) Hewan and Nanmenshan breccia pipes nearby the NW-striking deep fault are shaped relatively early, belonging to the acidic siliceous ingredient production of ultra-hypabyssal magmatic in the late magma stage.
2) Tietangdong and Nanmenshan breccia pipes that are shaped relatively late are the ultrahypabyssal auriferous fluid productions composed mainly of the skarn in the hydrothermal stage after the magma period.
3) Both root in the unified deep magmatic chamber, belonging to differentiation derivatives at different stages in the deep magmatic chamber.
4) The magmatic gummy breccia pipes of Hewan and Nanmenshan are on behalf of the vanguard in magmatic emplace. The area of its surrounding contact zone system is the most favorable zone of mineralization.
5) The skarned gummy breccia pipes of Tietang- dong and Jinjiling are on behalf of the center of fluid pooling largely. The area of the developed cracks in its internal and its surrounding is the best venue for the orebody positioning, it is also the best target area that needs to be inspected for prospecting.
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(Edited by YANG Hua)
Foundation item: Project(2007CB416608) supported by the Major State Basic Research Development Program of China; Project(2006BAB01B07) supported by the National Scientific Project for Tackcling Key Problems
Received date: 2007-06-02; Accepted date: 2007-10-18
Corresponding author: SHAO Yong-jun, PhD; Tel:+86-731-8660015; E-mail: shaoyongjun@126.com
