I. Geological evolution and major geological events
1. Pre-Devonian base
According to the relevant data in the surrounding areas, it is speculated that there may be a pre-Devonian metamorphic basement in the Taiwan Province Strait and western Taiwan, because the pre-Devonian aojiao group metamorphic rocks exposed in Pingtan, Jinjiang and Dongshan along the coast of Fujian may extend to the Strait and western Taiwan and become its early metamorphic basement. In addition, metamorphic rocks with isotopic age of 1 860ma were discovered by drilling in Lingfeng1well in Oujiang sag of the East China Sea in the northeast of Taiwan Province Strait, which can also provide evidence for this.
2. Permian basin deposits.
In Permian, Taiwan Province Province was in a basin sedimentary environment, at least in the central mountainous area. This has been confirmed by the stratum of Da 'nan 'ao Group, and its lithologic combination includes gneiss (migmatite), marble, crystalline limestone and various schists (even coal seams). Among them, Permian family and coral fossils have been found in marble, and the Rb-Sr isotopic age is 200 ~ 240 Ma (which should be metamorphic age). The greenschist in schist may also be metamorphic from basic volcanic rocks (in addition, it is generally believed that some mafic-ultramafic rocks are not in-situ rock series, but are formed by mixed accumulation of oceanic crust). The lithostratigraphic assemblage of this Da 'nan 'ao Group has many similarities with the Carboniferous-Permian strata in Fujian mainland. If there are carbonate rocks, coal-bearing clastic rocks and basic volcanic rocks. Although it can't be concluded that the central mountainous area at that time belonged to Yongmei Depression, it was at least a marine product of the same period. In addition, from the analysis of erosion source areas, it seems that there are no other erosion source areas in southeast Fujian except Wuyi uplift and coastal fault uplift zone.
3. The compressional metamorphic uplift of the central mountain area in the early and middle Mesozoic.
The isotopic age of marble in Da 'nan 'ao Group is 200 ~ 240 ma, which is equivalent to Triassic. The Da 'nan 'ao Group lacks reliable Triassic stratigraphic records. It can be inferred that from Late Permian to Middle Jurassic, regional metamorphism occurred in the central mountainous area (even in western Taiwan) under the influence of subduction collision, forming another metamorphic volcanic basement in Taiwan Province Province. Taroko uplift also began to form during this period.
4. Small-scale sedimentation and magmatism occurred in Jurassic-Cretaceous.
The Jurassic sedimentary strata only see continental black shale in deep wells in the southwest of Taiwan Province Basin off Kaohsiung. Cretaceous sedimentary strata, called Yunlin Formation, are composed of clastic rocks, limestone and basalt, mainly distributed in the Taiwan Province Strait. Magmatic activity did not develop during the Yanshan Movement in Taiwan Province Province. In addition to basalt, gabbro and granodiorite intruded in a few areas and were migmatized. In addition, there are only some granite pegmatite, quartz porphyry and porphyrite dikes. During this period, regional metamorphism further occurred, forming Taroko high-pressure transitional metamorphic belt.
5. Plate movement at the end of Cretaceous
Plate tectonics in Taiwan Province area was very active in the late Yanshan period. Affected by the subduction of the Pacific plate, on the one hand, the Yuli metamorphic belt and the high-pressure and low-temperature melange belt were formed to the east of Taroko, and on the other hand, the Da 'nan 'ao melange belt was formed by the insertion of ocean crust fragments into the Da 'nan 'ao Group.
6. Extensive transgression in Eogene.
Under the influence of Himalayan movement, transgression occurred extensively in the Early Tertiary from the ridge west of Taroko Uplift to the Taiwan Province Strait, forming thick marine deposits accompanied by eruption of intermediate-basic volcanic rocks. At the same time, there are small-scale intrusions of intermediate-basic rocks such as diabase and diorite. These are probably related to the back-arc tension produced in the intermittent relaxation stage of plate subduction.
7. Plate Movement in Late Tertiary
At present, it is believed that the plate movement in this period was an arc-continental collision between the Philippine sea plate and the Eurasian continental plate, which led to the formation of flysch-type marine volcanic-sedimentary rocks, as well as Chimei igneous complex and Duluanshan Formation volcanic rocks extending from Luzon arc. In particular, arbitrage gibbsite ophiolite melange is formed at the suture line of the plate; Tertiary sedimentary strata in the Lingxueshan area west of the central mountain range have been metamorphic. Some important faults in Taiwan Province Island, such as Quchi-Chaozhou fault, Zhongyangshan fault and coastal mountain fault, may have developed and formed during this period. Due to the collision between the central mountain fault and the coastal mountain fault, the longitudinal valley of Taidong was formed at this time.
8. Quaternary volcanic arc activity
During the Pleistocene, the Ryukyu (or Luzon) volcanic arc extended to the northern part of Taiwan Province Province, forming volcanic groups in Keelung and Datun. The rock assemblage consists of (basaltic) andesite-dacite volcanic lava and subvolcanic rocks (porphyrite). During this period, intense compressional folding, inversion and overthrust occurred in coastal mountainous areas. Some scholars have drawn the plate action model after Pleistocene, as shown in Figure 4-32. Dynamic evolution process of lithosphere in X-Y section on the right. This area was originally a collision process (3Ma), and with the movement of the Philippine Sea Plate, it became the Ryukyu subduction zone (2Ma). Since 2Ma, the Ryukyu Trench has retreated and the lithosphere of the collision orogenic belt has become thinner. The collision orogenic belt collapsed into a rift, and the rift developed from an intermontane basin into a deep Okinawa trench (Loui S.Teng).
Two. Regional mineralization, metallogenic series and metallogenic types
The regional mineralization in eastern Taiwan Province Province is mainly related to subvolcanic porphyry and regional metamorphism, thus forming a set of metallogenic series and types with relatively new metallogenic age characteristics, as shown in Table 4- 1 1.
Figure 4-32 Late Cenozoic geodynamic evolution process in southern Ryukyu.
(Louis Teng, 1996)
Table 4- 1 1 Brief Table of Mineralization, Metallogenic Series and Metallogenic Types of Tectonic Magmatic Belt in Eastern Taiwan Province Province
The new metallogenic era has advantages and disadvantages for mineralization and prospecting in this area. The advantage is that the deposit can be basically preserved after being eroded for a short time, and the blind ore hidden underground has a greater chance of being discovered because it has not been stripped. The disadvantage is the lack of necessary time conditions for the metallogenic types that need to be accumulated through long-term multi-stage and multiple mineralization.
Three. Description of each metallogenic belt
Up to now, there are not only few endogenetic metal mines found on the island of Taiwan Province Province, but also few with industrial value, which may be related to the lack of research and exploration. Lead-zinc-silver deposits in endogenetic metallic minerals have not been reported, which seems to be a feature of regional mineralization.
According to the distribution of gold and copper deposits in the area and their geological structural units, * * is divided into four metallogenic belts, and there are no metallogenic units in the belts. Only gold mines can be reached, and the rest are still ore occurrences (Figure 4-33).
Figure 4-33 Division Map of Metallogenic Belt of Tectonic Magmatic Belt in Eastern Taiwan Province Province
1. Taroko uplift metallogenic belt (Ⅳ-1)
The uplift of the Taroko uplift metallogenic belt is located on the eastern slope of the central mountain range, and the main body is composed of (Carboniferous)-Permian in the Taroko belt, with ophiolite melange. The west is separated from the tertiary depression belt of Jiliangshan-Xueshan by faults, and the eastern coastal mountains are fractured, and the longitudinal valley of Taidong is connected with the coastal mountains. A set of ophiolite melange composed of schist and oceanic ophiolite suite is developed near the longitudinal valley of Taitung, which is considered to be the product of Cretaceous plate acting on suture line.
There are many copper mines in greenschists in this belt, such as Tongmen in Hualien and Dabaishan in Yilan, among which Tongmen in Hualien is the representative.
The ore body of Tongmen copper mine in Hualien is lenticular, integrated in greenschist and sericite schist, closely related to greenschist and diabase, and the metallogenic type belongs to metamorphic basic volcanic rock stratabound type. The ore composition is copper-bearing pyrite, with a copper grade of 0.5% ~ 2.8%, which is disseminated and galena occurs.
In addition, there is a similar occurrence in Tongshan, Yilan County, where the grade of copper can reach 3% ~ 4%.
2. Jiliangshan-Xueshan sag metallogenic belt (Ⅳ-2)
The metallogenic belt is located in the west of the central mountain range and consists of the tertiary system with slight metamorphism. The eastern mountainous area is deeply metamorphic, and its rocks are mainly slate and phyllite, while the western snowy mountain area is slightly metamorphic, mainly argillaceous slate. This zone is separated from the Late Tertiary-Quaternary subsidence zone in western Taiwan by the Quchi-Chaozhou fault in the west, and is in fault contact with the Taroko fault uplift zone in the east.
There are many gold deposits in the shallow metamorphic rocks in this zone, such as Taibai Mountain in Yilan and Hehuan Mountain in Nantou. Gold-bearing quartz veins occur in argillaceous slate, phyllite and metamorphic sandstone. Some of its occurrences are consistent with surrounding rocks, showing synchronous folds, and some are through bedding. Thick veins can reach 1m, and thin veins are only a few centimeters. The metallic minerals are natural gold, galena and sphalerite, and the grade range is (10 ~1000) ×10-6. Natural gold occurs in the uplift, bifurcation or morphological change of the timely vein. Wall rock alteration includes pyritization, chloritization, kaolinite and calcite. The metallogenic type belongs to metamorphic rock structure-altered rock type.
3. Coastal Mountainous Metallogenic Belt (Ⅳ-3)
The metallogenic belt is located in the coastal zone on the eastern edge of Taiwan Province Island, and the western part is connected with the longitudinal valley of Taitung by coastal mountain faults. This terrane is a tectonic mosaic terrane, which is considered to be the product of the collision between Luzon Island Arc at the front of Philippine Sea Plate and Eurasia Plate in the early Cenozoic. The Miocene Chimei andesite igneous complex (with an exposed area of 22km2) and Miocene Duluanshan Formation andesite marine pyroclastic rocks originally belonged to Luzon volcanic arc, and the Pliocene-Miocene Dagangkou Formation sedimentary clastic rocks (including a small amount of pyroclastic rocks) on it belonged to flysch deposits in the pre-arc basin.
Chimei copper deposit is located in Chimei igneous complex in this area, and Duluanshan copper deposit is located in volcanic-intrusive rock of Duluanshan Formation.
(1) Chimei copper mine
Chimei igneous complex is a medium-basic eruption-intrusion complex composed of basaltic andesite, porphyritic andesite, basalt, diorite and various dikes. Mineralization is related to diorite porphyry. Among them, pyroxene diorite is an early intrusive rock with K-Ar age of (22.5±3.5)Ma, porphyritic pyroxene diorite age of (17.0 2.5) Ma and alteration age of (18.01.8) Ma. Andesite is relatively late, with zircon age of 15.4 ~ 16.4Ma. The age of the mineralized altered diorite measured by He He 1969 is (9.0±0.7)Ma, which is considered as the metallogenic age. Five mineralized outcrops were found, all of which are porphyrite, chalcocite, sky blue, vitrinite, staghorn, copper-bearing limonite and patina. Alteration types include clay mineralization (montmorillonite, illite), green rock formation (epidote, chlorite, pyrite, calcite) and anhydrite formation. Among them, the third mineralized area consisting of outcrops No.3 and No.4 is 650 meters long from north to south and 400 meters wide from east to west. After drilling, if the copper content is higher than 0.3%, there are 7.33 million tons of ore reserves. More than 0.5% is 6.5438+0.86 million tons (gold content is 0.4 ~ 0.5g/t), and the metallogenic type belongs to porphyry type (Figure 4-34).
(2) Duluanshan Copper Mine
The Miocene Duluanshan Formation is pyroclastic rocks such as andesite lump, tuff and tuffaceous sandstone, among which there are limestone lenses. Andesite is a kind of subvolcanic rock, which intrudes in the form of bedrock, bedrock and tongue. The ore body is formed in the contact zone inside and outside andesite, with 6 outcrops, all of which are controlled by structural faults. The ore is composed of pyrite and chalcopyrite, with a small amount of chalcocite and celestite. Because of chalcedony, the locals call it Duluanshan gem. Mineralization is disseminated or massive, and the surrounding rocks are greenstone belt and argillaceous alteration. * * * There are 6 outcrops, among which the largest outcrop No.4 is a breccia tube with a diameter of 10 ~ 15cm, which is sub-circular, composed of clayey andesite, which often runs through andesite veins, and is presumed to be the center of volcanic eruption. The ore contains 0. 1% ~ 6.3% copper, with an average of 0.4%, about 4.6× 10-6 gold and 5×10-6 silver. The metallogenic type is tentatively classified as subvolcanic hydrothermal type, and it is speculated that there may be porphyry copper deposits under the breccia tube.
Figure 4-34 Distribution Map of Point Mineralization Outcrop of Chimei Copper Mine in Hualien, Taiwan
(According to Gao Zhenmin and Li Chaoyang, 2000)
4. Keelung Volcanic Arc Metallogenic Area (Ⅳ-4)
There are two volcanic groups in the north of Taiwan Province Province, Datun Mountain and Keelung, which are composed of Pleistocene basic-neutral volcanoes and subvolcanic rocks. The former covers an area of 290km2, with an isotopic age of 0.37 0.04 (2.56 0.5) Ma, while the latter covers an area of1.1~1.7ma. In the past, it was generally believed that these volcanic groups were the westward extension of Ryukyu volcanic arc, but recently, according to the comparison of Sr and nd isotopes, they were different. Therefore, it is considered that the Keelung Volcano Group is not the Ryukyu Volcano Arc, but the product of the extension environment after the Luzon Island Arc collided with the Eurasian continental margin 2Ma ago. Among them, Benshan rock body intruded along the axis ridge of sedimentary anticline at 1.7 ~ 1.4 Ma. The rest rocks are formed between 1.0 ~ 0.9 Ma.
Jinguashi gold-copper deposit is located in Keelung volcanic group, and five sets of large dacite subvolcanic rocks intrude into Miocene Ruifang Group and Miocene-Oligocene yehliu Group coal-bearing sand shale strata. Submerged volcanic rocks are produced in bedrock, sheet, wall and rock flow, and explosive breccia tubes are formed in some areas. In the metallogenic area, two large-scale deposits, Jinguashi and Ruifang, and dozens of surrounding ore bodies constitute the Jinguashi ore field. The ore field can be divided into four ore sections: Benshan, Changren, Jiufen and Wudankeng, as shown in Figure 4-35. The following focuses on two mineral deposits: Jinguashi and Ruifang.
Figure 4-35 Distribution Map of Mineral Deposits and Breccia Tubes in Jinguashi Area
(According to Fang Jianneng, 1984)
Jinguashi deposit is the largest deposit in Benshan ore block. Because the outcrop of the ore body protrudes from the ground in a silicified hat shape, it is commonly known as large golden melon. Orebodies occur in steep veins or steep pipes in An Ying rock mass in Benshan, and extend to Miocene of small rock mass (forming Shumeikeng ore body group). The isotopic age of the rock mass is 1.4 ~ 1. 1 Ma. The main ore body of Jinguashi is vein-shaped, extending over 1200m and over 650m, and the surface branch veins often flow down into the main veins. The main metal minerals are chalcopyrite, chalcopyrite, Luzon copper ore (isomorphous with chalcopyrite, its composition is: Cu content is 48.3%, As 19%, S32.6%) and a small amount of jamesonite (jamesonite). Gangue minerals include quartz, alunite, chlorite and kaolinite, followed by calcite, barite and kaolinite. The surrounding rocks are altered into silicification, pyritization, clayey, alum petrochemical, chloritization, etc. Ore has many structural structures, such as vein, veinlet network, disseminated and breccia. Gold occurs in pyrite (limonite), chalcopyrite and alunite, and it is common in large gold nuggets. Gold nuggets weighing more than 60 taels have been collected. According to the isotopic age of alunite is (1.0 0.1) Ma, it is known that the mineralization time is slightly later than that of Ying 'an pluton. The mineralization temperature of minerals is concentrated in the range of 200 ~ 250℃. The mineralization depth measured by fluid inclusion method is about 2400 ~ 3500m, and inferred by geothermal gradient is about1000m. In addition, tubular ore bodies are distributed in the eastern edge of dacite. From south to north, a rock tube belt with a length of about 3km is composed of a plurality of explosive breccia tubes, most of which are produced in the surrounding rock of sand shale, and a few are produced in dacite (rough rock mountain). Basically, it is all-tube mineralization, most of which are concealed minerals, and there are only signs of silicification and clayey alteration on the surface. The exposed rock pipe has a depth of 200 ~ 700 m and a diameter ranging from several meters to 20 meters. See Figure 4-36 for the general pattern of the breccia tube drawn by Fang Jianneng and Yu Bingsheng. The deposit has obvious vertical mineralization zoning, with silicified caprock and oxidation zone on the surface, gold-bearing limonite and clay minerals, as well as barite and quartz. The grade of gold is positively correlated with barite; The primary ore is about 200 meters underground, of which 200 ~ 450 meters underground is a low-grade gold belt and 450 ~ 600 meters underground is a gold-rich belt. Below 600m, it evolved into a copper-bearing gold belt dominated by copper. Fang Jianneng and others drew a vertical zoning map according to the mineral types of iron sulfide and the changes of ore-forming elements contained in it (Figure 4-37). Alteration is also vertically zonal, with silicification, pyritization, alum fossilization and clay mineralization from top to bottom.
Ruifang deposit Ruifang deposit is the main deposit in Jiufen ore section, also known as small melon because of the outcrop of silicified cap on the surface. Ore bodies occur in veins on two concealed dacite bodies, Jiufen and Xinshan, and some of them also occur in the surrounding rocks of the external contact zone. Xiaojingua ore body has 200 or 300 veins, of which the main four are vein A, vein B, vein C and vein Yong. Ore minerals include pyrite, white iron ore, galena, sphalerite and trace stibnite. Gangue minerals are mainly calcite, as well as kaolinite, diaspore and Yingshi. Weathering outcrop of vein becomes gold-bearing clay vein, which contains coarse natural gold, mostly silver. Wall rock alteration includes bluestone, clay, carbonation, pyritization and silicification.
See Table 4- 12 for the brief information of Changren ore section and wudan pit ore section of other deposits.
Table 4- 12 Brief introduction of Changren ore section and Wudankeng ore section in Jinguashi ore field
Figure 4-36 Overall Model of Jinguashi Breccia Tube
(According to Fang Jianneng, Yu Shousheng 1984)
The gold in the Jinguashi ore field mainly exists in the form of natural gold and silver-gold ore, and its forms are granular, scaly, needle-like and hairy, and octahedral crystal can also be seen. The particle size is generally less than 0.3mm, and some submicroscopic gold occurs in pyrite (Benshan). The fineness of gold minerals in different ore blocks varies greatly, with Jiufen being 665,438+00 ~ 750, Wudanshan being 720 ~ 865,438+00 and Benshan being 690 ~ 930. This change also occurs at different heights of the same deposit. For example, the gold purity at the middle section of Jiufen ore block is 6 10, while the gold purity at the middle section of 33 1m rises to 7 10. This shows that the fineness of gold increases with the increase of metallogenic temperature. Gold minerals contain a variety of trace elements, and their contents also change regularly. Taking Jiufen ore block as an example, Al, Te, Mn and Pb in natural gold increase from bottom to top, while V decreases, while Mg, Si and Ti change. Te and V in the above elements exist in the form of inclusions in gold minerals, while Fe, Mn and Pb exist in the crystal lattice of gold. This may reflect that Al, Fe, Mn and Pb tend to be enriched in low-temperature gold, while V is enriched in high-temperature gold. In addition, gold in different areas contains different trace elements. For example, four gold samples in Jiufen contain high Bi and Pb, while the gold samples in Dakukeng, Wudanshan and Benshan contain high Cu, Hg and Ni. These trace elements all exist in the crystal lattice of natural gold, and the difference of their compositions should be related to the different geochemical conditions at the time of formation, which reflects that the mineralization of Jinguashi ore field is not one-off, but multi-stage and multi-stage at different temperatures and pressures.
Figure 4-37 Vertical Zoning of Jinguashan Gold (Copper) Deposit
According to various methods, the overall metallogenic temperature is 160 ~ 400℃, the highest metallogenic temperature in Benshan ore block is 160 ~ 300℃, the concentrated value is 200 ~ 250℃, and the highest value after pressure correction is 350℃. The metallogenic temperature of Jiufen and Wudanshan ore blocks is about 200℃. As a result, a horizontal zoning centered on Benshan ore block was formed, which evolved eastward into Changren copper-bearing gold belt and westward into Wudanshan and Jiufen gold mines. At the same time, there are vertical zoning with gold-rich upper part and copper-rich lower part. Some scholars have also studied the properties of ore-forming fluids. Juan et al. (1959) thought that the first hydrothermal activity occurred before mineralization, initially from weak alkaline to neutral, and then from neutral to weak acidic. The second hydrothermal activity was in the metallogenic period, and the hydrothermal properties were alkaline first and then acidic. According to the isotopic composition of sulfur, Folinsbee et al. (1972) think that hot water and sulfur come from intrusive rocks, while metal elements come from altered andesite. Yen( 1976) points out that most vein ore bodies contain only 3 ‰ ~ 14 ‰ NaCl, which may mean that water is mixed in. Some scholars have put forward some views on mineralization after mineralogical research. Tan Liping (1992) thinks that the metasomatism and filling of hydrothermal solution are repeated many times according to the phenomenon that high-temperature chalcopyrite penetrates into low-temperature chalcopyrite in veins. Hwang & Meyer (1982) discovered tellurium gold deposit in Jiukeng, Benshan, and thought that there was a magma chamber in the lower part of Jinguashi deposit, which was the main power to provide hot water circulation, and its depth might be below 6km.
According to the fact that the mineralization of Jinguashi deposit is related to dacite subvolcanic rocks, there are explosive breccia tubes in the deposit, and the deposit was formed in a hypabyssal environment, containing sulfate minerals such as alunite and barite, and low-temperature minerals such as Cinnabar and stibnite. It can be determined that its metallogenic type belongs to hypabyssal hydrothermal sulfate type (high sulfur type) deposit related to intermediate-acid subvolcanization. This deposit is essentially the same as the Zijinshan deposit in Shanghang, Fujian. Wang Shaoxiong (1995) explained that the degree of erosion and exposure of the deposit were different. The Zijinshan deposit was formed in the Early Cretaceous 1 100 million years ago. The denudation has been very deep, and it is estimated that at least 300m has been stripped, so the upper part of the deposit has not been preserved. The Jinguashi deposit was formed in the Pleistocene, but it is only about one million years ago, and the erosion is shallow, so the deposit is well preserved. Therefore, the present Jinguashi deposit can be regarded as the denuded upper part of Zijinshan deposit, while the present Zijinshan deposit can be regarded as the lower part or even deeper part of Jinguashi deposit. The complementary results of the two can establish a more complete metallogenic system and model of subvolcanic epithermal deposits.
The Jinguashi ore field, especially the large and small Jinguashi deposits, is the only large-super gold deposit in Taiwan Province Province so far. According to official statistics, 83t gold has been mined, 1898 ~ 196 1 year, and 8.75x copper104t. If it reaches 1987, the cumulative gold production will be 100t, copper 10× 104t and silver 170t. However, according to the estimation of Tan Liping of Taiwan Province Provincial University and Lin Chaobang of Jiufen Sun Company, the actual gold production is far more than that. There should be 300 tons of gold in the Jinguashi ore section, 200 tons in the Jiufen ore section and 600 tons in the Wudanshan ore section 100 ... The untapped and undiscovered resources have great potential.