广东紫金铁嶂锡矿床地质特征及矿床成因探讨

铁嶂锡矿床赋矿围岩为下侏罗统金鸡组下部的砂页岩,其锡含量较高,为矿源层。矿体分布于北东向和北西向断裂构造中,可分为大脉型与细脉带型矿体。锡石有两类:一类为沉积的胶状锡石和由胶体变晶生成的微晶锡石,分布于富动植物化石和碳质的千枚状页岩和富锡的砂岩结核中;第二类为粒状锡石,与石英、硫化物一起呈脉状产出。矿床系沉积变质热液改造成因。

A Study On Geology And Genesis Of The Tiezhang Tin Deposit, Zijin County, Guangdong Province

The Tiezhang tin deposit occurs in a suite of sandstone and shale horizontally equivalent to lower part of Lower Jurassic Jinji Formatiou divisible into seven layers, of which Jij5 and Jij7 are greyish black phyllitic shale conta:ining relatively abundant fossils and higher tin partly in the form of visible colloidal cassiterite, and Jij4 is greyish feldspar-quartz sandstoiae with plenty of sandstone nodules enclosed that contain Sn, Pb, Zn, Sn and Ag. No granitic rocks are observed in the ore district. Nevertheless, 1.5-2 km south and north of the ore district there occur respectively fine-grained pyroxene-hornblende-biotite monzonitic granite and fine-grained porphyritic pyroxene-hornblende-biotite monzonitic granite. These two sorts of granite are considered to be probably linked up with each other at depth; they contain some 0.01% Sn and have formed greisen type Sn deposits at part of the endocontact zone. Albitite veins are frequently seen, and tend to become increasingly developed towards the depth. The Tiezhang tin deposit consists of quite a few tin orebodies which might be grouped into large vein type ones and veinlet type ones according to form or size of the orebodies or into NE-trending ones and NW-trending ones coincident spatially with the corresponding faulted zones. The orebodies have basically consistent mineral constituents, ore textures and structures as well as wall rock alterations, usually with more quartz and sulfides in the upper part and albite and chlorite in the lower. Cassiterites might be divided into two types. The first type includes sedimentary colloidal cassiterite and cryptocrystalline-microcrystalliue cassiterite formed from metacrystal of colloidal cassiteiite, with the former assuming drop-like, cotton fibre-like and, occasionally, oolitic forms distributed in phyllitic shale containing abundant fossils and carbonaceous substance, and the latter in the crypotocryst-alline-microcrystalline forms present in Sn-, Pb-, Zn-, Cu-bearing sandstone nodules, locally so concentrated as to become the cement of the sandstone. The second type is granular cassiterite filled along structural fissures together with quartz-sulfides. the early-formed cassiterite has dark color, coarse grains, highly enhedral crystals in the form of thick brachyprism, well-developed twin crystals and zonal structure, and is crushed ubiquitously with quartz and sulfides filled along the fissures; the late-formed cassiterite is light-colored, fine-grained and xenomorphic, generally having no zonal structrue and twin crystals. Wall rock alterations are mainly sericitization and pyritization and, to a less extent, chloritization, carbonatization, silicification and albitization, with localized tourmalinization. The sandstone and shale of Jinji Formation usually contain 0.001-0.1% Sn, partly 0.2-0.46%, and occasionally ever as high as l*6%} 46 samples Om phyllitic shale give an average Sn of 0.063%; 74 samples from sandstone and siltstone show an average Sn of 0.047%. It is therefore conceivable that this suite of stanniferous formation might have served as an ore source bed. The ore deposit lies or] the saddle of the monzonitic granite formed by granitization. During the granitization, the stanniferous ore-source bed might have been partly melted, and such ore-forming substance as tin was likely to enter the magma or even the lattice of rock-forming minerals. When the solidified granite was subjected to leaching by steams, tin was extracted again and traveled into ore-forming solutions. At the same time, the extremely hot steams and alkaline metasomatism enabled tin in the sandstone and shale to become active, thus further concertrating tin. Under the influence of a series of geological actions, the ore-bearing fluids migrated upward, and eventually precipitated cassiterite to form tin deposit where physical and chemical conditions were favorable. This deposit is therefore considered to be of sedimentary-metamorphic a and hyhrothermally-transformed type.