四川大铜厂表生硫化物铜矿床成因探讨

The Ore Genesis of the Datongchang Ag-rich Copper Deposit, Sichuan

四川大铜厂铜矿床产于中生代晚白垩世红层砾岩中。铜的次生硫化物占全铜含量的93.40%。铅同位素Pb/204Pb为18.054～18.815,207Pb/204Pb为15.528～15.644,208Pb/204Pb为38.030～38.722,模式年龄出现负组成:206值。δ34S平均值黄铁矿>辉铜矿>斑铜矿,硫酸盐的δ34S值为较大正值,硫化物矿物大多为负值:有较大平衡分馏系数,说明成矿是在开放系统、低温低压、非平衡的表生条件下进行的。矿物流体包裹体细小(多数小于3μm)。成矿金属来源于小坝组红色砂岩层,硫来源于小坝组顶部膏盐层,为一表生硫化物矿床。

Ore mineral compositions in Datongchang Ag-rich copper deposit are predominant with copper and iron sulfide minerals, with little amount copper and iron oxide minerals. It's showed that minerals were formed in an oxidation-reduction transitional belt. It has been found out by phase analytical results of ore copper, that secondary sulfide copper accunt for 93.40% of total copper. So the ore deposit belong to, obviously, a secondary enrichment sulfide deposit.Studies on geochemistry of trace elements, REE, lead and sulfur isotopes, indicated that ore-forming elements were derived from red sandstone bed of Xiaoba Formation,and that sulfur were derived from gypsum and salt bed of the top of Xiaoba Formation.The range of lead isotopic compositions are as follows: ^(206)Pb/^(204)Pb 18.054～18.815,^( 207)Pb/^(204)Pb 15.528～(15.644,) ^(208)Pb/^(204)Pb 38.030～38.722.A negative value was obtained from the lead model age.All of those showed that ore leads are very young. And it is clearly distinguished from extremely enrich anomalous leads of copper deposits in older beds. The mean δ^(34)S values of sulfide minerals are as follows: chalcocites(19), -3.13‰, bornites(2)(-4.07‰,) pyrites(4), -0.98‰, with an ascending order as:δ^(34)S_(Py)>δ^(34)S_(Cc)>δ^(34)S_(Bn)which turned out to be contrary to the order of ^(34)S enrichment among some sulfide minerals. It is showed that large equilibrium fractionation factors occurred between sulfide minerals and gypsum-salt. Above mentioned facts showed that ore deposit were formed under an open, low-temperature, low-pressure and nonequilibrium system. Fluid inclusions with very small (mostly<3 μ) sizes were fairly strong evidence, too.