Lucara Diamond Corporation's Karowe Mine was commissioned in mid-2012 and it has since turned out to be a very profitable operation with a nominal internal rate of return (IRR) of 58%. We extracted information from Karowe Mine's annual financial statements and constructed a financial model to evaluate the average effective tax rate (AETR) that was realized up until December 2019. We then assessed whether the variable rate income tax formula, which was intended for less profitable base metal mines, was effective in taxing the mineral resource rent from Karowe Mine. Finally, we ran scenarios for government participation in order to test what the effective rate would have been had the Government of Botswana acquired up to 50% paid equity. We find that the AETR for Karowe Mine is 71.9%, which exceeds the recommended range of 40% to 60% for non-diamond mines but falls slightly short for the case of large scale diamond mines as a 50% paid equity results in an AETR to 79.2%. We conclude that the current variable rate income tax formula fails to capture the mineral rent in profitable diamond mining projects based on the robust project economics and therefore should be used only for the purpose it was originally intended.

Lucara Diamond Corporation的Karowe矿于2012年中期投产，此后已证明是一项非常有利可图的业务，名义内部收益率（IRR）为58％。我们从Karowe Mine的年度财务报表中提取了信息，并构建了一个财务模型来评估直至2019年12月实现的平均有效税率（AETR）。然后，我们评估了可变利率所得税公式是否旨在降低利润基础金属矿山有效地征税了Karowe矿的矿产资源租金。最后，我们运行了政府参与的方案，以测试如果博茨瓦纳政府获得最多50％的有偿股权将产生的有效率。我们发现Karowe矿的AETR为71.9％，对于非钻石矿，该值超出了建议的40％至60％的范围，但对于大型钻石矿，则略有不足，因为50％的有偿权益导致AETR为79.2％。我们得出的结论是，基于稳健的项目经济学，当前的可变税率所得税公式无法捕获获利的钻石开采项目中的矿产租金，因此应仅将其用于最初的目的。