Calculating/approximating complete Mantel statistic from bootstrap estimates

I have two (sparse) large matrices (~ 1million by 1million) and want to compute Mantel statistic to find correlation between them. To counter memory problems, I have computed the mantel statistic between their submatrices by random sampling. I now have a set of Mantel statistic and P-value scores for these pairs. How can I compute the final Mantel statistic between the two original matrices given these bootstrap estimates? Can I compute their mean? This be a trivial question, but I am not a statistician and any help in this regard is much appreciated. Thanks.